Intramembranous ossification involves the formation of bone directly on or with loose fibrous connective tissue membranes. Which bones of the body develop by intramembranous ossification?

Hint: The compact and spongy bone grows directly from sheets of mesenchymal (undifferentiated) connective tissue during intramembranous ossification. By intramembranous ossification, the flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are developed.

Complete answer:
The phase of bone formation from fibrous membranes is intramembranous ossification. It is involved in the development of the skull, the mandible, and the clavicles of flat bones. Ossification starts when a model of the future bone is created by mesenchymal cells. They then distinguish at the centre of ossification into osteoblasts. The extracellular matrix is secreted by osteoblasts and calcium is deposited, which hardens the matrix. Around blood vessels, the non-mineralized part of the bone or osteoid continues to form, becoming a spongy bone. In the foetus, the connective tissue in the matrix differentiates into the red bone marrow. On the surface of the spongy bone, the spongy bone is remodelled into a thin layer of compact bone.

Additional information: The mechanism of bone formation by osteoblasts is ossification or osteogenesis. Ossification is distinct from the process of calcification; while calcification takes place during the ossification of bones, it may also occur in other tissues. Ossification starts roughly six weeks after an embryo is fertilised. The embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage before this time. Intramembranous ossification is the formation of bone from fibrous membranes; hyaline cartilage formation is called endochondral ossification. Bone development lasts until the age of 25 or so. Throughout life, bones may expand in thickness, but after age 25, ossification works mainly in bone remodelling and repair.

Note: Intramembranous ossification is considered the direct conversion of mesenchymal tissue into bone. This phase happens predominantly in the skull's bones. The mesenchymal cells divide into cartilage in other cases, and this cartilage is later replaced with bone.