Stimulation of Neurosecretion in Shark Embryos by Thyroid Hormones.

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Abstract
Differentiation of the hypothalamic neurosecretory system was studied in normal and thyroid hormone injected embryos of Squalus suckleyi removed from the uterus and kept in flowing sea water. Untreated and treated "pup" stage 23 month-old embryos had well differentiated preoptic nuclei, and neurosecretion visible in the preoptico-hypophyseal tract and neurohypophysis. Untreated "candle" stage 11 month-old embryos had only traces of stainable neurosecretion in the neurohypophysis. Injections of 1-thyroxine, 1-triiodothyronine, or triiodothyroacetic acid stimulated a large increase in neurosecretory material in the neurohypophysis, and the early appearance of secretory droplets in the axones of the preoptico-hypophyseal tract of candle embryos.