Decidualization of human endometrial stromal cells in vitro: effects of progestin and relaxin on the ultrastructure and production of decidual secretory proteins*

Abstract

Decidual cells arise by proliferation and differentiation of endometrial stromal cells of the uterus after appropriate stimulation by ovarian hormones. Previously we have shown that progestin and relaxin stimulate the secretion of several decidual-cell-specific secretory proteins in a long-term primary cell culture system. We now report the effects of progestin and relaxin on the morphology of stromal cells in association with the production rate of two major decidual secretory proteins, prolaction and insulin-like growth factor binding protein-1 (IGFBP-1). Stromal cells were cultured in RPMI 1640 and 2% fetal bovine serum for 22 days under control conditions (no hormone), with relaxin or medroxyprogesterone acetate (MPA), or MPA plus relaxin. Cells treated with MPA alone or MPA plus porcine relaxin grew to a high density with many areas of heaping while control cells and cells grown in medium containing relaxin alone formed discontinuous layers. The cytoplasm was distinguished by aggregates of rough endoplasmic reticulum and secretory granules. Surfaces of cells treated with MPA plus relaxin had clusters of short blunt processes containing secretory granules. The processes were rarely seen in cells exposed to MPA alone and completely absent in control cells or cells exposed to relaxin alone. Intercellular space was greatly widened in cells treated with MPA alone or MPA plus relaxin. There were many extended gap junctions in MPA and relaxin-treated cells in contrast to controls. The production rates of prolactin and IGFBP-1 measured by radioimmunoassay and enzyme-linked immunosorbent assay respectively increased from 0.004 to ∼0.7 μg for prolactin and 0.01 to 44 μg for IGFBP-1 per 106 cells/day after 20 days incubation with MPA and relaxin, in agreement with the previous findings. The parallel biochemical events and morphological changes are similar to those of decidualization of endometrium from the secretory phase to the early gestation in vivo.