Ultrastructural study of in vivo produced ovine embryos: characterization of secretory vesicles

Abstract

The ultrastructure of in vivo produced ovine embryos, at the morula, early blastocyst and late blastocyst stages, was evaluated using transmission electron microscopy. In vivo produced embryos (n=14) were obtained from Portuguese Black Merino ewes, superovulated during Spring. Embryos were recovered by abdominal laparotomy, under general anesthesia, on day 6 or 7 after sponge removal. Embryos were processed for electronic microscopy observation and ultrathin sections (700A) were observed in a transmission electron microscope JEOL 100CXII. Embryonic cells presented 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 with one or more reticular nucleolus. The most noticeable aspect of the cytoplasm of ovine embryos was the presence of abundant cytoplasmic small light vesicles with heterogeneous contents. These light vesicles seem associated to small cisternae of Golgian and endoplasmic reticulum origin and their labile membrane enabled them to rapidly coalesce into medium size vesicles that began to engulf mitochondria and lipid droplets, forming giant vacuoles mostly filled with fat. Incomplete matured secretory vesicles were observed to exocytate into the perivitelline space of morulae, whereas fully matured secretory vesicles appeared only in trophectoderm cells, being exocytated into the blastocoelic cavity.