Environmental Effects on the Embryos and Fetuses of Experimental Animals

In examining the offspring of experimental animals or man in relation to exposures to environmental agents, account should be taken of certain principles developed over the past five decades. TERATOGENESIS Before the Period of Implantation Experimental embryologic studies have shown that before implantation there is a so-called omnipotential period of development, when all cells have the ability to form a complete embryo. During this interval it is difficult, if not impossible, to induce congenital malformations. This interval extends to 6½ days in the mouse, 8 days in the rat, and about 12 to 13 days in man. Teratogenic Period The interval during which teratogenesis may be induced in man is short relative to the total gestational period, in contrast to the period of susceptibility in the mouse or rat, in which it is about one third of the gestational period. Fetal Period Subsequently, classical teratogens do not produce gross malformations that are easily observable at birth. Exposures to hazardous agents may still produce other effects throughout the later period of gestation, namely, cell deletions or tissue hypoplasia. This is a new area about which we do not yet understand very much, even with respect to high doses. When the mouse embryo is exposed late in gestation to 200 rad, a marked depletion has been observed in the cerebral cortex. This easily visible difference from normal upon inspection of the brain has little effect on behavior. In the same way, x-irradiation of the mouse testes produced hypoplasia, but no defect in morphogenesis.