Ultrastructural and morphometric analysis of cytoplasmic structures in preimplantation pig embryos

Preimplantation pig embryos (1-, 2-, 3/4-, 16-cell stage, blastocysts) were analysed morphologically and/or morphometrically by means of electron microscopy and computer-assisted image analysis. Qualitative and quantitative changes in localization, organization, number, and area of cytoplasmic inclusions were analysed on thin sections. The structure and number of mitochondria, number of endoplasmic reticulum profiles were recorded in several individual developmental stages. Three types of cytoplasmic inclusions were described (vesicles with yolk globules, vesicles with homogeneous light inclusions, lipid droplets), each having a different topographic distribution. Quantitatively, a gradual increase in the number of endoplasmic reticulum profiles and mitochondria occurred between 4-cell stage blastomeres and blastocyst trophectoderm cells. The significant differences recorded in number, morphology and distribution of organelles in different embryonic developmental stages are discussed in relation to physiological phenomena known to occur during cleavage and compaction.