Osteoblast activity in the goldfish scale responds sensitively to mechanical stress

The adaptive response of bone to mechanical loading in teleosts is not well understood. We recently developed a new assay system using teleost scales, which consists of osteoblasts, osteoclasts, and bone matrix protein. In this system, alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) were used as markers of osteoblasts and osteoclasts, respectively. Using this assay system, we examined the effects of mechanical loading on ALP and TRAP activity in goldfish scales. ALP activity in the scales was significantly elevated (p < 0.01) by ultrasound stimuli (1 MHz, 50% duty factor, 0.5 Hz pulse repetition frequency, 60 mW/cm2 [ISATA] and 6 min) after both 18 h and 24 h of incubation while TRAP activity remained unchanged. In addition, mRNA expression of both insulin-like growth factor-I (IGF-I) and estrogen receptors (ER) increased significantly, as did ALP activity. After the goldfish had been swimming for 3 days (speed: 2 body lengths/second, duration: 3 h/day), the scales' ALP activity increased significantly (p < 0.01) but TRAP activity did not change. These in vitro and in vivo results strongly suggest that osteoblasts in the goldfish scale respond sensitively to mechanical stress and may be important in promoting bone formation.