Crystalline inclusion bodies in rabbit embryos.

Crystalline inclusion bodies (CIB) may be found as prominent ultrastructural components of the trophoblast cells of rabbit blastocysts and of progestational uterine endometrium. In the work reported here we have sought to describe developmental steps in crystal formation, to correlate these events with embryonic age and to determine if the uterus is essential (either as a source or an environment) for crystal formation in the embryo. CIB, which are of a size and periodicity to make them appear to be clusters or packages of microtubules, are first detectable in embryos 4 days 6 h post coitum and by 41/2 days are well established in significant numbers. Another structural component, granular vesicles, may be seen in embryos as early as 21/2 days post coitum, and decrease in number during the same time the CIB are increasing. We believe that the CIB originate from the pre-existing granular vesicles and present electron micrograph evidence of crystal formation progressing from such vesicles. CIB formation does not occur in 41/2- to 5-day-old embryos which have been locked in the oviduct by a suture around the utero-tubal junction. However, when such tube-locked embryos are transplanted into the uterus, they develop crystals within 36 h thereafter. We conclude that the uterus is essential for CIB formation to occur in the rabbit embryo.