Thyroid Hormone Control of Tail Regeneration: Differential in Loco and Systemic Effects and Seasonal Variation

Thyroid hormones have been implicated in the control of vertebrate appendage regeneration. In lizards, thyroid hormones have been reported to induce ependymal outgrowth and also exercise control over adaptive systemic metabolic activities. However, there have been no attempts to correlate the seasonal differences in regenerative performance with thyroid activity. The present study has evaluated the effect of induced thyroid hormone excess or deficiency (by T4 administration or methimazole treatment, respectively) on tail regeneration in H. flaviviridis on a seasonal basis in summer, monsoon and winter months. The experiments revealed a retardative influence of hypothyroidism in tail regeneration in both summer and winter months; however blastema formation occurred in the normal time course in the summer months. Hyperthyroidism induced by daily T4 administration, either systemically or in loco, hastened the formation of blastema and provided an early growth spurt but, ultimately, retarded regenerative growth in the summer months. However, T4 administration daily for the first 15 days and every other day thereafter favored a better regenerative growth. In contrast, during the monsoon months, both daily administration and administration every alternate day, either systemically or in loco, delayed blastema formation as well as retarded linear growth. Neither hypothyroidism nor hyperthyroidism exerted any influence on the sluggish performance characteristic of winter months. It is concluded that thyroid activity and thyroid hormone responsiveness vary on a seasonal basis with maximum activity at higher temperatures and minimal at lower temperatures. It can also be concluded that there is differential sensitivity to thyroid hormone during summer and monsoon seasons.