No abstract

Despite the common structure of vertebrates, the development of the vertebral column differs widely between teleosts and tetrapods in several respects, including the ossification of the centrum and the function of the notochord. In contrast to tetrapods, vertebral development in teleosts is not fully understood, particularly for large fish with highly ossified bones. We therefore examined the histology and gene expression profile of vertebral development in fugu, Takifugu rubripes, a model organism for genomic research. Ossification of the fugu centrum is carried out by outer osteoblasts expressing col1a1, col2a1, and sparc, and the growing centra completely divide the notochord into double cone-shaped segments that function as intercentral joints. In this process, the notochord basal cells produce a thick notochord sheath exhibiting Alcian-blue-reactive cartilaginous properties and composing the intercentral ligament in cooperation with the external ligament connective tissue. Synthesis of the matrix by the basal cells was ascertained by an in vitro test. Expression of twist2 indicates that this connective tissue is descended from the embryonic sclerotome. Notochord basal cells express sox9, ihhb, shh, and col2a1a, suggesting that the signaling system involved in chondrocyte proliferation and matrix production also functions in notochord cells for notochord sheath formation. We further found that the notochord expression of both ntla and shh is maintained in the fugu vertebral column, whereas it is turned off after embryogenesis in zebrafish. Thus, our results demonstrate that, in contrast to zebrafish, a dynamic morphogenesis and molecular network continues to function in fugu until the establishment of the adult vertebral column.

No abstract

Circadian rhythms enable organisms to coordinate multiple physiological processes and behaviors with the earth's rotation. In mammals, the suprachiasmatic nuclei (SCN), the sole master circadian pacemaker, has entrainment mechanisms that set the circadian rhythm to a 24-h cycle with photic signals from retina. In contrast, the zebrafish SCN is not a circadian pacemaker, instead the pineal gland (PG) houses the major circadian oscillator. The SCN of flounder larvae, unlike that of zebrafish, however, expresses per2 with a rhythmicity of daytime/ON and nighttime/OFF. Here, we examined whether the rhythm of per2 expression in the flounder SCN represents the molecular clock. We also examined early development of the circadian rhythmicity in the SCN and PG. Our three major findings were as follows. First, rhythmic per2 expression in the SCN was maintained under 24 h dark (DD) conditions, indicating that a molecular clock exists in the flounder SCN. Second, onset of circadian rhythmicity in the SCN preceded that in the PG. Third, both 24 h light (LL) and DD conditions deeply affected the development of circadian rhythmicity in the SCN and PG. This is the first report dealing with the early development of circadian rhythmicity in the SCN in fish.

Pleuronectiform fish develop marked external asymmetry in eye location and skin color at metamorphosis. The bamboo sole, Heteromycteris japonica, also exhibits loss of the pectoral fins at metamorphosis. Because of its small body size, short generation time, and long spawning season, we focused on the bamboo sole as an experimental model to investigate metamorphic asymmetry and pectoral fin loss during development. In the present study, we utilized a small-scale culture system to evaluate bamboo sole larvae and larval development, and a microinjection system for fertilized eggs. The culture system described here uses an 18 L culture tank for rotifers (the first diet for larvae) and 5 L plastic beakers for larval culture. Under this system, most larvae completed metamorphosis, including one-eye migration and pigmentation of the ocular side, by 23 days postfertilization (dpf) at 25°C. Larvae at density of 120-150 per liter were grown from hatching to 23 dpf with a survival ratio of 60-75% per beaker. Pectoral fins, including coracoid and disk cartilage, formed but were completely lost in late metamorphosis without formation of proximal radials and fin rays. The microinjection system designed here is adequate for the bamboo sole and allows injection of 100 one-cell-stage embryos per day. We expect that the culture and microinjection systems described here will facilitate the use of the bamboo sole as an experimental model organism in developmental biology.