Fine structure of bovine morulae and blastocysts in vivo and in vitro

The ultrastructure of bovine morulae and blastocysts developed from in vitr o-matured and -fertilized oocytes in a serum-supplemented medium was compar ed with that of morulae and blastocysts collected non-surgically from super ovulated cows. In the in vivo-derived morulae, two characteristic cells typ es could be identified by the electron-density of their cytoplasm and by th eir ultrastructural features. One type appeared light in color with low ele ctron-dense cytoplasm. These cells were located in the peripheral layer of the cluster of blastomeres, possessed numerous cellular organelles such as mitochondria and Golgi apparatus and had microvilli projecting into the per ivitelline space. The other cell type was distinguished by cytoplasm that s tained more densely than that of the lighter-appearing cells. The darker-ap pearing cells generally possessed fewer organelles than the lighter cells, but many lysosome-like structures were present in the cytoplasm. The in vit ro-developed morulae also contained two types of cells similar to those obs erved in the in vivo morulae. However, most of the in vitro-developed cells possessed numerous lipid droplets and contained fewer lysosome-like struct ures than the cells of the in vivo-derived morulae. The blastocysts, both i n vivo and in vitro, showed a clear differentiation of trophoblast cells an d inner cell mass (ICM)-cells. In the in vivo-derived blastocyst, the apica l membrane of trophoblast cells was covered with large, numerous microvilli and well-developed junctional complexes were observed, Lipid droplets were present in the cytoplasm of trophoblast and ICM-cells but were not abundan t. In vitro-developed blastocysts showed less well-developed junctional com plexes between trophoblast cells, less well-developed apical microvilli on the trophoblast cells, and contained large numbers of lipid droplets. This accumulation of lipid droplets was higher in the trophoblast cells than in the ICM-cells. The zonae pellucidae of in vitro-developed embryos were thin ner than that of the in vivo-derived embryos. This study demonstrates consp icuous differences in the ultrastructural features between the in vivo-deri ved and in vitro-developed embryos, suggesting that the ultrastructure may reflect the various physiological anomalies observed in previous studies.