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Osteoclast activity and the growth of cranial lateral line canals in zebrafish (Danio rerio)

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Description: Lateral line system development and morphology have been studied in a variety of fish species over the past 100 years. Recently, the pattern of cranial lateral line canal development has been described in the zebrafish, Danio rerio and the cichlid, Archocentrus nigrofasciatus using SEM and histological analyses. These studies showed that following initial ossification of the tubular lateral line canals, they continue to expand in diameter as the dermal bones in which they are embedded Continue to grow. With an understanding of the general patterns of canal development, came new questions about the nature of the cellular level processes that result in this canal expansion. Such questions are fundamental to understanding skull development and functional integration of sensory systems and the dermal skeleton in fishes. The current hypothesis is that the lateral line canals are enlarged through the excavation of the internal canal surface by osteoclasts that resorb bone as part of a process of bone remodeling. This study is the first to examine how cranial lateral line canals grow after they have initially ossified, by examining osteoclast distribution, as reflected by the distribution of the enzyme tartrate-resistant acid phosphatase (TRAP). Two alternative hypotheses were evaluated: (1) bone resorption occurs around the entire inner canal surface, and (2) bone resorption occurs only at the initial point of fusion of the canal roof, the locations of the last step in the initial ossification of the canals. Osteoclast activity, as reflected by TRAP distribution, was examined in three lateral line canals (supraorbital, mandibular, infraorbital) in three fish in each of six different age classes (32, 42, 58, 81, 90, 100 dpf). Results indicate that osteoclasts are present in fish in all six age classes and in all three lateral line canals examined, but the number of osteoclasts among canals and among age classes demonstrated a great deal of variation. Bone resorption occurs around the entire circumference of the entire inner surface in all three canals, thus supporting hypothesis one (as above). Bone resorption as indicated by osteoclast activity was observed around the entire inner surface of the canals. However, there was considerable variation in osteoclast distribution among canals and among fishes of different ages. Osteoclasts were less frequently found along the outer canal surface (10--28% of total osteoclasts observed); they were usually located along the edge of the nerve foramen and/or the canal pore. These results suggest that canals are enlarged through resorption of the inner canal surface along with the bone excavation at nerve foramen and at canal openings (pores). This work is the first to show the pattern of osteoclast-mediated bone resorption in developing lateral line canals. This provides a description of the relationship between developing cranial lateral line canals and the dermal bones with which they are associated, thus giving us insights into the pattern of developmental integration of the lateral line canal system in the dermatocranium during growth.
Language: English
Format: Degree Work