The regulation of the release of granulocytes from normal marrow

Figure 16 synthesizes the various aspects of marrow egress. The central anatomical relationship of the hematopoietic compartment to vascular sinus is shown above. The hatched blocks represent the sinus wall, capable of developing narrow migration channels. In the marrow, immature granulocytes alter their biophysical characteristics by developing motility, nuclear and cytoplasmic deformability and surfaces which facilitate egress. Humoral factors contribute to proliferation and maturation. Also, other humoral agents, releasing factors, may act on mature cells, for example, as cytoattractants, and on the sinus wall to reduce its adventitial cover and thereby to enhance egress. In the sinus, flow or discharge of sinus contents may be regulated by humoral agents or neural messages which may affect terminal sphincters or other structures. Although the marrow in situ is a difficult organ to study, future innovations are to be expected and our understanding of the delicate balance between hematopoietic cells, stroma and vasculature will be enhanced. Corrections of inferential errors due in part to the semi-quantitative and qualitative nature of much of our current data should be expected.