Features of the clear zone of odontoclasts in the Chinook salmon (Oncorhynchus tshawytscha)

An adult osteopetrosis model in medaka reveals the importance of osteoclast function for bone remodeling in teleost fish

Osteoclasts play important roles during bone growth and in maintaining bone health and bone homeostasis. Dysfunction or lack of osteoclasts leads to increased bone mass and osteopetrosis phenotypes in mouse and human. Here we report a severe osteopetrosis-like phenotype in transgenic medaka fish, in which membrane bound EGFP (mEGFP) was expressed in osteoclasts under control of the cathepsin K promoter (ctsk:mEGFP). In contrast to reporter lines with GFP expression in the cytoplasm of osteoclasts, adult fish of the mEGFP line developed bone defects indicative for an osteoclast dysfunction. Activity of tartrate-resistant acid phosphatase (TRAP) was down-regulated and excess bone was observed in most parts of the skeleton. The osteopetrotic phenotype was particularly obvious at the neural and haemal arches that failed to increase their volume in growing fish. Excess bone caused severe constriction of the spinal cord and the ventral aorta. The continuation of tooth development and the failure to shed teeth resulted in severe hyperdontia. Interestingly, at the vertebral column vertebral body arches displayed a severe osteopetrosis, while vertebral centra had no or only a mild osteopetrotic phenotype. This confirms previous reports from cichlids that, different from the arches, allometric growth of fish vertebral centra initially does not depend on the action of osteoclasts. Independent developmental mechanism that shapes arches and vertebral centra can also lend support to the hypothesis that vertebral centra and arches function as independent developmental modules. Together, this medaka osteopetrosis model confirms the importance of proper osteoclast function during normal skeletal development in teleost fish that requires bone modeling and remodeling.

A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Resorption and remodelling of skeletal tissues is required for development and growth, mechanical adaptation, repair, and mineral homeostasis of the vertebrate skeleton. Here we review for the first time the current knowledge about resorption and remodelling of the skeleton in teleost fish, the largest and most diverse group of extant vertebrates. Teleost species are increasingly used in aquaculture and as models in biomedical skeletal research. Thus, detailed knowledge is required to establish the differences and similarities between mammalian and teleost skeletal remodelling, and between distantly related species such as zebrafish (Danio rerio) and medaka (Oryzias latipes). The cellular mechanisms of differentiation and activation of osteoclasts and the functions of teleost skeletal remodelling are described. Several characteristics, related to skeletal remodelling, distinguish teleosts from mammals. These characteristics include (a) the absence of osteocytes in most species; (b) the absence of haematopoietic bone marrow tissue; (c) the abundance of small mononucleated osteoclasts performing non-lacunar (smooth) bone resorption, in addition to or instead of multinucleated osteoclasts; and (d) a phosphorus- rather than calcium-driven mineral homeostasis (mainly affecting the postcranial dermal skeleton). Furthermore, (e) skeletal resorption is often absent from particular sites, due to sparse or lacking endochondral ossification. Based on the mode of skeletal remodelling in early ontogeny of all teleosts and in later stages of development of teleosts with acellular bone we suggest a link between acellular bone and the predominance of mononucleated osteoclasts, on the one hand, and cellular bone and multinucleated osteoclasts on the other. The evolutionary origin of skeletal remodelling is discussed and whether mononucleated osteoclasts represent an ancestral type of resorbing cells. Revealing the differentiation and activation of teleost skeletal resorbing cells, in the absence of several factors that trigger mammalian osteoclast differentiation, is a current challenge. Understanding which characters of teleost bone remodelling are derived and which characters are conserved should enhance our understanding of the process in fish and may provide insights into alternative pathways of bone remodelling in mammals.

Unique and shared gene expression patterns in Atlantic salmon (Salmo salar) tooth development

To validate the use of Atlantic salmon (Salmo salar L.) as a model species in research on the mechanism of continuous tooth replacement, we have started to collect data on the molecular control underlying tooth formation in this species. This study reports expression patterns in the lower jaw dentition of a number of key regulatory genes such as bmp2, bmp4, and sox9 and structural genes such as col1alpha 1 and osteocalcin (= bgp, Bone Gla Protein) by means of in situ hybridization using salmon-specific, digoxygenin-labeled antisense riboprobes. We compare expression of these genes to that in other skeletogenic cells in the lower jaw (osteoblasts, chondroblasts, and chondrocytes). Our studies reveal both expression patterns that are in accordance to studies on mammalian tooth development and patterns that are specific to salmon, or teleosts. The epithelial expression of sox9 and a shift of the expression of bmp2 from epithelium to mesenchyme have also been observed during mammalian tooth development. Different from previous reports are the expressions of col1alpha 1 and osteocalcin. In contrast to what has been reported for zebrafish, osteocalcin is not expressed in odontoblasts, nor in the osteoblasts involved in the attachment of the teeth. At the lower jaw, osteocalcin is expressed in mature and/or resting osteoblasts only. As expected, col1alpha 1 is expressed in odontoblasts. Surprisingly, it is also strongly expressed in the inner dental epithelium, representing the first report of ameloblast involvement in collagen type I transcription. Whether the collagen is translated and secreted into the enameloid remains to be demonstrated.