In a series of investigations carried out in recent years on intramembranous ossification centers (Ferretti et al., 2002; Palumbo et al., 2004), we pointed out the existence of two types of bone deposition we respectively named static osteogenesis (SO) and dynamic osteogenesis (DO). SO always occurs first; it is characterized by stationary osteoblasts, arranged in cords, which form at a rather constant distance from the network of blood capillaries and, without moving, they transform into osteocytes in the same site where they differentiated. DO is the more familiar mechanism of bone deposition. DO is characterized by movable osteoblasts arranged in pseudoepithelial laminae, which move away from the mineralizing surface during preosseous matrix secretion and osteocyte transformation. In the case of intramembranous ossification, we observed that DO takes place only after SO on the surface of the bony trabeculae laid down by SO. The main speculation we drew from these investigations was that SO is needed in soft mesenchymal tissue to provide a more rigid framework for DO and DO can only occur on a preexisting rigid mineralized surface.
To verify the validity of our working hypothesis, the process of endochondral ossification was studied to determine the sequence of ossification. Specifically, to compare the SO and DO previously described in intramembranous ossification. Contrary to intramembranous ossification, endochondral ossification begins with a rigid scaffold of calcified cartilage.
MATERIALS AND METHODS
Metaphyseal plates from the tibiae and humeri of three newborn rabbits and four growing rabbits aged 8–10 days were used for light and electron microscopy. The care and use of animals were in accordance with the National Institutes of Health guidelines. All specimens were fixed for 2 hr with 4% paraformaldehyde in 0.13 M phosphate buffer, pH 7.4, postfixed for 1 hr with 1% osmium tetroxide in 0.13 M phosphate buffer at pH 7.4, dehydrated in graded ethanol, and embedded in epoxy resin (Durcupan ACM). Decalcified specimens were prepared by placing the tissue in 2.5% EDTA (0.13 M phosphate buffer, pH 7.2) until soft and then postfixing in osmium tetroxide prior to embedding in epoxy resin. The epoxy blocks were sectioned with a diamond knife using an Ultracut-Reichert Microtome. The methaphyseal plates were longitudinally and transversely sectioned with respect to the main axis of the shaft. Semithin sections (1 μm) were stained with toluidine blue and examined with a Axiophot-Zeiss light microscope (LM). Ultrathin sections (70–80 nm) were mounted on Formvar- and carbon-coated copper grids, stained with 1% uranyl acetate and lead citrate, and examined with a Zeiss EM109 transmission electron microscope (TEM).
In all metaphyseal plates, the following sequence of events of endochondral ossification were observed: columns of proliferating chondrocytes undergoing gradual hypertrophy towards the midshaft level; cartilage mineralization; chondrolysis and partial calcified cartilage disruption by osteoclasts; and the formation of osteocartilaginous trabeculae.
Our LM and TEM analyses were particularly focused on the surface of the trabeculae of calcified cartilage reabsorbed by osteoclasts (Fig. 1) and the early stages of bone matrix deposition. Prior to the appearance of osteoblasts, a consistent observation was the presence of flattened or spindle-shaped cells, similar to those of a reversal phase, lining the cartilage lacunae eroded by osteoclasts (Figs. 2 and 3). After the disappearance of the spindle-shaped cells, they were replaced by osteoblasts arranged in laminae from their inception (Fig. 4). Osteoblasts were sometimes irregularly grouped in such laminae, particularly where the eroded surface of calcified cartilage was irregularly indented by small and narrow lacunae. Most osteoblasts appeared to be very active, being rounded in shape and displayed a well-developed endoplasmic reticulum and Golgi apparatus. All osteoblasts were functionally polarized toward the cartilaginous or the osteoid seam surface, as shown by the position of their organelles with respect to the nucleus (Fig. 5). Gap junctions were sometimes observed between adjacent osteoblasts (Fig. 6).
The bone covering the remnants of calcified cartilage contained osteocytes, whose cell body displayed a globous or ovoid shape, enclosed in single lacunae. Confluent lacunae were never observed (Fig. 2). An interesting and fairly frequent finding was the presence of osteocyte cell bodies very close to the calcified cartilage (Figs. 7 and 8; see also Figs. 1 and 2). Osteocyte dendrites showed an asymmetrical arborization with short mineral dendrites radiating toward the calcified cartilage and longer vascular dendrites directed toward the osteoblastic laminae and the vessels (Fig. 8). Gap junctions or simple contacts were observed between the cytoplasmic processes of adjacent osteocytes and between osteocyte dendrites and osteoblasts (Fig. 9).
In intramembranous ossification centers that surround the developing shafts of long bones, transverse periosteal bone growth occurs by progressive extension of osteoblastic cords inside the surrounding mesenchyme at about midway between blood capillaries (mean distance of cords from vessels, 28 ± 0.4 μm). The cords are made up of 2–3 layers of active plump osteoblasts, all functionally polarized, but in a different direction with respect to the adjacent ones. Additionally, these osteoblasts are stationary since they transform into osteocytes without moving from the site where they appeared to differentiate. It is from these observations that we proposed the term “static osteogenesis” to describe this process (Ferretti et al., 2002). As a consequence, the bony trabeculae, laid down in this manner, contain several globous-shaped osteocytes, often irregularly grouped inside confluent lacunae and displaying short dendrites having about the same length all around the cell body (Palumbo et al., 2004). Afterward, typical monostratified laminae of movable osteoblasts differentiate along the surface of these trabeculae laid down by static osteogenesis. These osteoblasts form new bone by dynamic osteogenesis that results in thicker trabeculae that cause the narrowing of the enclosed primitive Haversian spaces (primary osteons). Osteocytes derived from movable osteoblasts generally are ovoid and display an asymmetrical dendrite arborization. It should be noted that the network of dendrites extending throughout all osteocytes, derived by both static and dynamic osteogeneses, forms a functional syncytium since they were found to be connected by gap junctions (Ferretti et al., 2002; Palumbo et al., 2004).
In endochondral ossification, static osteogenesis never seems to take place. In fact, the osteoblasts in contact with the remnants of calcified cartilage are directly arranged in movable laminae and all appear to be functionally polarized in the same directions, i.e., toward the calcified cartilage. Additionally, the osteocytes inside the bone surrounding the calcified cartilage are never grouped inside confluent lacunae and all display an asymmetrical arborization of dendrites. Thus, in endochondral ossification, dynamic osteogenesis is not preceded by static osteogenesis. We wish to stress, however, that one must not confuse static osteogenesis with stationary osteoblasts. In endochondral ossification, the presence of osteocytes close to the calcified cartilage clearly indicates that such osteocytes are derived from stationary osteoblasts that remain in the same site where they differentiated. The thin layer of bone matrix separating the osteocyte cell bodies from the calcified cartilage generally contained osteocyte mineral dendrites. Therefore, this layer does not indicate that the parental osteoblasts moved; it instead reflects the diminution in size of osteoblast protoplasms and the radiation of cytoplasmic processes that occurred during the differentiation of the osteoblast into an osteocyte. Osteocytes derived from stationary osteoblasts can frequently be observed close to the reversal lines of secondary Haversian systems; hence, stationary osteoblasts are not a distinctive feature of static osteogenesis. At the onset of bone formation, it is necessary for some stationary osteoblasts to be present, independently of whether static or dynamic osteogenesis is taking place, to ensure the presence of osteocytes, thus providing a network of strain-sensitive cells throughout the developing bone tissue. Without these stationary osteoblasts, the first layers of bone tissue would not contain osteocytes. We wish to stress that a continuous osteocyte functional syncytium was observed throughout the bone laid down in both intramembranous and endochondral ossification and that such syncytium comes into contact with the osteoblasts covering the bone growing surfaces. Such a cellular organization suggests that the bone is capable of sensing and answering mechanical signals since from its inception.
Osteoblast differentiation seems to depend on a variety of inductive factors; in intramembranous ossification, it probably depends on endothelial cell-derived cytokines, such as endothelin-1 (Sasaki and Hong, 1993; Kasperk et al., 1997; Inoue et al., 2000) and growth factors (EDGF) (Guentheret al., 1986; Canalis et al., 1989; Streeten and Brandi, 1990; Villanueva and Nimni, 1990). Since static and dynamic osteogeneses were observed to occur in the same manner and sequence during bone repair (Marotti, 2004), osteoblast differentiation could also depend on platelet-derived growth factor (PDGF) (Zheng et al., 1992; Lind, 1998; Chaudhary et al., 2004) in bone healing. In endochondral ossification, osteoblast differentiation can depend on endothelial-cell derived cytokines as in intramembranous ossification; however, it seems likely that it might also depend on coupling factors released by the cells of the reversal phase, as has been suggested in the bone remodeling cycle (Baron, 1976, 1989). Though the origin of these cells is still unknown, they may be stromal cells in a preosteoblast phase. The main difference between intramembranous and endochondral ossification is that in the latter, osteoblasts differentiate only after osteoclastic resorption of calcified cartilage followed by a transient reversal phase.
In conclusion, this study of endochondral ossification supports the hypothesis that dynamic osteogenesis needs a rigid mineralized surface to occur and static osteogenesis only occurs in soft tissues where a rigid framework is lacking. This is not only true in the mesenchyme during intramembranous bone histogenesis but also in soft callus during bone repair (Marotti, 2004).