Ultrastructure of porcine embryos following development in vitro versus in vivo

The present study examined the ultrastructural appearance of porcine embryos from the four‐cell stage to the blastocyst grown either in vivo or in vitro. Embryos were collected at slaughter from superovulated gilts and were fixed for transmission electron microscopy either immediately or after various periods of in vitro culture. In general, the morphology of in vivo and in vitro grown embryos was similar. In vivo grown four‐cell stages contained dense fibrillar nucleoli. At the eight‐cell stage the nucleoli possessed increasing amounts of chromatin and granules. In both stages the mitochondria were spherical or ovoid in shape and had only few cristae. In morulae and blastocysts the nucleoli were mainly of the fibrillogranular type, and the mitochondria were filamentous and possessed more cristae, of which many were tubular. Two major ultrastructural deviations were observed in about half of the in vitro cultured embryos. First, nucleolus‐like structures were found outside the nuclei in the cytoplasm of blastomeres. These structures were spherical and composed of chromatin‐like material containing characteristically a single large and several small vacuoles. The structures were frequently associated with profiles of smooth endoplasmic reticulum (SER). A second type of deviation was aggregates of SER appearing as spiral coils or multiangular complexes. Some embryos displayed both types of deviations. The physiological significance of these deviations remains speculative. They may be involved in the considerably reduced capability of porcine embryos to develop to piglets following in vitro culture.