Nonrandom distribution of mouse spermatogonial stem cells surviving fission neutron irradiation.

Colony formation by surviving spermatogonial stem cells was investigated by mapping pieces of whole mounted tubuli at intervals of 6 and 10 days after doses of 0.75 and 1.50 Gy of fission neutron irradiation. Colony sizes, expressed in numbers of spermatogonia per colony, varied greatly. However, the mean colony size found in different animals was relatively constant. The mitotic indices in large and small colonies and in colonies in different epithelial stages did not differ significantly. This finding suggests that size differences in these spermatogenic colonies are not caused by differences in growth rate. Apparently, surviving stem cells start to form colonies at variable times after irradiation. The number of colonies per unit area varied with the epithelial stages. Many more colonies were found in areas that during irradiation were in stages IX-III (IX-IIIirr) than in those that were in stages IV-VII (IV-VIIirr). After a dose of 1.50 Gy, 90% of all colonies were found in areas IX-IIIirr. It is concluded that the previously found difference in repopulation after irradiation between areas VIII-IIIirr and III-VIIIirr can be explained not by differences in colony sizes and/or growth rates of the colonies in these areas but by a difference in the number of surviving stem cells in both areas. In area XII-IIIirr three times more colonies were found after a dose of 0.75 Gy than after a dose of 1.50 Gy. In area IV-VIIirr the numbers of colonies differed by a factor of six after both doses. This finding indicates that spermatogonial stem cells are more sensitive to irradiation in epithelial stages IV-VII than in stages XII-III. In control material, spermatogonia with a nuclear area of 70-110 micron2 are rare. However, especially 6 days after irradiation, single cells of these dimensions are rather common. These cells were found to lie at random over the tubular basement membrane with no preference for areas with colonies. It is concluded that the great majority of these cells were not or do not derive from surviving stem cells. These enlarged cells most likely represent lethally injured cells that will die or become giant cells (nuclear area greater than 110 micron2).