Spatially Separated Distribution and Highly Flexible Expression of Adhesion Molecules Facilitates Dynamic Hematopoiesis

In bone marrow niches, direct interactions of Hematopoetic Stem Cells (HSCs) with supportive stromal cells and the extracellular matrix are essential for regulation of hematopoiesis. These interactions are mediated by several sets of adhesion molecules. In vivo, different adhesion molecules were spatially separated and showed specific distributions for the different adhesion molecules. These patterns for VLA-2, VLA-3, ALCAM, VLA-1, VLA-4, VCAM-1 and ICAM-3, varied in size, ranging from a single cell to smaller or larger cell clusters. ICAM-1 and VLA-5 were expressed by virtually all cells. The results are similar in seven individual steady state bone marrows. In vitro, adhesion molecule expression changed for several stromal cell types by exposing them to different inductors as present in different animal sera. Besides the normal blood cell turn over recruitment of more blood cell developmental centers is needed during increased demands for hematopoietic cells. During infection far more granulocytes are demanded by the organism than during normal turnover. Such adaptation to changing conditions is easily accomplished by highly flexible spatio-temporal distribution of adhesion molecules since it provides bone marrow stromal cells with the capacity to deliver the specific lineages that the organism demands.