Histology of bones
When examined under microscope, the bone tissue seen to be composed of extracellular matrix and cell. There are four types of cells, present in bone tissue
1. Osteogenic cell: Bone stem cells, derived from mesenchyme and responsible for the formation of all connective tissue. They are only the bone cells those undergo cell division and divided into the osteoblasts.
2. Osteoblasts: They are the bone building cells that synthesize the collagen fiber and other extracellular components and also initiate the calcification. After the formation of extracellular matrix they are converted into the osteocytes.
3. Osteocytes: The mature bone cells those maintain the metabolism and exchange of nutrients, waste product with blood.
4. Osteoclasts: They are formed by the fusion of more that 50 monocytes and form the ruffle border. These types of cells are capable of secreting the lysosomal enzymes that digest the protein and mineral components of the extracellular matrix of bone (bone erosion or resorption). This process is essential in repair of bones. They also help to maintain the blood calcium level.
The extracellular matrix of bone tissue is consisting water (15%), collagen fibre (30%) and crystallized mineral salts (55%). The abundant minerals salt is calcium phosphate along with other salts (calcium hydroxide, calcium carbonate) and few ions (magnesium, fluoride, potassium, and sulphate). The deposition of mineral salts in between the framework of collagen fiber during bone formation is called the calcification.
Compact (cortical) bone tissue:
This is strongest bone tissue, found beneath the Periosteum of all bone and make up the bulk of diaphyses. It contains large no of osteons that contains a canal composed of blood and lymph vessels, nerves, surrounded by concentric rings or plates of bone (lamellae). Between these spaces tissue fluid and spider shaped osteeocytes (matured bone cells) are present. Lamellae arer interconnected with each other by canaliculi.
Spongy or trabecular or concellus bone tissue:
Does not contain osteons. These are composed of a framework formed of trabeculae or little beams, which consists of lamellae and osteocyted interconnected by canaliculi. The spaces between trabeculae are composed of red bone marrow.
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