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Immunocytochemical demonstration of T4 content and TSH-binding by cells of the thyroid of the developing chick embryo
R C Thommes1, E J Fitzsimons, M Davis
1Laboratory of Developmental Endocrinology, Sarasota, Florida 34239.
Insights
Thyroid hormone (T4) and TSH-binding cells appear early in chick embryo development. Their numbers increase significantly, suggesting pituitary regulation of thyroid function.
Area of Science:
- Developmental Biology
- Endocrinology
- Cell Biology
Background:
- The thyroid gland's development and function are crucial for embryonic growth.
- Understanding the initial appearance and regulation of thyroid hormones is vital.
Purpose of the Study:
- To quantify the numerical densities of thyroxine (T4)-positive and TSH-binding cells in developing chick embryos.
- To investigate the developmental timing of T4 presence and TSH binding in embryonic thyroid cells.
Main Methods:
- Immunohistochemistry was used to detect T4-positive and TSH-binding cells.
- Numerical densities (Nv) were calculated for these cells across various embryonic developmental stages (Day 5.5 to Day 17.5).
Main Results:
- T4 and TSH binding were detected in "prefollicular" thyroid cells as early as Day 5.5.
- The Nv of T4-positive cells increased significantly between Day 10.5 and Day 11.5.
- The Nv of TSH-binding cells showed significant increases between Day 11.5 and Day 12.5, followed by a decline on Day 13.5.
Conclusions:
- Thyroid cells in chick embryos express T4 and bind TSH early in development.
- Observed changes in cell densities suggest pituitary regulation of the embryonic thyroid gland.
- The yolk may contribute to circulating T4 levels during development.
Abstract:
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.