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The ultrastructure of the endometrial epithelium of the pig was studied during the estrous cycle and early pregnancy up to implantation. Special attention was given to the luminal epithelium and morphological indications of protein synthesis. Although the general morphology of the luminal and glandular epithelia is similar (both tissues consist of secretory cells and ciliated cells at all the stages studied), it appears that the two epithelia should be considered as two functionally different units in the pre-implantation period. Morphological evidence suggests the presence of at least three different secretory products within luminal epithelial cells; they are released at different times, i.e. at estrus, between day 8 and 10 and after day 11. The glandular epithelium shows release of secretory products from day 10-11. Increasing amounts of glycogen were found within epithelial cells, especially in pregnant gilts from day 12. The possible significance of secretory activity of the epithelium is discussed in relation to the development of the embryos.

Between days 8 and 11 of pregnancy spherical blastocysts from 0.3 to 10 mm in diameter were flushed from the uterine horns of Dutch Landrace pigs. A description of their ultrastructure is given, and the uptake of horseradish peroxidase and ferritin is demonstrated. The ultrastructure of the trophoblast was similar at all ages studied. The trophoblast which has many apical microvilli is able to take up and digest the macromolecules which were offered in the in vitro incubation medium. The hypoblast consists of flattened cells. In blastocysts 2 mm and larger, compact cells bearing microvilli are found below the embryoblast. Cell organelles indicating protein synthesis are found within hypoblast cells of such blastocysts. In the embryoblast, local concentrations of cell organelles are visible, indicating that differentiation has started. After the disappearance of Rauber's layer, which takes place when the blastocyst reaches a diameter of about 2 mm, superficial embryoblast cells develop short microvilli. The cells do not absorb ferritin or peroxidase but are dependent on the trophoblast for their food requirements. All cell layers in the blastocyst contain mitochondria that have characteristics of those found in steroid-producing cells. The significance of the uptake and digestion of macromolecules by trophoblast cells, the synthesis of protein by hypoblast cells and the possible synthesis of steroids is discussed with respect to the relationship between the cell layers of the blastocyst and in the context of concepto-maternal relationships.