ABSTRACT
References .......................................................................................................................... 371
The processes underlying formation of new blood vessels have traditionally been divided
into two categories: vasculogenesis and angiogenesis [1]. In vasculogenesis, new vessels
form by the differentiation of mesenchymal cells (angioblasts) into endothelial cells, a process
that occurs in parallel with the formation of blood islands, which are co-derived from
hematopoietic progenitors. Very likely, an earlier precursor (hemangioblast) gives rise to
both populations of cells. In angiogenesis, endothelial cells lining previously formed vessels
proliferate and form sprouts, which give rise to new vascular structures. Whereas there is good
evidence that angiogenesis occurs both during development and after birth, until a few years
ago it was widely accepted that vasculogenesis, in the broader sense of an undifferentiated cell
type giving rise to endothelial cells, took place only in the embryo. Recent findings have
challenged this dogma [2], however. There is now evidence that supports the idea that
circulating vascular progenitors are involved in blood vessel formation after birth, a pro-
cess that has been referred to as neoangiogenesis [3]. Moreover, it has also been newly
recognized that hematopoietic cells contribute to the maintenance and initiation of these
processes, and that co-mobilization of both vascular and hematopoietic elements may be an
essential part of neoangiogenesis [4].
