4. Categories of cell populations.

Cell populations can be divided into three categories:

Stem cell population.

Transit cell population.

Static population.

A stem cell population is one whose sole purpose is to divide to (1) first maintain its own population (i.e., self-renewal), and (2) produce cells for another population. Stem cells are undifferentiated. Classic examples of stem cell populations are basal cells in the epidermis of the skin, cells in the bone marrow, the cells in the crypts of Lieberkuhn in the intestine, and spermatogonia in the testis. Tissues and organs containing stem cell populations are referred to as self-renewing.

Transit cell populations are cells on their way from one place (stem cell compartment) to another place (end cell compartment). While in transit these cells may or may not divide. An example of a cell that divides while in transit is the nucleated red cell. A cell that does not divide is the reticulocyte in the bone marrow.

Static population are cells that loses cells throughout the life of the organism. These cells are fully differentiated and exhibit no, or at least little, detectable mitotic activity. Examples of such populations are found in adult nervous tissue and muscle.

5. Terms “vim”, “dim”, Multipotential Connective Tissue Cells, “rpm”, “fpm”.

Rubin and Casarett have defined five categories of cell populations in terms of their radiation sensitivities.

Vegetative Intermitotic Cells (VIM).

Differentiating Intermitotic Cells (DIM)

Multipotential Connective Tissue Cells.

These cells divide irreg­ularly, are more differentiated than either VIM or DIM cells, and are intermediate in sensitivity to radiation. Cells included in this category are endothelial cells (blood vessel liners) and fibroblasts (composing connective tissue).

Reverting Postmitotic Cells (RPM)

Fixed Postmitotic Cells (FPM)

6. Tissue organization (compartments).

Tissues and organs are made up of two compartments:

• the parenchymal compartment, containing the cells characteristic of that individual tissue or organ

• the stromal compartment, composed of connective tissue and vasculature, which makes up the supporting structure of the organ.

The parenchymal compartment of tissues and organs may be com­posed of one or more than one category of cells, as defined by Rubin and Casarett. The testis is an example of an organ that contains more than one category of cells: stem cells — type A spermatogonia (VIM cells); intermediate cells — type B spermatogonia, spermatocytes, and spermatids (DIM cells); and mature, functional cells — spermatozoa (FPM cells). Another example is the hemopoietic system: the bone marrow contains the undifferentiated stem cells, and the circulating blood contains the mature end cell. Two other examples are the skin and the intestinal tract.

Examples of tissues and organs whose parenchymal compartments are composed of only RPM cells or FPM cells are the liver, muscle, brain, and spinal cord.