10.11:
Bone Formation by Intramembranous Ossification
The human embryo begins bone formation around the eighth week of development.
The embryonic skeleton is initially composed of fibrous mesenchymal membranes and hyaline cartilage on which the bone matrix is deposited.
Bone formation occurs through two processes. While most bones develop from the cartilage by endochondral ossification, flat bones, such as the cranial bones, develop from the mesenchyme by intramembranous ossification.
During intramembranous ossification, clusters of mesenchymal cells differentiate into osteoblasts, which then secrete the unmineralized organic matrix.
This organic osteoid then calcifies into a hard bone matrix. The bone grows outward from such ossification centers and around the surrounding blood vessels.
These vessels provide oxygen and nutrition for further bone formation.
As the matrix grows further, it also surrounds some osteoblasts, which differentiate into mature osteocytes.
Eventually, the growth from various ossification centers interconnects, forming the spongy bone.
Towards the cortex, the osteoblasts secrete bone matrix layers that arrange into compact bone.
At the surface, the mesenchyme develops into the periosteum, resulting in a flat bone.
10.11:
Bone Formation by Intramembranous Ossification
Intramembranous ossification is one of the two processes involved in the development of bones within an embryo. The flat bones of the face, most of the cranial bones, and the clavicles are formed via this process. During intramembranous ossification, the bones develop directly from sheets of undifferentiated mesenchymal connective tissue.
The process begins when mesenchymal cells in the embryonic skeleton gather together and differentiate into osteogenic cells, which then develop into osteoblasts. These early osteoblasts assemble in clusters termed ossification centers.
Formation of Bone Matrix
The osteoblasts secrete the uncalcified osteoid matrix, which calcifies within a few days as mineral salts are deposited. As the matrix hardens, it traps some of the osteoblasts within it. Once entrapped, the osteoblasts mature into osteocytes, and the osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts. Thus the bone grows outward from these ossification centers.
Formation of Spongy and Compact Bone
The osteoid secreted around the capillaries results in a trabecular matrix forming the spongy bone. As the trabeculae form around the blood vessels, the intertrabecular spaces are filled with red marrow derived from the vasculature. Meanwhile, the osteoblasts surrounding the spongy bone produce a protective layer of compact bone. The osteogenic cells at the surface, together with the mesenchymal membrane, form the periosteum, thus shaping the final bone structure.
Bone Development with Age
Intramembranous ossification begins in the uterus during fetal development and continues into adolescence. At birth, the skull and clavicles are not fully ossified, nor are the skull sutures closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 6.4: Bone Formation and Development