Intramembranous ossification occurs during mammalian development within the womb and is the process by which flat bones such as some of the bones of the skull and the collarbones are created from connective tissue. It also is the process by which injured bones heal and occurs when bones are broken or damaged in order to reconstruct the bone. Also occurring during embryonic development is endochondral ossification, a process in which bones are produced from cartilage. Endochondral ossification occurs in the development of long bones such as the arms and legs. In intramembranous ossification, there is no cartilage present, and the bones develop from other connective tissues instead.

The process of intramembranous ossification begins with mesenchymal stem cells (MSCs), which are cells that have yet to be differentiated into a particular cell type. MSCs develop into osteoblasts, cells that generate bone tissue, both the spongy inner tissue of the marrow and the mineralized compact bone tissue that surrounds the marrow. Several steps occur during intramembranous ossification as the MSCs change, differentiate and become specific types of bone cells. A similar process occurs to repair broken or damaged bone, with bone tissue developing from membranous or connective tissue in much the same way that it develops within the womb.

In the basic process, MSCs form layers of relatively primitive connective tissue, and others develop into bone-producing cells. The bone-producing cells then create a bone matrix of calcium that eventually collects into bone spicules. Bone spicules grow in size as more of the bone matrix is excreted, eventually gathering together into a larger formation called a trabeculae. These structures continue the process, some building upon each other and eventually forming woven bone, and other trabeculae remain as a spongy tissue that becomes bone marrow.

Bones or parts of bones that develop through the intramembranous ossification process include the collarbones; the patella or kneecap; the parietal, frontal, occipital and temporal bones in the skull; and the upper and lower jawbones. Other specialized structures, such as a turtle's shell, also develop through intramembranous ossification. Intramembranous ossification also contributes to bone growth, especially in the growth of short bones and thickening in long bones as they develop.

This complex process of bone development occurs within all mammals, both in the womb and when bone repair is necessary. Even in mammals other than humans, the same sets of bones are formed through intramembranous ossification. In some cases, as with the turtle, animals develop other bones or structures in addition to the cranial, clavicle and other flat bones.