Ultrastructural characterization of in vivo-produced ovine morulae and blastocysts.

The ultrastructure of in vivo-produced ovine embryos, at the morula, early blastocyst and late blastocyst stages, was evaluated using transmission electron microscopy. Embryonic cells were characterized by the presence of intact intercellular junctions, numerous mitochondria, smooth endoplasmic reticulum cisternae and light vesicles. Polyribosomes, rough endoplasmic reticulum cisternae, secondary lysosomes, Golgi complexes and lipid droplets were also observed in the cytoplasm. The nucleus was well defined and organized, with an intact envelope rich in nuclear pore complexes, and one or more reticular nucleoli. Microvilli were present in external blastomeres of morulae and became more abundant in trophectoderm cells of early and late blastocysts. Light vesicles seemed to be associated with small cisternae of Golgi and endoplasmic reticulum origin. These cisternae fused and created light vesicles with engulfed heterogeneous cytosolic structures, small cisternae and vesicles. Their labile membrane enabled them to rapidly coalesce into medium-sized vesicles that began to engulf mitochondria and lipid droplets, forming giant vacuoles mostly filled with fat. Incomplete matured secretory vesicles were observed to exocytose into the perivitelline space of morulae, whereas fully matured secretory vesicles appeared only in trophectoderm cells, being exocytosed into the blastocoelic cavity. These observations suggested that these endoplasmic-/Golgi-derived vesicles behave as active autophagic organelles presenting probably a maturation process from compact morulae to blastocyst.