MadSci Network: Anatomy
Query:

Re: What do cranial bones develop from?

Date: Tue Feb 16 10:11:32 1999
Posted By: Leslie Gartner, Faculty Histology/Anatomy, University of Maryland, Baltimore, MD
Area of science: Anatomy
ID: 918870981.An
Message:

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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