Inner cell mass-specific expression of a cell adhesion molecule (PECAM-1/CD31) in the mouse blastocyst

Platelet/Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) is thought to be a vascular-specific protein, but its function has not been clearly de fined. Here, we demonstrate by using confocal immunofluorescence microscopy that PECAM-1 is first detected in the mouse blastocyst, which contains no vascular cells, and its expression is restricted to the pluripotent inner c ell mass (ICM) cells. Expression is localized to cell-cell borders of the I CM and is detected at the very first signs of blastocoel formation. Consist ent with these observations is that embryonic transcripts of PECAM-1 mRNA, as detected by RT-PCR, greatly increase during the morula-to-blastocyst tra nsition and seven of the eight known alternatively spliced isoforms of PECA M-1 are expressed in the blastocyst. The synthesis of PECAM-1 is independen t of compaction, cytokinesis, and DNA replication, as it is detected in emb ryos that are chronologically at the blastocyst stage following culture of 8-cell embryos in Ca2+-free medium, or medium containing cytochalasin D or aphidicolin, respectively. By the late blastocyst stage, PECAM-1 expression is restricted to the pluripotent epiblast, at which point it has a mutuall y exclusive expression pattern to that of type IV collagen, a basement memb rane marker. The reduction in PECAM-1 transcripts in retinoic acid-induced differentiation of F9 teratocarcinoma cells, a model of epiblast-to-primiti ve endoderm differentiation, confirmed the epiblast-specific expression of PECAM-1. By the egg cylinder stage of development, at which point the epibl ast is no longer pluripotent, PECAM-1 is not detected. This ICM-specific pa ttern of expression suggests a novel developmental role of PECAM-1 that is independent of its function in vascular ontogeny. (C) 2001 Academic Press.