Ultrastructural and histochemical studies of the epiphyseal plate in normal chicks

A Second Career for Chondrocytes-Transformation into Osteoblasts

PURPOSE OF REVIEW

The goal of the review is to summarize the current knowledge on the process of chondrocyte-to-osteoblast transdifferentiation during endochondral bone formation and its potential implications in fracture healing and disease.

RECENT FINDINGS

Lineage tracing experiments confirmed the transdifferentiation of chondrocytes into osteoblasts. More recent studies lead to the discovery of molecules involved in this process, as well as to the hypothesis that these cells may re-enter a stem cell-like phase prior to their osteoblastic differentiation. This review recapitulates the current knowledge regarding chondrocyte transdifferentiating into osteoblasts, the developmental and postnatal events where transdifferentiation appears to be relevant, and the molecules implicated in this process.

Characterization of the development of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells during osteoinduction by rhBMP-2: a histochemical and ultrastructural study

OBJECTIVE

To investigate the characteristics of ectopic chondroid/bone matrix and chondrogenic/osteogenic cells induced by recombinant human bone morphogenetic protein-2 (rhBMP-2).

MATERIALS AND METHODS

rhBMP-2 (5 microg) combined with atelocollagen was implanted into calf muscles of rats and removed on days 7, 10, 14, 21, or 28. Tissue sections were examined using: (i) hematoxylin/Alcian blue/Sirius red stain, (ii) enzyme histochemistry for alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase activity, (iii) immunohistochemistry for types I, II, and X collagen, and (iv) electron microscopy.

RESULTS

On day 7, numerous fibroblast-like cells with ALP activity were present on the pellet rim. On day 10, chondroid matrix (CM) had formed, contained both type I collagen and proteoglycans, and often continued into the BMP pellet. On day 14, bone-like matrix formed around hypertrophic chondrocytes simultaneously with endochondral ossification. Coexpression of types I and II collagen within chondrocytes and osteocytes was observed throughout the time course of the experiment.

CONCLUSION

These results suggest that fibroblast-like cells invading the pellet differentiate into chondrocytes and form CM under the scaffold of the carrier component. It appears that some chondrocytes change their phenotype to produce the bone-like matrix and remain within the endochondral bone. This process enables rapid osteogenesis to occur.

An immuno-light- and electron-microscopic study of the expression of bone morphogenetic protein-2 during the process of ectopic bone formation in the rat

Immunolocalization of endogenous bone morphogenetic protein-2 (BMP-2) was investigated during the process of ectopic bone formation induced by recombinant human BMP-2 (rhBMP-2). Pellets consisting of 5 microg of rhBMP-2 and 6 mg of atelopeptide type I collagen (AC) were implanted into the calf muscles of 6-week-old rats. On days 7, 10, 14, 21 and 28 after implantation, tissue specimens were removed and examined immunohistochemically by light and electron microscopy after incubation with anti-BMP-2 monoclonal antibodies. Immunolocalization by light microscopy showed BMP-2 in chondrocytes at the pellet rim on days 7 and 10, in osteocyte-like cells in the chondroid matrix on day 14, and in osteocytes in the newly formed bone on days 21 and 28 after implantation. Ultrastructurally, on days 7 and 10 after implantation, immunolabelling for BMP-2 was aggregated in vesicle-like matrices released from mature chondrocytes in the chondroid matrix. On day 14, immunolabelling against BMP-2 had accumulated in vesicle-like matrices embedded in the calcified cartilage, in the cytoplasmic vacuoles of chondroclasts absorbing the matrix, and at the resorption surface of the calcified cartilage. On days 21 and 28, BMP-2 immunolabelling was seen in the osteoid layer and osteocyte lacunae. These results suggest that the chondrocytes and osteocytes induced by rhBMP-2 produce endogenous BMP-2. It seems that part of the endogenous BMP-2 that accumulated in the chondroid matrix was absorbed by chondroclasts and then participated in the osteoblastic differentiation of immature mesenchymal cells. This study indicates that, in addition to the implanted exogenous rhBMP-2, endogenous BMP-2 plays an important part in the maintenance of the bone-formation cascade during ectopic osteoinduction.

Ultrastructural aspects of cartilage formation, mineralization, and degeneration during primary antler growth in fallow deer (Dama dama)

Due to their rapid growth, regular replacement and easy accessibility, deer antlers are considered a useful model for the study of cartilage and bone differentiation and mineralization in mammals. The present study describes, for the first time, the cellular and extracellular matrix changes associated with cartilage formation, mineralization and degeneration in primary antlers on the ultrastructural level. Growing primary antlers of 3 to 4 cm length were obtained from six fallow bucks, aged about 10 months. It was shown that the chondroblasts were derived from progenitor cells of the antler perichondrium and differentiated into mature chondrocytes that subsequently underwent hypertrophic changes. Concomitant with cell hypertrophy, formation of a lacunar and a perilacunar extracellular matrix was observed, the latter containing numerous collagenous fibers. Mineralization of the extracellular matrix occurred via matrix vesicles and the formation of apatite crystals at distinct sites of the collagenous fibers. The hypertrophic chondrocytes of the mineralized cartilage then degenerated, a process that was also occasionally observed in more distally located cells surrounded by still unmineralized matrix. No morphological indications of a transdifferentiation of hypertrophic chondrocytes into bone forming cells, i.e., co-occurrence of a degenerating chondrocyte and a viable osteogenic cell in intact lacunae, were found. The cellular and extracellular matrix changes seen in primary antlers resemble those described for secondary antlers. Our results further indicate that the hypertrophic chondrocytes of primary antlers eventually undergo apoptosis, thereby providing further evidence that metaplastic conversion of cartilage into bone does not play a role in antler growth.