Bone is broken down by osteoclasts, and rebuilt by osteoblasts, both of which communicate through cytokine (TGF-β, IGF) signalling.<unk>  (or osteogenesis or bone mineralization..) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the formation of normal, healthy bone tissue: Intramembranous<unk>  is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral<unk>  involves cartilage as a precursor. In fracture healing, endochondral osteogenesis is the most commonly occurring process, for example in fractures of long bones treated by plaster of Paris, whereas fractures treated by open reduction and internal fixation with metal plates, screws, pins, rods and nails may heal by intramembranous osteogenesis. Heterotopic<unk>  is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location. Calcification is often confused with<unk> . Calcification is synonymous with the formation of calcium-based salts and crystals within cells and tissue. It is a process that occurs during<unk> , but not necessarily vice versa. The exact mechanisms by which bone development is triggered remains unclear, but growth factors and cytokines appear to play a role. Time period Bones affected Third month of fetal development<unk>  in long bones beginning Fourth month Most primary<unk>  centers have appeared in the diaphyses of bone. Birth to 5 years Secondary<unk>  centers appear in the epiphyses 5 years to 12 years in females, 5 to 14 years in males<unk>  is spreading rapidly from the<unk>  centers and various bones are becoming ossified. 17 to 20 years Bone of upper limbs and scapulae becoming completely ossified 18 to 23 years Bone of the lower limbs and os coxae become completely ossified 23 to 26 years Bone of the sternum, clavicles, and vertebrae become completely ossified By 25 years Nearly all bones are completely ossified
Ossification
Calcification
Calculus 
Granulation
Necrosis
Demineralization 
Glomerulation
Infibulation