How many bones are there in the skeleton? What are the types and composition of bone
• The human skeleton (from the Greek word, skeletos , “dried up”) consists of 206 bones (126 appendicular bones, 74 axial bones, and 6 ossicles).
• Bone is a mineralized connective tissue, with two subtypes: cortical (or compact) bone and cancellous (or trabecular) bone. Cortical bone forms 80% of the skeleton weight and is increased in long bone shafts. Cancellous bone is in contact with bone marrow cells and is enriched in the vertebral bodies, pelvis, and proximal ends of femora, all of which are subject to osteoporosis and fractures. Bone remodeling normally replaces 25% of the trabecular bone and 3% of the cortical bone each year.
• Bone is comprised mainly of type I collagen and contains three cell types: osteoclasts, which resorb mineralized bone; osteoblasts, which synthesize the proteins of the bone matrix; and osteocytes, which are probably osteoblasts that have secreted bone matrix and become buried within it. Osteocytes communicate with each other through a canalicular system and play a role in response to mechanical loading. The skeleton contains 99% of the total calcium, 80%–85% of the total phosphorus, and 66% of the total magnesium in the body.