CORRELATION OF PINOPOD DEVELOPMENT ON UTERINE LUMINAL EPITHELIAL SURFACE WITH HORMONAL EVENTS AND ENDOMETRIAL SENSITIVITY IN RAT

Intrinsic role of preovulatory and nidatory estrogen and progesterone and presence, of viable blastocysts in utero in pinopod development on the uterine luminal epithelial surface and correlation between time o f their development and onset of endometrial sensitivity were investig ated. In adult rats, pinopods were observed on the entire epithelium e ven before secretion of nidatory estrogen, i.e. at 14.00 h on day 4 po st-coitum (p.c.). Apparently, their number increased, more so on the a ntimesometrial than the mesometrial side, at 10.00 h on day 5, but wer e fewer and mostly collapsed at 10.00 h on day 6. Pinopods on day 4 we re located within epithelial depressions and foldings, but protruded f rom the surface on days 5 and 6. Normal pinopods were also present on day 8 p.c. in rats under delayed implantation, but an implantation-ind ucing dose of estradiol-17 beta administered about 18 h earlier caused their collapse like that on day 6 in intact rats. Development and app earance of pinopods in intact or delayed rats was unaffected when nati ve preimplantation embryos were prevented from entering the uterus. No rmal pinopods were seen in immature rats receiving progesterone for at least 3 days or cyproterone acetate for 4 days, but not after estradi ol alone. In animals receiving progesterone or priming/sensitizing est radiol in addition to progesterone, the decidual response was suboptim al, irrespective of the presence of pinopods on the day of stimulation . In animals in which a condition mimicking preimplantation had been p roduced by suitable hormone supplementation, optimal endometrial sensi tivity and decidual response were elicited, even though most pinopods appeared collapsed, resembling those on day 6 in intact rats and about 18 h after estradiol in implantation-delayed rats. Findings confirm t hat pinopod development on uterine luminal epithelium was dependent on progesterone alone and demonstrate that: (i) preovulatory (priming) o r nidatory (endometrial sensitizing) estrogen or viable blastocysts in utero have no role in their development. Nidatory estrogen, instead, appears to limit pinopod development by causing their collapse; (ii) p inopod development/presence on the endometrial surface might indicate the uterus coming into a period of sensitivity rather than actually be ing in it and might thus serve as a useful marker of ''transfer window '' rather than ''implantation window''; (iii) in the rat, pinopod deve lopment might serve as an alternate assay for evaluation of progestati onal activity of newer test agents.