Chondrocyte stacks near the epiphyseal plate represent a zone rapidly growing cartilage. Such 'hypertrophic' cartilage will soon be degraded by phagocytic cells. Epiphyseal plate, low magnification to show the relationships of the hypertrophic cartilage and newly forming endochondral bone. Osteoblasts line the surface of newly forming bone matrix. Osteocytes are within new bone lacunae.
Giant osteoclasts are specialized bone phagocytic cells.
One has to understand the structure of the developing tissue in order to interpret how secreted proteins that serve as intercellular signals are acting. Figure 7 in the cited paper shows patterns of expression of the genes encoding these proteins in preparations just like the ones we are examining.