The color of the ventral skin of the Japanese adult newt Cynops pyrrhogaster is red, whereas that of the small juvenile newts at metamorphosis is creamy. Xanthophores in the red skin have many ring carotenoid vesicles (rcv) and a few homogenous carotenoid granules (hcg), as reported earlier. To understand the reason for this change in coloration of the ventral skin of the newt, we carried out histological and biochemical studies to see whether the size and the number of carotenoid granules (hcg and rcv) in the xanthophores and also carotenoid content in the ventral skin change during the growth of this animal. By electron microscopic observation, only hcg were observed in the creamy skin of larvae at stage 59. The diameter of the hcg in the skin of the larvae was approximately 0.85 μm, but significantly decreased to 0.35 μm in the skin of the small juvenile newt. However, the number of the hcg/100 μm2 of a xanthophore in the ventral skin was very low in the larva at stage 59, but increased in the small juvenile. The carotenoid content was very low in the creamy skin of small juveniles, but dramatically high in the red skin of the adult newts. In the red skin of the adult newt, many rcv (85%) and a few hcg (15%) were observed. However, the number of carotenoid granules (rcv and hcg)/100 μm2 of a xanthophore in the red skin of adult newts was not different from that of hcg/100 μm2 of a xanthophore in the creamy skin of small juveniles. The results, taken together, suggest that the increase in the size and the number of carotenoid granules and also carotenoid content in the ventral skin is very important for red body coloration during the growth of the Japanese newt Cynops pyrrhogaster.
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