Basement membrane composition of cartilage canals during development and ossification of the epiphysis

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Abstract
Background: Cartilage canals are perichondral invaginations of blood vessels and connective tissue that are found within the epiphyses of most mammalian long bones. Functionally, they provide a means of transport of nutrients to the hyaline cartilage, a mechanism for removal of metabolic wastes, and a conduit for stem cells that are capable of initiating and sustaining ossification of the chondroepiphysis. Morphological and biomolecular changes of the chondroepiphyses appear to potentiate vascular invasion and enable regional formation of secondary centers of ossification within the chondroepiphyses of developing bones. Methods: As both cell migration and vascular invasion are anchorage dependent processes, antibodies to laminin and Type IV collagen were used to assess compositional changes in the basement membrane of cartilage canals accompanying epiphyseal ossification. Results: Differences in chronological appearance, as well as, in distribution between the two components were noted in the chondroepiphysis. Laminin was distributed throughout the connective tissue of cartilage canal at all stages of developement, and not limited to an association with the vascular lumen. Type IV collagen was not Present during the initial perichondral invagination. Although staining for Type IV collagen was later acquired, its distribution was restricted to a discontinuous rimming of the periphery of the canal, and a diffuse presence within the intra-canalicular mesenchyme. Conclusions: Concurrent with chondrocyte hypertrophy and mineralization of the hyaline matrix, rapid changes in both the morphology of the vessel and distribution of the antibodies were detected. In addition to the presence of laminin at the interface of the endothelium and the hyaline matrix, a wide distribution within the connective tissue components of the newly ossifying matrix of epiphyseal bone could be detected. Type IV collagen remained closely associated with the lumens of the intra-canalicular vessels throughout the transition. Following ossification of the secondary center, staining for Type IV collagen could then be detected in the boneforming regions of transforming matrix as well, clearly delineating the individual vessels within the newly formed marrow spaces. This suggests that bone formation is intimately related to vessel staining for collagen type IV, and that acquired vessel competence is a facet of endochondral bone formation that results from provisional matrix changes. Furthermore, the data suggests that during bone formation under tension, basement membrane deposition can be demonstrated without an intermediary hyaline matrix hypertrophic chondrocyte phase. This data was interpreted to suggest that chondrocyte hypertrophy at the growth plate may be a reaction to vascular invasion, that in turn, stimulates adjacent chondrocyte proliferation.