- An undifferentiated cell that is found in differentiated adult tissue, can renew itself, and can (with certain limitations) differentiate to yield all the specialized cell types of the tissue from which it originated.

- Any substance, usually a protein, that stimulates an immune response.

- Transplanted tissue that is derived from the intended recipient of the transplant. Such a transplant helps avoid complications of immune rejection.

- A preimplantation embryo of 30-150 cells and 4-7 days of age. 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).

- The soft, living tissue that fills most bone cavities and contains hematopoietic stem cells from which all red and white blood cells evolve. The bone marrow also contains mesenchymal (stroma) stem cells that a number of cells types come from, including chondrocytes, which produce cartilage.

- Refers to both hematopoietic cells and mesenchymal (stromal) cells.

- Refering to one of at least two types of multipotent stem cells: hematopoietic stem cell and mesenchymal stem cell.

- Nucleic acid-protein structures in the nucleus of a cell. Chromosomes are composed chiefly of DNA, the carrier of hereditary information. Chromosomes contain genes, working subunits of DNA that carry the genetic code for specific proteins, interspersed with large amounts of DNA of unknown function. A normal human body cell contains 46 chromosomes; a normal human gamete (egg or sperm), 23 chromosomes.

- The contents of a cell, other than the nucleus cytoplasm, consists of a fluid containing numerous structures, known as organelles, that carry out essential cell functions.

- Extension of a nerve cell, typically branched and relatively short, that receives stimuli from other nerve cells.

- The process whereby an unspecialized early embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell.

- A chemical, deoxyribonucleic acid, found primarily in the nucleus of cells. DNA carries the instructions for making all the structures and materials the body needs to function.

- The upper, outermost of the three primitive germ layers of the embryo that will give rise to the skin, hair, nails, nerve, and brain including the retina of the eye.

- 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.

- Cells found in a specific part of the embryo/fetus called the gonadal ridge, and normally develop into mature gametes.

- Primitive (undifferentiated) cell from the embryo that have the potential to become a wide variety of specialized cell types.

- The lower, inner of the three primitive germ layers of the embryo that will give rise to the epithelial layers of the lungs and bronchi, pharynx, gastrointestinal tract, liver, pancreas, and urinary bladder.

- The process whereby male and female gametes unite.

- A functional unit of heredity that is a segment of DNA and located in a specific site on a chromosome. Genes generally direct the formation of an enzyme or other protein.

- The complete genetic material of an organism.

- A gamete, that is, a sperm or egg, or a cell that can become a sperm or egg. All other body cells are somatic cells.

- The three initial tissue layers arising in the embryo— endoderm, mesoderm, and ectoderm—from which all other somatic tissue-types develop.

-Anatomic site in the early fetus where primordial germ cells are formed.

- A condition that occurs following bone marrow transplant in which the donor's immune cells, in the transplanted marrow, make antibodies against the host's tissues.

- A stem cell from which all red and white blood cells evolve.

- Relating to the liver.

- 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.

- White blood cells or leukocytes that originate from the bone marrow. They include antigen-presenting cells, such as dendritic cells, T and B lymphocytes, and neutrophils, among many others.

- From the Latin for, “in glass”; in a laboratory dish or test tube; an artificial environment.

- An assisted reproduction technique in which fertilization is accomplished outside the body.

- In the living subject; a natural environment.

- The cluster of cells inside the blastocyst. These cells give rise to the embryonic disk of the later embryo and ultimately the fetus. They are the source of embryonic stem cells.

- Any one of a group of fats or fat-like substances characterized by their insolubility in water and solubility in fat solvents such as alcohol, ether, and chloroform.

- A type of white blood cell that is part of the body's cellular immune system; present in the blood and lymphatic tissue.

- A lymphocyte that has left the circulation and settled and matured in a tissue. Because of their placement in the lymphoid tissues, macrophages serve as the major scavenger of the blood, clearing it of abnormal or old cells and cellular debris as well as pathogenic organisms.

- Connective tissue arising from multiple germ layers consisting of unspecialized cells. A number of cell types come from the mesenchyme, including the cells that give rise to collagen, muscle, cartilage, and bone.

- The middle of the three primitive germ layers of the embryo. These cells occur between the ectoderm and endoderm and give rise to most of the cardiovascular system, blood cells and bone marrow, the skeleton, smooth and striated muscles, and parts of the reproductive and excretory system.

- A solid mass of 12 or more cells that resembles a mulberry, occurring at 3 to 4 days after fertilization and that results from the cleavage of the zygote.

- Capable of differentiating into multiple cell types associated with different organs.

- Having the characteristic of potentially malignant growth.

- A stem cell that can give rise to the different types of cells of the nervous system. Neural stem cells are found in certain areas of the adult brain, in embryos, fetuses, newborns, and juveniles.

- The key data-processing cell of the nervous system. Each neuron has a cell body and one or more processes (extensions) called dendrites and axons. Neurons function by the initiation and conduction of electrical impulses that are transmitted to other neurons or cells.

- An egg, the female germ, or sex, cell produced in the ovaries.

- The oval or discoid spongy structure in the uterus from which the fetus derives its nourishment and oxygen

- The ability of a cell to differentiate into a cell type beyond the tissue in which it normally resides.

- Any cell of a plant or animal other than a germ cell or germ-cell precursor.

- The transfer of a cell nucleus from a somatic cell into an egg from which the nucleus has been removed.

- A cell that has the ability to divide for indefinite periods in culture and to give rise to specialized cells.

- A non-blood cell that is derived from blood organs, such as bone marrow or fetal liver, which is capable of supporting growth of blood cells in vitro. Stromal cells that make up the matrix within the bone marrow are derived from the mesenchyme and give rise to fat and cartilage.

- Proteins on the surface of cells that are capable of detection by antibodies or other means. These may stimulate an immune response.

- A type of white blood cell that is of crucial importance to the immune system. Immature T cells (termed T-stem cells) migrate to the thymus gland in the neck, where they differentiate into various types of mature T cells and become active in the immune system. T cells that are potentially activated against the body's own tissues are normally killed or changed (“down-regulated”) during this maturation process.

- Growth of tissue in vitro on an artificial media for experimental research.

- Not having changed to become a specialized cell type.

- Also known as a leukocyte. These cells normally protect against infection by, for example, ingesting bacteria or secreting antibodies. White blood cells are formed from the undifferentiated stem cell that can give rise to all blood cells. Those in the bone marrow may become any of the five types of white blood cells. Those in the spleen and lymph nodes may become lymphocytes, or monocytes, and those in the thymus can become lymphocytes (T-lymphocytes).

- The cell formed by the union of male and female germ cells (sperm and egg, respectively).