Female germ cell loss from radiation and chemical exposures.

Female germ cells in some mammals are extremely sensitive to killing by ionizing radiation, especially during development. Primordial oocytes in juvenile mice have an LD50 of only 6-7 rad, and the germ cell pool in squirrel monkeys is destroyed by prenatal exposure of 0.7 rad/day. Sensitivity varies greatly with species and germ cell stage. Unusually high sensitivity has not been found in macaques and may not occur in man, but this has not been established for all developmental stages. The exquisite oocyte radiosensitivity in mice apparently reflects vulnerability of the plasma membrane, not DNA, which may have implications for estimating human genetic risks. Germ cells can be killed also by chemicals. Such oocyte loss, with similarities to radiation effects, is under increasing study, including chemotherapy observations in women. More than 75 compounds have been tested in mice, with in vivo toxicity quantified by oocyte loss; certain chemicals apparently act on the membrane.