The effect of prolactin on the seasonal cyclicity of newt forelimb regeneration

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Abstract
A cyclic seasonal variation in forelimb regeneration in the newt, Notophthalmus viridescens, was noted under controlled environmental conditions at 20°C in previous experiments. Regeneration was enhanced in the summer months and reached its maximum linear growth rate in late spring‐early summer. It was suggested that this variation might be due to hormonal influences.The pituitary hormone prolactin was selected for study because it has been shown to support regeneration in hypophysectomized newts and to enhance regeneration in the intact newt, in the intact newt more effectively than growth hormone. Experiments in winter and summeri were performed to determine whether prolactin alone enhanced regeneration and what effect it might have on the seasonal variation in regeneration.The results clearly show enhancement of regeneration by intraperitoneal injections of prolactin in both summer and winter. The hormone increases the rate of linear growth and initiates early finger differentiation. The increase in linear growth rate persists throughout the 90‐day observation period. Prolactin does not enhance regeneration beyond the spontaneous optimum regenerative period in late spring‐early summer noted in previous experiments. Prolactin eliminates the seasonal variation in regeneration, and it is possible that the seasonal cycles are due to changes in output of this hormone.