Abstract
The present study examined in vivo the dynamic changes of hypocellular bone marrow during increased hemopoietic activity induced by phlebotomy or administration of erythropoietin. During increased hemopoietic activity, large venules and venous sinusoids located within the marrow spaces were replaced by dilated or polygonal networks of sinusoids supplied by small arterioles or capillaries that coursed from surrounding spicules of bone into the marrow spaces. An increased cellularity (hemopoietic and fat cells) was associated with this change in vascularity. The results suggest the presence of two functional parts of the microvascular system in bone marrow. The first is associated with hemopoietic areas of the marrow and is erythropoietin-sensitive; the second, with regeneration and resorption of bone. The presence of cancellous bone and the morphology of the microvascular system may determine the extent of hemopoietic activity of bone marrow. During increased hemopoietic activity, the intimate relationship between the microvascular system, bone spicules, and the developing cellular compartment suggests local factors resident in bone are transported to these proliferating cells to exert a hemopoietic effect.
