Light and electron microscopic analysis of bovine embryos derived by in vitro and in vivo fertilization

The aim of this study was to examine the morphological features of in vitro fertilization-derived bovine embryos (IVt) and compare them with those of in vivo fertilization-derived (IVv) ones.

Light microscopy showed the blastomeres of IVv embryos to have a tendency to be rounder up to the 16-cell stage and to form a more compact mass at the morula stage than in their IVt counterparts. Electron microscopy revealed that as development progressed, some structures (such as microvilli, phagosomes/lysosomes, intercellular junctions and intermediate filaments) appeared or reappeared while others (such as lipid droplets, vesicles with flocculent materials, cortical granules, nuclear annulate lamellae, nuclear envelope blebs) decreased or disappeared. These changes were observed about one cell stage later in IVt than in IVv embryos. Other structures were present in both IVt and IVv embryos, and they morphologically either changed (such as mitochondria, endoplasmic reticulum, Golgi apparatus and nucleoli) or did not change (cytoplasmic annulate lamellae). In contrast to previous reports, vacuolated nucleoli in both IVt and IVv embryos were observed from the two-cell stage.

It was concluded that (1) the development of bovine embryos to the blastocyst stage is like that of other mammalian embryos; (2) IVt and IVv embryos did not show consistent differences in morphological features; (3) although IVt embryos appear delayed in development, this may reflect different definition of age in vivo and in vitro.