Cell Populations
For our purposes here, cell populations can be divided into three categories:
Stem cell population
Transit ceil population
3.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 arc referred to as self-renewing.
Transit cell populations are defined precisely by their name — they are cells on their way from one place (stem cell compartment) to another placc (end cell compartment). While in transit these cells may or may not divide. An example of a ccll that divides while in transit is the nucleated red cell. A cell that does not divide is the reticulocyte in the bone marrow; it simply receives precursor cells and sends them on their way into the peripheral blood.
The third population, a static population, 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.
3.Radiation Response of Cells
Based on the aforementioned cell population categories, Rubin and Casarett have defined five categories of cell populations (Table 7-1) in terms of their radiation sensitivities. The criteria used were histologic signs of cell death, not the loss of proliferative potential.
3I. Vegetative lntermitotic Cells (VIM).
VIM cells are rapidly dividing, undifferentiated cells that have a short lifetime. According to Bergonie and Tribondeau, these cells comprise the most radiation-sensitive group of cells in the body. Examples of VIM cells arc basal cells of the epidermis, crypt cells of the intestines, type A spermatogonia, and erythroblasts.
3.2Differentiating lntermitotic Cells (DIM).
DIM cells are produced by division of VIM cells and, although actively mitotic, they are more differentiated than VIM cells. Therefore, these cclls are less sensitive (or more resistant) to radiation than are the VIM cells. Examples of DIM cells are intermediate and type В spermatogonia.
3.3.Mnltipotential 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).
3.4.Reverting Postmitotic Cells (RPM).
Cells in this category normally do not undergo mitosis; however, they retain the capability of division under specific circumstances. RPM cells are long-lived as individuals and are more differentiated than cells of the previous categories; therefore, RPM cells are relatively radioresistant. Examples of RPM cells are liver cells and the mature lymphocyte. The mature lymphocyte is included in this category because of its mitotic characteristics—it does not usually divide, but has the capability of dividing when a stimulus is present. The lymphocyte also is a differentiated cell; however, in contrast to other RPM cells that arc relatively radioresistant, the mature lymphocyte is very radiosensitive. It is one important exception to the general law of Bergonie and Tribondeau.
3.5.Fixed Postmitotic Cells (FPM).
FPM cells do not divide. These cells are highly differentiated both morphologically and functionally; therefore, they are resistant to radiation. In fact, this category comprises the group of cells most resistant to radiation. Some of the cells in this category have long lives, whereas others are relatively short-lived. When the short-lived cells die, they are replaced by differentiating (DIMj cells. Other cells in this category may not be replaced if cell death occurs. Examples of cells in this category include some nerve cells, muscle cells, erythrocytes (RBCs), and spermatozoa.
Table 7-2 gives a classification of cells according to decreasing radiosensitivity, again using histologic signs of cell death as the deter- ч mining factor.