Responses to thyrotropin during development in Japanese quail

Area of Science:

• Endocrinology
• Developmental Biology
• Comparative Physiology

Background:

• The ontogeny of thyroid hormone regulation is crucial for development.
• Japanese quail (Coturnix japonica) exhibit precocial development, making them a suitable model for studying embryonic thyroid function.
• Understanding thyroid-pituitary feedback mechanisms and responses to thyroid-stimulating hormone (TSH) is key to deciphering developmental endocrinology.

Purpose of the Study:

• To investigate the developmental maturation of thyroid responses to TSH in Japanese quail embryos and chicks.
• To determine the timing of thyroid-pituitary negative feedback maturation during embryonic development.
• To characterize developmental changes in serum thyroid hormone (T3 and T4) levels and ratios following TSH administration.

Main Methods:

• Administration of thiourea to quail embryos to assess thyroid stimulation and feedback maturation.
• Measurement of thyroid 32P uptake, serum T3 and T4 concentrations, thyroid weights, and body weights.
• Intravenous administration of TSH to embryos and chicks at various developmental stages to evaluate dose- and time-dependent responses.

Main Results:

• Thyroid-pituitary negative feedback matures between days 9 and 10 of the 16.5-day incubation period.
• Embryonic TSH administration leads to parallel increases in serum T3 and T4 without altering the T3:T4 ratio.
• Posthatching TSH administration results in increased T4 but decreased T3:T4 ratios, indicating developmental shifts in hormonal response and peripheral conversion.

Conclusions:

• Hypothalamic or higher brain control appears to dominate pituitary regulation of the thyroid in late-term quail embryos.
• Developmental changes in the hormonal response to TSH occur, with peripheral conversion of T4 to T3 likely accounting for the perinatal increase in the T3:T4 ratio.
• TSH-mediated thyroid effects alone do not explain the perinatal surge in the T3:T4 ratio, highlighting the importance of peripheral deiodination.