BONE HISTOGENESIS
Histogenesis designates the formation of a tissue. In the case of the bone tissue, this process is also called osteogenesis or ossification.
There are two basic modes of ossification: intramembranous and endochondral ossification, both processes that initiate with cells of mesenchymal origin. In humans osteogenesis begins around the sixth and seventh weeks of embryonic development.
Intramembranous ossification occurs inside a mesenchymal tissue — the embryonic connective tissue. It contains stem-cells that can give rise to various types of connective tissue — cartilage, bone, adipose tissue, etc. as well as blood cells.
In various parts of the body, the mesenchyme is arranged as layers or membranes. The intramembranous ossification happens within these layers.
This type of ossification gives rise to many flat bones, for example, the flat bones of the skull.
In the second type of ossification – endochondral ossification – mesenchymal cells occupy pre-formed models of the future bones made of cartilage and differentiate into osteoblasts. As the endochondral ossification process is ongoing the cartilaginous skeleton is gradually replaced by bones.
This type of ossification gives rise to long bones as well as some other types of bones, such as the vertebrae.