Ultrastructural studies of PECAM-1/CD31 expression in the developing mouse blood-brain barrier with the application of a pre-embedding technique.

The aim of this study was to investigate the time of expression of the adhesion molecule platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) in the developing mouse blood-brain barrier (BBB). Ultrastructural studies employing a preembedding technique described herein demonstrate that PECAM-1 is initially expressed on the luminal and abluminal endothelial cell surfaces in the newborn animals. This adhesion molecule expression appears to increase in intensity at 7-10 days post partum and then decreases to a weak labelling of the luminal endothelial cell surfaces at two weeks after birth. Our results present immunocytochemical detection of active angiogenesis during early brain development in the mouse. Moreover, because of the important role that adhesion molecules play in immune responses in the central nervous system, upregulation of PECAM-1 prior to structural maturity of the BBB may suggest that the development of an "immune BBB" manifests prior to anatomical closure of the BBB in the mouse.