Anatomical development of the thyroid gland in the Egyptian toad, Bufo regularis was studied during larval, pro-metamorphic, metamorphic climax, and post-metamorphic stages using histo-morphometric analysis. The thyroid follicles were firstly observed at the larval stage 38. Consequently, four developmental stages as representatives for the mentioned phases, namely, 40, 56, 60, and 66 were selected where significant morphometric changes were recorded in terms of the development of the thyroid gland. Morphometric parameters were employed for tracing the developmental changes in the thyroid gland histology including number of thyroid follicles, follicular cell height, gland volume as well as follicular lumen area. The present study revealed that the architecture of the thyroid gland progressively changes to accommodate the developmental demands. At metamorphic climax stage 60, the thyroid gland exhibited highest values in terms of the four investigated parameters compared to other stages, whereas significant regression in these values occurs at the post-metamorphic stage 66. The study also demonstrates that the peaks in thyroid activity expressed via the colloid are correlated with morphological changes that are directly comparable to metamorphic changes and remodeling associated with the biphasic life history characterizes most anuran amphibians. Different anatomical implications that are related to the developmental changes in the structure of the thyroid gland are discussed.
Key words: Thyroid gland; Development, Metamorphosis, Bufo regularis, Amphibians
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