Mesenchymal cells on the outer surface of the newly formed bone from the periosteum. These vessels eventually form the red bone marrow. As osteoblasts continue to secrete osteoid, these surround the blood vessels and form trabecular (or cancellous) bone. At the edge of the newly formed and growing bone, the osteogenic cells differentiate into new osteoblasts. Then, osteoblasts become osteocytes, which are involved in the routine turnover of the bony matrix. As this secreted matrix becomes calcified with the binding of calcium, the matrix hardens and entraps the osteoblasts. Here, osteoblasts synthesise and secrete the osteoid, which is the unmineralised, organic portion of the bone matrix. The osteoblasts aggregate in what is called the ossification centre. Some cells differentiate into blood vessels, while others differentiate into osteoblasts and osteogenic cells. Intramembranous ossification begins when mesenchymal stem cells within the matrix differentiate into a variety of different specialised cells. Mesenchymal connective tissue is found mostly during embryonic development and develops into the tissues of the lymphatic, circulatory and musculoskeletal systems. This is important because it allows the skull and shoulders to deform during the passage through the birth canal. So, at birth, the skull and clavicles are not completely ossified and the cranial sutures (junctions between the skull bones) are not closed. Intramembranous ossification begins in-utero and continues into adolescence. Intramembranous ossification is a process that forms flat bones such as the skull and the clavicle, through the remodelling of mesenchymal connective tissue.
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