Epithelial cell function during blastocyst implantation
In response to the ovarian secretion of progesterone and estrogen during early pregnancy, the mammalian uterus develops the capacity to perform complex cellular activities which occur before and after blastocyst implantation. Luminal epithelial cells participate in regulation of the metabolism of the blastocyst through the control of its humoral environment, provide an appropriate matrix for changes to occur at the interface between trophoblast and epithelium, and appear to transmit information from the blastocyst to the underlying stroma to initiate decidualization. With the completion of these functions during implantation in rodents, the epithelial cells self-destruct and are removed by phagocytic activity of the trophoblast. Control of both the endocytotic and secretory activity of luminal epithelial cells and their eventual self-destruction would require regulation of the Golgi-endoplasmic reticulum-lysosomes system within these cells. Progesterone secretion during early pseudo-pregnancy increases levels of cathepsin D, a lysosomal proteinase, in luminal epithelial cells by increasing the rate of enzyme synthesis. Progesterone pretreatment of ovariectomized rats followed by estradiol treatment results in the development of uterine sensitivity to deciduogenic stimuli. The number of proteins which are synthesized by luminal epithelial cells in response to estradiol to achieve this sensitivity has been determined. Epithelial cytosol proteins from rats treated with medroxyprogesterone acetate (3.5 mg sc) or medroxyprogesterone acetate plus estradiol (200 ng sc) were separated by two dimensional polyacrylamide gel electrophoresis. The synthesis of two proteins increased after 8 h of estradiol treatment and the synthesis of another three was increased by 12 h. The increased synthesis of these proteins could be related to changes in the capacity of the luminal epithelial cell for prostaglandin synthesis. The epithelial capacity for prostaglandin synthesis increases during pseudopregnancy to maximum levels at the time of maximum sensitivity to deciduogenic stimuli. Epithelial prostaglandin synthetic capacity may also depend upon the accumulation of prostaglandin precursors within these cells. Estradiol treatment of medroxyprogesterone acetate pretreated ovariectomized rats increased the arachidonic acid content and composition of epithelial phosphatidyl choline but the increases were not statistically significant. These changes in protein and lipid synthesis controlled by progesterone and estrogen would appear to contribute to the cellular activities of the luminal epithelium during early pregnancy.