The notochord in Atlantic salmon (Salmo salar L.) undergoes profound morphological and mechanical changes during development

Histopathology of the Intervertebral Disc of Nothobranchius furzeri, a Fish Model of Accelerated Aging

INTRODUCTION

Osteoarthritis is a classical age-related disease, which affects millions of patients worldwide. To further understand the pathophysiology and to develop therapeutic strategies for this disease, animal models play a significant role. Nothobranchius furzeri is an established model for accelerated aging that spontaneously develops spinal deformities. Although the bone properties of N. furzeri are well described, characteristics of the intervertebral discs are still unknown. The aim of this study was to investigate the characteristics of the intervertebral discs of healthy and deformed N. furzeri.

MATERIAL AND METHODS

Intervertebral properties of healthy and deformed N. furzeri were investigated in 8-, 12-, 18- and 21.5-week-old male fish of the GRZ strain. For histological evaluations the fish were decalcified, paraffin-embedded and stained with (1) hematoxylin and eosin, (2) toluidine blue and (3) alcian blue/picrosirius red.

RESULTS

8-week-old and deformed N. furzeri showed spongy-like tissue containing vacuolated notochord cells and a beginning formation of fibrous tissue in the central area. Older healthy fish showed fibrous tissue in the central region and a spongy-like tissue in the peripheral region.

CONCLUSION

Our study revealed age- and disease-related alterations of the vertebral discs in N. furzeri. Further studies should investigate the utility of N. furzeri as a model for degenerative spine diseases.

Heads and tails: The notochord develops differently in the cranium and caudal fin of Atlantic Salmon (Salmo salar, L.)

While it is well known that the notochord of bony fishes changes over developmental time, less is known about how it varies across different body regions. In the development of the Atlantic salmon, Salmo salar L., cranial and caudal ends of the notochord are overlaid by the formation of the bony elements of the neurocranium and caudal fin, respectively. To investigate, we describe how the notochord of the cranium and caudal fin changes from embryo to spawning adult, using light microscopy, SEM, TEM, dissection, and CT scanning. The differences are dramatic. In contrast to the abdominal and caudal regions, at the ends of the notochord vertebrae never develop. While the cranial notochord builds a tapering, unsegmented cone of chordal bone, the urostylic notochordal sheath never ossifies: adjacent, irregular bony elements form from the endoskeleton of the caudal fin. As development progresses, two previously undescribed processes occur. First, the bony cone of the cranial notochord, and its internal chordocytes, are degraded by chordoclasts, an undescribed function of the clastic cell type. Second, the sheath of the urostylic notochord creates transverse septae that partly traverse the lumen in an irregular pattern. By the adult stage, the cranial notochord is gone. In contrast, the urostylic notochord in adults is robust, reinforced with septae, covered by irregularly shaped pieces of cellular bone, and capped with an opistural cartilage that develops from the sheath of the urostylic notochord. A previously undescribed muscle, with its origin on the opistural cartilage, inserts on the lepidotrich ventral to it.

Serial blockface SEM suggests that stem cells may participate in adult notochord growth in an invertebrate chordate, the Bahamas lancelet

Background

The cellular basis of adult growth in cephalochordates (lancelets or amphioxus) has received little attention. Lancelets and their constituent organs grow slowly but continuously during adult life. Here, we consider whether this slow organ growth involves tissue-specific stem cells. Specifically, we focus on the cell populations in the notochord of an adult lancelet and use serial blockface scanning electron microscopy (SBSEM) to reconstruct the three-dimensional fine structure of all the cells in a tissue volume considerably larger than normally imaged with this technique.

Results

In the notochordal region studied, we identified 10 cells with stem cell-like morphology at the posterior tip of the organ, 160 progenitor (Müller) cells arranged along its surface, and 385 highly differentiated lamellar cells constituting its core. Each cell type could clearly be distinguished on the basis of cytoplasmic density and overall cell shape. Moreover, because of the large sample size, transitions between cell types were obvious.

Conclusions

For the notochord of adult lancelets, a reasonable interpretation of our data indicates growth of the organ is based on stem cells that self-renew and also give rise to progenitor cells that, in turn, differentiate into lamellar cells. Our discussion compares the cellular basis of adult notochord growth among chordates in general. In the vertebrates, several studies implied that proliferating cells (chordoblasts) in the cortex of the organ might be stem cells. However, we think it is more likely that such cells actually constitute a progenitor population downstream from and maintained by inconspicuous stem cells. We venture to suggest that careful searches should find stem cells in the adult notochords of many vertebrates, although possibly not in the notochordal vestiges (nucleus pulposus regions) of mammals, where the presence of endogenous proliferating cells remains controversial.

Developmental expression patterns of six6: A gene linked with spawning ecotypes in Atlantic salmon

The Atlantic salmon has been studied extensively, particularly as a model for understanding the genetic and environmental contributions to the evolution and development of life history traits. Expression pattern analysis in situ, however, is mostly lacking in salmon. We examine the embryonic developmental expression of six6, a candidate gene previously identified to be associated with spawning ecotypes and age at sexual maturity, in Atlantic salmon. Six6 is a member of the sine oculis homeobox family of transcription factors and is known to regulate eye and brain development in other vertebrates. We assay the expression of this gene in embryonic Atlantic salmon Salmo salar by whole-mount in situ hybridization. In line with earlier studies in other vertebrate species, we find conserved expression in the developing brain and sensory organs, including optic and olfactory primordia. However, we also find previously unreported domains of expression that suggest additional roles in axial and appendicular development, cardiovascular, intestinal, and sensory organogenesis. Each of these systems are important in the sensory ecology of Atlantic salmon, suggesting it is plausible that six6 may have pleiotropic roles in this complex phenotype.

Notochord vacuoles absorb compressive bone growth during zebrafish spine formation

The vertebral column or spine assembles around the notochord rod which contains a core made of large vacuolated cells. Each vacuolated cell possesses a single fluid-filled vacuole, and loss or fragmentation of these vacuoles in zebrafish leads to spine kinking. Here, we identified a mutation in the kinase gene dstyk that causes fragmentation of notochord vacuoles and a severe congenital scoliosis-like phenotype in zebrafish. Live imaging revealed that Dstyk regulates fusion of membranes with the vacuole. We find that localized disruption of notochord vacuoles causes vertebral malformation and curving of the spine axis at those sites. Accordingly, in dstyk mutants the spine curves increasingly over time as vertebral bone formation compresses the notochord asymmetrically, causing vertebral malformations and kinking of the axis. Together, our data show that notochord vacuoles function as a hydrostatic scaffold that guides symmetrical growth of vertebrae and spine formation.

Radiological changes during fracture and repair in neural and haemal spines of Atlantic cod (Gadus morhua)

Although spinal injuries in fish have been associated with electric stimuli applied during electrofishing and electrotrawling, bone fracture and repair in the axial skeleton have yet not been studied. To study this, we radiographed a group (n = 64) of individually tagged farmed cod twice, with a 1-year interval (∼36 cm at first and ∼ 50 cm at second inspection). The study focus was on the neural and haemal spines. These structures are un-paired and are not covered by other bones laterally, making them useful for radiological studies on axial skeletal fracture in live fish. At the first examination, four animals showed radiological changes in their neural and haemal spines. Two animals had fractures, and two had callus formations. One year later, at the second radiological examination, the fractures had developed into calluses or into normal morphology, and calluses either remained as calluses or had developed into normal morphology. A further 14 animals that were all normal at the first inspection had developed changes in their neural and haemal spines, both fractures and callus formations. This is the first record of spontaneous bone fracture in fish; the fractures observed occurred under normal farming conditions and were not induced. The results show that cod have a functional fracture healing mechanism in their neural and haemal spines. The findings are discussed in relation to fish hyperostosis.

The external phenotype-skeleton link in post-hatch farmed Chinook salmon (Oncorhynchus tshawytscha)

Skeletal deformities in farmed fish are a recurrent problem. External malformations are easily recognized, but there is little information on how external malformations relate to malformations of the axial skeleton: the external phenotype-skeleton link. Here, this link is studied in post-hatch to first-feed life stages of Chinook salmon (Oncorhynchus tshawytscha) raised at 4, 8 and 12°C. Specimens were whole-mount-stained for cartilage and bone, and analysed by histology. In all temperature groups, externally normal specimens can have internal malformations, predominantly fused vertebral centra. Conversely, externally malformed fish usually display internal malformations. Externally curled animals typically have malformed haemal and neural arches. External malformations affecting a single region (tail malformation and bent neck) relate to malformed notochords and early fusion of fused vertebral centra. The frequencies of internal malformations in both externally normal and malformed specimens show a U-shaped response, with lowest frequency in 8°C specimens. The fused vertebral centra that occur in externally normal specimens represent a malformation that can be contained and could be carried through into harvest size animals. This study highlights the relationship between external phenotype and axial skeleton and may help to set the framework for the early identification of skeletal malformations on fish farms.