Histomorphological Changes in the Thyroid Gland of Guinea Fowl (Numida meleagris) Exposed to Different Photoperiod Regimens and Exogenous Melatonin

Investigations established that birds used photoperiod to predict and adjust seasonal changes in their environment through predictive changes. Understanding the interaction of photoperiods and exogenous melatonin will provide information on physiological activities and guide in better poultry management. In this study, thirty matured male helmeted guinea fowls were utilized. They were randomly assigned to three groups (n = 10) of different photoperiodic regimes: short-day (SD; 8L:16D), control (CTR; 12L:12D) and long-day (LD; 16L:8D). Each of these groups was further divided into two groups: melatonin (Mel; 1 mg/kg) or without melatonin. The experiment lasted for a period of eight weeks. The results obtained from the SD + Mel group showed that the thyroidal follicles were lined by simple squamous follicular cells and serrated edges, while the SD without Mel had smooth follicular edges with similar lining. The LD + Mel and LD without Mel appeared normal, with similar histological features observed in the CTR; the lining was dominated with simple cuboidal epithelium with very few squamous cells. The follicular diameter across the groups was statistically significant (p < 0.0001). The LD + Mel group had the largest follicular diameter, while the lowest was in SD without melatonin. The thyroid glands were both positive for Periodic Acid Schiff and Alcian Blue with varied percentage of positive areas. Overall, the activity of the thyroid gland in the guinea fowl is influenced by long photoperiod and exogenous melatonin. Its influence on the thyroid glands mimics the reproductively active phase of the guinea fowl.