The thyroid gland and thyroid hormones in Senegalese sole (Solea senegalensis) during early development and metamorphosis

We here describe the ontogeny and morphology of the thyroid gland in Senegalese sole (Solea senegalensis), and correlate these with whole body concentrations of thyroid hormones during early development and metamorphosis. Under our rearing conditions at 19.5 °C, most larvae entered metamorphosis in stage 1 at 15 days post-hatching (dph), and completed metamorphosis in stage 4 at 25 dph. The onset of metamorphosis coincided with surges in whole body T4 and T3 concentrations. Crossmon’s trichrome stain colored the lumen of follicular structures brightly red, and this co-localized with a T4-immunoreactivity. Thyroid follicles were first observed in stage 0 pre-metamorphic larvae at 5 dph of age, and were detected exclusively in the subpharyngeal region, surrounding the ventral aorta. Increases in whole body thyroid hormone levels coincided with a 2½-fold increase in the total thyroidal colloid area in stage 1 larvae (aged 15 dph) compared to stage 0 larvae (12 dph). This was preceded by an approximately 40%-increase in the follicles’ epithelial cell height in stage 0 larvae at 12 dph compared to larvae at 5 dph, and by an increase in the whole body T3/T4 ratio, indicative of an increase in outer ring deiodination. We conclude that in S. senegalensis there is a clear chronology in the activation of the thyroid gland that starts in early pre-metamorphic larvae.