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

Histomorphological analysis of the effect of rigid fixation on growing sutures in the rabbit

The effect of internal rigid fixation (IRF) on bone growth was studied in an experimental model set up in the rabbit. The frontonasal suture of the right side was surgically bridged by a microplate. As reference for bone growth, four screws were placed symmetrically in the four bony segments including the frontonasal suture on both sides. The suture development was followed dynamically for 40 days on the basis of the position of the screws established radiographically. The rate of bone formation along the sutural bones was evaluated by means of the tetracycline labelling technique. The structure of the newly formed bone and its degree of mineralization were respectively analysed under polarized light and with microradiography. It was demonstrated that IRF prevents growth of the sutural membrane but not of the osteogenic process; as a consequence the constrained sutures soon undergo synostosis. This fact must be taken into consideration when IRF is employed in children in order to avoid delayed removal of the plate irreversibly stopping the growth of the constrained suture

The mechanism of transduction of mechanical strains into biological signals at the bone cellular level

As appears from the literature, the majority of bone researchers consider osteoblasts and osteoclasts the only very important bony cells. In the present report we provide evidence, based on personal morphofunctional investigations, that such a view is incorrect and misleading. Indeed osteoblasts and osteoclasts undoubtedly are the only bone forming and bone reabsorbing cells, but they are transient cells, thus they cannot be the first to be involved in sensing both mechanical and non-mechanical agents which control bone modeling and remodeling processes. Briefly, according to our view, osteoblasts and osteoclasts represent the arms of a worker; the actual operation center is constituted by the cells of the osteogenic lineage in the resting state. Such a resting phase is characterized by osteocytes, bone lining cells and stromal cells, all connected in a functional syncytium by gap junctions, which extends from the bone to the vessels. We named this syncytium the Bone Basic Cellular System (BBCS), because it represents the only permanent cellular background capable first of sensing mechanical strains and biochemical factors and then of triggering and driving both processes of bone formation and bone resorption. As shown by our studies, signalling throughout BBCS can occur by volume transmission (VT) and/or wiring transmission (WT). VT corresponds to the routes followed by soluble substances (hormones, cytokines etc.), whereas WT represents the diffusion of ionic currents along cytoplasmic processes in a neuron-like manner. It is likely that non-mechanical agents first affect stromal cells and diffuse by VT to reach the other cells of BBCS, whereas mechanical agents are first sensed by osteocytes and then issued throughou

Osteocyte dendrogenesis in static and dynamic bone formation: An ultrastructural study

The present ultrastructural investigation into osteocyte dendrogenesis represents a continuation of a previous study (Ferretti et al., Anat. Embryol., 2002; 206:21-29), in which we pointed out that, during intramembranous ossification, the well-known dynamic bone formation (DBF), performed by migrating osteoblast laminae, is preceded by static bone formation (SBF), in which cords of stationary osteoblasts transform into osteocytes in the same site where they differentiated. The research was carried out on the perichondral center of ossification surrounding the mid shaft level of various long bones of chick embryos and newborn rabbits. Transmission electron microscope observations showed that the formation of osteocyte dendrites is quite different in the two types of osteogenesis, mainly depending on whether or not osteoblast movement occurs. In DBF, osteoblasts transform into small ovoidal/ellipsoidal osteocytes and their dentrites form in an asynchronous and asymmetrical manner in concomitance with, and depending on, the advancing mineralizing surface and the receding osteogenic laminae. In SBF, stationary osteoblasts give rise to big globous osteocytes, located inside confluent lacunae, with short and symmetrical dendrites that can radiate simultaneously all around their cell body because they are completely surrounded by unmineralized matrix. Contacts and gap junctions were observed between all osteocytes (both SBF- and DBF-derived) and between osteocytes and osteoblast's. Finally, a continuous osteocyte network extends throughout the bone, regardless of its static or dynamic origin. This network has the characteristic of a functional syncytium, potentially capable of modulating, by wiring transmission, the cells of the osteogenic lineage covering the bone surfaces. (C) 2004 Wiley-Liss, Inc

Collagen texture and osteocyte distribution in lamellar bone.

The osteocyte lacunae are only located inside loose lamellae

Utrastruttura della placca di accrescimento fibrosa nella metafisi prossimale della tibia di coniglio

No riassunt

The problem of bone lamellation: An attempt to explain different proposed models

Collagen texture and osteocyte distribution were analyzed in human woven- and lamellar-bone using scanning and transmission electron microscopy. We provide data substantiating the concept that lamellar bone is made up of an alternation of dense-acellular lamellae and loose-cellular lamellae, all exhibiting an interwoven texture of collagen fibers. An attempt is also made to explain how the present findings might conform to those of authors whose models propose orderly, geometric arrangements of collagen fibers inside bony lamellae. Such a comparison is possible because the present investigation analyzes split loose lamellae and tangentially-sectioned dense lamellae. It emerged that only loose lamellae can be dissected, revealing a loose interwoven collagen texture and halved osteocyte lacunae. Dense lamellae cannot be split because of their compactness. The analysis of tangentially sectioned dense lamellae demonstrates that they consist of a network of interwoven collagen fiber bundles. Inside each bundle, collagen fibers run parallel to each other but change direction where they enter adjacent bundles, at angles as described by other authors whose TEM investigations were performed at a much higher magnification than those of the present study. Consequently, what these authors consider to be a lamella are, instead, bundles of collagen fibers inside a lamella. There is discussion of the role played by the manner of osteocyterecruitment in the deposition of lamellar- and wovenbone and how the presence of these cells is crucial for collagen spatial arrangement in bone tissues

MORPHOLOGICAL-STUDY OF INTERCELLULAR-JUNCTIONS DURING OSTEOCYTE DIFFERENTIATION

The study shows, by the morphological and ultrastructural viewpoints, the persistence of intercellular junctions during all the process of osteocyte differentiation

Quantitative investigation on osteocyte canaliculi in human compact and spongy bone.

By means of the two techniques (reflected polarized light and SEM) used in previous investigations (Remaggi et al., Calcif. Tissue Int., suppl. 3b, 26, 1983), the number of canaliculi departing from osteocyte lacunae was counted in compact and spongy bone of human tibiae from normal subjects of different ages.The statistical analysis of the results indicates that in both compacta and spongiosa the extension of the canalicular network is significantly greater in woven than in lamellar bone. In the latter, moreover, the number of canaliculi per unit lacunar surface appears to be proportional to the size of the lacunae and greater in lacunar walls facing the vascular source than in the opposite walls facing the cement line. This occurs in secondary osteons and in fragments of spongy trabeculae outlined by reversal lines.These results strongly suggest the hypothesis that the metabolic activity of the osteocytes differs according to the type of bone tissue and in different skeletal regions

A SEM study of osteocyte orientation in alternately structured osteons.

Previous morphometric analyses, carried out by means of the light microscope show the existence of a close relationship between size, shape, orientation of osteocytc lacunae and collagen texture in both woven bone and longitudinally structured secondary osteons (Marotti 1979, 1980). However these investigations, owing to the technique applied, failed to demonstrate the orientation of the triaxial osteocyte ellipsoids in lamellar bone with alternate orientation of fibers. In the present study the SEM was used to measure the sectional area and the axes of osteocyte lacunae in secondary osteons with different types of lamellation. Compact bone from tibiae of normal men of various age was etched with N/10 HCl for 90", and treated with Trypsin (80 µg/ml, pH 7.4 for 41h at 37°C) before being gold-palladium coated for the SEM analysis. The results obtained are in close agreement with those of the previous investigations as regards the orientation of osteocytes in woven bone and in longitudinally structured osteons. Tn addition they show that in lamellar osteons with alternate orientation of fibers the osteocytes are mainly located in longitudinally structured lamellae with their major axis at an angle of 26-27° with respect to the longitudinal axis of the Haversian canal