Attachment, outgrowth, invasion and formation of the egg cylinder in mouse half embryos in vitro.

To investigate the effect of the decrease in the number of embryonic cells on the development and morphogenesis of mouse embryos at the peri-implantation stage, half embryos which developed from bisected morulae were morphologically classified into eu-blastocysts (eu-blasts), pseudo-blastocysts (pseudo-blasts), trophectodermal vesicles (TVs) and non-integrated forms (NIFs) and then cultured on plastic substratum, uterine-epithelial-cell monolayer and type I collagen gel. When half embryos were cultured on plastic substratum and cell monolayer, the rates of attachment and trophoblast outgrowth in the eu- and pseudo-blasts were not significantly different from those of the controls. The TVs and NIFs showed significantly lower rates of outgrowth than the controls (P < 0.01). When half embryos were cultured on type I collagen gel, no significant difference was observed in the rate of primitive endoderm formation between the eu-blasts and controls after 36 hr of culture. In the eu-blasts, however, the developmental rate to the egg cylinder at stage 8 was significantly lower than in the controls after 72 hr (P < 0.05). The pseudo-blasts revealed significantly lower rates of endoderm formation and development to the egg cylinder than the controls after 36 and 72 hr, respectively (P < 0.05). In the TVs and NIFs, the rates of outgrowth were significantly lower than those of the controls (P < 0.05) and no egg cylinder was observed. The invasion of type I collagen gel by the cytoplasmic protrusions of trophoblast cells was observed regardless of the type and developmental stage of the embryos. The results show that a decrease in the number of embryonic cells affects the formation of the primitive endoderm and the development to the egg cylinder in vitro.