Growth of elementary blood vessels in diffusion chambers. I. Process of formation and conditioning factors

1. An electron microscopical analysis of the development of new capillary structures from vascularized (omentum) or avascularized (cardiac valves) rat tissue expiants in diffusion chambers placed in rat peritoneal cavity, has shown that the solid precapillary structures previously described (Aloisiet al., 1970) are always multicellular in nature, even when they appear as syncytial to the light microscopy. 2. The nature of the cells composing the solid pre-capillary structures appears to be different from that of common fibroblasts, an evolution of the latter to endothelial-like cells being nevertheless not excluded. 3. In these pre-capillary structures cellular differentiations take place precociously and lead to pericyte formation on the outer side and to degeneration of the inner cells. 4. The formation of a capillary lumen is therefore associated to such degenerative changes, and to the shedding and dissolution of the inner cells, a process which, combined with the apposition of new endothelial cells from outside may also account for the widening of the newly formed elementary vessel. 5. The morphogenetic value of these regressive phenomena in the formation of capillary lumina is discussed, as analogous to other phenomena in the embryo and particularly reminiscent of the formation of embryo vasculature from angioblast and from other mesenchymal anlagen.