Immunocytochemical demonstration of T4 content and TSH-binding by cells of the thyroid of the developing chick embryo.

The numerical densities (Nv) of immunostained thyroxine (T4)-positive cells and thyrotropin-stimulating hormone (TSH)-positive cells were determined in chick embryos on Day 5.5 through Day 11.5 and Days 5.5, 7.5, 9.5, 10.5, 11.5, 12.5, 13.5, 14.5, 15.5, 16.5, and 17.5, respectively. The data demonstrate that selected cells of the "prefollicular" thyroid contain T4 and bind TSH at least as early as Day 5.5. The Nv of immunocytochemically demonstrable T4-positive cells increases gradually from Day 5.5 to Day 10.5, exhibiting a statistically significant increase (P less than 0.05) between Day 10.5 and Day 11.5. Similarly, the Nv of TSH-binding cells (Nv TSH) rises slowly from Day 5.5 until Day 11.5, followed by a statistically significant (P less than 0.05) increase between Day 11.5 and Day 12.5. The peak Nv TSH value on Day 12.5 is followed by a decline (P less than 0.05) on Day 13.5, and from Day 14.5 through Day 17.5, Nv TSH values remain relatively constant. Changes in the Nv of T4-positive and TSH-binding cells over developmental time are discussed with regard to pituitary regulation of the chick embryo thyroid and the possible contribution of the yolk to circulating T4 levels.