The endosteum refers to a layer of connective tissue that line the inner surfaces of bones including both the medullary (marrow) cavity of compact bone and the thin spicules or trabeculae of cancellous (spongy) bone. The inner surfaces of the Haversian canals within compact bone are also lined with endosteum, which is referred to as Haversian (osteonal) endosteum.
The endosteum is a single cell layer thick and is composed of osteoprogenitor cells that can differentiate and mature into bone matrix–secreting cells (active osteoblasts), bone-lining cells (inactive osteoblats) and osteoclasts. These diverse cell types enable the endosteum to carry out its vital processes of bone growth, remodelling, and repair.
The endosteum forms the deepest layer of the inner circumferential lamellae within compact bone. Upon stimulation, bone-lining cells which are flat quiescent osteoblasts transform into large, active osteoblats which synthesize a fresh layer of osteoid (unmineralized bone matrix). Following mineralisation, this layer transforms into a new layer of inner circumferential lamella. A similar process occurs with endosteum that envelops the trabeculae of cancellous bone leading to the deposition of new osteoid.
|Structure||Single cell layer of osteoprogenitor cells, osteoblasts, osteoclasts
|Function||Bone growth, remodeling, and repair.|
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