Matrix Gla protein (Mgp) was selected based on its involvement in the calcification of ECM, and mgp deficiency in mice results in overcalcification of bone and cartilage( Luo et al., 1997), suggesting that the protein functions, to some extent, as an inhibitor of calcification. Alkaline phosphatase (Alp) was selected because of its involvement in the mineralizing function of the osteoblasts, and the expression of its gene is commonly used in mammalian species to measure the level of extracellular matrix mineralization (ECM) ( Rawadi et al.,2003). In this study the gene expression of the gene sonic hedgehog( shh) was selected as a marker of osteoblast cell proliferation since the protein product is crucial for development and growth of bone in mouse( Chiang et al., 1996 St-Jacques et al., 1999) and ectopic expression of shh results in excess dermal bone formation in regenerating zebrafish ( Danio rerio) caudal fins( Quint et al., 2002). The molecular signals that bring about change in bone mass are unknown, but it is likely that light treatment stimulates a change in the formation rate of osteoid tissue and/or the mineralization ability of the tissue. Seasonality in bone mass has also been reported in the teleost Atlantic salmon ( Wargelius et al., 2005a). Studies in mammals have reported that light manipulation changes the pattern of osteoblast proliferation and bone mass in response to changes in circadian pattern. Recent reports support the hypothesis that osteoblasts can receive a direct innervation by sympathetic neurons controlled by the circadian rhythm in the suprachiasmatic nucleus(reviewed by Patel and Elefteriou,2007). However, the action of melatonin on bone can be mediated directly by action on membrane-bound receptors on osteoblast cells or through the function of melatonin in setting the suprachiasmatic circadian rhythm in the brain. The primary mechanism of pineal mediation of the effect on the vertebral column is unknown. Studies of Atlantic salmon after 6 months under continuous light( Fjelldal et al., 2005) and 1 year after pinealectomy ( Fjelldal et al.,2004) revealed a lower mineral content and mechanical strength in the bone of the vertebrae than under natural light, and after sham operation and under natural light. Pinealectomy has serious effects on the vertebral column in mammalians, birds and fish (reviewed by Pandi-Perumal et al., 2006). The pineal gland is the main source of melatonin production, and is the primary endocrine mediator of photoperiod changes. In Atlantic salmon ( Salmo salar L.), continuous light has been shown to suppress nocturnal melatonin production and flatten out the diurnal pattern in melatonin secretion observed under natural light conditions( Porter et al., 1999). Our results also suggest that shh can be used as a marker of osteoblast proliferation and col I a marker of osteoid incorporation, and that both alp and mgp expression could be associated with a rapid increase in mineralization in Atlantic salmon vertebrae. However, in this experiment light treatment did not promote the development of vertebral deformities. These results indicate that constant light affects mineralization and delays osteoid incorporation in Atlantic salmon during the spring. Stiffness, mineral incorporation and gene expression followed the same trend in both light groups in late spring, whereas an increase in the expression of mgp and alpwas detected in April, followed by significantly higher stiffness at last sampling in both light groups. Mineralization properties were measured with stiffness, mineral incorporation per day and expression of alkaline phosphatase ( alp) and matrix Gla protein ( mgp). It was also shown that fish on the continuous light regime had delayed activation of osteoid incorporation. Co-variation between yield-load and the expression of both shh and col I were detected in both groups. Changes in the organic phase were related to mechanical strength (yield-load) and the expression of the genes sonic hedgehog( shh) and collagen type I alpha 2 ( col I). During the experimental period,several parameters related to the inorganic (minerals) and organic (osteoid)phases were measured. In order to study the effects of photoperiod on fish bone, Atlantic salmon( Salmo salar L.) were exposed to two light regimes (natural and continuous light) from January until June.

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