Adipose tissue: A type of connective tissue that stores fat.

Adult stem cell: An undifferentiated cell found in a differentiated tissue that can renew itself and (with certain limitations) differentiate to yield all the specialized cell types of the tissue from which it originated. (NIH)

Allogeneic cell transplantation: Transplantation of cells from one individual to another of the same species.

Amniotic fluid: Fluid that fills the innermost membrane, the amnion, that envelopes the developing embryo or fetus.

Amnion: Innermost of the extra-embryonic membranes enveloping the embryo in utero and containing the amniotic fluid. (SMD)

Aneuploid: Having an abnormal number of chromosomes. (SMD)

Angiogenesis: Development of new blood vessels. (SMD)

Antigen: A substance that, when introduced into the body, stimulates the production of protein molecules called antibodies that can bind specifically to the substance.

Astrocyte: A type of nerve cell that has supportive and metabolic functions rather than signal conduction.

Autologous: In transplantation, referring to a graft in which the donor and recipient areas are in the same individual. (SMD)

Autosome: Any chromosome other than a sex chromosome, that is, any chromosome other than an X or a Y. (SMD)

Bacteria: Any of numerous unicellular microorganisms, existing either as free living organisms or as parasites, and having a broad range of biochemical, often pathogenic properties.

Blastocyst: (a)Name used for an organism at the blastocyst stage of development. (CR) (b) A preimplantation embryo of about 150 to 200 cells. The blastocyst consists of a sphere made up of an outer layer of cells (the trophectoderm), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass). (NIH)

Blastocyst stage: An early stage in the development of embryos, when (in mammals) the embryo is a spherical body comprising an inner cell mass that will become the fetus surrounded by an outer ring of cells that will become part of the placenta. (CR)

Bone marrow: The soft, fatty, vascular tissue that fills most bone cavities and is the source of red blood cells and many white blood cells.

Cardiomyoctes: Heart muscle cells.

Cartilage: A type of connective tissue that is firm but resilient. It is found in joints and also as supportive structure, for example in the ears.

Cell culture: Growth of cells in vitro on an artificial medium for experimental research. (NIH)

Cerebrospinal fluid: A blood serum-like fluid that bathes parts of the brain and the interior cavity of the spinal cord.

Chromosomes: Structures inside the nucleus of a cell, made up of long pieces of DNA coated with specialized cell proteins, that are duplicated at each mitotic cell division. Chromosomes thus transmit the genes of the organism from one generation to the next. (CR)

Clone: A line of cells that is genetically identical to the originating cell; in this case, a stem cell. (NIH)

Cord blood: Blood in the umbilical cord and placenta.

Cornea: Transparent tissue at the front of the eye.

Cryopreserved embryos: Embryos, generally those produced by in vitro fertilization exceeding the number that can be transferred for uterine implantation, that have been frozen.

Culture medium: The broth that covers cells in a culture dish, which contains nutrients to feed the cells as well as other growth factors that may be added to direct desired changes in the cells. (NIH)

Dental pulp: The soft part inside a tooth, containing blood vessels and nerves.

Diploid: Refers to the full complement of chromosomes in a somatic cell, distinct for each species (forty-six in human beings). (CR)

Diploid human cell: A cell having forty-six chromosomes. (CR)

Ectoderm: Upper, outermost layer of a group of cells derived from the inner cell mass of the blastocyst; it gives rise to skin nerves and brain. (NIH)

Edmonton protocol: A procedure (developed in Canada) for transplanting pancreatic islet cells to the liver of a patient with Type I diabetes.

Embryo: (a) In humans, the developing organism from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus. (NIH) (b) The developing organism from the time of fertilization until significant differentiation has occurred, when the organism becomes known as a fetus. An organism in the early stages of development. (CR)

Embryonic germ cells: Cells found in a specific part of the embryo/fetus called the gonadal ridge that normally develop into mature gametes. (NIH)

Embryonic stem cells: Primitive (undifferentiated) cells from the embryo that have the potential to become wide variety of specialized cell types. (NIH)

Embryonic stem cell line: Embryonic stem cells, which have been cultured under in vitro conditions that allow proliferation without differentiation for months to years. (NIH)

Endoderm: Lower layer of a group of cells derived from the inner cell mass of the blastocyst; it gives rise to lungs and digestive organs. (NIH)

Endometrium: The mucous membranes lining the uterus.

Endothelial: relating to a flat layer of cells lining the heart, for example, or blood vessels. (SMD)

Epidermal growth factor: A cell messenger protein that has effects including stimulation of epidermal development, in newborn animals it hastens eyelid-opening and tooth-eruption (SMD)

Ex vivo: Outside the body, frequently the equivalent of "in vitro."

Fate (of cell progeny): The normal outcome of differentiation of a cell's progeny.

Feeder layer: Cells used in co-culture to maintain pluripotent stem cells. Cells usually consist of mouse embryonic fibroblasts. (NIH)

Fertilization: The process whereby male and female gametes unite. (NIH)

Fetus: A developing human from usually two months after conception to birth. (NIH)

Fibroblast: A stellate (star-shaped) or spindle-shaped cell with cytoplasmic processes present in connective tissue, capable of forming collagen fibers. (SMD)

Gamete: Areproductive cell (egg or sperm). (CR)

Gamma-interferon: A type of small protein with antiviral activity, made by T lymphocytes.

Gastrulation:  The process whereby the cells of the blastocyst are translocated to establish three germ layers. Also sometimes used to mark the end of the blastocyst stage and the beginning of the next stage of embryonic development. (Based on SMD)

Gene: A functional unit of heredity that is a segment of DNA located in a specific site on a chromosome. A gene directs the formation of an enzyme or other protein. (NIH)  

Genome: The total gene complement of a set of chromosomes. (SMD)

Germ cells (or primordial germ cells): A gamete, that is, a sperm or egg, OR a primordial cell that can mature into a sperm or egg. (NRC)

Germ layers: The three initial tissue layers arising in the embryo-endoderm, mesoderm, and ectoderm-from which all other somatic tissue-types develop. (NRC)

Gonad: An organ that produces sex cells (testes or ovaries). (SMD)

Gonadal ridges: Embryonic structures arising in humans at about five weeks, eventually developing into gonads (either testes or ovaries).

Green Fluorescent Protein: A protein naturally occurring in some animals including jelly fish that spontaneously fluoresces. It can be used as a noninvasive marker in living cells by attaching it to different proteins and then letting it fluoresce so as to track the cell.

Haploid human cell: A cell such as an egg or sperm that contains only twenty-three chromosomes. (CR)

Hematopoietic stem cell: A stem cell from which all red and white blood cells develop. (NIH)

Hepatocyte: Liver cell.

Histocompatible: The immunological characteristic of cells or tissue that causes them to be tolerated by another cell or tissue; that allows some tissues to be grafted effectively to others. (NRC)

Hurler syndrome: A heritable condition involving deficiency of an enzyme (alpha-L-iduronidase), leading to abnormal accumulations of materials inside cells, then resulting in abnormal development of cartilage and bone and other systems. (SMD)

ICM cells: Cells from the inner cell mass, a population of cells inside the blastula that give rise to the body of the new organism rather than to the chorion or other supporting structures.

Immunodeficient: Unable to develop a normal immune response to, for example, a foreign substance.

Immunosuppressive drugs: Drugs that prevent or interfere with the development of an immunologic response. After a transplant, immunosuppressive drugs are usually necessary in order to prevent the recipient from rejecting the transplant.

Implantation: The attachement of the blastocyst to the uterine lining, and its subsequent embedding there. (Based on SMD)

In vitro fertilization (IVF): The union of an egg and sperm, where the event takes place outside the body and in an artificial environment (the literal meaning of "in vitro" is "in glass"; for example, in a test tube). (CR)

Inner cell mass: The cluster of cells inside the blastocyst. These cells give rise to the embryonic disk of the later embryo and, ultimately, the fetus. (NIH)

Karyotype: The chromosome characteristics (number, shape, etc) of an individual cell or cell line, usually presented as a systematized array in pairs. (SMD)

Leukemia inhibitory factor: A cell messenger protein originally noted for inhibition of mouse M1 myeloid leukemia cells that also has effects including inhibiting differentiation to maintain stem cells.

Lineage: The descendants of a common ancestor.

Long-term self-renewal: The ability of stem cells to renew themselves by dividing into the same non-specialized cell type over long periods (many months to years) depending on the specific type of stem cell. (NIH)

Lymphocyte: A motile cell formed in tissues such as the lymph nodes, that functions in the development of immunity.

Meiosis: A special process of cell division comprising two nuclear divisions in rapid succession that result in four cells (that will become gametes) with the haploid number of chromosomes. (Based on SMD)

Mesenchymal stem cells: Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage. (NIH)

Mesoderm: Middle layer of a group of cells derived from the inner cell mass of the blastocyst; it gives rise to.bone, muscle, and connective tissue. (NIH)

Metachromatic leukodystrophy: A heritable metabolic disorder, usually of infancy, characterized by myelin loss and other abnormalities of the white matter of the nervous system, leading to progressive paralysis and mental retardation or dementia. (SMD)

Mitochondria: Small, energy-producing organelles inside cells.

Mitochondrial DNA: Genetic material inside the mitochondria. Essentially all the mitochondria of an individual come from the cytoplasm of the egg, so all mitochondrial DNA is inherited through the maternal line.

Mitochondrial proteins: Proteins that are part of the mitochondria.

Mitosis: Cell division, resulting in two cells that each have the diploid number of chromosomes and are just like the original cell.

Morphology: Configuration or structure, shape.

Mutagenicity: Tendency to promote mutations, that is, genetic alterations.

Multipotent: As applied to stem cells, the ability to differentiate into at least two, more differentiated descendant cells.

Multipotent adult progenitor cells (MAPC): Cells isolated from bone marrow that can be differentiated into cells with characteristics of cartilage, fat, and bone.

Mycoplasma: A general category of microorganisms that shares some characteristics of bacteria.

Natural killer cell: A cell type of the immune system that destroys tumor cells and cells infected with some types of organisms.

Olfactory bulb: A part of the brain involved in detecting and discriminating among different smells.

Oligodendrocyte: A type of neuroglia, that is, a particular type of cell that is part of the nervous system with supportive and metabolic functions rather than signal conduction, this type forms the myelin sheath around nerve fibers.

Oocytes: Egg cells.

Osteogenesis imperfecta: A large and miscellaneous group of conditions of abnormal fragility and plasticity of bone, with recurring fractures on trivial trauma. (SMD)

Pancreas: An organ of the digestive system that secretes the hormones insulin and glucagon, as well as digestive enyzymes.

Pancreatic beta cells: Cells of the pancreas (located in pancreatic islets, or islets of Langerhans) that produce insulin.

Parkinson disease: A neurological syndrome usually resulting from deficiency of the neurotransmitter dopamine . . . ; characterized by rhythmical muscular tremors . . . (SMD)

Phenotypic characteristics: The genetically and environmentally determined physical characteristics of an organism.

Placenta: The oval or discoid spongy structure in the uterus from which the fetus derives it nourishment and oxygen. (NRC)

Pluripotent: having great developmental plasticity, as a pluripotent stem cell. Cells that can produce all the cell types of the developing body, such as the ICM cells of the blastocyst, are said to be pluripotent.

Polarity: The property of having two opposite poles, sides or ends (for example, humans have left-right polarity, also front-back polarity and head-tailward polarity).

Population doublings: The number of times cells growing in vitro have increased the total number of cells by a factor of 2 compared to the initial number of cells. Primitive streak: A band of cells appearing in the embryo at the start of the third week of development, that marks the axis along which the spinal chord develops.

Primordial germ cell: A gamete, that is, a sperm or egg, OR a primordial cell that can mature into a sperm or egg. (NRC)

Salivary gland: One of several pairs of glands in the mouth that secrete saliva.

Skin biopsy: Process of removing tissue, in this case skin, from living patients for diagnostic examination, or the tissue specimen obtained by that process. (SMD)

"Single-cell cloned": A procedure pertaining to cells in vitro in which the descendants of a single cell are physically isolated from other cells growing in a dish, and then expanded into a larger population.

Somatic cell nuclear transfer (SCNT): A method of cloning:  transfer of the nucleus from a donor somatic cell into an enucleated egg to produce a cloned embryo.

Somite: One of the longitudinal series of segments into which the body of many animals (including vertebrates) is divided. (Merriam-Webster on line)  

Stem cells: Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells and of undergoing differentiation into one or more specialized types of cells. (CR)

Stromal: Relating to the stroma of an organ or other structure, that is, its framework, usually of connective tissue, rather than its specific substance. (SMD)

Syngamy: The coming together of the egg and sperm at fertilization.

Thymus: An organ of the developing immune system, active mainly in childhood.

T-lymphocyte: A cell type of the immune system that matures in the thymus and is responsible for cell-mediated immunity.

Type-1A diabetes: A form of insulin dependent diabetes, usually becoming evident in childhood, resulting from an autoimmune reaction that destroys the pancreatic beta cells, so that the body cannot produce its own insulin. In those cases where the condition is not apparent until adulthood, it is called latent autoimmune diabetes of adulthood. (LADA)

Transcription factors: Specialized proteins that bind to specific sites on DNA and turn on or turn off the expression of different sets of genes.

Trophoblast: The extraembryonic tissue responsible for implantation, developing into the placenta, and controlling the exchange of oxygen and metabolites between mother and embryo. (NIH)

Twinning: Development of monozygotic twins, that is, when a very early embryo separates into two pieces, each of which continues development, so that two embryos actually come from one zygote.

Uterine: Pertaining to the uterus.

Virus: A submicroscopic pathogen composed essentially of a core of DNA or RNA enclosed by a protein coat, able to replicate only within a living cell.

Xenotransplantion: A transplant of tissue from an animal of one species to an animal of another species.

Zygote: The diploid cell that results from the fertilization of an egg cell by a sperm cell. (CR)