The myeloblasts reside extravascularly in the marrow. The hematopoiesis takes place in the extravascular cavities between the sinuses of the marrow. The wall of the sinuses is composed of two different types of cells, the endothelial cells and the adventitial reticular cells. The hemopoietic cells are aligned in cords or wedges between these sinuses, the myeloblasts and other granular progenitors are concentrated in the subcortical regions of these hemopoietic cords.