Bone marrow consists of specialised blood vessels and extravascular cells. They occupy the medullary cavity of long bones and the intertrabecullar spaces of cancellous bone. These cavities are lined by a membrane, the endosteum. The arteriole supply of the bone subdivides into broad sinuses before reconverging as venules.
The sinus wall consists of endothelial cells arranged as a simple squamous epithelium; they sit on a basement membrane and are surrounded by the adventitial layer of reticular cells. Around the outside of the sinus reticular layer resides a network of haemopoeitic cells arranged in cord-like strands.
In red marrow, the cords contain clusters of developing blood cells and megakaryocytes close to the sinus wall, with granulocyte progenitors slightly further away. Macrophages, mast cells and plasma cells are also present within the cords.
Mature blood cells pass from cords to sinuses by pushing aside the adventitial layer and passing through the basement membrane. The endothelial cell creates a special membrane-lined opening through which the new cell passes. Megakaryocytes extend part of their cell through such openings in order to shed platelets into blood. The opening seals after the blood element has traversed.
Yellow bone marrow differs from red in that its chief constituent is adipocytes; normal red marrow constituents are present in far smaller amounts.
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