Intramembranous preosteoblasts are cells in the inner layer of the periosteum that develop into osteoblasts. Theses cells secrete the osteoid, the organic non-mineralized portion of the bone matrix which is formed before maturation of the bone.
Only 10-20% of vertebrate bones take on the intramembranous ossification fate. Bones of the cranial vault (calvarium; flat bones) form through the process of intramembranous ossification.