Effect of thyroid stimulating hormone on the ultrastructure of the thyroid gland in the Mexican axolotl during metamorphic climax.

ABSTRACT

Anuran metamorphosis has long-served as a model of how thyroid hormones regulate post-embryonic development in vertebrates. However, comparatively little is known about urodele metamorphosis. Twofold approach was utilized in the present study including induction of metamorphosis in the obligatory neotenic Mexican axolotl (Ambystoma mexicanum) by applying thyroid stimulating hormone (TSH) treatment and investigating the alternations in the ultrastructure of the thyroid gland that occur during metamorphic climax. The results highlighted the difficulty of inducing metamorphosis in this obligatory neotenic urodele animal and indicated that there are huge variations in the response of experiment animals to the TSH treatment which ranged from complete insensitivity to death. During metamorphic climax induced by TSH treatment, the thyroid follicular cells exhibited evident signs of activation compared with both non-metamorphosed and control animals. These signs include the presence of abundant stacks of rough endoplasmic reticulum on the basement membrane side of the cell, large amount of Golgi complexes, intracellular droplets, dense lysosomal like vacuoles, mitochondria and free ribosomes are observed in the cytoplasm. Colloid vesicles and clear vesicles were also observed in the cytoplasm indicating the production of thyroid hormone.