- Key words: hematopoiesis, endothelial cells, hematopoietic progenitors.
- Page(s) : 510-7
- Published in: 1998
Major events in hematopoiesis involve interactions with endothelial cells (EC). During embryonic life, the onset of hematopoiesis occur in the yolk sac blood islands where the development of hematopoietic progenitor cells coincides with the generation of the first vascular cells. There after, close physical association of the hematopoietic progenitors can be found in the aorta endothelium-associated hematopoietic cells aggregates and in the long bone endothelium-delineated logettes during fetal hematopoiesis. A common precursor to EC and progenitors has long been postulated. In adult life, bone marrow sinus endothelium represents a morphologic barrier between the circulatory space and the hemopoietic compartment of the bone marrow. It forms a complete covering for the inner surface of the marrow sinuses. This location makes the bone marrow EC a critical element in the progenitor cells exit into circulation and in their selective homing in the bone marrow. The molecular mechanisms whereby hematopoietic progenitor cells egress or selectively home to the marrow are unclear. These mechanisms are likely to be a multistep adhesion process involving an interplay between adhesion molecules on bone marrow microvascular EC and the corresponding counter-structures on progenitors. Recognition of specific signals expressed by bone marrow EC are likely to be involved. Several groups have suceeding in isolating bone marrow microvascular EC. Most protocols involve a positive selection step using EC-specific markers such as the Ulex Europaeus-1 binding sites or EC-specific epitopes recognized by Mabs. These cells have been studied for their expression of adhesion molecules, the production of cytokines and their ability to support hematopoiesis. EC derived from the umbilical cord as well as bone marrow microvascular EC support the long-term proliferation and differentiation of myeloid and megakaryocytic progenitors. This support is essentially related to the production of cytokines by the EC. The obtention of pure preparations of bone marrow microvascular EC should facilitate the elucidation of the molecular mechanisms involved in marrow egress and reentry of hematopoietic progenitors.