No abstract

The morphological changes undergone by the osteoblast at the ultrastructural level, during its differentiation into osteocyte, were studied in the primary parallel-fibred bone of the newborn rabbit by means of incomplete three-dimensional reconstruction from partially serial-sectioned preosteocytes. The findings obtained suggest that the formation of osteocyte cytoplasmic processes is an asynchronous and asymmetrical phenomenon that seems to precede the mineralization of the organic matrix and to give rise to an asymmetrical mature osteocyte. The functions of cytoplasmic processes as regards bone formation, cell nutrition and osteoblast modulation are discussed. The mechanism by which the osteoblast ‘enters’ the bone matrix is hypothesized.

The onset and development of intramembranous ossification centers in the cranial vault and around the shaft of long bones in five newborn rabbits and six chick embryos were studied by light (LM) and transmission electron microscopy (TEM). Two subsequent different types of bone formation were observed. We respectively named them static and dynamic osteogenesis, because the former is characterized by pluristratified cords of unexpectedly stationary osteoblasts, which differentiate at a fairly constant distance (28+/-0.4 microm) from the blood capillaries, and the latter by the well-known typical monostratified laminae of movable osteoblasts. 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 osteoblastic laminae, stationary osteoblasts inside the cords are irregularly arranged, variously polarized and transform into osteocytes, clustered within confluent lacunae, in the same place where they differentiate. Static osteogenesis is devoted to the building of 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. In conclusion, it appears that while static osteogenesis increases the bone external size, dynamic osteogenesis is mainly involved in bone compaction, i.e., in filling primary haversian spaces with primary osteons.

A comparative polarized light (PLM), scanning (SEM), and transmission (TEM) electron microscopy study was carried out on cross- and longitudinal sections of human lamellar bone in the tibiae of four male subjects aged 9, 23, 45, and 70 years. SEM analysis was also performed on rectangular-prismatic samples in order to observe each lamella sectioned both transversely and longitudinally. The results obtained do not confirm the model hitherto suggested to explain the lamellar appearance of bone. In particular, the classic description by Gebhardt (still accepted by the majority of bone researchers), which suggests that collagen fibers alternate between longitudinal and transversal in successive lamellae, or that they have spiral paths of different pitches, appears to be no longer acceptable in the light of our findings. In fact, SEM and TEM observations here reported agree in demonstrating that lamellar bone is made up of alternating collagen-rich (dense lamellae) and collagen-poor (loose lamellae) layers, all having an interwoven arrangement of fibers. No interlamellar cementing substance was observed between the lamellae, and collagen bundles form a continuum throughout lamellar bone. Preliminary measurements of lamellar thickness indicate that dense lamellae are significantly (P < 0.001) thinner than loose lamellae. Compared with the classic model of Gebhardt, the dense lamellae correspond to the transverse lamellae and are birifringent under PLM, whereas the loose lamellae correspond to the longitudinal lamellae and are extinguished. Collagen-fiber organization in dense and loose lamellae is discussed in terms of bone biomechanics and osteogenesis.

No abstract