The effects of continuous prenatal exposure to ionizing radiation on germ cell survival and follicular development in the bovine

It is known that the germ cell of the prepuberal mammal is sensitive to acute doses of radiation; but only limited information is available on the effects of continuous irradiation. Gravid cows were continuously irradiated with either 8 or 15R of γ-rays for each of 10 days during select intervals of gestation. Fetuses absorbed approximately one-third of the air dose. Ovaries of prenatally irradiated heifers, aged 15 to 18 months, were removed at slaughter, serially sectioned, and prepared for microscopic analysis. Testes excised from newborn bull calves were prepared for microscopic analysis as well. Numbers of primary, growing, and vesicular follicles were determined from ovarian cross-sections, and primitive spermatogonia (gonocytes) were enumerated in seminiferous tubule cross-sections.

Reductions (18 to 21% of control) in number of primary follicles were observed only in heifers irradiated between 60 and 110 days of Gestation (P < 0.01). During this period the germ-cell population is comprised mainly of mitotically active oogonia, and it is, therefore, likely that this cell type is most susceptible to continuous irradiation. The process of follicular development, as reflected by counts of growing and vesicular follicles, was unimpeded (P < 0.01). Number of gonocytes was significantly reduced (10 to 40% of control) in bull calves that were irradiated between 60 and 270 days (P < 0.01); a developmental period that corresponds with the shift of the gonocyte from a mitotically active to a mitotically inactive state. The oocyte of the primordial follicle and the mitotically active gonocyte were relatively resistant to continuous irradiation.

Gonocyte losses in the male were probably insufficient to materially affect sperm production, but the reproductive capacity of females irradiated between 60 and 110 days of gestation would likely be reduced.