This is called interstitial growth. Bone always grows from the surface, appositionally, since the mineralized non-deformable matrix does not allow growth from within. Such 'hypertrophic' cartilage will soon be degraded by phagocytic cells 26. Osteoblasts line the surface of newly forming bone matrix. Osteocytes are within new bone lacunae.
Giant osteoclasts are specialized bone phagocytic cells.