As the name indicates, in this method of osteogenesis bones develop in membranes (or sheets) of con-densed mesenchyme. The flat bones of the skull vault and bones of the face | develop by this method of ossification.
In the intramembranous method of ossification formation of a bone is preceded by the formation of a sheet of J condensed mesenchyme, which is richly supplied by blood capillaries. Intramembranous ossification begins in this vascularized mesenchyme at one or more points which are called centers of ossification. At such centers osteoblasts appear and produce osteoid, which is soon converted into bone matrix. This newly formed bone tissue is laid down in the form of needle-like spicules. The expanding spicules unite into a meshwork of trabeculae which spread in all directions in a radiating fashion. Simultaneously, the entire pri-mordium of the developing bone becomes enclosed within a fibrous membrane,called periosteum, which is formed by the condensation of the local mesenchyme.
On the inner surface of the peri-mesenchymal cells differentiate Into osteoblasts, which deposit parallel plates (lamellae) of compact bone on lie inner and outer surface of the spongy bone. The spaces within the fcabeculae of spongy bone become IWed with bone marrow, which also develops from the mesenchyme. In a flat skull bone the central spongy bone is Called diploe, while the inner and outer periostea! layers of the compact bone are called inner and outer tables, respectively.