ENDOCRINE SYSTEM
The endocrine system of amphibians has been well studied as a representative model for the vertebrate world, on account of the organs being very similar to those in reptiles, birds, and mammals.
However, while the function of the various endocrine organs is similar to other vertebrates, the actual secretory products often have significant structural differences from their analogues in other vertebrates (Goin et al. 1978; Wright 2001c).The adrenal glands of amphibians are found in close association with the kidneys, although their exact location varies tremendously with the species. Like reptiles and birds, the adrenal gland appears homogenous on cut surface, and histologically it is comprised of intermingled cortical and medullary elements, rather than having the clear delineation between cortex and medulla seen in mammalian species. The adrenal gland produces corticosteroids, adrenaline (epinephrine), and noradrenaline (norepinephrine) (Goin et al. 1978; Wright 2001c).
The thyroid is primarily responsible for controlling metamorphosis of larval amphibians, and like other vertebrates produces tri-iodothyronine (T3) and tetra-iodothyronine (T4). The thyroid gland is also responsible for the control of ecdysis. The hypothalamus is responsible for controlling pituitary gland secretion of thyroid stimulating hormone (TSH), which in turn controls production of T3 and T4. Neoteny is due to the failure of the hypothalamus to produce releasing factors that stimulate the pituitary gland to produce and release TSH. In facultative neotenic species, such as the Tiger salamander (Ambystoma tigrinum), deteriorating environmental conditions will trigger metamorphosis by stimulating the hypothalamus to start producing releasing hormone. However, in obligate neotenic species, which never undergo metamorphosis in nature, such as the Mexican axolotl (Ambystoma mexicanum), only the administration of thyroxine will result in completion of metamorphosis (Goin et al. 1978; Mitchell et al. 1988).
The pituitary gland is also responsible for the production of adrenocorticotropic hormone (ACTH), antidiuretic hormone (ADH), arginine vasotocin (similar to vasopressin in mammals), follicle-stimulating hormone (FSH), luteinizing hormone, (LH), melanophore-stimulating hormone (MSH), oxytocin and prolactin. The other endocrine organs (and their associated secretory products) are the gonads (estrogen, progesterone, testosterone), pancreas (insulin), parathyroid glands (calcitonin, parathyroid hormone), pineal body (melatonin), ultimobranchial bodies (calcitonin), and the thymus (thymosin) (Holz et al. 2002; Wright 2001c).