5.3.3: Bone Tissue
- Page ID
- 53904
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There are two types of bone: spongy bone has spaces between bony trabeculae filled with bone marrow, and compact bone, which is more solid and composed of functional units called osteons.
Above: A femur (hip to knee) bone with a partial frontal section. Compact bone is located on the outer layers of the bone, particularly along the shaft. The interior of the bone is composed of spongy bone where bone marrow is located in the pockets between the trabeculae of bone.
The bone cells, or osteocytes, are located in lacunae (a space or gap inside the tissue; sing. lacuna). Osteocytes are protected from the extracellular matrix that surrounds them by being housed in lacunae, which are spaces free of mineralized extracellular matrix. Bone has mainly collagen protein fibers in and a ground substance rich in a mineralized calcium phosphate compound known as hydroxyapatite. The hydroxyapatite that surrounds the mostly collagen protein fibers in the ground substance of bone is not soluble in water and forms a mineral solid in which both the bone cells and the collagen fibers are embedded. It is usually difficult to see the collagen fibers in the extracellular matrix of bone due to the density of the ground substance that surrounds them. There are four types of bone cells: osteoprogenitor cells, osteoblasts, osteoclasts, and osteocytes. Osteocytes are the most abundant and the only ones found throughout the bone.
Osteocytes have numerous cytoplasmic extensions that project off of the main cell body. These extension connect up with the extensions from other nearby osteocytes. These projections, like the osteocyte cell body, are in tiny spaces free of the mineralized extracellular matrix. These spaces (but not the cytoplasmic projections themselves) are called canaliculi (singular: canaliculum) because, under the microscope, they look like tiny little canals.
Compact Bone
Osteocytes are found in concentric circles of mineralized extracellular matrix. Each circle is called a lamella (plural: lamellae) and the osteocytes are found along the edges of each lamella. In compact bone, groups of lamellae and osteocytes are arranged into individual osteons, the cylindrical arrangement of material that makes up the fundamental building block of the compact bone. Each osteon has a hollow central canal in its center that blood vessels and nerves can travel through.