Human breast milk is a rich source of growth factors, including erythropoietin (Epo), the endogenous hormonal stimulant of erythropoiesis. Recombinant human Epo (rhEpo) has been shown to stimulate 1) angiogenesis, the process of new blood vessel growth from preexisting vessels; 2) vasculogenesis, tubule formation from single-cell suspensions; and 3) endothelial cell proliferation in immortalized endothelial cells and vessel explants. We hypothesized that Epo would induce mitogenesis and stimulate vasculogenesis in primary cultures of microvascular endothelial cells (MVECs) from neonatal rat mesentery. Isolation, purification, characterization, and culture of MVECs were performed. Cell proliferative effects of rhEpo were studied by 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay in cultured MVECs. Vasculogenic effects of rhEpo were examined on cultured MVECs plated on either hormone-rich Matrigel substratum or the extracellular matrix protein, type I collagen. Our findings show that MVECs are isolated and purified, and that rhEpo stimulates MVEC proliferation, with maximal proliferation seen with a concentration of 50 IU/mL rhEpo. Tubule formation assays reveal that an rhEpo concentration of 50 IU/mL produces maximal tubule formation after 12 h on both Matrigel and the simple substratum, type I collagen. Our study is the first to examine the effects of rhEpo on the endothelium of the neonatal gastrointestinal tract. These data suggest that Epo may have a trophic effect on the vasculature of the gastrointestinal tract early in development. Furthermore, as Epo has been measured in breast milk, and its receptor has been shown to exist on the mucosa and gastrointestinal vasculature, Epo may be an endogenous stimulant of vessel growth during neonatal gastrointestinal development. Many growth factors and hematopoietic cytokines have been measured in human breast milk (1-4). Epo, the primary hormonal stimulant of red blood cell production, is among this group of substances (5). As it does for other growth factors, human milk protects Epo from proteolytic degradation (5). The EpoR is present in the brush border and vascular endothelium of the mucosa lining the small intestine (6). Because endothelial cells and erythropoietic cells are derived from the same progenitor cell, the hemangioblast, it is possible that Epo exerts similar trophic effects on endothelium as it does on erythropoietic cells (7). Milk-borne Epo has been shown to be absorbed without degradation (6). These data suggest that Epo may act as a stimulator of normal development of the digestive tract.Among its many potential effects, Epo may stimulate angiogenesis, the process by which preexisting blood vessels give rise to new vessels. During angiogenesis, proteins from vascular endothelial cells degrade the extracellular matrix, migrate into perivascular space, proliferate, align themselves into tu-
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