Simultaneous exocrine and endocrine secretion: trophoblast and glands of the endometrial cups

Invasion of trophoblast cells of the chorionic girdle into the endometrium and their subsequent dependence as endometrial cup cells on the maternal vascular system are unique features of equine gestation. This study of exocrine secretion by the endometrial glands and endocrine secretion by the cup cells was initiated to examine the relationships between and the effects of two disparate epithelial cell types sharing the endometrium. Endometrial cups were examined from day 36 to day 83 of gestation by light and electron microscopy, and immunohistochemical and lectin analysis. As the cup cells invade the endometrium they migrate along the basal lamina of glands, thus preserving the integrity of the glands. The hypertrophied cup cells develop an extensive network of cytokeratin-positive filaments and large Golgi zones that stain positively with antibodies to equine chorionic gonadotrophin and with the lectins SBA (agglutinin from Glycine max) and WGA (agglutinin from Triticum vulgaris). The endometrial glands within and immediately adjacent to the cup undergo substantial hypertrophy. The gland cells within the cup stain with alcian blue and with some lectins (agglutinins from Dolichos biflorus and lectin SBA), whereas gland cells in unmodified endometrium away from the cup stain positively with antibodies to transforming growth factor a, but only poorly with SBA. Secretory material in gland luminae in the cup stain heterogeneously with alcian blue, periodic acid Schiff and lectins. Later in gestation, glands within the cup become progressively disorganized, with some glands rupturing and releasing their contents into the stroma of the cup. Areas of lipid accumulation and apparent degeneration of cup cells were observed in the cups on day 83 of gestation only. In conclusion, it appears that cup cells substantially modify the integrity and secretion of adjacent endometrial glands, and that progressive disruption of gland integrity and isolation of endometrial vessels by extracellular matrix precedes deterioration of the cup cells. Necrosis and disintegration resulting from these factors appear more likely to limit cup lifespan than to cause programmed cell death of individual cup cells.