STROMAL MICROENVIRONMENT OF HUMAN FETAL HEMATOPOIESIS - IN-VITRO MORPHOLOGIC STUDIES

The stromal microenvironment is essential for the proliferation and di fferentiation of hematopoietic progenitors. The features of stroma whi ch contribute to normal hematopoietic stem cell ontogeny or to observe d differences in hematopoiesis between fetal and adult hematopoietic o rgans remain to be fully characterized. In this study, we used long-te rm culture conditions to grow human fetal liver, fetal bone marrow and adult bone marrow-derived stroma. The stromal layers in all cultures were observed to support multilineage hematopoiesis. Routine and elect ron microscopic evaluation of the stromal layers reveal the presence o f two distinct cell types: a large cell with extensive cytoplasmic pro jections, and a smaller cell resembling a macrophage. In contrast to s ome previous reports from in vitro stromal studies, there were no adip ocytes, endothelial cells, or cells which could conclusively be identi fied as fibroblasts in the stromal layer. These findings were further substantiated by negative findings on sections stained with oil-red-O for fat, trichrome for collagen, and factor VIII-related antigen for e ndothelial elements. There were no morphological differences in the st romal layers from fetal liver, fetal bone marrow, and adult bone marro w sources. This finding is important because it suggests that differen ces in the behavior of hematopoietic stem cells, when supported in the se various hematopoietic microenvironments, are less likely to be expl ained by obvious differences in the cytologic architecture of stroma t han by differences in stem cell biology or growth factor interactions.