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1⇒They are responsible for the make-up of the skeleton of vertebrates.
2⇒The remodeling of the bone through growths of new arms on the cell.
3⇒start bone cell replication and to help bone cells divide.

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Osteoblasts -They secrete osteoid, which forms the bone matrix. They also begin mineralization, and are unable to divide. 
Osteocytes-  A mature osteoblast which no longer secretes matrix, yet is surrounded by it. Maintains metabolism, and participates in nutrient/waste exchange via blood.
Osteoclasts  -These function in resorption and degradation of existing bone, the opposite of osteoblasts. Monocytes (a type of white blood cell) fuse together to create these huge cells, which are concentrated in the endosteum.
 Osteoprogenitors - Immature cells which differentiate to make osteoblasts. . 
bone-lining- quiescent osteoblasts covering the bone 
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my sisters answer
she is studying in 12th so i asked her