Stromal microenvironment of human fetal hematopoiesis: in vitro morphologic studies.

The stromal microenvironment is essential for the proliferation and differentiation of hematopoietic progenitors. The features of stroma which contribute to normal hematopoietic stem cell ontogeny or to observed differences in hematopoiesis between fetal and adult hematopoietic organs remain to be fully characterized. In this study, we used long-term 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 electron microscopic evaluation of the stromal layers reveal the presence of two distinct cell types: a large cell with extensive cytoplasmic projections, and a smaller cell resembling a macrophage. In contrast to some previous reports from in vitro stromal studies, there were no adipocytes, endothelial cells, or cells which could conclusively be identified 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 endothelial elements. There were no morphological differences in the stromal layers from fetal liver, fetal bone marrow, and adult bone marrow sources. This finding is important because it suggests that differences in the behavior of hematopoietic stem cells, when supported in these various hematopoietic microenvironments, are less likely to be explained by obvious differences in the cytologic architecture of stroma than by differences in stem cell biology or growth factor interactions.