TGF-BETA-1 REGULATION OF LAMININ SECRETION BY F9-DERIVED PRIMITIVE ENDODERM CELLS - A POTENTIAL ROLE IN CELL-MIGRATION WITHIN THE MOUSE BLASTOCYST

The first differentiation event occurring in the preimplantation mouse blastocyst is the formation of inner cell mass (ICM) derived primitiv e endoderm (PrE) cells, some of which migrate over the inner surface o f the trophectoderm to establish the parietal endoderm layer. Reichert 's membrane, a basement membrane located between the trophectoderm and the emerging endodermal layer, is implicated in this migration. In th is study, F9 murine embryonal carcinoma cells were used as an in vitro model for embryonic endoderm to investigate the regulation of laminin secretion by these cells during their differentiation under serum-fre e culture conditions. The formation of PrE-like cells was induced usin g retinoic acid, and the cells were then cultured with dibutyryl cAMP (dbcAMP), forskolin, or transforming growth factor (TGF) beta 1 or bet a 2, and the levels of secreted laminin were measured using ELISA. dbc AMP and forskolin stimulated (p < 0.01) laminin secretion, whereas TGF -beta 1 decreased (p < 0.01) the secretion of laminin by PrE-like cell s during a 72-h culture period (without influencing laminin deposition by these cells). In contrast, TGF-beta 2 did not significantly influe nce (p > 0.05) laminin secretion. The results of in vitro migration ex periments using mouse ICMs prepared by immunosurgery indicated that, u nlike fibronectin, neither laminin nor type IV collagen supported the outward movement of differentiating PrE-like cells. These findings sup port a potential role for TGF-beta 1 in influencing the establishment of the parietal endoderm cell layer within the mouse blastocyst by red ucing the extent of laminin deposition in Reichert's membrane during e ndoderm cell migration.