Control points to Charter 7

1. Factors that influence on organ or tissue response to radiation.

The response of an organ or tissue to radiation depends on two factors such us:

• The inherent sensitivity of the various cell populations in that tissue or organ.

• The turnover kinetics of each population in the tissue (Do they divide, and if so, how often?).

Inherent sensitivity as defined by the loss of reproductive capacity for all cells is similar both in vivo and in vitro. Normal tissues differ widely in their responses to radiation.

There are two factors that influence on organ or tissue response to radiation

• the biologic stress on the cell

• the conditions to which the cell is exposed pre-irradiation and postirradation.

The greatest influence on radiosensitivity is the biologic stress placed on the cell, and that the most important biologic stress on the cell is the necessity for division. Cells which divided quickly would simply express the damage sooner and appear "sensitive" compared with those that divided more slowly, and would express their damage later and thus appear "resistant."

This is expressed Ancel and Vitemberger based on a a series of extensive experiments in mammalian systems.

2. Radiosensitivity of a cell.

Two scientists Bergonie and Tribondeau investigated radiosensitivity of a cell in 1906. They performed experiments on rodent testicles to further define this observed selective effect of radiation. They chose the testes because they contain mature cells (spermatozoa), which perform the primary function of the organ, and also contain immature cells (spermatogonia and spermatocytes), which have no function other than to develop into mature, functional cells. Not only do these different populations of cells in the testes vary in function, but their mitotic activity also varies — the immature spermatogonia divide often, whereas the mature spermatogonia never divide.

After irradiation of the testes, Bergonie and Tribondeau observed that the immature dividing cells were damaged after lower doses than were the mature nondividing cells. Based on these observations of the response of the different cell populations in the testes, they formulated a hypothesis concerning radiation sensitivity for all cells in the body. In general terms, their hypothesis states that ionizing radiation is more effective against cells that are actively dividing, are undifferentiated, and have a long dividing future.

3. Differentiation.

A differentiated cell is one that is specialized functionally and/or morphologically (structurally); it can be considered a mature cell, or end cell, in a population. The spermatozoan is a differentiated cell; it is the end cell in the population. Another example of a differentiated cell is the erythrocyte (red blood cell, or RBC). The major function of the RBC is to transport oxygen to cells of the body; the RBC differs from other cells in the body in that it does not have a nucleus. The average lifetime of RBCs in the circulating blood is 120 days

An undifferentiated cell has few specialized morphologic or functional characteristics; it is an immature cell whose primary function is to divide. Undifferentiated cells can be considered precursor, or stem, cells in a population. The spermatogonium is an undifferentiated cell — the stem cell for the mature spermatozoan. . The stem cell the erythroblast, is present in the bone marrow and is an undifferentiated cell that divides and supplies cells which will differentiate to become erythrocytes.

The process by which immature spermatogonia become mature spermatozoa is termed differentiation.