Inter-specific Transfers of Eggs Between Rat and Mouse

Abstract

ABSTRACT Mouse 2- and 8-cell and rat 2-, 4-, and 8-cell eggs develop into blastocysts when reciprocally transferred to the oviduct of the other species. Some rat eggs, after attaining the blastocyst stage in the mouse oviduct, do not shed the zona pellucida at the normal time and continue to increase in volume beyond the normal. This is accompanied by spreading of cells of the inner mass beneath the trophoblast, leaving the blastocyst without any compact embryonic structure. Other eggs develop into normal blastocysts with an inner cell mass present. Mouse eggs in the rat give rise to normally built blastocysts. Both kinds of rat blastocysts—hollow and normal—and mouse blastocysts evoke a typical decidual reaction in the uterus of the foreign species. Normal crypts are formed with the blastocysts attached to the epithelium. These processes begin well before any changes in the uterine epithelium become visible. The blastocysts of neither species are able to secure a normal contact with the mucosa. Degeneration of the epithelium under the influence of the trophoblast is delayed, irregular, and only small areas are involved. In the mouse, the eventual denudation of the mucosa around the blastocyst is thought to be largely an effect of spontaneous degeneration of the epithelium due to oedomatous changes in the underlying mucosa. The survival of mouse and rat blastocysts after implantation is very limited, the upper time limit being, in the mouse, the 7th day and, in the rat, the 8th day. Those rat blastocysts which have succeeded in establishing a contact with the mucosa undergo some differentiation, i.e. form small egg cylinders. This is accompanied by growth which in some cases is, however, very slight. Mouse blastocysts in the most successful cases exhibit immense increase in size but neither differentiation nor growth have been observed in their inner cell masses. The kidney surface of both species provides a favourable environment for the development of rat and mouse blastocysts reciprocally transferred. The trophoblast gives rise to numerous giant cells. The cells presumably of ‘inner mass’ origin are also visible, but organized embryonic development has not been observed. A number of rat eggs which developed into blastocysts in the mouse were re-transferred to rat recipients. The subsequent mortality rate was very high. Three purely trophoblastic vesicles were encountered and it is believed that they have developed from hollow blastocysts. These findings are discussed in the light of embryology and reproductive physiology.