Figure 1a. H&E staining of a bucco-lingual section from the mandible of a three-week-old rat. Osteoblasts (ob) secrete unmineralized bone matrix, termed ‘osteoid’ (os, doubleheaded arrow), which is subsequently mineralized and becomes bone (mm, long arrow) when apatite crystals are deposited. Small arrows indicate osteocytes buried in the mineralized bone matrix. Bar = 40 µm.