The adventitial cell, also called a reticular cell, sends sheetlike extensions into the substance of the hemopoietic cords, which provide some support for the developing blood cells. In addition, adventitial cells produce reticular fibers. They also play a role in stimulating the differentiation of developing progenitor cells into blood cells by secreting several cytokines (e.g., CSFs, IL-5, IL-7). When blood cell formation and the passage of mature blood cells into the sinusoids are active, adventitial cells and the basal lamina become displaced by mature blood cells as they approach the endothelium to enter the sinusoid from the bone marrow cavity.
The bone marrow sinusoidal system is a closed circulation system; newly formed blood cells must penetrate the endothelium to enter the circulation.
As a maturing blood cell or a megakaryocyte process pushes against an endothelial cell, the abluminal plasma membrane is pressed against the luminal plasma membrane until they fuse, thus forming a transitory opening, or aperture. The migrating cell or the megakaryocyte process literally pierces the endothelial cell. Thus, migration across the bone marrow
FIGURE 10.22 • Bone marrow with active hemopoiesis. a. Schematic drawing of bone marrow shows the erythroblastic islets engaged in the formation of erythrocytes, megakaryocytes discharging platelets into the sinusoids, endothelial cells adjacent to a basal lamina that is sparse in places and absent where blood cells are entering the sinusoids, and adventitial or reticular cells extending from the basal lamina into the hemopoietic compartment. (Modified from Weiss L, ed. Cell and Tissue Biology: A Textbook of Histology, 6th ed. Baltimore: Urban & Schwarzenberg, 1988.) b. Photomicrograph of bone marrow stained with H&E shows active hemopoietic centers in a close proximity to bone marrow sinusoids. 420.
endothelium is a transcellular and not an intercellular event. Each blood cell must squeeze through an aperture to enter the lumen of a sinusoid. Similarly, a megakaryocyte process must protrude through an aperture so that the platelets can be released directly into the sinusoid lumen. The aperture is lined by the fused plasma membrane, thus maintaining the integrity of the endothelial cell during the transcellular passage. As the blood cell completes its passage through the aperture or the megakaryocyte that has extruded its platelets withdraws its process, the endothelial cell “repairs itself,” and the aperture disappears.
In active red bone marrow, the cords of hemopoietic cells contain predominately developing blood cells and megakaryocytes. The cords also contain macrophages, mast cells, and some adipose cells. Although the cords of hemopoietic tissue appear to be unorganized, specific types of blood cells develop in nests or clusters. Each nest in which erythrocytes develop contains a macrophage. These nests are located near the sinusoid wall. Megakaryocytes are also located adjacent to the sinusoid wall, and they discharge their platelets directly into the sinusoid through apertures in the endothelium. Granulocytes develop in cell nests farther from the sinusoid wall. When mature, the granulocyte migrates to the sinusoid and enters the bloodstream.
Bone marrow not active in blood cell formation contains predominately adipose cells, giving it the appearance of adipose tissue.