Ultrastructural studies on the origin and structure of matrix vesicles in bone of young rats

Abstract

Tibiae of young rats were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in order to study the origin and structure of matrix vesicles in periosteal bone. SEM studies have shown that periosteal osteoblasts have elongated processes with globular structures of 0.1 µm in diameter attached to the cell surface and processes. Similar structures were found to cover the inner surface of osteoblastic lacunae. The SEM studies have further shown that in the periosteal surface (forming bone) the above-described globules once mineralized, aggregated to form larger, nonhomogeneous mineralized spherules in which, by proper treatment with NaOCl, hydroxyapatite crystals could be exposed. Endosteal osteoblasts had fewer processes, devoid of the globular structures. Similarly, osteocytic and osteoclastic processes, although elongated and numerous, were not covered by the globular structures. In the matrix, collagen fibers of forming bone were randomly orientated, while in the deeper areas of bone they formed bundles with a longitudinal orientation. TEM studies have shown that the structures found on the osteoblastic surface and in the matrix are membrane-bound matrix vesicles which seem to be formed by budding from cell processes.Preformed membrane-bound vesicles were also observed by TEM inside sections of osteoblastic processes. These vesicles resembled the extracellular matrix vesicles in size and shape, thus giving the impression that at least some of the matrix vesicles are preformed cellular structures. While comparing SEM with TEM, it can be concluded that in bone, as in cartilage, matrix vesicles which probably serve as the initial locus of calcification, are formed directly by osteoblasts.

Calcospherulites11Calcospherulites (calco: calcium salt+spherulite: spherical crystalline body); calcium-containing, spherical bodies have also been referred to as calcified microspheres, mineral clusters, crystal ghost aggregates, calcification nodules, calcospherites, and small calcified spheres. isolated from the mineralization front of bone induce the mineralization of type I collagen

“…The data presented herein support the following conclusions about the structure and function of calcospherulites at the bone mineralization front. First, consistent with previous findings [10], the periosteal mineralization front of the juvenile rat tibial diaphysis exhibited numerous calcospherulites having an average diameter of ~0.5 micron and a Ca/P ratio of 1.3 in situ. Second, these spherulites were shown to actively participate in the mineralization process in vivo since they were intensely labeled with a calcium-binding fluorochrome (calcein) within 24 h after injection into live rats.…”

“…It has also been suggested that these structures may represent simultaneous, parallel reactions that exhibit some purposeful or coincidental interaction [4,7]. Relevant to the present study, matrix vesicles and calcospherulites have been observed at the periosteal surface of tibial diaphyses in young rats, and some calcospherulites were reported to be in close contact with the collagen fibers at the mineralization front [10].…”

Electron microscopy of calcification during high-density suspension culture of chondrocytes

“…Budding from the cell surface [31,32] and cellular degeneration/disintegration [33] are supported by electronmicroscopical studies. Budding and cellular disintegration are the most commonly observed phenomena in the genesis of matrix vesicles of the growth cartilage.…”

Isolation of Calcospherulites from the Mineralization Front of Bone

“…Prior studies have reported the presence of calcified, spherical- or fusiform-shaped structures (calcospherulites) all along the mineralization front of bone [Boyde and Sela, 1978; Sela and Bab, 1979; Ornoy et al, 1980; Aaron et al, 1999]. It has been reported that calcospherulites cluster together during periods of rapid bone formation, and that this focal aggregation is defined by an extracellular matrix assembly of bone acidic glycoprotein-75 [Gorski et al, 2004; Midura et al, 2004].…”