Red cell delivery and the function of the marrow-blood barrier: a review.

The last step in mammalian erythropoiesis consists of nuclear elimination and delivery of the non-nucleated red cell (reticulocyte) into the circulation. This is accomplished by migration of reticulocytes through the wall of the vascular sinuses which forms an anatomical barrier between the hemopoietic compartment and the circulation. This barrier is instrumental in a complicated mechanism to ensure orderly traffic of new cells in response to the requirement of the organism. Thus, it exerts a control over which cells will migrate into the vascular lumen and when they will do so. Defective red cells and nucleated red cells are prevented from migration; even normal reticulocytes may be temporarily held back if their presence in the circulation would exceed the requirement of the body. Little is known about how this barrier operates to control the red cell delivery. Literature relevant to this subject has been reviewed and an attempt is made to formulate some basic questions in this area.