Summary
An electron microscopic study was made of the changes of the trophoblast and the uterine epithelium at early implantation. During delay of implantation in mice, only a small amount of secretion was present in the uterine lumen, and the trophoblast-epithelial space was narrow. The luminal part of the uterine epithelial cells contained a 0.1 μm deep layer of dense ground cytoplasm devoid of organelles and inclusions. Below this juxtaluminal layer many apical vesicles were seen. The luminal part of the trophoblast cells contained several endocytic vesicles, indicating that in spite of its low metabolic activity, the blastocyst is actively taking up substances at this stage. This also implies that the uterine epithelium is secreting, although at a low rate. At the luminal surface of the uterine epithelium, however, tests for alkaline phosphatase and glucose-6-phosphatase reactions were negative.
During early oestrogen-induced activation for implantation, uterine secretion was present in a moderate amount in the lumen and the blastocyst lay separated from the epithelial surface. The apical vesicles of the epithelial cells now also occupied the juxtaluminal layer of cytoplasm, which had previously been free of vesicles. The luminal part of the trophoblast cells still contained many endocytic vesicles. A positive reaction to the alkaline phosphatase test was seen at the epithelial cell surface between 4 and 8 h after injection of oestrogen and still persisted at 16 h. No positive reaction indicating glucose-6-phosphatase was seen. One single implantation site was found at the 16-h stage and this showed a positive reaction to the alkaline phosphatase test at the trophoblast-epithelial border in the embryonal part, but not abembryonically.
These findings suggest that a marked increase occurs in the secretory activity of the uterine epithelium only a few hours after initiation of blastocyst activation for implantation. Considering the mode of uterine control of blastocyst activity, the present results would seem to favour the view that the uterine control of implantation is dependent upon the amount and nutritional value of the uterine secretion made available to the blastocyst.
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Nilsson, B.O., Lundkvist, Ö. Ultrastructural and histochemical changes of the mouse uterine epithelium on blastocyst activation for implantation. Anat Embryol 155, 311–321 (1979). https://doi.org/10.1007/BF00317644
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DOI: https://doi.org/10.1007/BF00317644