MadSci Network: Anatomy |
Hi Cara, The cranial bones develop during the fetal stages and most begin their development in a connective tissue membrane. Because of their membranous origin this type of bone formation is known as "intramembranous bone formation" and these bones are referred to as "membrane bones." Some of the bones of the lower region of the skull begin their formation around a cartilage template. This cartilage will then be replaced by bone and this type of bone formation is referred to as "endochondral bone formation." The process of intramembranous bone formation ocurs as follows: 1. In the region of the future bone mesenchymal cells (primitive cells that have the capability of forming many mature cell types) congregate and differentiate into fibroblasts. These fibroblasts manufacture a lott of collagen fibers and form a membranous sheet. 2. The membrane becomes vascularized by the formation of new blood vessels as well as by ingrowth of blood vessels. 3. Additional mesenchymal cells differentiate into "osteoprogenitor cells" and "osteoblasts." Osteoprogenitor cells have the ability to undergo cell division to form more of themselves as well as to form osteoblasts. Osteoblasts are cells that manufacture bone and are also responsible for the process whereby the bone becomes mineralized. 4. As small chunks of bone are formed in the region of the future mature bone, they begin to fuse with each other and thus become larger and larger. These bones of the skull continue to grow as the individual grows and all of the bones join each other at immoveable sutures by the time the child is about 3 to four years of age.
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