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Acidophil. A cell that stains with an acid dye. Example: the . acidophils of the anterior pituitary Acidophilic. A substance within a cell or tissue that stains with an . acid dye (such as eosin) Acinus (plural: acini). A grape-shaped secretory unit, composed of acinar cells, found in a variety of secretory glands such as the salivary glands and pancreas. Acrosome. A massive lysosome located on the head of a . spermatozoon that facilitates penetration of the egg Actin. A filamentous protein, composed of globular subunits, that composes the "thin filaments" of muscle and the microfilaments of . motile, nonmuscle cells

Acidophil. A cell that stains with an acid dye. Example: the . acidophils of the anterior pituitary Acidophilic. A substance within a cell or tissue that stains . with an acid dye (such as eosin) Acinus (plural: acini). A grape-shaped secretory unit, composed of acinar cells, found in a variety of secretory glands such as the salivary glands and pancreas. Acrosome. A massive lysosome located on the head of a . spermatozoon that facilitates penetration of the egg Actin. A filamentous protein, composed of globular subunits, that composes the "thin filaments" of muscle and .the microfilaments of motile, nonmuscle cells

Action Potential. An electrical signal that consists of an ionic current, or "wave of depolarization," that passes across the plasma membrane of certain nerve and muscle cells. Action potentials do not lose strength over distance. Active Transport. The active "pumping" of small dissolved substances, such as certain ions, sugars, and amino acids, across . a biologic membrane against a concentration gradient Adenohypophysis (synonym: anterior lobe). That portion of the pituitary gland that contains the pars distalis, the pars tuberalis, . and the pars intermedia .Adipocyte. A fat cell Adrenal Cortex. The outer portion of the adrenal gland; it

surrounds the adrenal medulla and consists of three regions, the zona glomerulosa, zona fasciculata, and zona reticularis, which secrete steroid hormones. Adrenal Medulla. The inner portion of the adrenal gland, surrounded by the cortex, that . secretes epinephrine and norepinephrine . Adrenaline. See Epinephrine Adrenocorticotropic Hormone (ACTH). A hormone released . by the pituitary gland that stimulates the adrenal cortex Adventitia. The outer covering, composed of loose connective tissue, that surrounds a variety of organs such as blood vessels . and intestines Afferent. An adjective describing a structure, be it a blood

vessel or a nerve, that carries materials or information in a .central direction (inward) Agranulocyte. A broad category of white blood cell that lacks cytoplasmic granules visible by light microscopy. Includes . lymphocytes and monocytes Aldosterone. A steroid hormone, secreted by the adrenal cortex, that stimulates cells of the proximal convoluted tubule of .the nephron to pump sodium ions back into the bloodstream Alveolus (plural: alveoli). A thin-walled, air-filled sac within the lung that permits gas exchange across its wall between air and . circulating blood Amacrine Cell. A type of interneuron within the retina that

connects ganglion cells; one of the "horizontal" components of . the retina that promotes intercellular communication Ameloblast. An epithelial cell, located within a developing .tooth, that secretes enamel Amino Acid. An organic acid that is a building block of . protein Ampulla. A saccular dilation of a canal or duct. Example: the ampulla of the oviduct, which lies between the infundibulum . and the isthmus Amylase. A starch-digesting enzyme that is a component of . saliva . Androgen. Testosterone; a male sex steroid hormone

Androstenedione. A precursor of the female steroid .hormone, estrogen Antibody. A class of protein, secreted by the plasma cell, that binds antigen. Antibodies are immunoglobulins endowed with specific amino acid sequences that bind specifically with the . antigens that induced their biosynthesis Antigen. A substance, usually foreign, that is recognized by the organism's immune system. Antigens tend to stimulate formation . of antibodies that bind to them Antrum. The space in the ovarian follicle filled by follicular fluid . (liquor folliculi) Apocrine Sweat Gland. A class of large sweat gland, usually

located in the groin and armpits, that produces odoriferous secretions. Unlike eccrine sweat glands, they are not concerned with temperature regulation Arrector Pili. A smooth muscle that raises a hair. Example: when a large number of these muscles are stimulated, one ." gets "goose bumps m in diameter, that m in diameter, that Arteriole. A tiny artery, less than 10 . leads to a capillary bed ATP. Adenosine triphosphate, a molecule that serves as an . energy source for many of the cell's biochemical processes Atrium (plural: atria). An auricle; a chamber of the heart that . receives venous blood

Auerbach's Plexus. A group of autonomic neurons, located between the muscle layers of the gut, that permit peris Autonomic Nervous System. That portion of the nervous system concerned with the involuntary activities of cardiac muscle, smooth muscle, and glands; often subdivided into the sympathetic . and parasympathetic nervous systems Axon. The cytoplasmic extension of a neuron by which impulses . travel (usually) away from the cell body Axoneme. The cytoskeletal part of a cilium or flagellum, responsible for motility, that is characterized by a 9 + 2" . pattern of microtubular organization Axoplasmic Transport. The movement of materials through an

.axon B-Lymphocyte. A class of lymphocyte, thought to be derived from the bone marrow, that can differentiate into an antibody. secreting plasma cell Basal Body. An organelle, identical in appearance to a centriole, that contains nine triplets of microtubules and is found at the base . of a cilium or flagellum Basal Cells. Small stem cells, located at the base of a variety of stratified and pseudostratified epithelia, that undergo mitosis and provide new cells to replace those periodically shed from the . epithelium Basal Lamina. A filamentous, 800-A-thin subdivision of the . basement membrane

Basal Striations. Thin, acidophilic, vertical striations, located at the basal pole of many epithelial and glandular cells, that represent long, thin mitochondria positioned to provide ATP for .active transport Basalis. That part of the endometrium of the uterus that is not shed at menstruation, but remains to participate in the regeneration of the . functionalis (which is shed at menstruation) Basement Membrane. A thin layer that underlies epithelia and separates muscle and nerve fibers from surrounding connective tissue. vBasilar Membrane. A sheet of connective tissue, found within the cochlea of the ear, that supports the mechanoreceptive "hair cells." vBasket Cell. A myoepithelial cell whose contractions

express (squeeze) the contents out of secretory cells. Basophil. A substance within a cell or tissue that stains with basic dyes such as . hematoxylin Bile. A fluid, secreted by the liver and released into the duodenum, . that serves to emulsify fats Bile Canaliculus (plural: canaliculi). A thin channel between the plasma membranes of adjacent liver cells through which bile . flows en route to a bile duct .Blastocyst. A very early mammalian embryo Blastula. An early stage of the embryo formed by the rearrangement of the cells of the morula; a fluidfilled sphere . surrounded by a single layer of cells

Bowman's Capsule. A simple squamous epithelium, intimately associated with the glomerulus of the nephron, that consists of . outer (parietal) and inner (visceral) layers Brunner's Glands. Mucoserous glands located within the submucosa of the duodenum . Brush Border. See Striated border Calcitonin. A hormone secreted by parafollicular cells of the thyroid gland that inhibits bone resorption by . osteoclasts and lowers blood calcium levels Callus. A hard, bonelike material that covers the ends of . a fractured bone during the healing process Canaliculi. Small channels, or canals, whose margins

. are frequently defined by cell membranes Capacitation. The maturation of spermatozoa, thought to occur within the epididymis, during which sperm . Cardiac Muscle Cell. See Cardiac muscle fiber Cardiac Muscle Fiber. The contractile unit of cardiac muscle; a single branched cardiac muscle cell, usually having one or two . nuclei Catecholamine. A class of neurosecretory substances including . epinephrine and norepinephrine Caveolae. Small, membrane-limited invaginations of the cell . surface that are especially conspicuous in smooth muscle cells Cell Membrane (synonyms: plasma membrane,

thin biologic membrane that -- plasmalemma). An 80 . surrounds the cell Cementum. A hard, bonelike substance that covers the root of .the tooth . Central Nervous System. The brain and spinal cord Central Vein. A blood vessel, located in the center of a liver lobule, that collects blood from the venous sinuses that pass . between hepatocytes Centriole. An organelle, characterized by nine triplets of microtubules, that sits in the cell center (cytocentrum) of nondividing cells and is found at the poles of the mitotic spindle . of dividing animal cells

Centroacinar Cell. A clear cell, found in the center of secretory acini of the pancreas, that forms the wall of the duct of the . acinus Chief Cell. A generic term used to describe the major (chief) cell type found within an organ. Example: the enzyme-secreting .(zymogenic) cells of the stomach are called chief cells Cholecystokinin. A hormone, elaborated by neurosecretory cells of the intestinal mucosa, that has many effects - including . contraction of the gallbladder following ingestion of fats . Cholesterol. The most abundant steroid in animal tissue Chondroblast. A young cartilage cell active in the secretion of . extracellular matrix

Chondrocyte. A mature cartilage cell necessary for the . maintenance of matrix Chondromucoprotein. A generic term referring to the major .macromolecular components of cartilage matrix Choriocapillaris. A system of small blood vessels in the choroid . of the eye Choroid. A highly vascular layer of loose connective tissue . situated between the sclera and retina in the wall of the eyeball Chromaffin Cells. Neurosecretory cells of the adrenal medulla that contain small granules rich in epinephrine and/or . norepinephrine Chromatin. The genetic material (DNA) of the nucleus and

. associated proteins Chromophobes. Cells of the anterior pituitary that appear clear when stained with H&E. Although their function is unknown, they .may represent spent (degranulated) acidophils or basophils Chromosome. A structural unit within the nucleus into which the genetic material and associated proteins are packaged chromosomes, of which 46 exist in man, stain heavily with basic . dyes Chyme. The mass of partially digested food that passes from the . stomach into the duodenum Cilium. A long, thin organelle, usually motile, that extends from the free surface of many cells. The cilium, which arises from a

basal body, consists of a supporting axoneme surrounded by an . extension of the plasma membrane Circumvallate Papilla. A large, circular structure, located near . the base of the tongue, that contains many taste buds Cisternae. Flattened, membrane-limited sacs found in the rough .endoplasmic reticulum and the Golgi apparatus Clara Cell. A nonciliated, club-shaped cell, probably secretory in function, found in the epithelium of the terminal . bronchiole Collagen. A fibrous protein, unusually rich in the amino acid hydroxyproline, that is an essential constituent of connective . tissue

Collagenous Fibers. Connective tissue fibers, visible with the light microscope, that consist of aggregations of collagen . fibrils Collecting Duct. A large duct in the nephron that collects . urine for eventual excretion Colloid. A viscous, gel-like substance such as the material .within the follicles of the thyroid gland Columnar Epithelium. An epithelium characterized by surface cells taller than they are wide. Example: the epithelium of the . small intestine Compact Bone. The dense bone that forms the walls of hollow . long bones and the outer and inner tables of flat skull bones

Complement. A series of proteins in blood plasma, associated with the immune response, that when activated attracts . neutrophils that become phagocytic Cone. A class of retinal photoreceptor that participates in color . vision . Cords of Billroth. See Splenic cords .Corium. See Dermis Corona Radiata. A "radiating crown" of cells and extracellular . material that adhere to the perimeter of the ovulated egg Corpus Luteum. A large, yellowish, progesterone-secreting body, found in the ovary, formed from an ovarian follicle . following rupture and release of the egg

. Cortical Sinus. See Subcapsular sinus Cortisol (synonym: hydrocortisone). A steroid hormone; one of the major glucocorticoids secreted by the zona fasciculata of . the adrenal cortex Cortisone. A glucocorticoid (a class of steroid), secreted by the adrenal cortex, often used as an anti-inflammato Cristae. Little baffles formed from infoldings of the inner . mitochondrial membrane . Crown. The exposed part of a tooth above the gumline hn (synonym: intestinal glands). Crypts of Lieberk Simple tubular glands, extending below the lumenal surface of the intestine, lined by epithelium coexten

Cuboidal Epithelium. An epithelium characterized by surface cells approximately equal in height and width. Example: the . epithelium of the thyroid gland Cumulus Oophorus. A group or "cloud" of granulosa cells that surround the oocyte in a growing (tertiary) folli Cytokinesis. Part of the process of cell division in which the cytoplasm of one cell is subdivided to give rise to two daughter cells. Usually accompanies mitosis (nuclear . division) Cytoplasm. The contents of the cell exclusive of the . nucleus Cytoskeleton. A generic term that describes a wide variety

of fibrous and tubular elements, such as microfilaments and microtubules, that support the structure and maintain the .shape of the cell Dark Cells. A class of dark-staining cell within the taste bud to . which no definite functions have yet been ascribed Dendrite. One of many slender cytoplasmic extensions of a . neuron that carry electrical excitation toward the cell body Dense Irregular Connective Tissue. Connective tissue . consisting of cells and densely packed, nonparallel fibers Dense Regular Connective Tissue. Connective tissue . consisting of cells and densely packed, parallel fibers Dentin. A hard, bony substance that constitutes the bulk of the

tooth; dentin is covered by enamel in the crown and by . cementurn in the root Dermis (synonym: corium). The connective tissue that .underlies the epidermis of the skin Desmosome (synonym: macula adherens). An intercellular junction that provides a "spot weld" between the plasma . membranes of neighboring cells Diastole. That phase of the heartbeat during which the left ventricle (which pumps blood to the body but not the lungs) . relaxes Distal Convoluted Tubule. That portion of the nephron that . connects the loop of Henle with the collecting duct

. DNA. Deoxyribonucleic acid; the genetic material .Ductus Deferens. See Vas deferens . Dust Cell. See Pulmonary alveolar macrophage Dynein Arm. A small projection from the microtubular doublets of the ciliary axoneme that provides the motive force for ciliary . motility Eccrine Sweat Gland. A class of sweat gland associated with . temperature regulation Efferent. An adjective describing a structure, be it a blood vessel, nerve, or duct, that carries materials or information in a peripheral . direction (outward)

Elastic Cartilage. A resilient cartilage, similar in histologic structure to hyaline cartilage; its matrix is rich in elastic fibers. Example: cartilage found in the external ear. Elastic Fiber. A class of connective tissue fiber, made largely of the protein elastin, that can recoil elastically after being stretched. Example: elastic . fibers are abundant in the arterial walls .Elastin. The protein component of elastic fibers Enamel. The hardest substance in the human body, found . covering the crown of the tooth Endocardium. The lining of the chambers of the heart; the innermost tunica of the heart that contains endothelium and some . loose connective tissue

Endometrial Glands. Secretory glands within the lining of the . uterus, or endometrium Endometrium. The inner lining of the uterus into which the trophoblast implants; failing implantation, the endometrium is shed . at menstruation Endomysium. The thin connective tissue sheath surrounding a .muscle fiber Endoneurium. A delicate sheath of connective tissue that . surrounds a single axon or other extension of a single neuron Endoplasmic Reticulum. A system of intracellular membranes that comes in two varieties: rough ER (with ribosomes) and . smooth ER (without ribosomes)

Endothelium. A layer of flat cells that line the inner surfaces of . blood and lymph vessels and the heart Enterochromaffin Cells. Neurosecretory cells in the gastric and intestinal glands that frequently release serotonin, a substance . that promotes smooth muscle contraction .Entropy. The tendency of everything to fall into disorder Enzyme. A protein molecule that serves as a catalyst in a . biochemical reaction Eosinophil. A class of white blood cell, thought to be active in the phagocytosis of antigen-antibody complexes, that contains conspicuous cytoplasmic granules (specialized lysosomes called . specific granules) that stain red with eosin

. Epicardium. The outermost region of the wall of the heart Epidermis. The keratinized stratified squamous epithelium that . covers the skin Epididymis (synonym: ductus epididymidis). A thin coiled tube connected to the posterior surface of the testis that carries sperm, made in the seminiferous tubules, to the vas deferens. The epididymis contributes to the maturity and acquisition of motility .(capacitation) of spermatozoa Epinephrine (synonym: adrenaline). The major hormone released by the adrenal medulla. A catecholamine, epinephrine is also a neurotransmitter in the sympathetic nervous

. system Epineurium. A sheath of connective tissue that surrounds a whole nerve, such as the sciatic . nerve . Erythrocyte. A red blood cell Estradiol. The most potent of all naturally . occurring estrogens Estrogen. A class of female steroid sex hormone . Epicardium. The outermost region of the wall of the heart Epidermis. The keratinized stratified squamous epithelium . that covers the skin

Epididymis (synonym: ductus epididymidis). A thin coiled tube connected to the posterior surface of the testis that carries sperm, made in the seminiferous tubules, to the vas deferens. The epididymis contributes to the maturity and . acquisition of motility (capacitation) of spermatozoa Epinephrine (synonym: adrenaline). The major hormone released by the adrenal medulla. A catecholamine, epinephrine is also a neurotransmitter in the sympathetic . nervous system Epineurium. A sheath of connective tissue that surrounds a .whole nerve, such as the sciatic nerve . Erythrocyte. A red blood cell

Estradiol. The most potent of all naturally occurring . estrogens . Estrogen. A class of female steroid sex hormone Euchromatin. Fine, light-staining strands of dispersed . chromatin present in the interphase nucleus Excitable Membrane. A kind of plasma membrane, found on nerve and muscle cells, capable of conducting an action potential. Most axons are covered by excitable . membrane; most dendrites are not Exocrine Gland. A gland that releases its secretion into a .duct Exocytosis. An active transport process by which secretory

. granules are released from a cell Extracellular Matrix. That material, found outside of (between) cells and produced by cells, that gives tissues . Fallopian Tube. See Uterine tube Fascicle. A generic term meaning bundle, as in a bundle (fascicle) of muscle or nerve fibers surrounded by a common . connective tissue sheath Fenestrated Capillary. A capillary with small holes in its wall that permit the passage of certain macromolecules, but not . cells, across the capillary's wall Fiber. A long, thin structure, made either of cells (as in nerve . fiber, muscle fiber) or by cells (as in collagen fiber)

Fibril. A thin, filamentous structure, often made of protein, that is a component of a fiber. Example: a collagen fiber, visible by light microscopy, is made up of tiny fibrils visible only by .electron microscopy . Fibroblast. A connective tissue cell that makes collagen Fibrocartilage. A cartilage rich in collagenous fibers that has relatively few chondrocytes and lacks a perichondrium. . Example: cartilage found in the intervertebral disks Fila Olfactoria. Bundles of olfactory receptors' axons that . coalesce to form the olfactory nerve (Cranial Nerve I) Filiform Papillae. Tiny projections atop the tongue, devoid of . taste buds, that give the tongue its feltlike texture

Flagellum. A long, thin, whiplike, motile extension of the surface of certain cells (such as spermatozoa); it has an .axoneme like that of a motile cilium Folia. A broad, thin, leaflike structure. Example: the . branched folds of the mucosa of the oviduct Follicle. A generic anatomic term for a small sac. . Examples: thyroid follicle; hair follicle Follicle-Stimulating Hormone (FSH). A hormone secreted by the anterior pituitary that stimulates . development of the ovarian follicle Follicular Cells (synonym: follicle cells). The cells that surround the ovum during development and

. maturation of the ovarian follicle .Follicular Fluid. See Liquor folliculi Formed Elements of the Blood. The cells and platelets of blood; the elements of the blood that remain . when the plasma is removed Fructose. A simple sugar found in fruits and honey; a . product of sucrose hydrolysis Functionalis. That part of the endometrium of the . uterus that is shed at menstruation Fungiform Papillae. Small, mushroom-shaped projections atop the tongue that usually bear one or . more taste buds

Ganglion. A cluster of nerve cell bodies in the peripheral .nervous system Gap Junction (synonym: nexus). A point of contact between adjacent cells that allows ions (hence electric currents) and . certain small molecules to pass from one cell to another Gastric Gland. An invagination of the surface epithelium of the stomach that contains parietal (HCI-secreting) cells and chief . (enzyme-secreting) cells, among others Gastric juice. The digestive fluid found in the stomach that consists of secretions of gastric glands; it contains proteolytic . enzymes and hydrochloric acid Gastric Pits. Funnel-shaped indentations in the mucosa of the

. stomach that lead to the gastric glands Gastrin. A hormone secreted by the mucosa of the pyloric region of the stomach that stimulates parictal cells within .gastric glands to produce HCl . Gastrocnemius. The calf muscle of the leg Germinal Center. The central region of an activated lymph nodule (within a lymph node or other lymphoid tissue, such as the tonsil) that is mitotically active in the production of new . lymphocytes Germinal Epithelium. The simple cuboidal epithelium that lines the outer surface of the ovary. Despite its misleading . name, it does not produce germ cells

Glomerular Filtrate. The solution remaining after blood has passed through the walls of the fenestrated capillaries of the glomerulus, through the slits created by podocytes, and into the urinary space of Bowman's capsule; it contains no blood . cells Glomerulus. A ball of capillaries, surrounded by Bowman's . capsule, in the renal corpuscle of the nephron Glucagon. A hormone secreted by alpha cells of the islets of Langerhans in the pancreas that mobilizes liver glycogen. An . insulin antagonist Glucocorticoid. A class of steroid hormone, secreted by the adrenal cortex, that affects carbohydrate metabolism. Often

. used clinically as an anti-inflammatory agent Glucose. A simple sugar, the most common form in which . carbohydrate is absorbed by mammals Glycogen. The major storage form of sugars in mammalian cells. Readily converted into glucose Glycoprotein. A macromolecule made of protein and . carbohydrate components Goblet Cell. A mucus-secreting epithelial cell named for . its shape Golgi Apparatus (synonym: Golgi complex). A stack of flattened, membrane-limited sacs and vesicles, found in most cells, that is intimately involved in the process of

. secretion and in the production of cytoplasmic organelles . Golgi Complex. See Golgi apparatus Graafian Follicle. A large, mature ovarian follicle that . contains the ripe egg ready for ovulation Granulocyte. A class of white blood cell that contains cytoplasmic granules visible by light microscopy. Includes . neutrophils, eosinophils, and basophils Granulosa Cell. One of the cells of the epithelial lining of a . growing or mature ovarian follicle . Greater Alveolar Cell. See Pneumocyte type II Ground Substance (synonym: matrix). The amorphous (as . opposed to the fibrillar) component of the intercellular matrix

. Growing Follicle. See Tertiary follicle Growth Hormone (synonyms: somatotrophic hormone; somatotropin; STH). A polypeptide hormone, secreted by the anterior pituitary, that has .several functions including the regulation of growth Haploid. A cell with half the normal number of chromosomes. . Example: sperm and egg cells are haploid Hassall's Corpuscle. A conspicuous structure of unknown function, peculiar to the thymus, that resembles a ball of . concentrically wrapped flat epithelial cells Haversian System (synonym: osteon). The structural unit of

organization of compact bone found in the cortex of long bones . of large animals Hemoglobin. The oxygen-bearing protein molecule of the red . blood cell Hemosiderin. A yellow-brown insoluble protein produced by the phagocytic digestion of hemoglobin-packed Heparin. An anticoagulant released by degranulation of . mast cells Hepatic Artery. An artery that brings fresh, oxygenated . blood to a liver lobule Hepatocyte. The liver cell; the major cell type that . constitutes the mass of the liver

Heterochromatin. Densely staining material, found in the interphase nucleus, that consists of super-coiled DNA and . associated proteins Histamine. A biologically active compound that, among other functions, causes increase in capillary permeability, .bronchial constriction, and vasodilation Holocrine Secretion. A secretory process in which whole cells are the secretory product. Example: holocrine secretion . occurs in sebaceous glands Horizontal Cell. A small interneuron in the vertebrate retina thought to interconnect spatially separated photoreceptors. Howship's Lacuna. A depression in bone, created by the

. erosive action of an osteoclast, in which the osteoclast sits Hyaline Cartilage. A glassy, smooth cartilage found in structures such as the trachea and the articular surfaces of . joints . Hydrocortisone. See Cortisol Hydrocortisone. A chemical process by which a compound .is cleaved into several simpler compounds Hydroxyapatite. The major crystalline component of . mineralized bone matrix Hypertrophy. Growth or enlargement of a unit such as a . cell or organ . Hypophysis. The pituitary gland

Hypothalamic-Hypophyseal Portal System. A special system of blood vessels that carry materials from the . hypothalamus to the pituitary gland Hypothalamus. A basal part of the forebrain that contains vital autonomic regulatory centers and, among many other functions, secretes "releasing factors" that . stimulate release of hormones from the pituitary gland Immunoglobulin. A class of serum proteins, found in blood . plasma, that includes antibodies Implantation. The attachment of the fertilized ovum . (blastocyst) to the endometriurn (lining of the uterus) Infundibulum. A funnel or funnel-shaped structure or

passage; a generic term that applies to structures found in a variety of organs such as the kidney, oviduct, and pituitary . gland Inner Nuclear Layer. A histologically distinct region of the retina that contains the nuclei of the bipolar, amacrine and . horizontal cells Inner Plexiform Layer. A histologically distinct region of the retina, situated between the inner nuclear layer and the layer .of ganglion cells, that contains nerve fibers and synapses Inner Segment. That part of the rod or cone photoreceptor that . includes the cell body and ellipsoid, but not the outer segment Insulin. A hormone secreted by beta cells of the pancreatic islets

. of Langerhans that promotes passage of glucose into cells Interalveolar Septum. The thin wall that separates adjacent . airsacs (alveoli) of the lung Intercalated Disk. A series of intercellular junctions that . interconnect branches of adjacent cardiac muscle fibers Intercalated Duct. The tiny duct that connects an acinus with a secretory duct in a gland such as the pancreas or the salivary . gland Intermediate Junction (synonym: zonula adherens). An intercellular junction, often part of a "junctional complex," that surrounds the apical pole of many epithelial cells and helps them .to adhere to one another

Internal Elastic Membrane. A perforated sheet of elastic tissue found between the inner layer (tunica intima) and middle layer . (tunica media) of arteries Interstitial Cells (synonym: Leydig cells). Large, foamy-appearing, androgen-secreting endocrine cells found between seminiferous . tubules in the testis Interstitial Fluid. Tissue fluid found in the . spaces between and around cells Interstitial Space. The space between and around cells normally filled with interstitial . fluid

Intestinal Glands. See Crypts of Lieberk .hn Islets of Langerhans. Groups of endocrine cells, located in the . pancreas, that produce the hormones insulin and glucagon Isthmus. A narrow anatomic passage that connects two larger structures or cavities. Example: the isthmus of the oviduct . projects from the uterus Junctional Complex. A group of intercellular junctions, located at the apical poles of cells in an epithelial sheet, that bind the cells together and control passage of materials between them. Includes the tight

junction (zonula occludens), intermediate junction (zonula adherens), desmosome (macula adherens), .and often the gap junction (nexus) Keratin. A tough, fibrous, waterproof protein found in skin, . nails, and hair . Keratinocyte. An epidermal cell that makes keratin Keratohyalin Granules. Dense granules found in the . stratum granulosum of the epidermis Killer T Cell. A T-lymphocyte that has become actively phagocytic; it recognizes and destroys specific antigens and . participates in cell-mediated immunity Kupffer Cells. Phagocytic cells found in the lining of liver

.sinusoids Lacteal. A lymphatic capillary in the core of a villus of the . small intestine Lacuna (plural: lacunae). A small space, cavity, or . depression in a tissue or organ . Lamella (plural: lamellae). A thin sheet or layer Lamina Propria. A highly cellular loose connective tissue, located beneath the epithelium, that provides a supporting framework for the epithelium of mucous . membranes Layer of Ganglion Cells. A histologically distinct region of the retina containing the ganglion cells, whose axons

coalesce to form the optic nerve that connects the retina .with the brain . Leukocyte. A white blood cell . Leydig Cell. See Interstitial cells Ligament. A tough strap of dense regular connective tissue that connects adjacent structures. Example: . ligaments connect adjacent bones in a joint Light Cells. A class of light-staining cell within the taste . bud to which no definite functions have yet been ascribed Limbus Spiralis. A structure within the cochlea of the inner ear, a bony spiral, extending inward from the . modiolus, that supports the organ of Corti

.Lipase. A class of enzyme that splits fats Lipids. Molecules-including fats and waxes-that are soluble in . nonpolar organic solvents Lipofuscin Granules. Pigment-containing granules, found in a variety of cells and especially common in old neurons, that represent remnants of lysosomes filled with indigestible . material. Also called wear-and-tear granules Liquor Folliculi (synonym: follicular fluid). Fluid that fills the . antrum of growing and mature ovarian follicles Littoral Cell. A type of cell that lines the lymphatic sinuses of . lymph nodes and blood sinuses of bone marrow Lobule. A generic term referring to a unit of histologic

:organization that is often a subunit of a lobe. Example Loop of Henle. A thin tube of the nephron that connects the . proximal and distal tubules Luteinizing Hormone (LH). A hormone secreted by the anterior pituitary that stimulates development of the corpus . luteum from an ovarian follicle following ovulation Lymphoblast. A large, immature lymphocyte often found in the . germinal center of a lymphatic nodule Lymphocyte. A small white blood cell, classified as an agranulocyte. It is involved in the immune response in several ways, depending, in part, on whether it arises in the thymus (T. lymphocyte) or the bone marrow (B-lymphocyte) Lymphokines. Substances released by sensitized lymphocytes

(those that have contacted specific antigens) that stimulate .activity of monocytes and macrophages Lysosome. A small, membrane-limited cytoplasmic organelle, filled with a wide array of hydrolytic enzymes, that fuse with and destroy unwanted foreign material or . senescent intracellular components Lysozyme. An enzyme that destroys bacterial cell walls. Macromolecule. A generic term referring to large and complex biologic molecules. Examples: proteins, . polysaccharides, nucleic acids . Macrophage. A large, ameboid, phagocytic cell

.Macula Adherens. See Desmosome . Marginal Sinus. See Subcapsular sinus Mast Cell. A large connective tissue cell characterized by conspicuous cytoplasmic granules thought to contain heparin, an anticoagulant, and histamine, which increases . capillary permeability . Matrix. See Ground substance Medulla. A general term referring to a region deep within an organ. Examples: the medulla of the kidney; the adrenal . medulla Megakaryocyte. A large, multinucleate cell, found in the bone marrow, from which cytoplasmic fragments break off and

. become platelets Meiosis. A highly complex series of "cell divisions" that include a reduction division in which the number of chromosomes is halved during sperm and egg (gamete) . formation Meissner's Corpuscle. A class of mechanoreceptor, found in the skin and some mucous membranes, thought to be . sensitive to light touch . Meissner's Plexus. A plexus of autonomic neurons, located in the submucosa of the gut, that affects contraction of smooth muscle fibers in the muscularis

. mucosae Melanin. A dark pigment that gives coloration to skin, hair, . and other body parts Melanocyte. A cell that makes the dark pigment called . melanin Menarche. The onset of menstruation in the female at . puberty Merkel's Cell. A type of cell located in the epidermis of the skin thought to participate in mechanoreception Meromyosin. A product of the tryptic digestion of the myosin molecule, further classified as heavy meromyosin and light . meromyosin

Mesenchyme. Embryonic connective tissue that can give rise to such structures as connective tissues, blood, lymphatics, . cartilage, and bone Mesothelial Cell. A type of flattened cell that lines the body . cavity and many of the organs found within the body Metachronal Wave. The coordinated wave of ciliary beating of . a field of ciliated cells Metaphase. That stage of cell division in which the chromosomes are aligned along the equatorial metaphase . plate (prior to their migration to opposite poles at anaphase ) Microfilament. A thin cytoplasmic filament, some 50 in diameter, often composed of actin and

associated with cell motility. An important component . of the cytoskeleton Micrometer (abbreviation: m). A unit of measure, formerly called the micron, equal to 1/1000 of a .millimeter Microtubule. A thin tube in the cytoplasm whose wall is made up of proteinaceous subunits called tubulin. Microtubules make up the framework of the mitotic spindle, are a vital part of the cytoskeleton, and are . often associated with motility Microvilli (singular: microvillus). Tiny, membranelimited, finger-shaped projections of the apical surface of many cells. Supported by core filaments made of actin, microvilli greatly increase the surface area

. available for secretion and absorption Mitochondria (singular: mitochondrion). Long, slender, membrane-limited cytoplasmic organelles that are active in the . production of ATP Mitosis. The process of nuclear division in which the nucleus divides to form two daughter nuclei, each with an identical complement of chromosomes. Mitosis usually is accompanied by cytoplasmic division (cytokinesis) that gives rise to two daughter cells. The major phases of mitosis are prophase (chromosomal condensation), metaphase (alignment of chromosomes on the equator of the mitotic spindle), anaphase (migration of one of each pair of chromosomes to opposite poles of the mitotic spindle), and

. telophase (formation of daughter nuclei) Monocyte. A large white blood cell that is the immature circulating . form of a macrophage Morula. A mass of cells, resulting from the early cleavage divisions of the zygote, that precedes the blastula in early embryoni Motor End Plate (synonym: myoneural junction). A chemical . synapse between a nerve and a striated muscle fiber Motor Neuron. A large neuron, with its cell body in the central nervous system, that sends an axon out to innervate an effector. usually a skeletal muscle fiber Mucopolysaccharide. A general term for large macromolecules that consist of a complex of protein(s) and polysaccharide(s) in

which the polysaccharide component is often the major part. Example: mucus and cartilage matrix contain large amounts of . mucopolysaccharides Mucosa (synonym: mucous membrane). The inner lining of many organs; consists of an epithelium, the lamina propria that . supports the epithelium, and the muscularis mucosae .Mucous Cells. Cells that produce mucus . Mucous Membrane. See mucosa Mucous Neck Cells. Epithelial cells, found in the . necks of the gastric glands, that secrete mucus Mucus (adjectival form: Mucous). A class of viscous, slippery secretion, rich in mucins, secreted by mucous

. membranes and mucous glands Microglia Cells. Specialized neuroglial cells within M . the retina .Multinucleate. Having more than one nucleus . Muscle Fiber. A muscle cell Muscularis Externa. The thick layers of smooth muscle, situated between the submucosa and adventitia of the digestive tract, that provides the motive force for peristaltic . movements of the gut Muscularis Mucosae. A loosely woven sheet of smooth muscle fibers located beneath (outside of) the lamina propria . of the alimentary canal

Myelin Sheath. A fatty sheath, composed of the concentrically wrapped cell membrane of a glial cell, that surrounds some axons of the central and peripheral nervous system. An effective electrical insulator, it permits the rapid "saltatory .conduction" characteristic of myelinated nerves . Myoblast. An embryonic cell that develops into a muscle fiber Myocardium. The middle layer of the heart, located between the enclocardium and epicardium, that contains a large mass of . cardiac muscle fibers Myoepithelial Cell. An epithelial cell, rich in myofilaments, that can contract. Example: myoepithelial cells surround secretory . acini of salivary glands

Myofibril. A long, slender, cylindric structural component of a muscle fiber. Each myofibril, which consists of a series of sarcomeres joined end to end, is surrounded by membranes of the sarcoplasmic reticulum. Many myofibrils, oriented parallel .and side by side, fill the cytoplasm of a muscle fiber Myofilament. A small, filamentous component of the sarcomere in skeletal muscle. Myofilaments are classified as either thick filaments (made of myosin) or thin filaments . (made of actin) .Myometrium. The muscular portion of the wall of the uterus . Myoneural Junction. See Motor end plate Myosin. The major protein of the thick filaments of the

. sarcomeres of striated muscle Myotube. A stage in the formation of a skeletal muscle fiber from embryonic muscle cells (myoblasts); the myoblasts .align end to end to form a myotube Neuron. A nerve cell. Most neurons consist of a cell body . (soma), an axon, and a group of dendrites Neurosecretory Cell. A neuron that makes, stores, and releases a substance that is eventually secreted from an axon terminal. Example: chromaffin cells of the adrenal medulla. Neurotransmitter. A chemical, released from an axon terminal at a synapse, that usually exerts an effect on the ionic . conductance of the membrane of the postsynaptic cell

Neutrophil (synonyms: polymorphonuclear leukocyte; PMN). The most common of the white blood cells; classified . as a granulocyte; phagocytic in function . Nexus. See Gap junction .Nissl Bodies. See Nissl substance Nissl Substance (synonym: Nissl bodies). A basophilic substance, prominent in the cytoplasm of nerve cell bodies, consisting of the rough endoplasmic reticulum and free . ribosomes Node of Ranvier. Located along the length of a myelinated nerve fiber, nodes of Ranvier are "naked" areas of the axon between segments of the myelin sheath laid down by adjacent

. glial cells Nomarski Interference Microscopy. A technique using a specially modified light microscope that enables one to see unstained cells of sectioned material in great detail. The image . produced seems set in relief, much like the head of a coin Norepinephrine. A hormone, classified as a catecholamine, produced by the adrenal medulla; also a neurotransmitter at .many synapses in the sympathetic nervous system Nuclear Envelope. A double membrane that surrounds the nucleus, the nuclear envelope is a specialized, perinuclear . cisterna of the encloplasmic reticulum in diameter, Nuclear Pore. A tiny perforation, some 700

in the nuclear envelope. Nuclear pores are numerous; they provide for exchange of materials between nucleus and . cytoplasm Nucleolus. A small, darkly staining mass within the nucleus . wherein ribonucleoprotein is produced Nucleus. A large, membrane-limited compartment within the cell that contains the genetic material, DNA, in the form . of chromosomes Odontoblast. A large cell that lines the pulp cavity of the tooth .and secretes dentin Olfactory Cilia. Small sensory cilia, projecting from the olfactory vesicle of the ciliated olfactory receptor neuron, thought to be a

. site of sensory transduction of olfactory stimuli Olfactory Nerve. A nerve (Cranial Nerve I) consisting of axons of the olfactory receptor neurons that travels from the olfactory . mucosa to the brain Olfactory Vesicle. The swollen terminal of the dendrite of an olfactory receptor (which is a bipolar neuron) that bears the . olfactory cilia Oligodendrocyte. A glial cell that makes the myelin sheaths that envelop the axons of many neurons in the central nervous system. (Note: Schwann cells myelinate axons in the peripheral nervous system.) Oocyte. The immature ovum, or egg

Optic Nerve. A major cranial nerve (Cranial Nerve II) that connects the retina with higher centers in the brain. It is made up of a bundle of axons from ganglion cells in the . retina Organ of Corti. That region in the cochlea of the inner ear that contains the "hair cells" - mechanoreceptors sensitive . to sound vibrations Organelle. A small, discrete, structural and functional unit within a cell. Example: the mitochondrion. Osmosis. Movement of a solvent through a semi-permeable membrane in the direction of the greater concentration of solute. Example: in a vial of sugarwater, capped with a semipermeable membrane and immersed in a beaker of

water, water will move from the beaker across the . membrane and into the vial Osteoblast. A cell that actively secretes unmineralized bone matrix, or osteoid. An . immature osteocyte Osteoclast. A large, multinuclear, phagocytic cell . that eats bone Osteocyte. A mature bone cell encased in . mineralized bone matrix Osteoid. Newly secreted, unmineralized bone matrix, made largely of collagen, that provides a site for deposition of hydroxyapatite crystals

. during the mineralization process .Osteon. See Haversian system Outer Nuclear Layer. A histologically distinct region of the . retina containing nuclei of the rod and cone photoreceptors Outer Plexiform Layer. A histologically distinct region of the retina, located between the outer and inner nuclear layers, . that contains nerve fibers and synapses Outer Segment. That part of a vertebrate rod or cone photoreceptor consisting of a stack of membranes containing . the photosensitive pigment Ovary. One of the paired organs in the female reproductive system in which the egg matures and from which it is

. released at ovulation Oviduct. See Uterine tube. Ovulation. The release of the egg from the ovary, preceded by rupture of the ovaria . Ovum. The unfertilized egg . Oxyntic Cell. See Parietal cell Oxytocin. A hormone, produced in the hypothalamus and released from the neurohypophysis (posterior pituitary), that promotes milk letdown and contraction of .uterine smooth muscle Pacinian Corpuscle. A large mechanoreceptor, found deep

in the dermis of the skin and in other connective tissues, that . is sensitive to deep pressure Pancreatic Duct. The duct that carries secretions of the exocrine pancreas (i.e., pancreatic juice) from the pancreas to the duodenum. Pancreatic juice. The collective secretions of the exocrine pancreas, including digestive enzymes and the . buffer, bicarbonate Paneth Cells. Large cells, situated at the base of the intestinal glands of the small intestine, characterized by conspicuous eosinophilic cytoplasmic granules thought to contain the antibacterial enzyme, lysozyme. May be . phagocytic Parafollicular Cells (synonyms: clear cells; C-cells). Lightstaining cells, located at intervals beneath the follicular

epithelium of the thyroid gland, that secrete the hormone .calcitonin (which lowers blood calcium) Parasympathetic Nervous System. A subdivision of the autonomic nervous system, consisting chiefly of cholinergic fibers, that tends to stimulate secretion, increases tone and . contractility of smooth muscle, and causes vasodilation Parathyroid Gland. A gland located near the thyroid gland; its secretion, parathyroid hormone, activates osteoclasts and . raises blood calcium levels . Parathyroid Hormone. See Parathyroid gland Parietal Cells (synonym: oxyntic cells). Large, round, eosinophilic cells, found in the epithelium lining the gastric

. glands of the stomach, that secrete hydrochloric acid Parietal Layer. A general term relating to the cellular lining of the wall of a cavity. Example: the parietal layer lines the inner .surface of the outer wall of Bowman's capsule Parotid Gland. One of a pair of large bilateral serous . salivary glands located below and in front of the ear Pars Distalis. The anterior lobe of the pituitary gland; . the largest subdivison of the pituitary gland Pars Intermedia. A thin strip of tissue sandwiched between the pars distalis and the pars nervosa of the . pituitary gland . Pars Nervosa. See Neurohypophysis

Pars Tuberalis. A collar of cells that surrounds the .infundibulum of the pituitary gland Pedicle. A foot-process of the podocytes that wrap around the capillaries of the glomerulus in Bowman's capsule of the . nephron Pepsin and Pepsinogen. The proenzyme pepsinogen is secreted by chief cells of the stomach's gastric glands and, when released into the acidic stomach lumen, is cleaved to . form the active proteolytic enzyme pepsin Perichondrium. A tough membrane of dense connective tissue that surrounds hyaline and elastic cartilage (except at joint surfaces); it contains immature cartilage cells, or

chondroblasts, that secrete matrix and form mature . chondrocytes during cartilage development Perikaryon. The cytoplasm surrounding the nucleus; usually . used in reference to the cell bodies of nerve cells Perimetrium. The serosal covering of the uterus continuous .with the peritoneal lining of the body cavity Perineurium. A sheath of connective tissue that surrounds a . bundle of nerve fibers within a large nerve Periodontal Ligament. The ligament made of dense connective tissue that surrounds the root of the tooth and . anchors the tooth to the walls of its bony socket Periosteum. A tough membrane of dense connective tissue

that surrounds bones; it contains the osteoblasts that secrete bone matrix during bone growth. Peripheral Nervous System. That part of the nervous system not enclosed within the skull . or spinal column Peristalsis. The undulating, wavelike motion of the intestines, generated by rhythmic smooth-muscle contractions, that . moves material during the digestive process Peyer's Patches. Large, conspicuous lymphatic nodules in .the wall of the ileum of the small intestine Phagocytosis. The process by which one cell engulfs and . frequently digests a bacterium, another cell, or other material Phalangeal Cells. Supporting cells located within the organ of

Corti of the cochlea that, together with the pillar cells, hold the hair . cells in place Pheromone. A chemical substance, secreted by the male or female of a species, that attracts members of the opposite sex; . often airborne and detected by the chemosensory system Pigment Epithelium. The outermost layer of the retina; a layer of pigment cells that absorbs photons and often phagocytoses worn. out rod outer segments Pillar Cells. Supporting cells located within the organ of Corti of the cochlea that, together with the phalangeal cells, hold the hair .cells in place Pinocytosis. The process by which a cell "drinks" small amounts of material by pinching off tiny vesicles of its plasma

membrane that contain the material and taking them into its . cytoplasm Pituicytes. Fusiform cells, similar to neuroglia, associated with . neurosecretory axons in the neurohypophysis Placenta. The highly vascular organ attached to the interior of the uterus that joins mother and fetus during intrauterine life and . provides for metabolic interchange Plasma. The noncellular, fluid phase of the blood in which the . blood cells and platelets are suspended Plasma Cell. A type of cell in connective tissues that makes .and secretes antibodies; a mature B-lymphocyte . Plasma Membrane. See Cell membrane

. Plasmalemma. See Cell membrane Platelet. An anucleate fragment of the cytoplasm of a megakaryocyte, found in circulating blood, that functions in . blood clot formation Plicae Circulares. Large folds of the intestinal wall that . include the submucosa Pneumocyte Type II (synonyms: septal cell; greater alveolar cell). A cuboidal epithelial cell in the wall of the alveolus that secretes pulmonary surfactant, a substance that reduces surface tension and functions to prevent .collapse of alveoli . Plasma Membrane. See Cell membrane

. Plasmalemma. See Cell membrane Platelet. An anucleate fragment of the cytoplasm of a megakaryocyte, found in circulating blood, that functions . in blood clot formation Plicae Circulares. Large folds of the intestinal wall that . include the submucosa Pneumocyte Type II (synonyms: septal cell; greater alveolar cell). A cuboidal epithelial cell in the wall of the alveolus that secretes pulmonary surfactant, a substance that reduces surface tension and functions to prevent .collapse of alveoli Podocyte. An epithelial cell, found in the visceral (inner) layer of

Bowman's capsule of the nephron, endowed with elaborately . branched foot processes that ensheath glomerular capillaries Poliomyelitis. A debilitating disease in which viruses infect and frequently kill spinal motor neurons, thereby causing paralysis and degeneration of the muscles innervated by those motor . neurons . Polymorphonuclear Leukocyte. See Neutrophil Polysaccharide. A carbohydrate that can be broken down into two or more monosaccharide molecules. Examples: glycogen, . cellulose Portal Triad. A structural and functional unit of a liver lobule that contains a branch of the hepatic artery, por

Portal Vein. The vein that brings blood from the gut and spleen . to the liver Predentin. Organic fibrillar matrix of dentin secreted by . odontoblasts. Predentin becomes calcified and forms dentin Primary Follicle. An early stage in the development of the ovarian follicle in which the ovum is surrounded by a single layer . of cuboidal (follicular) epithelial cells Primary Nodule. A region within lymphoid tissue that actively produces lymphocytes by mitotic division of stem cells in the . germinal center of the primary nodule Primary Spermatocytes. Large cells derived from spermatogonia that undergo meiosis and give rise to second

Primordial Follicle. A very early stage in the development of the ovarian follicle in which the ovum is surrounded by a single . layer of squamous epithelial cells Principal Cell. A generic term referring to the most numerous cells within a gland or organ. Example: the principal cells of the . thyroid gland secrete thyroid hormone Progesterone. A steroid hormone secreted by the corpus luteum of the ovary that prevents maturation of ovarian follicles . and prepares the uterus for implantation of the fertilized egg Prolactin. A proteinaceous hormone secreted by the anterior . pituitary that stimulates milk secretion Proliferative Phase. That phase of the menstrual cycle in which the functionalis of the endometrium lining th

Prostaglandins. A general term describing a variety of cyclic fatty acids that perform hormone-like functions, including . modulation of smooth-muscle contraction and blood pressure Prostate Gland. A gland of the male reproductive system, located near the bladder, that secretes a viscous fluid that is a . major component of the seminal fluid Prostatic Concretions. Hard, mineralized bodies found within . the prostate gland of older males Prostatic Fluid. Fluid secreted by the prostate gland that is . propelled into the urethra during ejaculation Prostatic Fluid. A macromolecule made up of long .sequences of amino acids

Proximal Convoluted Tubule. The first tubular portion of the nephron, directly connected to Bowman's capsule, into which glomerular filtrate flows. It transports useful materials such as salts, amino acids, and sugars from the glomerular . filtrate back into the blood Pseudopodia. Extensions of a cell s plasma membrane and cytoplasm ("false feet") that often engulf material during the process of phagocytosis; they also . participate in cellular locomotion Ptyalin. An amylase, found in saliva, that converts starches . into sugars Pulmonary Alveolar Macrophage (synonym: dust cell). A

wandering macrophage, found in the alveoli of the lung, that "sweeps" the lining of the lung of particulate matter and . debris Pulp. The substance within the pulp cavity in the core of the ,tooth that contains connective tissue, nerves Radial Spokes. Components of the ciliary axoneme that connect the central sheath that surrounds the central pair of . microtubules with the nine outer microtubular doublets Red Pulp. Regions of the spleen that contain venous . sinuses and associated cells Renal Corpuscle. That part of the nephron that includes . Bowman's capsule and its contained glomerular capillaries

Renal Pelvis. The connection between the kidney and the ureter; the funnel-shaped end of the ureter that receives . urine from the kidney Residual Bodies. Membrane-limited cytoplasmic inclusions that contain material indigestible by the cell's ly Reticular Fiber. A class of thin connective tissue fiber, based on the collagen molecule, that reacts with certain silver stains. Reticular fibers are commonly found in . lymphatic organs such as the spleen and lymph nodes Reticulocyte. This term describes each of two dramatically different kinds of cells: a connective tissue cell, similar to the fibroblast, that secretes reticular fibers;

and an immature, circulating erythrocyte, within whose . cytoplasm wisps of rough endoplasmic reticulum persist Reticuloendothelial System. A diffuse system of cells, found in lymphoid organs and many connective tissues, that includes all of the body's phagocytic cells (with the . exception of circulating leukocytes) Ribonucleoprotein. A combination of protein and .ribonucleic acid Ribosome. A small dense cytoplamic inclusion that functions as the site of assembly of protein from its amino acid . components RNA: Ribonucleic acid -- an abundant class of nucleic acids

associated, among other things, with the translation and . transcription of genetic information Rod. The class of retinal photoreceptor that mediates vision . in dim light; rods "see" black and white, but not color Romanovsky's Stain. A class of stains, commonly used on smears of blood and bone marrow, that include a basic dye such as methylene blue, an acid dye such as eosin, and . alcohol, which fixes the cells. Example: Wright's stain .Root. The portion of the tooth located below the gumline Romanovsky's Stain. A class of stains, commonly used on smears of blood and bone marrow, that include a basic dye such as methylene blue, an acid dye such as eosin, and

. alcohol, which fixes the cells. Example: Wright's stain . Root. The portion of the tooth located below the gumline Rough Endoplasmic Reticulum (abbreviation: RER). A system of intracellular membranes, commonly associated with the synthesis of "protein for export," that appears in the cytoplasm of the cell as flattened sacs, or cisternae, whose .membranes are heavily encrusted with ribosomes Sarcolemma. The cell membrane surrounding a skeletal or . cardiac muscle fiber Sarcomere. A unit of structure and function within a myofibril of a skeletal or cardiac muscle fiber bordered by two . adjacent Z bands

Sarcoplasmic Reticulum. The modified smooth endoplasmic reticulum of a skeletal or cardiac muscle fiber that sequesters calcium and releases it under appropriate . conditions of stimulation Schmidt-Lantermann Cleft. An irregularity in the myelin . sheath of a nerve Schwann Cell. A class of neuroglial cell that functions in wrapping the myelin sheath around a peripheral Sclera. The outermost layer of the eyeball, rich in connective tissue, that presents itself as the "whites" of . the eyes Sebaceous Gland. A gland of the skin, usually

associated with a hair, that secretes an oily substance . called sebum Secondary Follicle. That stage in the development of the ovarian follicle in which the ovum is surrounded by . several layers of cuboidal (follicular) epithelial cells Secondary Papillae. Projections of the papillary layer of . the dermis upward into the basal layer of the epidermis Secondary Spermatocytes. Small cells, derived from primary spermatocytes that undergo meiosis, that give Secretory Duct (synonym: striated duct). A medium-sized . intralobular duct within a salivary gland Secretory Granules (synonym: secretory vesicles).

Membrane-limited packets of material within the cytoplasmfrequently found near the apical pole of the cell-destined to be . released into the lumen of a gland, duct, or organ Secretory Phase. That phase of the menstrual cycle in which . the endometrium of the uterus prepares for implantation . Secretory Vesicles. See Secretory granules Semen. Material that contains seminal fluid and spermatozoa released from the penis of the male during ejaculat Seminal Fluid. The fluid component of semen in which . spermatozoa are suspended Seminal Vesicle. An outpocketing of the vas deferens, lined by a . secretory epithelium, that stores seminal fluid prior to ejaculation Seminiferous Tubule. One of several long, coiled tubules within

. the testis that contains the germinal epithelium . Septal Cell. See Pneumocyte type II Septum (plural: septa). A generic term referring to a thin wall . that divides two cavities or masses of soft tissue Serosa. The layer that forms the outer lining of organs, such as the duodenum, that consists of an extension of the inner wall of . the body cavity Serous Cells. Cells that produce a watery secretion rich in proteins and glycoproteins. Example: cells of serous acini of the . parotid gland Serous Demilune. A half-moon-shaped cap of serous cells . fitted around a mucous acinus within a salivary gland

Sertoli Cell (synonym: nurse cell). A larger cell within the seminiferous tubule to which spermatids attach during . spermiogenesis Sinusoid. A class of large, leaky capillaries, such as those in the liver, that permit passage of large objects through their . perforated walls Skeletal Muscle. A class of striated muscle whose large, multinucleate fibers, under control of motor neurons, generate .the contractile forces responsible for voluntary movements . Skeletal Muscle Cell. See Skeletal muscle fiber Skeletal Muscle Fiber. The contractile unit of skeletal muscle; a skeletal muscle cell; a large, multinucleate cell that is a

. syncytium Smooth Endoplasmic Reticulum (abbreviation: SER). A system of intracellular membranes in the cytoplasm of the cell. Devoid of ribosomes, the SER is organized as an anastomosing network of tubules and participates in a variety of cellular functions such as glycogen metabolism and steroid . biosynthesis Smooth Muscle. A class of muscle whose small, fusiform, mononucleate fibers lack the striations characteristic of skeletal and cardiac muscle. Innervated by the autonomic nervous system, smooth muscles power involuntary movements such as intestinal peristalsis and constriction of airways and blood . vessels

Somatotropes. Cells of the anterior pituitary that . secrete somatrotrophic (growth) hormone . Somatotrophic Hormone. See Growth hormone . Somatotropin. See Growth hormone Spermatid. A haploid cell, derived from a secondary spermatocyte, that differentiates to form a spermatozoon . in the process of spermatogenesis Spermatocyte. A cell in the germinal epithelium of the seminiferous Soma. A general term meaning "body" that often refers to the cell body of a neuron. tubule, classified as either a primary spermatocyte or a secondary spermatocyte. A

primary spermatocyte, derived from a spermatogonial cell, divides to form two secondary spermatocytes; each Spermatogenesis. The formation of a spermatozoon from a . stem cell, the spermatogonium Spermatogenic Cells. A general term referring to cells in the spermatogenic series: spermatogonia, primary . spermatocytes, secondary spermatocytes, and spermatids Spermatogonia. Stem cells located at the periphery of the germinal epithelium in the seminiferous tubule that undergo . mitotic division and give rise to primary spermatocytes Spermatozoon. A mature sperm cell, equipped with a head . and a tail, designed to fertilize the egg

Spermiogenesis. The morphogenetic transformation of . spermatids into spermatozoa Spicules. Small spikes that traverse the marrow spaces . within spongy bone Splenic Cords (synonym: cords of Billroth). Dense regions of tissue within the red pulp of the spleen that contain . venous sinuses and associated cells Spongy Bone (synonyms: cancellous bone; trabecular bone). A kind of bone, located within the marrow spaces of large bones, that consists of a network of spicules (spikes) . and trabeculae (plates) Squames. Flattened, scalelike, dead keratinized cells

. periodically shed from the surface of the skin Squamous Epithelium. An epithelium characterized by flat . surface cells Stellate Reticulum. Loose connective tissue associated with the developing tooth. Stereocilia. Long microvilli, such as those found extending from the surface of cells of the epididymis and hair cells . of the cochlea. Stereocilia are in no way related to cilia Steroid. A large class of chemical substances, including many vitamins, hormones, drugs, and cellular components, based on a . four-ring phenanthrene skeleton Stratified Squamous Epithelium. An epithelium consisting of several layers of cells; the uppermost layer contains flattened, or

. squamous, cells Stratum Basale (synonym: stratum germinativum). The lower layer of the epidermis containing mitotically active stem cells that . supply cells for the upper layers of the skin Stratum Corneum. The superficial layer of the epidermis that . consists of flattened, keratinized cells . Stratum Germinativum. See Stratum basale Stratum Granulosum. A layer of cells in the epidermis, located above the stratum spinosum, that contains conspicuous . keratohyalin granules Stratum Lucidum. A clear layer of epidermal cells located between the stratum corneum and stratum granulosoum of thick

. skin, such as the sole of the foot Stratum Spinosum (synonyms: spiny layer; prickle cell layer). A layer of cells in the epidermis, just above the stratum germinativum, characterized by a multitude of intercellular . attachments called desmosomes Striated Border (synonym: brush border). A felt-work of closely packed, parallel microvilli that extend from the apical cell surface of epithelia such as those lining the intestine and proximal . convoluted tubule of the kidney . Striated Duct. See Secretory duct Subcapsular Sinus (synonyms: cortical sinus; marginal sinus). The space beneath the capsule covering a lymph node, it receives

. incoming lymph from small afferent lymphatic vessels Sublingual Gland. One of a pair of bilateral salivary glands situated . beneath the tongue; has both mucous and serous acini Submandibular Gland (synonym: submaxillary gland). One of a pair of large bilateral salivary glands located beneath the lower jaw, . or mandible; has both mucous and serous acini . Submaxillary Gland. See Submandibular gland Submucosa. A layer of dense irregular connective . tissue found beneath the mucosa of many organs Supporting Cell (synonym: sustentacular cell). A generic term for a kind of cell, usually within an . epithelium, that supports, or sustains, other cells

Surface Mucous Cells. Epithelial cells found on the surface of the gastric mucosa that have a protective . apical cap of mucus beneath the cell surface Surfactant. A wetting agent, secreted by the pneumocyte type II, that lowers surface tension and . prevents collapse of the alveoli . Sustentacular Cell. See Supporting cell Sympathetic Nervous System. A subdivision of the autonomic nervous system that contains mainly adrenergic fibers and frequently depresses secretion, decreases the contractility of smooth muscle, and causes . vasoconstriction

Synapse. A functional point of contact between two nerve cells or processes that permits the passage of an . impulse Synaptic Cleft. The narrow intercellular (extracellular) space between the two adjacent nerve cell membranes in . a synapse Synaptic Vesicle. A tiny, membrane-limited vesicle, located at the synapse, bearing the chemical that functions as a neurotransmitter that, when released from the presynaptic nerve terminal, excites (or inhibits) the . postsynaptic neuron Syncytium. A large, multinucleate cell, derived from many cells, whose cell membranes have fused to form

Synovial Fluid. The fluid within a joint, secreted by the epithelium that lines the joint capsule, that serves to . lubricate the articulating surfaces within the joint Systole. The strong pulse generated by the contraction of . the left ventricle of the heart T-Lymphocyte. A class of lymphocyte that matures in the . thymus and participates in cell-mediated immunity T-System. A system of tubular invaginations of the cell membrane in skeletal and cardiac muscle fibers that carry electrical excitation inward to all the sarcomeres of the myofibrils. The transverse tubules of the T-system are usually

. closely associated with the sarcoplasmic reticulum Taste Bud. A teardrop-shaped cluster of cells associated with the epithelium of circumvallate, foliate, and some fungiform papillae of the tongue, containing chemoreceptors that . mediate gustation . Taste Pore. The opening at the apical surface of the taste bud Tectorial Membrane. A thin sheet of connective tissue in the organ of Corti of the cochlea that makes contact with the . stereocilia of the mechanoreceptive hair cells Tendon. A strap of dense regular connective tissue that connects . muscle to bone Terminal Cisternae. Dilated cisternae of the sarcoplasmic

reticulurn of skeletal and cardiac muscle fibers. Two terminal . cisternae usually surround a single tubule of the T-system Tertiary Follicle (synonym: growing follicle). A large, welldeveloped ovarian follicle that contains a fluid-filled cavity, the antrum. A tertiary follicle can continue to grow to form a mature . Graafian follicle . Testicle. See Testis Testis (synonym: testicle). The organ of the male . reproductive system in which spermatogenesis occurs . Theca. An enclosing case or sheath Theca Externa. The outer, fibrous region of the theca . folliculi of the ovarian follicle Theca Folliculi. A sheath of connective tissue that

. surrounds an ovarian follicle Theca Interna. The inner, highly cellular layer of the theca . folliculi of the ovarian follicle Theca Lutein Cells. Progesterone-secreting cells of the corpus . luteum Thymus. A glandular structure located in the chest and made of lymphoid tissue that participates in the function and . development of the immune system Thyroglobulin. A glycoprotein macromolecule, contained within the colloid in the lumen of a follicle of the thyroid gland, that is . complexed with thyroid hormone Thyroid Hormone (synonym: thyroxine). The hormone

secreted by the thyroid gland that, among other functions, is . important in the control of metabolic rate Thyrotrope. A cell (basophil) of the anterior pituitary that secretes thyroid-stimulating hormone (thyrotropin Thyrotropin. A hormone, secreted by the anterior pituitary, that . stimulates secretion of thyroid hormone by the thyroid gland Thyrotropin-Releasing Factor. A hormone, secreted by the hypothalamus of the brain, that stimulates the anterior pituitary to . secrete thyrotropin. Thyroxine. See Thyroid hormone Tight Junction (synonym: zonula occludens). A type of intercellular junction, commonly found at the apical pole of epithelial cells, in which the closely apposed cell membranes

prevent the flow of materials into the extracellular (intercellular) . space between two neighboring cells Trabeculae. A generic term referring to thin plates of supporting material within a tissue or organ. Example: trabeculae of spongy . bone Transcription. The process by which genetic information contained in DNA produces a complementary sequence Transfer Vesicles. Membrane-limited vesicles that transfer material from the rough endoplasmic reticulum to the Golgi . apparatus Transitional Epithelium. A class of epithelium, several cell layers thick, that stretches or contracts (hence changes in

appearance) when the container it lines is filled or emptied. . Example: the epithelium lining the urinary bladder Trophic Hormone. A hormone secreted by one gland that stimulates secretion of a hormone by another gland. Example: thyroid-stimulating hormone (TSH), released by the anterior pituitary, stimulates secretion and release of thyroid hormone . from the thyroid gland Trypsin. A digestive enzyme, secreted by the pancreas into the duodenum of the small intestine, that breaks down proteins . (is proteolytic) Tunica Adventitia. An envelope of loose connective tissue that surrounds blood vessels larger than arterioles or venules

Tunica Albuginea. A tough capsule of dense connective . tissue that envelops organs such as the testis Tunica Intima. The innermost layer of a blood vessel; lined by endothelium, the tunica intima is in contact with . blood in the lumen Tunica Media. The thick middle layer of the wall of a blood vessel, rich in smooth muscle and, in the case of . arteries, elastic tissue Unit Membrane. A term used to describe the trilaminar, "railroad-track" structure displayed by many biologic membranes when fixed in osmium tetroxide, cut in cross . section, and viewed with the electron microscope Ureter. The tube that carries urine from the kidney to the

. bladder Urethra. The canal that carries urine from the bladder to . outside the body Urinary Space. The space within Bowman's capsule of the nephron, continuous with the lumen of the proximal convoluted . tubule, that receives the glomerular filtrate Uterine Tube (synonyms: fallopian tube; oviduct). The tube through which the egg, following ovulation, passes en route . from ovary to uterus Uterus. The organ of the female reproductive system that . contains the developing fetus prior to birth

Vas Deferens (synonym: ductus deferens). A thick-walled tube that carries spermatozoa from the testis to the urethra Vasa Recta. A system of small, straight blood vessels, located in the medulla of the kidney, that are closely associated with tubules . of the nephron Vasopressin (synonyms: antidiuretic hormone; ADH). A hormone made in the hypothalamus and released by the . neurohypophysis that promotes water retention by the kidneys Venous Sinus. A large, very leaky capillary, such as those found . in the liver and spleen Villus (plural: villi). A small infolding of the intestinal wall that projects into the lumen; the villus is lined by

simple columnar epithelium and supported by the . lamina propria Visceral Layer. The inner epithelial lining of Bowman . s capsule of the nephron Vitreous Humor. The viscous, dense, jellylike material that fills the posterior chamber of the eyeball (behind the lens White Pulp. That part of the tissues of the spleen that consists of arterioles and surrounding clusters of . lymphocytes Woven Bone. A kind of bone found in embryos, young children, and certain pathological states in which orderly

. lamellae of collagen fibers are not present Zona Fasciculata. A layer of the adrenal cortex, just internal to the zona glomerulosa, that secretes . glucocorticoids such as cortisone Zona Glomerulosa. The outermost secretory layer of the adrenal cortex that secretes mineralocorticoids such as . aldosterone Zona Pellucida. A dark-staining zone surrounding the egg within the developing ovarian follicle, consisting of cytoplasmic extensions of the oocyte and granulosa cells . and the material in which they are embedded Zona Reticularis. The innermost layer of the adrenal cortex

. that secretes steroids similar to sex hormones . Zonula Adherens. See Intermediate junction . Zonula Occludens. See Tight junction . Zygote. The fertilized egg Zymogen Granule. A secretory vesicle, usually found in the apical pole of a secretory cell, . containing enzyme precursors

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