An immunofluorescence study of chondrogenesis in murine mandibular ectomesenchyme

In situ localization of gelatinolytic activity during development and resorption of Meckel's cartilage in mice

Degradation of Meckel's cartilage in the middle portion is accompanied by hypertrophy and death of chondrocytes, calcification of the cartilaginous matrix, and chondroclastic resorption. We hypothesize that the gelatinolytic activity of matrix metalloproteinases (MMPs) largely contributes to the degradation of extracellular matrix (ECM) in the process. The activity in Meckel's cartilage of mouse mandibular arches at embryonic days 14-16 (E14-E16) was examined by a combination of in situ zymography (ISZ), using quenched fluorescent dye-labeled gelatin as a substrate, with CTT (a selective inhibitor of MMP-2 and -9) or with EDTA (a general MMP inhibitor). On E14 and E15, ISZ showed fluorescence in the perichondrium, in the intercellular septa between chondrocytes, and in the nucleus of chondrocytes. CTT attenuated fluorescence, and EDTA eliminated it. On E16, calcified cartilaginous matrix showed intense fluorescence, and dot-like fluorescence was observed in as-yet uncalcified intercellular septa, even after CTT treatment. EDTA inhibited fluorescence, but unexpectedly intense fluorescence was found in the cytoplasm of hypertrophic chondrocytes facing the resorption front. MMP-2, -9, and -13 immunoreactivity was detected in the perichondrium and chondrocytes of Meckel's cartilage. These findings suggest that MMPs and other proteinases capable of degrading gelatin play an integral role in the development, calcification, and resorption of Meckel's cartilage through ECM reconstitution.

Maintenance of regional histodifferentiation patterns and a spatially restricted expression of type X collagen in rat Meckel's cartilage explants in vitro

The major, central portion of Meckel's cartilage undergoes fibrous transformation and contributes to the sphenomandibular ligament, whereas its distal end undergoes endochondral ossification ultimately giving rise to inner-ear ossicles. This regional histodifferentiation of Meckel's cartilage is known to be associated with the spatially restricted expression of type X collagen. The objective of this study was to determine if this unique histodifferentiation is regulated by local environmental factors or by a preprogrammed genetic mechanism. Meckel's cartilage, and condylar cartilage used for comparison, were isolated from 17-day-old rat embryos and from newborn rats, respectively. The cartilage explants were maintained in vitro for 50 days with or without supplementation with 10% fetal bovine serum. When the explants were cultured under serum-free conditions, well-regulated cartilage development was observed. Expression of type X collagen, a differentiation marker for hypertrophic cartilage, was restricted to the distal end of Meckel's cartilage, whereas type II and IX collagens were found uniformly along the entire explant. Matrix calcification was examined histochemically using alizarin red S staining and found to be restricted to the distal end of Meckel's cartilage. Both Meckel's and condylar cartilage cultured with 10% fetal bovine serum developed unregulated dysmorphogenesis. These data suggest that, although Meckel's cartilage has an intrinsic potential to differentiate to its terminal stage, external regulatory factors can significantly influence its normal development at the molecular level.

Distribution of type I collagen, type II collagen and PNA binding glycoconjugates during chondrogenesis of three distinct embryonic cartilages

Previous studies of chondrogenesis have been focused on limb bud cartilage, whereas little is known about chondrogenic processes of other cartilages with different developmental fates. We hypothesize that cartilages with various developmental fates might show identical characteristics of chondrogenesis. The chondrogenic processes in the nasal septum, the mandible, and the limb bud of the mouse were examined by means of PNA-binding glycoconjugate, and types I and II collagen expression. Swiss-Webster mouse embryos of 11 days (E11) to 14 days (E14) gestation were fixed and processed for immuno- and lectin histochemistry. The blastema of mesenchymal cell aggregates stained positively with anti-type I collagen, but very weakly with anti-type II collagen in all three models at E12, whereas PNA bound to the blastema in the limb bud but not in nasal septum or mandible. Types I and II collagens coexisted in cartilages at E13. Type II collagen was predominant in E14; type I collagen was confined to the peripheral region. The synchronized transitional expression of the collagen phenotypes in all three embryonic cartilages may be systemically regulated. The presence or absence of the PNA-binding glycoconjugates may be involved in characterizing the nature of the cartilages.

Development of cartilage and bone tissues of the anterior part of the mandible in cichlid fish: a light and TEM study

The present paper presents ultrastructural details of chondrogenesis of Meckel's cartilage and of ossification of its associated peri- and parachondral bones in a teleost fish, the cichlid Hemichromis bimaculatus. We have distinguished four stages during chondrogenesis, each of which is characterized by specific cellular and matrix features: blastema, primordium, differentiated cartilage and cartilage surrounded by perichondral bone. The blastema is characterized by prechondroblasts and the lack of cartilage matrix; the primordium by chondroblasts and the onset of secretion of matrix of fibrillar and granular nature; differentiated cartilage is characterized by chondrocytes and larger amounts of typical hyaline cartilage matrix. Once perichondral bone is laid down, the chondrocytes show degenerative features but not true hypertrophy. Differentiation of the cartilage cells is attended with cytoplasmic changes indicative of an increasing secretory activity. There is a regional calcification of the cartilage matrix by fusion of calcospherites. Chondrogenesis of the symphyseal area is continuous with that of the rami but starts slightly later. Formation of perichondral bone at the cartilage surface is attended with the deposition of a transitional zone apparently containing a mixture of the two matrices. The role of the perichondral cells is discussed and it is proposed that they may contribute to the formation of the two matrices. The transitional zone may then result either from a diffusion process or from the simultaneous deposition of elements of the two matrices. Growth of the cartilage is argued to be largely the result of matrix secretion, except in the symphyseal area where appositional growth probably occurs until the region is completely covered by perichondral bone. This paper provides a basis for further studies on the developmental interactions between cartilage, bone and teeth during mandibular development in cichlids.

Temporal and spatial expression of genes for cartilage extracellular matrix proteins during avian mandibular arch development

We have examined the temporal expression of genes for extracellular matrix proteins (type I collagen, type II collagen, and the cartilage specific proteoglycan core protein) during the development of the avian mandibular arch. We detected low levels of type II collagen mRNA in the mandibular arch as early as stage 15. Type II collagen mRNA remained low but increased slightly as development progressed from stage 15 to stage 25. More dramatic increases occurred after stage 25 coincident with overt chondrogenesis. In contrast, mRNA for the core protein of cartilage specific proteoglycan was not detected prior to the onset of chondrogenesis, appeared at stage 25, and increased thereafter. Type I collagen mRNA was also present as early as stage 15 and dramatically increased after stage 28/29, coincident with initiation of osteogenesis. Using in situ hybridization, we found that type II collagen mRNA became detectable in the center of the mandible around stage 24/25 coincident with the initiation of chondrogenesis. At later stages (26-32) type II collagen mRNA was localized in the cartilaginous rudiment. The pattern of hybridization observed with the proteoglycan core protein probe at later stages of development was essentially identical to that observed with the type II collagen probe. In contrast, the probe for the alpha 1 (I) collagen mRNA was localized over the perichondrium, over differentiated bone, and in areas within the mandibular arch where bone formation had been initiated.

Regulation of alkaline phosphatase and alpha 2(I) procollagen synthesis during early intramembranous bone formation in the rat mandible

We have studied intramembranous bone formation in the developing rat mandible. In this system discrete developmental stages can be readily distinguished: mesenchymal condensation, osteoid deposition, and mineralization. In mandibles of 14-day rat embryos avascular condensed mesenchymal cells can be discerned in a region lateral to Meckel's cartilage and anterior to the first molar bud. In 18-day embryos primary bone structures with mineral deposition are evident, and at 2 days postnatally the mandible is extensively mineralized. In the developing mandible we investigated the pattern of bone/liver/kidney/placenta (BLKP) alkaline phosphatase (ALP) and alpha 2(I) procollagen expression in the differentiating osteoblasts. The level of ALP activity in loose mesenchymal tissue is close to background levels. In contrast, the condensed mesenchymal cells in 14-day embryos, which will subsequently form bone, display intense ALP activity prior to discernible osteoid or mineral deposition. ALP activity in the condensed mesenchymal cells can be inhibited by levamisole, indicating activity of the BLKP gene product. We could not detect a corresponding increase in transcript level for either ALP or alpha 2(I) in the condensed mesenchyme in 14-day embryo using in situ hybridization, probably due to low message abundance. At 18 days, cells throughout the developing mandible express ALP activity, and intense in situ hybridization to BLKP ALP probes is evident in cells lining the developing bone trabeculae. Alpha 2(I) procollagen transcripts have accumulated in cells of the developing mandibular bone, but are not specifically localized to osteoblastic cells. Our results demonstrate that ALP activity is a very early marker of differentiation of cells of the osteogenic lineage, since a marked increase in ALP enzyme activity is clearly detectable in condensed mesenchymal cells prior to osteoid or mineral deposition. In contrast, Wright and Leblond, using the same model system and immunohistochemistry, could not localize type I collagen to preosteoblastic cells surrounding the developing bone trabeculae, and demonstrated localization of type I collagen to osteoblasts bordering developing trabeculae, indicating a substantial increase in type I collagen expression (at least at the protein level) during preosteoblast to osteoblast differentiation. These results indicate a discrete pattern of regulation for both the ALP and alpha 2(I) genes during osteogenic differentiation, which may involve both transcriptional and posttranscriptional regulation.

The fate of Meckel's cartilage chondrocytes in ocular culture

Modulation of the chondrocyte phenotype was observed in an organ culture system using Meckel's cartilage. First branchial arch cartilage was dissected from fetal rats of 16- and 17-day gestation. Perichondrium was mechanically removed, cartilage was split at the rostral process, and each half was grafted into the anterior chamber of an adult rat eye. The observed pattern of development in nonirradiated specimens was the following: hypertrophy of the rostral process and endochondral-type ossification, fibrous atrophy in the midsection, and mineralization of the malleus and incus. A change in matrix composition of the implanted cartilage was demonstrated with immunofluorescence staining for cartilage-specific proteoglycan (CSPG). After 15 days of culture, CSPG was found in the auricular process but not in the midsection or rostral process. In order to mark the implanted cells and follow their fate, cartilage was labeled in vitro with [3H]thymidine [3H]TdR). Immediately after labeling 20% of the chondrocytes contained [3H]TdR. After culturing for 5 days, 20% of the chondrocytes were still labeled and 10% of the osteogenic cells also contained radioactive label. The labeling index decreased in both cell types with increased duration of culture. Multinucleated clast-type cells did not contain label. Additional cartilages not labeled with [3H]TdR were exposed to between 20000 and 6000 rad of gamma irradiation before ocular implantation. Irradiated cartilage did not hypertrophy or form bone but a fibrous region developed in the midsection. Cells of the host animal were not induced to form bone around the irradiated cartilage. Our studies suggest that fully differentiated chondrocytes of Meckel's cartilage have the capacity to become osteocytes, osteoblasts, and fibroblasts.