dendritic cells, T and B lymphocytes, and neutrophils, among many others.

In vitro

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

In vitro fertilization (IVF)

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

In vivo

- In the living subject; a natural environment.

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. They are the source of embryonic stem cells.

Lipid

- 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.

Lymphocyte

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

Macrophage

- 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.

Mesenchyme

- 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.

Mesoderm

- 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.

Morula

- A solid mass of 12 or more cells that resembles a mulberry,