In human beings, as well as in most vertebrate animals, an embryonic skeleton is firstly made of cartilage, which will eventually mineralise to originate the bones. These process is called ossification.
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Although ossification varies from one species to the other, the main events in this process are shared among mammals.
Flat bones, such as the ones that form the skull, develop by intramembranous ossification, directly from connective tissue sheets instead of cartilage.
Unspecialised cells become osteoblasts, which start producing the bone extracellular matrix, originating a very porous bone. Eventually, these osteoblasts will get stuck within the hard matrix and will later mineralise, becoming osteocytes (bone cells).
Long bones are the classic example of endochondral ossification, which will always originate from hyaline cartilage.
In this case, the ossification process starts with the death of chondrocytes (cartilage cells) to give space to osteoblasts. Then, the ossification will start in the shaft, moving toward the bone's extremities. In human embryos, this will occur between prenatal weeks five to 12.
Skeleton and Blood Production
The marrow of bones is a real factory of blood cells. At birth, almost all bones are a source of blood cells, this being restricted to a few specific bones in the postnatal period, says Dr. Mark Hill of the Department of Embryology of the University of New South Wales, Australia.
Bone tissue, which is the mineral matrix that forms the rigid parts of the organs, consists of 95 per cent collagen (Type I) and only 5 per cent proteoglycans and noncollagenous proteins (osteopontin and osteocalcin).
Bones are organs made of bone tissue and other components, such as marrow, blood vessels and nerves.
Post-Natal Bone Growth
Children's bones grow in a similar way to the embryonic endochondral ossification.
The only cartilage that remains in their bones is in the extremities, where the epiphyseal disks are located. The cells that form these discs will be continuously dividing during the growing period.
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- Dr. Mark Hill; Department of Embryology of the University of New South Wales, Australia: Musculoskeletal Development - Axial Skeleton; 2010
- Dawn A. Tamarkin, Ph.D.; Springfield Technical Community College; Massachusetts: Bone Development and Growth: 2008
- Anne Martin; Brown University; Providence: Appendicular Skeleton: Ossification