Vbone marrow

Red bone marrow lies entirely within the spaces of bone, in the medullary cavity of young long bones and the spaces of spongy bone

Bone marrow consists of blood vessels, specialized units of blood vessels called sinusoids, and a sponge-like network of hemopoietic cells. In sections, the hemopoietic cells appear to lie in "cords" between sinusoids or between sinusoids and bone.

The sinusoid of red bone marrow is a unique vascular unit. It occupies the position normally occupied by a capillary; i.e., it is interposed between arteries and veins. It is believed to be derived from vessels that have just nourished the cortical bone tissue. The sinusoids arise from these vessels at the corticomedullary junction. The sinusoid wall consists of an endothelial lining, a basal lamina, and an outer adventitial cell layer (Fig. 9.21). The endothelium is a simple squamous epithelium.

The adventitial cell, also called a reticular cell, sends sheet-like 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 CSFs (e.g., 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; 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. Each blood cell must squeeze through such 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 hemopoietic sinusoid endothelium arterial capillary arteriole sinusoid adipose cell

sinusoid penetration of megakaryocyte by white cell adventitial cell endothelial cell

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