These vessels will eventually form the red bone marrow. As osteoid continues to be secreted by osteoblasts, it surrounds blood vessels, forming trabecular/cancellous/spongy bone. This entrapment results in the transformation of osteoblasts to osteocytes. The binding of calcium to osteoid results in the hardening of the matrix and entrapment of osteoblasts.

Osteoblasts begin secreting osteoid, an unmineralized collagen-proteoglycan matrix that can bind calcium. Osteoblasts group into clusters and form an ossification center. It begins when neural crest-derived mesenchymal cells differentiate into specialized, bone-forming cells called osteoblasts. This process involves the direct conversion of mesenchyme to the bone. By the time of birth, the majority of cartilage has undergone replacement by bone, but ossification will continue throughout growth and into the mid-twenties. This framework determines where the bones will develop. This template is mostly cartilage, derived from embryonic mesoderm, but also includes undifferentiated mesenchyme (fibrous membranes) in the case of intramembranous ossification. The lateral plate mesoderm forms the long bonesīone formation requires a template for development.

Somites form the remainder of the axial skeleton. Cranial neural crest cells form the flat bones of the skull, clavicle, and the cranial bones (excluding a portion of the temporal and occipital bones. The development of the skeleton can be traced back to three derivatives : cranial neural crest cells, somites, and the lateral plate mesoderm.

What are flat bones