The ultrastructure of four roe-deer blastocysts at different stages of embryonic development were studied. During delayed implantation, the outer surface of the trophoblast possessed numerous microvilli and periodic invaginations or caveolae. There was a marked lack of organelles such as mitochondria or endoplasmic reticulum in the cytoplasm of the trophoblast cells, though many lipid droplets, granular inclusions and a lamina of fine fibrillae were present. Elongation of the blastocyst was associated with a decrease in the size and number of the microvilli, the disappearance of lipid droplets and granular inclusions, a reduction in the amount of fibrillar material and a dramatic increase in the development of mitochondria, granular endoplasmic reticulum, ribosomes and Golgi apparatus.
The histology of the ovaries and uterus was studied in thirty-one roe deer. No prominent changes occurred in the ovaries at any stage of development; all ovaries possessed active CL and showed signs of follicular growth and atresia. Changes in the degree of mitotic activity, epithelial cell height, endometrial vascularity and stromal oedema were observed in the uterus throughout the period of delayed implantation and during the phase of rapid embryonic growth. Elongation of the embryo was associated with a marked decline in the height of the glandular epithelium and an increase in endometrial vascularity.
The most important ultrastructural changes in the uteri of six roe deer were observed in the endometrial glands. Delayed implantation was associated with the accumulation of numerous supranuclear vesicles derived from the Golgi apparatus, while the resumption of embryonic growth was correlated with their sudden disappearance. When elongation had been completed, there was a sudden decrease in the cellular activity of these endometrial glands.