Spontaneous differentiation of trophoblast cells along the spongiotrophoblast cell pathway: expression of members of the placental prolactin gene family and modulation by retinoic acid.

The purpose of this study was to examine part of the trophoblast cell multilineage pathway and its modulation by retinoic acid. A method for studying trophoblast cell differentiation along the spongiotrophoblast cell pathway in vitro was established and characterized. Cells were isolated from junctional zones of Day 13 rat chorioallantoic placentas via mechanical dissection, enzymatic digestion, and enrichment through a Percoll cushion. The cells were cultured up to 8 days and analyzed for their purity, morphology, and ability to express members of the placental prolactin (PRL) family. Cell preparations contained minimal mesenchymal contamination as estimated by immunocytochemical analysis for vimentin. The cells expressed PRL-like protein-A (PLP-A), PLP-B, PLP-C, and placental lactogen-I variant (PL-Iv) indicative of their differentiated spongiotrophoblast cell phenotype. Expression of members of the PRL family increased markedly during culture. Temporally the increase in PLP-A expression preceded the increased expression of PLP-B (0.9 kb), PLP-C, and PL-Iv. These in vitro observations paralleled the behavior of spongiotrophoblast cells developing in situ. Some differences were evident, including the immediate activation of PLP-B (1.2 kb) following enzymatic isolation of the cells. These cells were also susceptible to experimental manipulation. Exposure to retinoic acid influenced the morphology of the cells and the profile of members of the placental PRL family expressed by in vitro differentiated cells. In summary, a culture system has been devised to examine the control of spongiotrophoblast cell differentiation and the regulation of expression of members of the placental PRL gene family. Spongiotrophoblast cells spontaneously differentiate in vitro through discrete developmental phases that are susceptible to modulation by retinoic acid.