Abstract
The capabilities of the thyroid and adenohypophysis of the developing chick embryo to respond to exogenous thyrotropin (TSH) and thyrotropin-releasing hormone (TRH), respectively, were evaluated by means of radioimmunoassay measurements of plasma total thyroxine (T 4) levels. Chick embryos were treated with bovine TSH (2.5 mIU) on Days 6.5, 7.5, and 8.5 of incubation. The embryonic chick thyroid was shown to be sensitive to TSH as early as Day 6.5, as evidenced by an increase in plasma T 4 levels ( P < 0.05). Additionally, the adenohypophyses of 6.5-day-old chick embryos were shown to respond to synthetic TRH (400 μg/100 g body weight) as indicated by a statistically significant rise in circulating thyroxine levels ( P < 0.05). Possible underlying mechanisms to explain the increase in thyroidal activity that normally occurs during the Day 10.0–12.0 incubation interval in the chick embryo are discussed.
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