FIGURE 18.

Models for the potential sources of endothelial cells that contribute to specialized vascular structures. Model A: a branch forms from an existing vessel in response to a local cue, and differentiates into a specialized vascular niche, potentially following an initial de-differentiation step. Model B depicts a scenario in which bone marrow (BM)-derived precursor cells are recruited to integrate into an existing vessel so that they can participate in formation of a new vascular branch. Models C and D: de novo vasculogenesis gives rise to a small vascular plexus adjacent to an existing vessel. The source of these endothelial cells could be a hematopoietic organ that is adjacent to the vessel (model C) or, as also indicated in model B, endothelial cells recruited from the BM. Alternatively, angioblasts (green) could develop locally from mesenchymal precursors (blue) through a mechanism conceptually similar to the development of arterial precursor cells from mesenchymal precursors during embryonic vasculogenesis (model D). Model E: vascular defects caused by inactivation of endothelial Notch signaling could also be caused by disruption of blood vessel maturation (e.g., due to inadequate recruitment of mural and other nonendothelial cell types).