Biology Reference
In-Depth Information
Figure 7.1 Central and peripheral organization of the thyroid and stress endocrine axes
controlling amphibianmetamorphosis. A schematic representationof thehypothalamo-
pituitary-thyroid (HPT) and hypothalamo-pituitary-adrenal (HPA; stress) axes in am-
phibian tadpoles, their regulation by input from the external environment, transduction
of this input byneural andneuroendocrinepathways, and synergistic interactions among
thyroid hormones and corticosteroids in target cells leading to the promotion of meta-
morphosis. The two endocrine axes are controlled centrally by corticotropin-releasing
factor (CRF) which acts on the anterior pituitary gland (AP) to stimulate the release of
thyrotropin (TSH) and corticotropin (ACTH). TSH acts on the thyroid gland to stimulate
release of thyroxine (T
4
) and 3,5,3
0
-triiodothyronine (T
3
). Thyroid hormones are trans-
ported in the blood bound by serum binding proteins (transthyretin, TBG, and albumin).
ACTH acts on adrenal cortical cells in the interrenal glands to stimulate biosynthesis and
