Development of early PCLP1-expressing haematopoietic cells within the avian dorsal aorta.

The first haematopoietic stem cells (HSC) develop in the dorsal aorta as haematopoietic intra-aortic clusters (HIAC). To evaluate the initial steps of definitive haematopoiesis, we have studied the emergence and the expression profile of podocalyxin-like protein 1 (PCLP1)-expressing cells in early chick embryos. Here we demonstrate that at embryonic day 2 (E2), the PCLP1(+) cells are present in the splanchnic mesoderm and in the ventral lining of the paired dorsal aorta. Following aortic fusion at E3, the PCLP1-expressing cells are exclusively found in the aortic floor and as the development proceeds, both the haematopoietic clusters and the aortic endothelial cells express PCLP1. In parallel with the early PCLP1 expression, bone morphogenetic protein 4 (BMP4) expression was detected in the splanchnopleura and thereafter in the densely packed mesenchymal cells beneath the HIAC. The microarray analyses of early E3 PCLP1(+) cells revealed elevated expression of genes known to be involved in the stem cell function. These data suggest that splanchnopleura-derived PCLP1-expressing cells give rise to the earliest definitive haematopoietic progenitors.