Early expression of endomucin on endothelium of the mouse embryo and on putative hematopoietic clusters in the dorsal aorta

Endomucin is a recently identified sialomucin that is specifically expresse d on endothelium of the adult mouse. Here, we have analysed the expression of endomucin during development of the vascular system by immunohistochemis try by using three monoclonal antibodies (mAb). We demonstrate that two of the mAb, V.5C7 and V.1A7, recognize epitopes on the nonglycosylated protein , because they recognize the antigen when it is synthesized as a bacterial fusion protein and when it is in vitro translated in a membrane-free reticu locyte lysate. During in vitro differentiation of embryonic stem cells to e ndothelial cells, endomucin is expressed at day 6 after onset of differenti ation, 1 day later than PECAM-1. During differentiation of the mouse embryo , endomucin is first detected at E8.0 in all embryonic blood vessels detect able at this stage but is absent in blood islands of the yolk sac. Analysin g the paraaortic-splanchnopleura. (P-SP) region and the aorta-gonad-mesonep hros (AGM) region as sites of intraembryonic hematopoiesis, we found that e ndothelium of the dorsal aorta is brightly positive for endomucin at E8.5-9 .0 and at E11.5. At later stages and in the adult aorta, endothelial staini ng is strongly reduced and confined to focal areas. Cell clusters associate d with the luminal surface of the endothelium. of the dorsal aorta could be stained for endomucin and for CD34. At a later stage (E15.5) single leukoc ytes in the lumen of large venules were stained for endomucin. We conclude that endomucin is an early endothelial-specific antigen that is also expres sed on putative hematopoietic progenitor cells. (C) 2001 Wiley-Liss, Inc.