The hematopoietic stroma.

The stroma of the hematopoietic tissues includes a number of diverse cell types, i.e., bone-lining cells and related capsular and trabecular structures, epithelial-reticular cells and other epithelial cells, reticular cells, macrophages, and antigen-presenting cells. These stromal cells possess a variety of mechanical and metabolic functions. They support and confine the hematopoietic tissues and their vasculature. They regulate the migrations of blood and hematopoietic cells. Stromal cells contribute to the microenvironment that induces the differentiation of stem cells into the several blood-cell lines. The nature of the interactions of stromal and hematopoietic cells depends upon long- and short-range humoral factors, and upon such cell-surface compounds as immunoglobulins and major histocompatability antigens. Blood cells themselves, as neutrophils and lymphocytes, play regulatory roles in hematopoiesis along with stromal cells. Moreover, certain stromal cells, such as macrophages, originate as blood cells and have migratory phases. Distinctions between blood cells, their derivatives, and stromal cells may therefore be difficult to draw.