Ultrastructural Studies of the Endometrial Stromal Cells in Rats During Estradiol-Induced Implantation After an Experimental Delay

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Abstract
The ultrastructural features of the subepithelial endometrial stromal cells were studied during estradiol-induced implantation after a progesterone-maintained (5 mg s.c. daily) delay in rats. Blastocyst-free (control) and blastocyst-containing sites were compared at 4 h intervals up to the pontamine blue reaction (PBR), which became visible between 20 and 24 h after administration of estradiol-17.beta. (0.1 .mu.g s.c.). The ultrastructure of the stromal cells was influenced by both the estradiol and the blastocysts. Administration of estradiol rapidly brought about pronounced qualitative nucleolar changes. These were present in both control and blastocyst-containing sites at 4 and 8 h after the injection of estradiol (h.a.e.), but had disappeared in most of the controls examined 12 h.a.e. and in all those examined 16-24 h.a.e. The appearance and disappearance of the nucleolar alterations in the controls appeared to be accompanied by a transient numerical increase in cytoplasmic polyribosomes. In the blastocyst-containing sites, the initial estradiol-induced nucleolar and cytoplasmic changes were maintained and further developed from 12 h.a.a. onwards. In the experimental system used, the blastocysts influence the surrounding stromal cells as early as 8-12 h before the PBR. This is in accordance with biochemical findings at normal implantation. The stromal cell changes seen around the blastocysts from 12 h.a.e. onwards could be early signs of the decidual reaction. The observed estradiol- and blastocyst-induced nucleolar alterations are assumed to be characteristic of enhanced RNA synthesis in the stromal cells.