Potten, C. S. Radiation, the Ideal Cytotoxic Agent for Studying the Cell Biology of Tissues such as the Small Intestine. Radiat. Res. 161, 123–136 (2004). Epithelial tissues are highly polarized, with the proliferative compartment subdivided into units of proliferation in many instances. My interests have been in trying to understand how many cellular constituents exist, what their function is, and what the intercommunicants are that ensure appropriate steady-state cell replacement rates. Radiation has proven to be a valuable tool to induce cell death, reproductive sterilization, and regenerative proliferation in these systems, the responses to which can provide information on the number of regenerative cells (a function associated with stem cells). Such studies have helped define the epidermal proliferative units and the structurally similar units on the dorsal surface of the tongue. The radiation responses considered in conjunction with a wide range of cell kinetic, lineage tracking and somatic mutation studies together with complex mathematical modeling provide insights into the functioning of the proliferative units (crypts) of the small intestine. Comparative studies have then been undertaken with the crypts in the large bowel. In the small intestine, in which cancer rarely develops, various protective mechanisms have evolved to ensure the genetic integrity of the stem cell compartment. Stem cells in the small intestinal crypts are intolerant of genotoxic damage (including that induced by very low doses of radiation); they do not undergo cell cycle arrest and repair but commit an altruistic TP53-dependent cell suicide (apoptosis). This process is compromised in the large bowel by BCL2 expression. Recent studies have suggested a second genome protection mechanism operating in the stem cells of the small intestinal crypts that may also have a TP53 dependence. Such studies have allowed the cell lineages and genome protection mechanisms operating the small intestinal crypts to be defined.

Crypt survival curves were obtained for normal and mutant <tex-math>${\rm W}/{\rm W}^{{\rm v}}$</tex-math> mice. This latter have stem cell defects in their germ cells, melanocytes, and haematopoietic tissue. The crypt survival curves were similar in shape and slope for normal and <tex-math>${\rm W}/{\rm W}^{{\rm v}}$</tex-math> mice suggesting that the number and split-dose recovery of intestinal crypt stem cells are not affected by the mutation in the <tex-math>${\rm W}/{\rm W}^{{\rm v}}$</tex-math> mice. This was further confirmed by a split-dose assay designed to estimate the single-cell survival curve shoulder size and hence the cellular multiplicity in the crypt survival curve, i.e., the number of cryptogenic (stem) cells per crypt.

The response of mouse hair follicle melanocytes in four distinct phases of their life cycle, to X-rays or actinomycin D has been studied. X-rays were found to have a destructive effect on both G 0 and dividing amelanotic melanocytes, while actinomycin D appeared to have no effect. For melanocytes nearing the end of the division phase and the beginning of the melanogenic phase, X-rays are still destructive; however, the sensitivity at this stage is less than when the cells are dividing. Actinomycin D produces growing follicle pigmented cell distribution abnormalities when given to follicles containing melanocytes at the end of their dividing phase. These pigment abnormalities are characteristically scattered throughout the follicle in both the upper external sheath regions and the lower follicle bulb. Both X-rays and actinomycin D cause growing pigmented hair follicles to show gross abnormalities of lower follicle pigmented cell distribution. The growing follicle, nine days after plucking, is very sensitive to treatments with either agent. Pigmented cells are seen at abnormal locations 24-72 h after doses of 25 rads of X-rays or 25 μg/kg of actinomycin D. Some speculations are made regarding the local concentration of actinomycin after subcutaneous and intraperitoneal injections. The mechanisms involved in the pigment abnormalities are discussed with particular reference to gene derepression.

A study has been made of the depigmentation response of resting hair follicle melanoblasts to x-rays. The survival curve for these cells extrapolates to a value of about 6 and the D 0 value is about 200 rads. Three days after initiating hair growth the physiological state of the cells has altered and the survival curve shows an increase in the D 0 and $D_{{\rm q}}$ . This is accompanied by a probable increase in cell number. Split-dose experiments with cells believed to be in G 0 show a recovery factor of about 6 and a recovery curve which reaches a plateau at this value after time intervals of 6 hours or more. These results are obtained when the experiments are scored in the second postirradiation growth phase. The survival curve obtained when variable second doses are delivered 8 hours after a priming dose of 500 rads shows the same D 0 value as the single dose curve but extrapolates to about 2.7. These data can be taken to indicate that there are not more than two melanoblasts in the resting zigzag hair follicle. Histological data appear to support this view. The relevance of these data to split-dose studies in general is discussed.

Experiments were conducted on Strong F or DBA-1 mouse skin in telogen (resting stage) to determine the effect of local tissue oxygen tension on sensitivity to 300-kVp x-rays. Radiation response was measured by counting the proportion of pigmented follicles, and the results were expressed as dose-response curves. During irradiation, the vascular and environmental oxygen supplies were controlled by the use of a skin-fold technique and a vascular clamp. By applying a clamp and passing nitrogen over the skin for 15 minutes prior to irradiation, sensitivity could be reduced by a factor of 2.89 from the maximum obtained in pure oxygen. When the only supply of oxygen was by diffusion from pure oxygen outside the skin, sensitivity was near maximum, indicating appreciable diffusion to a depth of 200 to 300 microns. When air was the external source of oxygen, sensitivity was near minimum, indicating that little oxygen diffuses to this depth from air. The sensitivity under normal conditions, in air with no clamp, was slightly lower than that obtained in pure oxygen.