Ultrastructure of bovine embryos developed from in vitro-matured and -fertilized oocytes: Comparative morphological evaluation of embryos cultured either in serum-free medium or in serum-supplemented medium

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Abstract
The ultrastructure of bovine embryos developed from in vitro‐matured and ‐fertilized oocytes, cocultured with bovine cumulus/granulosa cells either in a serum‐free medium (IVMD101) or in a serum‐containing medium (TCM199+CS) was compared. Embryos up to the eight‐cell stage had many cellular organelles and cytoplasmic components that were randomly distributed in the cytoplasm. Mitochondria were spherical or ovoid and had only a few peripheral cristae. There were no obvious differences in the ultrastructure between embryos developed in IVMD101 and TCM199+CS up to the eight‐cell stage. However, conspicuous differences in the ultrastructural features between the embryos cultured in IVMD101 and TCM199+CS were observed at the morula and blastocyst stages. At the morula stage, embryos cultured in IVMD101 had cells containing elongated mitochondria, well‐developed Golgi apparatus, lipid droplets, and large vesicles resembling lysosomes. The lysosome‐like vesicles were partially filled with electron‐dense materials and were frequently fused with lipid droplets. The blastomeres of morulae cultured in TCM199+CS contained numerous large lipid droplets and fewer lysosome‐like vesicles than those cultured in IVMD101. In blastocysts cultured in IVMD101, lysosome‐like vesicles were frequently observed in the trophoblast cells and lipid droplets were present in the cytoplasm of trophoblast and inner cell mass (ICM)‐cells, but they were not abundant. On the other hand, the blastocysts developed in TCM199+CS contained fewer lysosome‐like vesicles and large numbers of lipid droplets. This accumulation of lipid droplets was higher in the trophoblast cells than in the ICM‐cells. This study showed major differences in the ultrastructural features between the morulae and blastocysts from serum‐free and serum‐supplemented cultures, suggesting that the ultrastructural differences may reflect physiological characteristics of embryos. Mol. Reprod. Dev. 53:325–335, 1999.