Bone Part 1 PDF

Bone Part 1 Definition of Bone Bone is a specialized type of dense supporting connective tissue composed of calcified intercellular substance; the bone matrix and bone cells. It is formed of: ❖1-Bone cells ❖ 2-Bone matrix ❖3- Periosteum ❖4-Endosteum. Bone matrix It is composed of inorganic and organ...

Bone Part 1 Definition of Bone Bone is a specialized type of dense supporting connective tissue composed of calcified intercellular substance; the bone matrix and bone cells. It is formed of: ❖1-Bone cells ❖ 2-Bone matrix ❖3- Periosteum ❖4-Endosteum. Bone matrix It is composed of inorganic and organic materials. A)The inorganic materials: - Form about 75% of the dry weight of bone - Consist mainly of calcium in the form of hydroxyapatite Decalcification of bone will result in flexible bone like a tendon. )The organic materials: - Form about 25% of the dry weight. - About 90% of the organic material is type I collagen fibers. Removal of the organic bone matrix will result in fragile bone on handling (egg- shell consistency). The periosteum It is the C.T that covers the bone. It consists of an outer vascularized dense fibrous layer and an inner osteogenic layer. Endosteum: It is a very thin layer of flattened osteogenic cells and osteoblasts that lines the bone marrow cavity and spaces in addition to the Haversian canals Bone cells. - bone formation and maintainance cells: Osteogenic cells, osteoblasts & osteocytes. could be considered as different functional phases of the same cell type, and -bone resorption cells; osteoclasts. Osteogenic cells They are found on or near all the free surfaces of bones in the endosteum and the inner layers of periosteum. They have pale oval nuclei and scanty, faintly basophilic cytoplasm. These cells are able to proliferate and differentiate into osteoblasts. Osteogenic cells are active during the growth of bone and repair of bone after its injury. Osteoblasts ( Bone forming cells). They are large oval cells with cytoplasm is highly basophilic. the nuclei appear rounded eccentric in position Secrete both type I collagen (which constitutes 90% of the protein in bone) and bone matrix proteins The osteoblast is also responsible for the calcification of bone matrix( activity of alkaline phosphatase) It has the appearance of secretory cells with well developed RER , Golgi and mitochondria and vesicles: 1-The procollagen molecules are present in secretory vesicles. 2-Small matrix vesicles contain alkaline phosphatase enzyme which liberate phosphate ions important for precipitation of calcium and formation of the hydroxyapatite crystals of the bone matrix. Osteocytes They are oval branched cells, present inside lacunae. Only one osteocyte is found in each lacuna Osteocytes extend cytoplasmic processes that traverse the matrix through the canaliculi which filled with extracellular fluid and originate from the lacunae. Processes of adjacent cells make contact through gap junctions. Osteocytes maintain the calcified matrix. Death of the osteocytes is followed by resorption of this matrix. Osteoclasts -They are large multinucleated cells containing 6-12 nuclei (up to50 nuclei). Osteoclasts are derived from the fusion of monocytes. The cytoplasm is acidophilic Osteoclasts occupy pits in the surface of the bone called Howship's lacunae. Have a striated border (ruffled border) which is at the surface exposed to bone. Function: Bone resorption &remodeling