Light micrograph of a flat bone of the skull in the process of direct, or intramembranous, ossification. Immature woven, or trabecular, bone tissue has formed directly from the mesenchyme, which is still in a very active phase of osteogenesis, as evidenced by the presence of very numerous osteoblasts in the periosteum. Among the trabeculae of immature bone tissue, there remains a mesenchyme with abundant vessels and osteoprogenitor cells. Unlike chondral ossification, mixed trabeculae with remains of calcified cartilage are not seen, nor is there a clear delimitation of the future cortical bone.