ABSTRACT
The last trimester of the embryonic life of chickens is marked by a steady increase in circulating thyroxine (T(4)) levels, reaching a maximum around hatching. We have measured thyroidal mRNA expression levels of several genes involved in the biosynthesis of T(4), namely sodium/iodine symporter (NIS), thyroglobulin (Tg), thyroid peroxidase (TPO), thyrotropin receptor (TSHR), and thyroid transcription factor 1 (TTF-1), during this period. Subsequently, we measured the expression of these genes in more detail during the entire hatching process and compared the gene expression profiles with concomitant changes in intrathyroidal and circulating thyroid hormone levels. We found that NIS and TPO mRNA expression increased significantly in the perinatal period, whereas Tg mRNA expression rose gradually throughout the last week of embryogenesis but was stable during hatching. TSHR and TTF-1 mRNA levels did not change significantly during the last week of embryonic development and hatching. Our results suggest that the elevated plasma T(4) levels observed in the developmental period studied are caused by an increased synthesis and secretion of T(4) by the thyroid gland. Augmented expression of Tg may play an important role in the increasing T(4) production during the last week of embryonic development, whereas increased NIS and TPO expression around hatching allows the thyrocytes to boost T(4) synthesis even further.
Subject(s)
Chick Embryo/metabolism , Chickens/metabolism , Gene Expression Profiling/veterinary , Thyroid Gland/embryology , Thyroid Gland/metabolism , Animals , Chick Embryo/growth & development , Iodide Peroxidase/genetics , Nuclear Proteins/genetics , RNA, Messenger/analysis , Receptors, Thyrotropin/genetics , Symporters/genetics , Thyroglobulin/genetics , Thyroid Nuclear Factor 1 , Thyroxine/biosynthesis , Thyroxine/genetics , Time Factors , Transcription Factors/geneticsABSTRACT
The ease of in vivo experimental manipulation is one of the main factors that have made the chicken embryo an important animal model in developmental research, including developmental endocrinology. This review focuses on the development of the thyrotropic, corticotropic and somatotropic axes in the chicken, emphasizing the central role of the pituitary gland in these endocrine systems. Functional maturation of the endocrine axes entails the cellular differentiation and acquisition of cell function and responsiveness of the different glands involved, as well as the establishment of top-down and bottom-up anatomical and functional communication between the control levels. Extensive cross-talk between the above-mentioned axes accounts for the marked endocrine changes observed during the last third of embryonic development. In a final paragraph we shortly discuss how genomic resources and new transgenesis techniques can increase the power of the chicken embryo model in developmental endocrinology research.
