Microscopic Anatomy (Bone Cells)

Microscopic Anatomy (Bone Cells)

The five major types of bone cells are osteogenic cells, osteoblasts, osteocytes, bone lining cells , and osteoclasts. All except osteoblasts originate from mesenchymal cells. Each is basically a specialized form of the certain cell type that becomes mature or functions in a certain process involved in bone growth. Like various connective tissue cells, bone cells are also surrounded by their own self-made extracellular matrix. The five types are explained in detail as:

■■ Osteogenic cells: Also known as osteoprogenitor­ cells, these mitotically active stem cells are found in the periosteum and endosteum. They are squamous or flattened cells when bones are growing. Stimulation of these cells causes them to often differentiate into osteoblasts or bone lin-ing cells; others may remain as osteogenic cells (FIGURE 7-4).

■■ Osteoblasts: These cells produce bone matrix and are related to osteoprogenitor cells, osteocytes­,fibroblasts, and chondroblasts. They are mitotic and become active with connective tissue layers, depositing bony matrix around them. Spongy bone tissue forms in all directions within the layers of connective tissues. They secrete an unmineralized bone matrix that includes colla-gen (which makes up the majority of bone pro-tein) and calcium-binding proteins that form the original unmineralized bone ( osteoid). They also aid in matrix calcification. Osteoblasts are cube-shaped when they are depositing matrix, but appear similar to flattened osteogenic cells when inactive. They may also differentiate into bone lining cells. Osteoblasts become osteocytes when they are totally surrounded by the matrix they are secreting.

■■ Osteocytes: These are mature osteoblasts that have become embedded in the bone matrix. They occupy small cavities (lacunae) in the bone and have protoplasmic projections con-nected with the same structure of other osteo-cytes. The osteocytes conform to the shapes of the lacunae. These connections form a system of tiny canals within the bone matrix and act to maintain it as needed. When osteocytes die, the matrix surrounding them is resorbed. They also react to strain or stress and respond to stimuli such as bone deformation, bone loading, and weightlessness. The osteocytes alert the osteo-blasts and osteoclasts to build up or degrade the bone matrix as needed. This preserves calcium homeostasis.

■■ Bone lining cells: These are flat cells on bone sur-faces where bone remodeling does not occur and are believed to also help maintain the bone matrix. On external bone surfaces, they are called periosteal cells , and when they line internal sur-faces, they are called endosteal cells.

■■ Osteoclasts: These are large, multinucleated bone cells found at sites of bone resorption, which is called osteolysis. They form from the hema-topoietic stem cells that also differentiate into macrophages. During fractures and bone heal-ing and certain disease processes, osteoclasts use enzymes to excavate passages (resorption bays) through the surrounding tissue, breaking down the calcified extracellular matrix. At this point, they have an irregular border that con-tacts bone directly. This border has deep plasma membrane infoldings that greatly increase the surface area for bone degradation via enzyme activity. The infoldings close off the surface area from the matrix surrounding it. Osteoclasts are also known as osteophages. They secrete an acid that dissolves the matrix. They resorb bone matrix throughout life, replacing it with osteoblasts­. These opposing processes (resorp-tion and deposition) are regulated by hormones that control blood calcium.

Compact Bone

Bone cells called osteocytes occupy small chambers (lacunae) that create concentric circles around cen-tral canals in bones (FIGURE 7-5). Cellular processes passing through canaliculi allow osteocytes to com-municate with other cells. Bone tissue is mostly made up of collagen and inorganic salts such as calcium phosphate. Calcium phosphate interacts with calcium hydroxide to form crystals of hydroxyapatite. These crystals incorporate various calcium salts as well as ions such as fluoride, magnesium, and sodium. Com-pact bones have a central canal that helps to make up cylinder-shaped osteons or ­Haversian systems. The osteons are parallel to the bone’s long axis, aid in weight bearing, and are the structural units of compact­ bone. Each osteon consists of a group of hol-low tubes of bone matrix that appear like the rings in a tree trunk. Each lamella (matrix tube) lends its name to the other description of compact bone, which is lamellar bone.

The collagen fibers of each lamella run in one direction,­ whereas those in nearby lamellae run in different directions. This alteration of collagen fiber placement strengthens compact bone and resists twisting motions. Between collagen fibrils, bone salt crystals also are aligned with directional alterations. Each central canal contains nerve fibers, blood vessels and the surrounding connective tissue. The central canals are connected by perforating Volkmann’s canals, which contain larger nerves and blood ves-sels. The Volkmann’s canals lie at right angles to the long axis of the bone. They are not surrounded by con-centric lamellae, but are lined with endosteum. At the junctions of the lamellae are spider-shaped osteocytes occupying the lacunae. Thin, hair-like canaliculi con-nect lacunae to each other and to the central canal. During bone formation, the osteoblasts that secrete bone matrix surround blood vessels and stay in con-tact with each other, as well as nearby osteocytes, via projections that extend outward. Each of these exten-sions contains gap junctions. As the matrix hardens, a system of canaliculi is formed, containing tissue fluid and the osteocytes’ extensions. A mature osteon is then bound together, and both nutrients and wastes can move from one osteocyte to the next. The bone matrix, therefore, allows bone cells to receive nour-ishment while it still remains hard and impermeable. However, some lamellae in compact bone are not part of complete osteons. Between osteons are incomplete interstitial­ lamellae that either fill gaps or are leftover­ ­structures of previous osteons that experienced­ bone remodeling­. Deep to the periosteum, just superficial to the endosteum, are circumferential­ lamellae, which extend completely around the diaphysis, help-ing the long bone to resist twisting.

Spongy Bone

Spongy bone is similarly composed as compact bone, but its cells do not aggregate around the central canals. The cells in spongy bone lie inside the trabeculae (supporting structures of dense tissue) and take their nutrients from diffused substances that enter the can-aliculi. The trabeculae are only a few cells thick. They have irregular lamellae and osteocytes,­ interconnected by canaliculi, and no osteons are present. Nutrients reach spongy bone osteocytes via diffusion through the canaliculi from capillaries in the endosteum that surround the ­trabeculae.

1. Compare the structure of compact bone with spongy bone.

2. List various types of bone cells and their functions.

3. Identify osteons and lamellae.