Intramembranous ossification takes place in the connective tissue membrane and produces the flat bones of the skull, part of the clavicle, and part of the mandible. It starts around the 8th week of development and ends at about 2 years of age. Before intramembranous ossification can take place, mesenchymal cells will condense into a soft sheet of connective tissue that is permeated with blood vessels. These mesenchymal cells will differentiate into osteochondral progenitor cells, and regions of the mesenchyme become a network of sheets called trabeculae.
Osteochondral progenitor cells gather on the surface of these trabeculae and differentiate into osteoblasts. The osteoblasts deposit an organic matrix called osteoid tissue. The osteoid tissue is contains collagen and carbohydrate-protein complexes such as proteoglycans, glycosaminoglycans, and glycoproteins. The osteoid tissue is similar to bone except for a lack of minerals. Calcium phosphate will be deposited into the matrix (hydroxyapatite).
When the matrix is mineralized it contains 85 percent hydroxyapatite, 1 percent calcium carbonate, and the remaining 5 percent is made up of magnesium, fluoride, sodium and potassium. As osteoblasts deposit more matrix, some of the will become trapped in the matrix and will develop into osteocytes that reside in lacunae. Osteocytes are important for maintaining the bone matrix. It is important to understand that the surface of the trabeculae remains uncalcified leading to the formation of the periosteum. The periosteum is divided into an outer fibrous layer, and an inner osteogenic layer. The inner layer contains three types of cells. They are osteochondral progenitor cells, osteoblasts, and osteoclasts.
Osteoblasts continue to deposit minerals forming a honeycomb of trabeculae. These tabeculae join together forming spongy bone. Osteoclasts start to resorb and remodel the trabeculae forming a bone marrow cavity.