Before you are born this mesenchyme is replaced by the mature connective tissues. All bones formed via intramembranous ossification are flat bones. Mesenchymal cells in the center of the membrane differentiate into osteoblasts, which then begin to secrete the organic bone matrix. The osteoblasts then secrete the mineral portion of the matrix. The matrix being formed is called woven bone, an immature bone tissue that will eventually become spongy, or compact bone, depending on the location.
Mesenchymal cells form around the ossified portion of the membrane and form a periosteum.
The periosteal osteoblasts begin to form bone plates around the spongy bone middle. Think of an oreo cookie - the filling is the spongy bone, and the cookies are the compact bone. Endochondral Ossification Remember that endo means within, and chon is our root word for cartilage. Endochondral ossification occurs within cartilage bone models, and occurs in most bones of the body. This process is somewhat more complicated than intramembranous ossification because the cartilage must be replaced with the bone.