In HA21, there is periosteum below the tendon (dense regular connective tissue, HA17). In the endosteum there is a layer of cuboidal osteoblasts along the bone surface. Underneath them is a layer of osteoid (HA21, bright pink). You can see one or two osteoblasts getting buried in the osteoid. On the other side of the marrow is calcified cartilage, which is gray and amorphous in appearance.