Ultrastructural aspects of the appositional stage of interstitial blastocyst implantation.

The ultrastructural aspects of the appositional phase of interstitial blastocyst implantation in the guinea pig have been investigated using scanning and transmission electron microscopy. Blastocysts measured 28-34 micron in diameter and were encased in the zona pellucida. Trophoblast pseudopodia which penetrated the zona made contact with the underlying uterine epithelium and served, along with uterine epithelial contact with the zona, to hold the blastocyst over the subsequent site of implantation. Pseudopodia emanated from the embryonic pole of the blastocyst measured 2-4 micron in diameter and initiated uterine-blastocyst contact. Pseudopodia contained a cytoplasm rich in tubular mitochondria, rough endoplasmic reticulum, and Golgi apparatus and usually were found directed toward a subepithelial capillary plexus. These results suggest that the trophoblast pseudopodia are very active metabolically and may serve to align the blastocyst over stroma regions possessing a rich subepithelial vascular supply and high nutrient availability.