Distribution of red blood cells throughout the embryo proper vasculature occurs by 35 sp (E10.5).

The pattern of the embryo proper endothelial and red blood cell distribution was visualized in stage-matched embryos by PECAM immunohistochemistry (A, C, E, G-I) and o-dianisidine staining of hemoglobin (B, D, F, J-L), respectively. To facilitate comparison, the 12th sp is indicated by a dot, the 20th sp by the letter x, and the 25th sp by a star (h indicates heart ventricle; lb, limb bud). (A, C, E) The extensive arborized vascular network evident in the head and ventricle and between the somite pairs at E9.5 (24 sp) became progressively more complex by E9.75 (28 sp) and E10.5 (35 sp). (B, D, F) Red blood cells initially more concentrated in a subset of vessels at 24 sp became more evenly dispersed by 35 sp. Additionally, high levels of hemoglobin-containing cells were seen in the fetal liver (arrow) as it became the primary site of hematopoiesis by E10.5. (G-L) Along the back, paired intersomitic arteries and veins meet at capillary networks of increasing complexity between somite pairs during this time period. Initially, red blood cells were observed in paired intersomitic vessels in the anterior back, at mid positions in deeper arteries only, and not at all between posterior somite pairs. By 35 sp, red blood cells had penetrated intersomitic vasculature throughout the back. Red and blue arrowheads indicate intersomitic arteries and veins, respectively. Original magnifications A-F, × 20; G-L, × 50.