Endosteum

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Bone is the basic unit of the human skeletal system.

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 central (or Haversian) canals of osteons within compact bone are also lined with endosteum, which is referred to as osteonal endosteum.

The endosteum is a primarily composed of single cell layer ofbone-lining cells and osteoblasts. In regions of active bone resorption, osteoclasts can also be present in the endosteum.

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.