Peripheral area of the diaphysis of an embryonic bone, light micrograph. The periosteum is at top right, showing two layers: an outer, or fibrous layer (pink), and an internal, or osteogenic layer, with a more cellular mesenchyme, where the osteoprogenitor cells are generated. In the more peripheral bone trabeculae, osteoid lines appear formed by osteoblasts that are increasing the thickness of the diaphyseal cortex. The mean diameter of the trabeculae of immature bone tissue increases from the periosteum to the inside, since, once formed in the periosteum, new layers of bone matrix continue to be attached as time passes. In the internal part, resorption takes place and progressively augments the size of the medullary cavity. At bottom left is the hematopoietic bone marrow.