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ORDOVICIAN PERIOD

488-444 million years ago

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THE ENDOCRINE SYSTEM

The endocrine advances of the early vertebrates included a better differentiated pituitary gland and receptors for estrogen and progesterone.


Primitive vertebrates increased the complexity of their endocrine systems with a distinction between the pars distalis and pars intermedius of the pituitary gland (Hoar, Vol. 2), F in endocrine pancreas (Youson, 1999), and innervation of endocrine pancreas (Jansson, 1998). Thyroid follicles were concentrated in ventral pharynx and had ample blood supply (Hoar, Vol. 2) and iodine was concentrated in the colloid around thyroid follicular cells (Hoar, Vol. 2). Three steroid hormone receptors evolved in primitive vertebrates, including those for estrogen and progesterone (Thornton, 2001).