intramembranous bone

Intramembranous bone is bone tissue that forms directly within mesenchyme rather than replacing another tissue. In the process that produces it, osteoblasts secrete a collagen-proteoglycan matrix that binds calcium salts and becomes calcified. The defining contrast is with endochondral ossification: no cartilage is present at any stage of intramembranous ossification, so there is no cartilaginous model to be eroded and replaced. This mode of formation accounts for the flat bones, among other structures, and eleven subordinate entries are recorded beneath it in the anatomical hierarchy. Bones are therefore classified in this scheme not by shape or location but by the developmental route through which their tissue arose.