embryo that have the potential to become a wide variety of specialized cell types.

Endoderm

- 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.

Fertilization

- The process whereby male and female gametes unite.

Gene

- 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.

Genome

- The complete genetic material of an organism.

Germ cell

- 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.

Germ layers

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

Gonadal ridge

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

Graft-versus-host disease

- 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.

Hematopoietic stem cell (HSC)

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

Hepatic

- Relating to the liver.

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.

Immune system cells

- White blood cells or leukocytes that originate from the bone marrow. They include antigen-presenting cells, such as