Static and dynamic osteogenesis.

Two subsequent different types of bone formation, we respectively named static osteogenesis (SO) and dynamic osteogenesis (DO), were observed in intramembranous ossification centers of newborn rabbits and chick embryos as well as during bone repair. In all cases the onset of intramembranous ossification is characterized by the appearance, around the vessels, of pluristratified cords of unexpectedly stationary osteoblasts that transform into osteocytes in the same site where they differentiated, whence the name of static osteogenesis (SO). Soon after, typical monostratified laminae of well known movable osteoblasts differentiate along the surface of the bony trabeculae laid down by SO and thicken them by DO. No significant structural and ultrastructural differences were found between stationary and movable osteoblasts, all being polarized secretory cells joined by gap junctions. However, unlike in typical movable osteoblast laminae, stationary osteoblasts inside the cords are irregularly arranged, variously polarized, and transform into osteocytes clustered within confluent lacunae. Briefly SO seems to be devoted to building the first trabecular bony framework having, with respect to the subsequent bone apposition by typical movable osteoblasts, the same supporting function as calcified trabeculae in endochondral ossification. SO-bone is a bad quality woven-bone, whereas DO-bone generally is a lamellar-bone and thus mechanically more resistant. The relevance of this fact in bone repair and clinical practice will be discussed.