Vertebrate endocrine systems

In contrast to the invertebrate endocrine system, the emphasis in the vertebrate endocrine system is on classical endocrine organs with many physiological processes controlled by these organs. However, the nervous system still exerts an influence over the endocrine system since some of the peripheral endocrine organs are under the control of the anterior pituitary, which will be described later. During vertebrate evolution, there has been much conservation in terms of endocrine function. This means that some hormones have found new roles ― for example, the hormone thyroxine controls metabolic rate in mammals, but in amphibians it is essential for the metamorphosis from tadpole to adult frog. In addition to this, as the vertebrates have evolved, new endocrine organs have emerged, such as the parathyroid glands that control Caz+ levels which first appeared in the teleosts {bony fish). The typical vertebrate endocrine system is seen to consist of three principal glands or groups of glands:

  • • the hypothalamus;
  • • the pituitary gland;
  • • peripheral endocrine glands.

The hypothalamus and pituitary gland

The hypothalamus is part of the vertebrate brain and sits beneath the thalamus. Its main function is as an interface between the nervous and endocrine systems. A major role of the hypothalamus is to control the pituitary gland ― the so-called master gland. The secretions of the hypothalamus are transported to the pituitary gland. There are two types of secretions ― those that are released into the posterior pituitary gland and those released into the anterior pituitary gland. Hormones secreted by the hypothalamus travel down axons extending from the hypothalamus to the posterior pituitary gland (the neurohypophysis). This region has a typical neuroendocrine role in that hormones are released from the posterior pituitary gland directly into the circulation. In mammals, the hormones released from die posterior pituitary gland are antidiuretic hormone {also known as vasopressin), which controls water absorption in the kidney, and oxytocin, which stimulates uterine smooth muscle contraction and milk ejection from the mammary glands. Both of these substances are peptides. Peptides with different amino acid compositions, but which have a similar biological role to either antidiuretic hormone or oxytocin, are found in all vertebrates. Another important group of secretions produced by the hypothalamus are the releasing hormones. These substances are released from axon terminals into capillaries which then pass to the anterior pituitary gland (adenohypophysis). Releasing hormones are thus delivered to the anterior pituitary gland indirectly via the blood system, rather than by direct release from axon terminals. The function of the releasing hormones, as the name suggests, is to influence the release of hormones from the anterior pituitary. The hormones released from the anterior pituitary then influence the secretions from other structures. Alternatively, release of hormones may be inhibited by release-inhibiting hormones secreted by the hypothalamus. The hypo-thalamic releasing and release-inhibiting hormones and the hormones whose release they control are shown in Table Hypothalamic hormones and the anterior pituitary hormones they influence. It is essential that the plasma concentrations of all the secretions in this system are maintained at acceptable levels.

Table Hypothalamic hormones and the anterior pituitary hormones they influence

Hypothalamic hormone Anterior pituitary hormone influenced
Releasing hormones
Growth hormone-releasing hormone (GHRH) Growth hormone
Thyrotropin-releasing hormone (TRH) Thyrotropin-stimulating hormone (TSH)
Protactin-releasing hormone (PRH) Prolactin
Luteinizing hormone-re I easing hormone (LHRH) Leutenizing hormone
Fotticle stimulating hormone-releasing hormone (FSHRH)a Follicle-stimulating hormone
Melanocyte-stimulating hormone releasing hormone (MSHRH) Melanocyte-stimulating hormone
Corticotropin-releasing hormone (CRH) Corticotropin (adrenocorticotropic hormone, ACTH)
Release-inhibiting hormones
Growth hormone release-inhibiting hormone (GHRIH, somatostatin) Growth hormone
Prolactin release-inhibiting hormone (PRIH) Prolactin
Melanocyte-stimulating hormone release-inhibiting hormone (MSHRIH) Melanocyte-stimutating hormone

a FSHRH and LHRH may be identical substances and are sometimes referred to as gonadotropin releasing hormone (GnRH)

Growth hormone promotes growth in all vertebrates. It has effects on carbohydrate, lipid and protein metabolism. It also induces the liver to release a compound called somatomedin which stimulates mitosis in bone tissue. Thyrotropin releasing hormone stimulates the thyroid gland to secrete thyroxine and triiodothyronine. The secretions of the thyroid gland have a variety of effects ― the control of metabolic rate in mammals and the control of metamorphosis in amphibians. Prolactin is a hormone which is well known for its effects on reproductive tissue, and its stimulatory effect on milk production. However, it also has other effects, influencing water and Na+ exchanges in amphibians. Follicle-stimulating and luteinizing hormones (FSH and LH) affect the gonads. FSH promotes gamete (i.e. egg and sperm) development, whilst LH, amongst many other functions, promotes steroid production. Melanocyte-stimulating hormone is involved in physiological color change in some of the lower vertebrates, e.g. amphibians and fish, whilst in some higher vertebrates it may be involved in osmotic and ionic regulatory processes. Adrenocorticotropic hormone stimulates the adrenal cortex to release the hormones produced there (i.e. the miner-alocorticoids, such as cortisol).

Peripheral endocrine organs

Within the vertebrate phyla, there are a vast array of organs which have an established endocrine function. The list is ever increasing; it is now known that the heart produces a hormone (atrial naturetic peptide, ANP), which is involved in the renal regulation of Na+. Table Some established endocrine organs and the functions of the hormones they produce shows a few examples of some of the more established endocrine organs and their secretions in the vertebrates. It should not be considered a complete list and the reader is directed to more comprehensive endocrinology texts for further information.

Table Some established endocrine organs and the functions of the hormones they produce

Endocrine organ Hormone released Hormone function
Parathyroid gland3 Parathormone/calcitonin Increase/decrease of blood Ca2+ levels
Stomachb Gastrin Regulation of acid secretion
Adrenal medullac Adrenaline Short term response to stress, e.g. increases blood sugar levels, increases cardiac output
Adrenal cortex Glucocorticoids, e.g. corticosterone Regulation of metabolism
Mineralocorticoids, e.g. aldosterone Regulation of electrolyte levels
Ovary Estrogens Initiates proliferation of endometrium
Progestogens Supports thickened endometrium
Testis Androgens, e.g. testosterone Maintains production of gametes and is involved in the development of secondary sexual characteristics

aExcept tish, there is no secretion of parathormone and a calcitonin-like substance called hypocalcin is secreted by the corpuscles of Stannius
bGastrin is not produced in the lower vertebrates, e.g. fish and amphibians
cln higher vertebrates the adrenal medulla and cortex constitute a single gland, in lower vertebrates they are separate