FIG. 2

Histological examination of in utero embryos obtained from YY1 heterozygous matings. The uteri of female YY1^+/− mice were dissected between 5.0 and 7.5 days after intercross mating as described in Materials and Methods. All uterine decidua (wild-type and mutant) were sectioned transversely at a 7-μm thickness and stained with hematoxylin and eosin. Wild-type or heterozygous embryos are shown in the column labeled wt, and presumptive YY1^−/− mutants are shown in the column labeled mut. (A and B) E5.0, early egg cylinder stage; (C and D) E5.5, egg cylinder stage; (E and F) E6.5, late egg cylinder stage; (G and H) E7.5, late primitive streak stage. The wild-type embryos show a differentiating early egg cylinder stage embryo at E5.0 (A) and E5.5 (C), whereas the mutant embryos show decreased proliferation and a lack of organized embryos (B and D) at these embryonic stages. Note the appearance of a proamniotic cavity in the E6.5 elongated late-egg-cylinder wild-type embryo (E) including the formation of the embryonic ectoderm (ee) and extraembryonic ectoderm (eee) and the lack of these clearly differentiated tissues in the mutant embryos (F). Wild-type embryos at the late primitive streak stage (G) show clearly defined yolk sac (ys), ectoplacental cavity (ec), extraembryonic coelomic cavity (eec), amniotic cavities (ac), and allantois (al). In contrast, the presumptive YY1^−/− mutant at E7.5 is nearly completely resorbed as shown by the appearance of an empty uterine crypt (H).