Inner Cell Mass-Specific Expression of a Cell Adhesion Molecule (PECAM-1/CD31) in the Mouse Blastocyst

AbstractPlatelet/Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) is thought to be a vascular-specific protein, but its function has not been clearly defined. 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 cell mass (ICM) cells. Expression is localized to cell–cell borders of the ICM and is detected at the very first signs of blastocoel formation. Consistent with these observations is that embryonic transcripts of PECAM-1 mRNA, as detected by RT-PCR, greatly increase during the morula-to-blastocyst transition and seven of the eight known alternatively spliced isoforms of PECAM-1 are expressed in the blastocyst. The synthesis of PECAM-1 is independent of compaction, cytokinesis, and DNA replication, as it is detected in embryos 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 mutually exclusive expression pattern to that of type IV collagen, a basement membrane marker. The reduction in PECAM-1 transcripts in retinoic acid-induced differentiation of F9 teratocarcinoma cells, a model of epiblast-to-primitive endoderm differentiation, confirmed the epiblast-specific expression of PECAM-1. By the egg cylinder stage of development, at which point the epiblast is no longer pluripotent, PECAM-1 is not detected. This ICM-specific pattern of expression suggests a novel developmental role of PECAM-1 that is independent of its function in vascular ontogeny