Hypothalamic regulation of the Pituitary-Thyroid Unit in the Developing Chick Embryo

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

“…Some regulatory heat production may be present in late embryonic life; chicken embryos near the end of incubation show an increased resistance to cooling or transient increases in heat production with cooling (review: Visser, 1998). Two studies suggest that these metabolic responses may be due to the increasing plasma thyroid hormones present at this time; 16.5-day chicken embryos increase plasma T 4 in response to cooling (Thommes et al, 1988), and chicken embryos treated with thiourea do not show the metabolic responses to cooling (Tazawa et al, 1989). However, it should be noted that in many studies precocial embryos have not shown metabolic responses to cooling (review: Visser, 1998).…”

Thyroid Function in Embryonic and Early Posthatch Chickens and Quail

“…Thyroidal radioiodine uptake studies suggested that thyroid hormone (TH) synthesis was initiated early in TG development, but sensitive methods for detecting such synthesis have been developed relatively recently. Measurement of protein bound iodine (PBI) concentrations in serum, although not accurately representative of serum TH in birds (Astier, 1980), indicated relative maturity of thyroid function prior to hatching and a burst of thyroid activity during the perinatal period (see reviews by McNabb, 1987McNabb, , 1988Thommes, 1987;Thommes et al, 1988).…”

“…During the perinatal period both serum TH rise dramatically, but the cause of the rise is different for each. The rise in T4 results from stimulation of the TG by the anterior pituitary via thyroid-stimulating hormone (TSH), whereas the rise in T3 results from an increase in peripheral deiodination of T4 to T3 (see following section and McNabb, 1987McNabb, , 1988Thommes, 1987;Thommes et al, 1988). The RIA for reverse-T3 (rT3) reveal that this product of T4 degradation, which is considered to be without physiological function, rises during late embryonic life and then decreases after hatching (Thommes and Hylka, 1977;Hylka et al, 1986).…”

“…Heterologous TSH antibodies, although insufficiently sensitive for RIA, have been used successfully for determining the presence and location of immunoreactive TSH in the pituitaries of chicken embryos (Thommes, 1987;Thommes et al, 1988). Although some caution must be used in the acceptance of these results, careful use of correlative information from hormone assays and pituitary ablation and hormone replacement studies support their accuracy.…”

“…Studies of TRH also have used immunocytochemical techniques, but in this case there is no question about the specificity of the antibody because this tripeptide hormone is identical in birds and other vertebrate classes. Much of our knowledge of avian hypothalamic TRH and its developmental pattern comes from the work of Thommes and coworkers (see reviews of Thommes, 1987;Thommes et al, 1988). Although TRH can be measured in serum by RIA, its rapid degradation in vivo casts some doubt on the values.…”