Histology of Bone
Like other connective tissues, bone, or osseous tissue, contains specialized cells surrounded by an extracellular matrix that contains fibers and various mineral salts. These salt crystals form when calcium phosphate [Ca3(PO4)2] and calcium carbonate [Ca(OH)2] combine to create hydroxyapatite [Ca10(PO4)6(OH)2] (hī-drok-sē-AP-a-tīt), which incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle. Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue:
- osteoblasts
- osteocytes
- osteogenic cells
- osteoclasts.
The osteoblast is the bone cell responsible for building new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide through mitosis , synthesize collagen and secrete the matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), little, canal-like channels within the bone matrix.
If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cell, the osteogenic cell. These osteogenic cells are stem cells with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption (meaning the breakdown or cutting away of bone) is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The continuous balance between osteoblasts building up bone and osteoclasts cutting it away is responsible for the constant but subtle reshaping of bone, so that over about 10 years you have a new skeleton.
Use the word "builder" to remember the function of Osteoblasts and "cutter" to remember that Osteoclasts cut away at the bone. Here is a table to fill out from the information above:
| Cell Type | Function | Location |
|---|---|---|
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Osteogenic |
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Osteoblast |
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Osteocyte |
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Osteoclast |
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