Osteoprogenitor cells are stem cells found in the endosteum and periosteum. These spindle-shaped cells have ovoid to elongate nuclei and unremarkable cytoplasm. Two types are distinguishable with the electron microscope, one gives rise to osteoblasts, the other to osteoclasts. Osteoblast precursors derive from embryonic mesenchyme and have sparse RER and Golgi complexes. Osteoclast precursors derive from blood monocytes and have abundant free ribosomes and mitochondria.
Osteoblasts, the major bone forming cells, are typically cuboidal, each with a large, round nucleus and basophilic cytoplasm. They form one-cell thick sheets resembling simple cuboidal epithelium on surfaces where new bone is being deposited. Osteoblasts exhibit high alkaline phosphatase activity and have the well-developed RER and Golgi complex typical of protein-secreting cells. They synthesize and secrete all the organic components of bone matrix and may be involved in bone mineralization. Once surrounded by matrix, osteoblasts are considered mature and called osteocytes.
Osteocytes are terminally differentiated bone cells found in cavities in the bone matrix called lacunae. Their long, thin cytoplasmic processes, called filopodia, radiate from the cell body in fine extensions of the lacunar cavity called canaliculi. Osteocytes are isolated from one another by the impermeable bone matrix and contact one another at the tips of their filopodia, often through gap junctions. This arrangement provides limited cytoplasmic continuity between the cells and explains how osteocytes obtain nutrients and oxygen and dispose of wastes at relatively great distances from the blood vessels. While incapable of mitosis, osteocytes retain some synthetic and resorptive capacity whereby they turn over and maintain nearby bone matrix. The death of osteocytes results in bone breakdown, or resorption. Osteocytes recently derived from osteoblasts are located near bone surfaces in rounded lacunae, older cells are found further from the surface in flattened lacunae.
Osteoclasts are bone-resorbing cells that lie on bony surfaces in shallow depressions termed Howship's lacunae. They are large and multinucleated (2-50 nuclei per cell), with acidophilic cytoplasm containing abundant lysosomes and mitochondria and a well-developed Golgi complex. The osteoclast surface facing the depression exhibits a ruffled border of plasma-membrane infoldings, which form many isolated compartments between the cell and the bone surface. The cells release acid, collagenase, and other lytic enzymes into the compartments, these break down bone matrix and release minerals, a process called bone resorption. Osteoclasts respond to Parathyroid hormone (PTH) by enlarging their ruffled borders and increasing their activity, resulting in increased blood calcium level. The effect of PTH may be indirect and mediated by a signal from the osteoblasts. Calcitonin, which decreases blood calcium, reduces surface ruffling and osteoclast activity. While their immediate precursors are found in the endosteum and periosteum, osteoclasts ultimately derive from the fusion of blood monocyte derivatives and are considered components of the mononuclear phagocyte system.
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Bone matrix. Bone matrix contains organic components, or osteoid, and inorganic components, or bone mineral.
Organic components. Osteoid constitutes about 50% of bone volume and 25% of bone weight. It is composed of fibers and unmineralized ground substance.
1. Fibers. Type I collagen fibers constitute 90-95% of the osteoid. The overlapping pattern of staggered tropocollagen results in periodic gaps (lacunar regions), which may contain up to 50% of the hydroxyapatite crystals (mineral) in bone.
2. Ground substance. Hydroxyapatite crystals and collagen fibers are embedded in the acidic ground substance, which is composed of proteins, carbohydrates, and small amounts of proteoglycans and lipids. The proteins are glycoproteins, phosphoproteins, sialoproteins. The carbohydrates (glycosaminoglycans) include chondroitin sulfates and keratan sulfate. Some ground substance components may be nucleation sites for hydroxyapatite crystals.
Inorganic components. Bone mineral makes up about 50% of bone volume and 75% of bone weight. It is composed primarily of calcium and phosphate, with some bicarbonate, citrate, magnesium, and potassium and trace amounts of other metals. Calcium and phosphate form needlelike crystals of hydroxyapatite, Ca10(PO4)6(OH)2. Hydrated ions at the crystal surface form an enveloping hydration shell, through which ions are exchanged between the crystal and surrounding body fluids.
Adult bone occurs in 2 basic organizational types, spongy and compact. 1. Spongy bone, also called cancellous bone, forms a fine 3-dimensional lattice with many open spaces. The branching and anastomosing slips of bone between the spaces, termed trabeculae or spicules, align along the lines of stress to which the bones are subjected, maximizing the weight-bearing capacity of this bone tissue. Spongy bone is found at the core of the epiphyses of mature long bones, at the core of short bones (eg, phalanges), and between the thick plates, or tables, of the flat bones of the skull, where it is called the diploe. It may be composed of either primary or secondary bone.
Compact bone, also called dense bone or cortical bone, lacks the large spaces and trabeculae. It forms the thick diaphyseal cylinder of long bones, a thin covering over the epiphyses, and the tables of the flat bones of the skull. Compact bone is always composed of secondary bone.
