Tissue components

Bone-lining cells are present on the surface of bone where no active growth or resorption is occurring. They are highly flattened cells and are thought to provide maintenance and nutritional support for osteocytes and may have osteoprogenitor capacity.

Osteoblasts lie on the surface of bone and produce the organic components of the extracellular matrix, both ground substance and collagen fibers. Osteoblasts are large, plump cells filled with the organelles necessary for protein production. They occur in the endosteum where bone deposition is occurring. Osteoblasts are also located on the external surfaces of bone associated with the periosteum.

Formation of the extracellular matrix of bone begins with deposition of the organic portion, osteoid, by osteoblasts. Osteoid appears as a pale band on the bone surface immediately beneath the active osteoblasts and consists mainly of type I collagen fibrils. Type I collagen fibrils comprise about 90% of the total protein of osteoid and thus the matrix is highly eosinophilic. In addition to collagen, osteoid also contains proteoglycan aggregates. Osteoid rapidly becomes mineralized with calcium phosphate (hydroxyapatite).

After osteoid is formed, it becomes mineralized with an inorganic matrix of calcium phosphate (hydroxyapatite). Hydroxyapatite is deposited along collagen fibers in the matrix, giving bone rigidity to serve as a framework for the body, support and as protection. The matrix also serves as a reservoir for calcium. The abundance of collagen and calcium produces an eosinophilically-stained extracellular matrix.

Osteocytes are osteoblasts that have surrounded themselves with matrix and, therefore, lie within (rather than on) bone. Although not visible here, numerous processes radiate out from each cell body, communicating with other osteocytes and with surface cells. Osteocytes lie within spaces called lacunae, seen here as a halo surrounding each cell body. Osteocytes maintain the bone matrix.

Osteoclasts lie on the surface of bone where they resorb bone, forming small depressions called Howship’s lacunae. Osteoclasts are large, multinucleated cells with elaborate plasma membrane infoldings that face the bone to increase surface area for bone. Osteoclasts are also located on the external surfaces of bone associated with the periosteum. Osteoclasts are derived from the monocyte/macrophage cell lineage.

Osteoclasts lie on the surface of bone where they resorb bone, forming small depressions called Howship’s lacunae. Osteoclasts are large, multinucleated cells with elaborate plasma membrane infoldings that face the bone to increase surface area for bone. Osteoclasts are also located on the external surfaces of bone associated with the periosteum. Osteoclasts are derived from the monocyte/macrophage cell lineage.

The spicule is surrounded by red bone marrow which produces all the circulating blood cells. Numerous blood vessels (sinusoids) in the bone marrow serve to transport newly formed blood cells into the general circulation.

The spicule is surrounded by red bone marrow which produces all the circulating blood cells. Numerous blood vessels (sinusoids) in the bone marrow serve to transport newly formed blood cells into the general circulation.