Polar auxin transport activity in the epicotyl region proximal to the cotyledons (proximal side) of 4.5-day-old etiolated pea seedlings has been found to be approximately twice that in the region distal to the cotyledons (distal side). The study aimed to determine whether the responsiveness to gravitropic stimulation differs between the proximal and distal sides, and to investigate the mechanism that induces gravitropic bending. When the seedlings were placed horizontally with the proximal side facing downward, the onset of bending occurred earlier and the bending angle was greater than when the distal side was facing downward. In the seedlings placed horizontally, tissue tension substantially decreased in the lower flank, while that in the upper flank remained the same as that in upright epicotyls. Decreased tissue tension in the lower flank was observed earlier in the seedlings with the proximal side facing downward, indicating that the asymmetrical distribution of tissue tension in the epicotyl provides a net gravitropic moment. Gravitropic stimulation increased cell wall extensibility in the lower flank, but not in the upper flank. These results suggest that differences in polar auxin transport between the proximal and distal sides is closely related to gravitropic responsiveness, and that changes in cell wall extensibility regulate asymmetrical tissue tension to cause gravitropic bending in etiolated pea seedlings.

Exposure to microgravity during spaceflight induces excessive osteoclast activation, resulting in bone loss and dysregulation of calcium metabolism. Accumulating evidence from spaceflight and ground-based studies suggests that endocrine factors involved in osteoclast inhibition may be altered under microgravity conditions; however, the underlying regulatory mechanisms remain incompletely understood.

Goldfish scales provide a unique experimental model for bone metabolism, as they contain functional osteoblasts, osteoclasts, and a calcified matrix, and exhibit osteoclast responses to microgravity comparable to those observed in mammalian bone. In previous spaceflight experiments, we demonstrated that microgravity decreases calcitonin (CT) mRNA levels in goldfish scales and that melatonin increases CT expression, thereby suppressing microgravity-induced osteoclast activation. These findings led us to propose a regulatory role for the melatonin–calcitonin axis in osteoclast activation under microgravity conditions.

In the present study, we sought to obtain mechanistic evidence supporting this concept under 1 g conditions using the goldfish scale model. We first identified the coexistence of immature and mature osteoclast populations in cultured goldfish scales, validating this system for evaluating osteoclast regulatory factors. We then demonstrated that exogenous calcitonin significantly suppresses osteoclast activity in cultured regenerating scales, similar to the effect observed with melatonin. Furthermore, intraperitoneal administration of exogenous melatonin significantly reduced plasma calcium levels in male goldfish, a physiological parameter reflecting osteoclast activity in the scales.

Taken together, when interpreted in the context of our previous spaceflight study, these findings provide mechanistic support for the concept that alterations in the melatonin–calcitonin regulatory axis contribute to abnormal osteoclast activation under microgravity conditions.