cycle /cy·cle/ (si´k'l) a succession or recurring series of events.
cell cycle,
cell, centrioles of (sen´trēōls),
carbon cycle the steps by which carbon (in the form of carbon dioxide) is extracted from the atmosphere by living organisms and ultimately returned to the atmosphere. It comprises a series of interconversions of carbon compounds beginning with the production of carbohydrates by plants during photosynthesis, proceeding through animal consumption, and ending and beginning again in the decomposition of the animal or plant or in the exhalation of carbon dioxide by animals.
cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and intervening pause. 
)
Events of the cardiac cycle.
cell cycle the cycle of biochemical and morphological events occurring in a reproducing cell population; it consists of: the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, a relatively quiescent period; the M phase, consisting of the four phases of mitosis; and the G1 phase of interphase, which lasts until the S phase of the next cycle.
citric acid cycle tricarboxylic acid c.
Cori cycle the mechanism by which lactate produced by muscles is carried to the liver, converted back to glucose via gluconeogenesis, and returned to the muscles.
γ-glutamyl cycle a metabolic cycle for transporting amino acids into cells.
Krebs cycle tricarboxylic acid c.
Krebs-Henseleit cycle urea c.
menstrual cycle the period of the regularly recurring physiologic changes in the endometrium, occurring during the reproductive period of female humans, culminating in partial shedding of the endometrium and some bleeding per vagina (menstruating). 
)
Changes in the menstrual cycle in the human female. Solid lines indicate the course of events when the oocyte is not fertilized; dotted lines indicate the course of events when fertilization occurs. Arrows indicate the actions of hormones of the pituitary and the ovary in regulating the cycle.
mosquito cycle that period in the life of a malarial parasite that is spent in the body of the mosquito host.
nitrogen cycle the steps by which nitrogen is extracted from the nitrates of soil and water, incorporated as amino acids and proteins in living organisms, and ultimately reconverted to nitrates: (1) conversion of nitrogen to nitrates by bacteria; (2) the extraction of the nitrates by plants and the building of amino acids and proteins by adding an amino group to the carbon compounds produced in photosynthesis; (3) the ingestion of plants by animals, and (4) the return of nitrogen to the soil in animal excretions or on the death and decomposition of plants and animals.
ornithine cycle urea c.
ovarian cycle the sequence of physiologic changes in the ovary involved in ovulation.
reproductive cycle the cycle of physiologic changes in the female reproductive organs, from the time of fertilization of the oocyte through gestation and parturition.
sex cycle , sexual cycle
1. the physiologic changes recurring regularly in the genital organs of nonpregnant female mammals; in humans, the menstrual cycle.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle the final common pathway for the oxidation to CO2 of fuel molecules, most of which enter as acetyl coenzyme A; it also provides intermediates for biosynthetic reactions and generates ATP by providing electrons to the electron transport chain. 
)
Tricarboxylic acid cycle. Diagrammatic representation of reactions by which carbon chains of sugars, fatty acids, and amino acids are metabolized to yield carbon dioxide. Water produced by the cycle and components of the high-energy phosphate pool generated by the associated electron chain are not shown.
urea cycle a series of metabolic reactions in the liver, by which ammonia is converted to urea using cyclically regenerated ornithine as a carrier. 
)
Urea cycle. Diagrammatic representation of reactions by which excess nitrogen in the form of ammonia is converted to soluble urea, using l-ornithine as a recyclable carrier.
uterine cycle the phenomena occurring in the endometrium during the menstrual cycle, preparing it for implantation of the blastocyst.
visual cycle the cyclic interconversion of 11-cis- retinal and all-trans- retinal and association with opsins, creating an electric potential and initiating the cascade generating a sensory nerve impulse in vision. 
)
Visual cycle of retinal rod cells; an analogous cycle occurs with iodopsins in the cones.
| cell cycle n. The series of biochemical and structural events involving the growth, replication, and division of a eukaryotic cell. |
cell cycle,
n divided into five stages, the resting phase (G0), the first phase of growth and protein synthesis (G1), the DNA synthesis phase (S), a second phase of growth and protein synthesis (G2), and a final phase in which the cell enters mitosis (M).
)
Cell cycle.
cell, centrioles of (sen´trēōls),
n.pl cylinder-shaped organelles that contain microtubules. Function is to organize spindle fibers during cell division.
cell, connective tissue,
n the fibroblast, which for purposes of clarity is characterized by such terms as
perivascular connective tissue cell or
young connective tissue cell.
perivascular connective tissue cell or
young connective tissue cell.
cell count,
n the number of cells contained in a unit volume; usually refers to red and/or white blood cells in a unit volume of blood.
cell culture,
n living cells that are maintained in vitro in artificial media of serum and nutrients for the study and growth of certain strains, experiments in controlling diseases, or study of the reaction to certain drugs or agents.
cell cycle,
n the sequence of events that occur during the growth and division of tissue cells.
cell, cytoplasm of
n the aqueous part of the cell in which are suspended all the organelles and inclusions. Site of all metabolic activities in the cell.
cell death,
n the point in the process of dying at which vital functions have ceased at the cellular level. It precludes the use of tissue or organs as transplant donors.
cell, defense,
n a cell, mobilized within inflamed, irritated, or otherwise diseased tissue, that acts as a protective element to neutralize or wall off the foreign irritant. Defense cells include plasma cells, polymorphonuclear leukocytes, and the cells of the reticuloendothelial system.
cell, dendritic (sel dendrit´ik),
n the immune cells involved in the activation of T cells and B cells. They are primarily found in exposed tissue such as skin, the lungs, the stomach and intestines, and the membranes of the nose, but they are also found in blood. Not to be confused with dendrites.
cell differentiation,
n the development of the cells into the various basic cell units of tissue: the epithelial cell and the nerve cell, which arise from the ectodermal tissue layer of the embryo; and the blood, muscle, bone, cartilage, and other connective tissue cells, which arise from the mesodermal tissue of the embryo. The mature tissue cell has many intermediary, transitional forms that are sequential in their development from the primitive, less differentiated anlage cell forms. These intermediary forms are evident clinically in disease in blood dyscrasias, tumors, and inflammation and in health in the normal processes of growth, development, healing, and repair.
cell, endoplasmic reticulum of,
n See endoplasmic reticulum.
cell, endosteal,
n a reticular cell that is modified and identified by its location; the endosteum is a condensation of the stroma of the bone marrow.
cell, filaments of,
n.pl threadlike structures the function of which is to support the cytoskeleton; also integral parts of intercellular junctions.
cell, germ,
n a cell of an organism the function of which is to reproduce an entity similar to the organism from which the germ cell originated. Germ cells are characteristically haploid.
cell, giant,
n a large cell frequently having several nuclei.
cell, Golgi complex in,
n See Golgi apparatus.
cell homeostasis,
n See homeostasis, cell.
cell, homeostasis of
n See homeostasis, cell.
cell, inclusions of,
n.pl nonliving bodies, by-products of cellular metabolism present in the cytoplasm.
cell, Langerhans,
n.pr star-shaped cells of unknown function that appear to be permanent residents of the epithelium.
cell, lysosomes in
n.pl membranous organelles produced from the Golgi complex; contain hydrolytic enzymes, which aid intracellular digestion.
cell membrane,
n the outer covering of a cell. The membrane controls the exchange of materials between the cell and its environment.
cell, membrane of, transport through,
n the movement of biomolecules into and out of cells. See diffusion, osmosis, active transport, phagocytosis.
cell, mesenchymal
n an embryonic connective tissue cell with an outstanding capacity for proliferation and capable of further differentiation into reticular cells or osteoblasts. When persisting in the adult organism, the cells are usually arranged in loose connective tissue along the small blood vessels or in reticular fibers. They are identified by their location and capacity to differentiate into other cell types, such as smooth muscle cells in the formation of new arteries, phagocytes in inflammatory processes, and bone cells in the formation of new bone tissue.
cell, microtubules of,
n.pl See microtubule.
cell, mitochondria of,
n.pl See mitochondria.
cell, mucous,
n a mucous-secreting cell.
cell, nucleus of,
n See nucleus.
cell, outer, of the dental papillae,
n an outer cell of the dental papilla within the concavity of the enamel organ that will differentiate into dentin-secreting cells or odontoblasts.
cell, plasma,
n a cell of disputed origin (lymphatic versus undifferentiated mesenchymal cell) that is seen in chronic inflammation and certain disease states and tumors but not normally in the circulating blood. The cell is larger than a lymphocyte and has a cartwheel-like, eccentric nucleus with basophilic nuclear chromatin peripherally located. The cells synthesize antibodies (immunoglobulins).
cell, progenitor,
n a cell that is able to transform into different types of cells through replication and differentiation.
cell, replication,
n See mitosis.
cell, reticular,
n a cell of reticular connective tissue, such as in the stroma of the bone marrow, that retains both osteogenic and hematopoietic potencies; it is identified by its location, morphology, potency, and direct origin from mesenchymal cells.
cell, serous,
n a specialized glandular epithelial cell that produces enzymatic secretions. These cells have a rounded nucleus and special secretory granules, or vesicles, in their cytoplasm. Serous cells include the acinar cells of the salivary glands and pancreas, gastric chief cells, and intestinal Paneth cells.
cell, somatic
n a cell that forms parts of the body, including the cells of the skin, bone, blood, connective tissue, and internal organs. From the Greek word
soma, meaning “body.”
soma, meaning “body.”
cell, stem,
n.pl the cells in the bone marrow from which all blood cells originate.
cell, typical,
n See cell.
cell wall,
n See cell membrane.
cell
1. the basic structural unit of living organisms.
2. a small more or less enclosed space.
All living cells arise from other cells, either by division of one cell to make two, as in mitosis and meiosis, or by fusion of two cells to make one, as in the union of the sperm and ovum to make the zygote in sexual reproduction.
All cells are bounded by a structure called the cell membrane or plasma membrane, which is a lipid bilayer composed of two layers of phospholipids. Each layer is one molecule thick with the charged, hydrophilic end of the lipid molecules on the surface of the membrane and the uncharged hydrophobic fatty acid tails in the interior of the membrane.
Cells are divided into two classes, eukaryotic cells and prokaryotic cells:
Eukaryotic cells have a true nucleus, which contains the genetic material, composed of the chromosomes, each of which is a long linear deoxyribonucleic acid (DNA) molecule associated with protein. The nucleus is bounded by a nuclear membrane, which is composed of two lipid bilayer membranes.
Prokaryotic cells, the bacteria, have no nucleus, and their genetic material, consisting of a single circular naked DNA molecule, is not separated from the rest of the cell by a nuclear membrane.
Eukaryotic cells are larger and more complex than prokaryotic cells. They also have membrane-bounded structures, such as mitochondria, chloroplasts, Golgi apparatus, endoplasmic reticulum and lysosomes, that prokaryotic cells lack.
The contents of a cell are referred to collectively as the protoplasm. In eukaryotic cells the contents of the nucleus are referred to as nucleoplasm and the rest of the protoplasm as the cytoplasm.
The lipid bilayer of eukaryotic cells is impermeable to many substances, such as ions, sugars and amino acids; however, membrane proteins selectively move specific substances through the cell membrane by active or passive transport. Water, gases such as oxygen and carbon dioxide, and nonpolar compounds pass through the cell membrane by diffusion. Materials can also be engulfed and taken into the cell enclosed in a portion of the cell membrane. This is called phagocytosis when solids are ingested and pinocytosis when liquids are ingested. The reverse process is called exocytosis. All of these processes permit the cell to maintain an internal environment different from its exterior. See also body fluids.
The cells of the body differentiate during development into many specialized types with specific tasks to perform. Cells are organized into tissues and tissues into organs. Embedded in the cell membrane are a wide range of molecules that vary with the cell type and are typically composed of proteins or glycoproteins that have a cytoplasmic transmembrane and external domains. These molecules serve as cell receptors and are involved in signal transduction for a wide range of ligands, including hormones, cytokines and incidentally serve as receptors for viruses and drugs.
See also betz cells, gaucher's cells, golgi's cells, hela cells, hürthle cell, kupffer's cells, merkel cell, mesangial cell, neuroendocrine cell.
)
Structure of the cell as seen by light microscopy. By permission from Guyton R, Hall JE, Textbook of Medical Physiology, Saunders, 2000
accessory c's
macrophages involved in the processing and presentation of antigens making them immunogenic.
acinar cell, acinous cell
any of the cells lining an acinus, especially applied to the zymogen-secreting cells of the pancreatic acini.
adherent cell
one that adheres to the glass or plastic container in cell cultures, to form the monolayer. See also cell culture.
alpha c's
1. cells in the islets of Langerhans that secrete glucagon.
2. acidophilic cells of the anterior pituitary.
APUD c's
see apud cells.
argentaffin c's
enterochromaffin cells containing cytoplasmic granules capable of reducing silver compounds, located throughout the gastrointestinal tract, chiefly in the basilar portions of the gastric glands and the crypts of Lieberkühn. They secrete serotonin.
band cell
an immature neutrophil in which the nucleus is not lobulated but is in the form of a continuous band, horseshoe shaped, twisted or coiled. Called also band-form granulocyte and stab cell.
basal cell
an early keratinocyte, present in the basal layer of the epidermis.
basket c's
cells in the cerebellar cortex whose axons carry basket-like groups of fibrils which enclose the cell body of each Purkinje cell.
beta c's
1. basophilic cells in the pancreas that secrete insulin and make up most of the bulk of the islets of Langerhans; they contain granules that are soluble in alcohol.
2. basophilic cells of the anterior pituitary.
blood cell
one of the formed elements of the blood. See also blood.
cell body
the nucleus of the cell and the adjacent cytoplasm in cells which have processes, e g. neurons which consist of a cell body, an axon and dendrites.
bone cell
a nucleated cell in the lacunae of bone. Called also osteocyte.
cartilage cell
chondrocyte.
chromaffin c's
cells whose cytoplasm shows fine brown granules when stained with potassium bichromate, occurring in the adrenal medulla and in scattered groups in various organs and throughout the body.
cleavage cell
any of the cells derived from the fertilized ovum by mitosis; a blastomere.
cell count
cell culture
see cell culture.
cell cycle
see cell cycle.
daughter cell
a cell formed by division of a mother cell.
cell dehydration
fluid loss from cells due to elevation of the osmotic pressure of blood and tissue fluid; a potent stimulus to thirst.
dendritic cell
macrophage-like cells with long, filamentous processes located in the cortex of lymph nodes and the skin. Important in antigen trapping, processing and presentation. See also langerhans' cell.
cell differentiation
the process whereby cells become specialized usually with concurrent loss of reproductive capacity.
embryonic stem cell
a stem cell of fetal origin. See stem cell (below).
epithelioid cell
enlarged macrophages with enlarged lysosomes and much endoplasmic reticulum. May fuse to form multinucleated giant cell (below).
epsilon cell
one of the groups of acidophilic cells in the adenohypophysis. Contains granules that stain with azocarmine dye.
foam cell
a cell with a vacuolated appearance due to the presence of complex lipoids; seen in xanthoma.
cell fusion
see syncytial giant cell.
ganglion cell
a large nerve cell, especially one of those of the spinal ganglia.
germ cell
see germ cell.
giant cell
a very large, multinucleate cell; applied to megakaryocytes of bone marrow, to giant cells formed by coalescence and fusion of macrophages occurring in infectious granulomas and about foreign bodies, and to certain cancer cells.
glial c's
neuroglial cells.
goblet cell
a unicellular mucous gland found in the epithelium of various mucous membranes, especially that of the respiratory passages and intestines.
granular cell
one containing granules, such as a keratinocyte in the stratum granulosum of the epidermis, when it contains a dense collection of darkly staining granules.
gustatory cell
see taste bud.
heart failure c's, heart lesion c's
iron-containing, rust-colored macrophages found in the pulmonary alveoli in congestive heart failure.
helmet cell
schistocyte.
helper cell
a subset of T lymphocytes which cooperate with B and other T lymphocytes for the synthesis of antibodies to many antigens; they play an integral role in immunoregulation.
hybrid cell
a mononucleate cell produced from a binucleate heterokaryon after the latter undergoes mitosis. Such cells are initially unstable, tending to lose randomly some of the double complement of chromosomes. Used for mapping genes to particular chromosomes. See also heterokaryon, hybridoma.
immunologically competent cell
see immunocyte.
interstitial c's
the cells of the connective tissue of the ovary or of the testis (Leydig's cells) which furnish the internal secretion of those structures, i.e. testosterone.
islet c's
cells composing the islets of Langerhans in the pancreas. See alpha cells, beta cells (above).
juxtaglomerular c's
specialized cells, containing secretory granules, located in the tunica media of the afferent glomerular arterioles. They cause aldosterone production by secreting the enzyme renin and play a role in the regulation of blood pressure and fluid balance.
K c's, killer c's
T lymphocytes or null lymphocytes that have cytotoxic activity against target cells coated with specific IgG antibody.
lacis cell
accumulation of cells between the arterioles at the glomerular hilus. Called also granular cell.
lacunar cell
precursor of the malignant interdigitating reticular cell in Hodgkin-like lymphoma in humans.
LE cell
a mature neutrophilic polymorphonuclear leukocyte characteristic of lupus erythematosus. See also lupus erythematosus (le) cell.
Leydig's c's
interstitial cells of the testis, which secrete testosterone.
cell line
see cell culture.
lutein c's
the plump, pale-staining, polyhedral cells of the corpus luteum.
lymph cell
lymphocyte.
lymphoid c's
lymphocytes and plasma cells.
mast cell
a connective tissue cell that has basophilic, metachromatic cytoplasmic granules that contain histamine, heparin, hyaluronic acid, slow-reacting substance of anaphylaxis (SRS-A), and, in some species, serotonin. Have Fc receptors specific for IgE in the cell membrane.
cell-mediated immune reaction
see cellular immunity.
cell migration
movement of cells from their place of origin to other tissues; one of the fundamental processes of development.
microglial cell
see microglia. See also neuroglia cells (below).
milk cell count
see milk cell counts.
mother cell
a cell that divides to form new, or daughter, cells.
Mott cell
a plasma cell with large, clear cytoplasmic pockets.
natural killer c's, NK c's
cells capable of mediating cytotoxic reactions without themselves being specifically sensitized against the target.
nerve cell
any cell of the nervous system; a neuron.
cell nests
see isogenous groups.
neuroglia c's, neuroglial c's
see neuroglia.
null c's
lymphocyte-like cells that lack specific antigen receptors and other surface markers characteristic of B and T lymphocytes; they include K and NK cells; their numbers are elevated in active systemic lupus erythematosus and other disease states.
olfactory c's
a set of specialized cells of the mucous membrane of the nose; the receptors for smell.
parafollicular c's
see c cell.
Pick's c's
round, oval or polyhedral cells with foamy, lipid-containing cytoplasm found in the bone marrow and spleen in Niemann-Pick disease.
plasma cell
a spherical or ellipsoidal cell with a single, eccentrically placed nucleus containing dense masses of chromatin in a wheel-spoke arrangement, an area of perinuclear clearing which contains the Golgi apparatus, and generally abundant cytoplasm. Plasma cells are produced by cell division of B lymphocytes following antigen stimulation and are involved in the synthesis and release of antibody. Called also plasmacyte and plasmocyte.
prickle cell
a dividing keratinocyte of the prickle-cell layer of the epidermis, with delicate radiating process connecting with other similar cells.
prokaryotic cell
see prokaryote.
Purkinje's c's
large branching cells of the middle layer of the cerebellar cortex.
red cell, red blood cell
erythrocyte.
Reed-Sternberg c's
giant histiocytic cells, typically multinucleate, which are the common histological characteristic of Hodgkin's disease in humans.
reticular c's
the cells forming the reticular fibers of connective tissue; those forming the framework of lymph nodes, bone marrow and spleen. They are weakly phagocytic, stromal in origin and are distinct from the monocyte-macrophage system.
reticuloendothelial cell
a cell of the reticuloendothelial system.
Schwann cell
any of the large nucleated cells whose cell membrane spirally enwraps the axons of myelinated peripheral neurons supplying the myelin sheath between two nodes of Ranvier.
Sertoli c's
elongated cells in the tubules of the testes to which the spermatids become attached; they provide support, protection and, apparently, nutrition until the spermatids are transformed into mature spermatozoa.
sickle cell
a crescentic or sickle-shaped erythrocyte seen in some humans and deer. The abnormal shape caused by the presence of varying proportions of hemoglobin S.
signet-ring cell
a cell in which the nucleus has been pressed to one side by an accumulation of intracytoplasmic mucin.
somatic c's
the cells of the body other than the germ cells.
cell sorting
see fluorescence-activated cell sorter.
cell specialization
conversion of a simple cell type into a specialized cell type capable of a special function, e.g. a secretory cell; a major part of the growth of an embryo and the differentiation of basic mesenchymal tissue into specialized organs.
spindle cell
spindle shaped cells of the dermis or subcutis; principal component of spindle cell tumors.
spur cell
spiculed mature erythrocyte.
squamous c's
flat, scalelike epithelial cells.
stab cell
see band cell (above).
stellate cell
any star-shaped cell, as a Kupffer cell or astrocyte, having many filaments extending in all directions.
stem cell
1. any precursor cell.
2. a primitive hematopoietic cell that is capable of self-replicating or differentiating into precursor cells of erythrocytes or any of the leukocytes.
stipple cell
an erythrocyte containing granules that take a basic or bluish stain with Wright's stain.
suppressor c's
a not well defined subset of T lymphocytes that are reported to inhibit antibody and cell-mediated immune responses. They may play a role in immunoregulation, and are believed to be abnormal in various autoimmune and other immunological disease states. See also T lymphocytes.
target cell
1. an abnormally thin erythrocyte showing, when stained, a dark center and a peripheral ring of hemoglobin, separated by a pale, unstained zone containing less hemoglobin; seen in various anemias and other disorders. Called also codocyte.
2. any cell selectively affected by a particular agent, such as a hormone or drug.
3. cell containing nonself antigens in its cell membranes that is a target for nonimmune and immune cytolysis, e.g. virus-infected or tumor cell.
taste c's
cells in the taste buds associated with the nerves of taste.
cell therapy
see glandular therapy.
totipotential cell
an embryonic cell that is capable of developing into any type of body cell.
Türk's cell
a lymphocyte with increased basophilia.
visual c's
the neuroepithelial elements of the retina.
white cell, white blood cell
leukocyte.
cycle
a succession or recurring series of events.
cardiac cycle
a complete cardiac movement, or heartbeat, including systole, diastole, and the intervening pause.
The cycle includes eight separate phases: (1) isovolumetric contraction; (2) maximum ejection; (3) reduced ejection; (4) protodiastole (onset of ventricular relaxation); (5) isovolumetric relaxation; (6) rapid flow; (7) diastasis (onset of atrial contraction); (8) atrial systole.
cell cycle
the cycle of biochemical and morphological events occurring in a dividing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, for gap 2, the interval between S and M; the M phase, for mitosis, consisting of the four phases of mitosis; and the G1 phase, which lasts from the end of M until the start of S phase of the next cycle. Fully differentiated cells are nondividing and are said to be in G0.
)
Cell cycle. By permission from Booth DM, Small Animal Clinical Pharmacology and Therapeutics, Saunders, 2000
citric acid cycle
estrus cycle
see estrous cycle.
Krebs cycle
ovarian cycle
the sequence of physiological changes in the ovary involved in ovulation. See also ovulation and reproduction.
reproductive cycle
the cycle of physiological changes in the reproductive organs, from the time of fertilization of the ovum through gestation and parturition. See also reproduction.
sex cycle, sexual cycle
1. the physiological changes recurring regularly in the reproductive organs of female mammals when pregnancy does not supervene.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle
urea cycle
a cyclic series of reactions that produce urea, a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney. See also urea.
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