The Radiosensitivity of Ooegonia and Ooecytes in the Chick Embryo.

NUMEROUS recent studies have clearly shown that in mammals, the radiosensitivity of germ cells changes markedly during the course of ovarian development (e.g. Rugh & Jackson, 1958; Russell, Badgett & Saylors, 1960; Beaumont, 1961, 1962). In the rat, for instance, female germ cells undergo two periods of high radiosensitivity: the first at 15 • 5 days post coitum, and the second some 5 days post partum. The first peak is correlated with a high incidence of oogonial mitoses, while the second coincides with the onset of the ' dictyate' phase of meiotic prophase (Beaumont, 1961, 1962; cf. Beaumont & Mandl, 1962). Oocytes at the leptotene, zygotene and pachytene stages are relatively radioresistant. Corresponding observations on oogonia and oocytes of the developing avian embryo are scanty, and some were made before the advent of accurate dosimetry. DantschakorT & Lacassagne (1932) claimed that a total dose of 900 r. of X-rays, administered in three sessions (on the 6th, 9th and 12th days of incubation) induced 'sterility' by the 17th and 19th days of incubation. In contrast, Gladstone & Colwell (1933) observed only minor histological changes in the ovaries of chick embryos 2 to 24 hr. after exposure to 4 pastille doses (Sabouraud) of X-rays on the 9th day of incubation (i.e. probably between 1500 and 2000 r.). According to Essenberg & Zikmund (1938), ovarian radiosensitivity is maximal at 5 to 7 days' incubation; it decreases between the 8th and 12th days, and following irradiation shortly before or after hatching, scarcely any changes could be detected in the ovaries. Warren & Dixon (1949) also postulated that the germ cells of chick embryos become more resistant to ionizing radiations as they mature. Injections of 47 • 5 /xC. of P into embryos at 4 to 8 days' incubation destroyed all the germ cells, whereas a dose of 200 ̂ C. given on the 14th day failed to do so. More recently, Moreng & Muller (1962) have shown that, as judged by the rate of egg-laying after the onset of sexual maturity, the ovary of the chick embryo is particularly radiosensitive on the 12th day of incubation.