Ultrastructural differences in bovine morulae classified as high and low qualities by morphological evaluation

Bovine morulae collected from superovulated cows were classified into four groups (excellent, good, fair, poor) using morphological evaluation under a phase contrast microscope. The number of blastomeres in morulae classified as excellent and good (high quality) was significantly higher than the number in fair and poor (low quality) morulae. The ultrastructure of morulae in each group was compared. In morulae classified as excellent and good, two characteristic cell types could be identified by the electron density of their cytoplasm and by their ultrastructural features. One type (presumptive trophoblast cells) was located in the peripheral layer of the embryo mass, was generally characterized by low electron density of cytoplasm, and possessed numerous cellular organelles such as mitochondria and Golgi apparatus and had numerous microvilli projecting into the perivitelline space. The other type (presumptive embryonic cells) was located in the interior of the embryo mass and distinguished by cytoplasm that stained more densely than that of the lighter-appearing cells. The darker-appearing cells generally possessed fewer organelles than the lighter cells, but many lysosome-like structures were present in the cytoplasm. All high quality morulae contained well-developed mature nucleoli. Many embryos classified as fair contained both types of cells, while cells in some fair and most poor quality morulae could not be distinguished by the electron density of their cytoplasm and ultrastructural features. The morulae classified as fair and poor showed less well-developed junctional complexes between cells, less well-developed apical microvilli on the blastomeres, and contained large numbers of lipid droplets, vesicles, immature mitochondria, and nucleoli with low transcriptional activity. This study demonstrates conspicuous differences in the ultrastructural features between the high quality and the low quality morulae, suggesting that these ultrastructural features may reflect the various physiological anomalies observed in previous studies.