Cellular and Molecular Biology of Endothelial Cell Differentiation during Embryonic Development

Abstract

How does the pattern of the rudiments of the major blood vessels establish itself in the developing embryo? To understand the cellular biology of these events we must know how the angioblasts, the precursors of endothelial cells, segregate from the mesoderm, migrate, and cohere to one another to form the cords and tubes which are the earliest embryonic blood vessels. We have been using a monoclonal antibody (QH-1) and microsurgery to determine where angioblasts originate and how they assemble into vessel rudiments (Coffin and Poole, 1988; Poole and Coffin, 1989; 1991). The extent and type of directed angioblast migration define three distinct modes of vessel morphogenesis (Poole and Coffin, 1991; Poole, 1993). Vessel rudiments may organize in place, a process termed vasculogenesis, either from angioblasts originating at the rudiment’s location (vasculogenesis type I) or from angioblasts which migrate as individual cells or small groups to that site from different locations (vasculogenesis type II). The dorsal aortae form by the first type of vasculogenesis (Coffin and Poole, 1988; DeRuiter et al., 1993; Pardanaud et al., 1987; Poole and Coffin, 1988; 1989; 1991). The endocardium, ventral aortae and posterior cardinal veins form by the second type (Coffin and Poole, 1991; DeRuiter et al., 1993; Drake and Jacobson, 1988; Poole and Coffin, 1991). New vessels may also form by sprouting from preexisting vessels, a process called angiogenesis. The intersomitic and vertebral arteries are the first vessels to form by angiogenesis, sprouting off the rudiments of the dorsal aortae (Coffin and Poole, 1988; Poole and Coffin, 1988; 1989; 1991). Figure 1 illustrates the different roles of endothelial cells in vasculogenesis and angiogenesis.