Endosteum
Endosteum is a thin, soft, connective tissue, lining the cavity of long bones like humerus and femur.
It acts as a coating for the inner compact bone and the trabeculae of the spongy (cancellous) bone.
The marrow stroma consists principally of a network of sinuses that originate near the endosteum from cortical capillaries and terminate in collecting vessels that enter the systemic venous circulation.
The trilaminar sinus wall is composed of endothelial cells; a thin basement membrane; and mesenchymal adventitial reticular cells that give rise to osteogenic-adipogenic cells. The endothelium and reticular cells are also sources of hematopoietic cytokines. Hematopoiesis occurs in the spaces between sinuses and is controlled by a complex array of stimulatory and inhibitory cytokines, cell–cell contacts, and extracellular matrix components.
Lymphohematopoietic stem cells can leave and reenter marrow as part of their normal circulation. Their extramedullary circulation can be increased by exogenous cytokines and chemokines.
Within the unique marrow environment, the hematopoietic stem cells differentiate into all the blood cell lineages. Mature cells are produced and released to maintain steady-state blood cell levels. T
he system can respond to meet increased demands for additional cells as a result of blood loss, hemolysis, inflammation, immune cytopenias, and other causes.
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Functions
Bone production
The trilaminar sinus wall is composed of endothelial cells; a thin basement membrane; and adventitial reticular cells that are progenitors of chondrocytes, osteoblasts and adipocytes.
The osteoprogenitor cells of the preosteoblasts present in the endosteum, differentiate into osteoblasts and later on to osteocytes, which are the bone forming cells. They also aid in the formation of bone-matrix secreting cells, also known as bone-lining cells.
The endosteum, along with the periosteum functions for the growth of bone in diameter.
Blood production
Endosteum contains hematopoietic stem cells.