Endocrine System in Vertebrates


Hormones of vertebrates are classically described in the context of the principal glands that produce them and there is a generally agreed homology of the endocrine structures throughout the vertebrates. Contemporary views of hormones as key regulators take account of their sometimes ubiquitous sources, their modes of delivery, and their cellular and molecular mechanisms of action.

Keywords: endocrinology; hormones; biosynthesis; molecular and cellular mechanisms

Figure 1.

Illustration of the modes of delivery of hormones (H) to their sites of action. Specific receptors (R) are involved in the cellular response to a hormone and these may span the plasma membrane or may be cytoplasmic and intranuclear with genomic interactions. Based on O’Malley (1989).

Figure 10.

A flow diagram to illustrate the general sequence of events associated with G protein reactions of intracellular second messengers after a hormone has occupied a G protein‐coupled cell surface receptor. (a) The receptor complex spanning the membrane. (b) The events that occur within or closely associated with the membrane. (c) The intracellular results. AC, ; DAG, diacylglycerol; PLC, .

Figure 11.

Flow diagram summarizing the sequence of events that follow activation of a surface receptor that interacts with inositol phosphates, diacylglycerol, calcium and calmodulin signalling pathways. ER, ; PI, ; DAG, diacylglycerol; PKC, protein kinase C; PLC, phospholipase C.

Figure 12.

Key events in the actions of those hormones that interact directly with the genome. Free hormone enters the cell to become associated with a cytoplasmic receptor or binding protein and on its entering the nucleus a well‐ordered sequence of events leads to the production of a new protein to give the biological response. HBP, .

Figure 13.

Global view of regulatory levels in the endocrine system. Level A represents the molecular and cellular components and may be represented as single cells or whole endocrine glands. The biosynthetic machinery comes under the control of trophic or inhibitory agents and will include enzyme–substrate interactions and posttranslational processing. Intracrine factors can be key regulators at this level, while the release may be inhibited for some hormones to be stored for autocrine, paracrine and endocrine purposes. Many factors may signal directions at this level. Level B involves transport to the sites of action where molecular interactions take place to elicit the biological response. At this level, modulation of receptor activities is key, with hormones sometimes autoregulating their own receptor affinities and numbers. Level C represents the final biological response, which may range from apoptosis to a behavioural response in the whole animal. The arrows on the right represent feedback control within levels A and B, while the vertical arrow on the left indicates the possibility that there may be feedback regulation from the final response across levels action from changing the intracellular cascades to activation or inhibition of the biosynthesis of the hormone. Horizontal arrows (left) indicate controls at the interfaces between the various levels (secretion and transport; molecular interactions on or within the cell).

Figure 2.

Biosynthesis of steroid hormones (top panel) and the interrelationships between adrenocortical, oestrogenic and androgenic steroids (lower panel). The substrate, cholesterol, may be synthesized de novo or be supplied in the diet. After side‐chain cleavage, the pathways towards the various steroid types are embarked upon. Cholesterol is also the substrate for vitamin D metabolism.

Figure 3.

Biosynthesis of catecholamines from the substrate tyrosine. PNMT, phenylethanolamine N‐methyltransferase; Dopa, dihydroxyphenylalanine. Dopamine, noradrenaline and adrenaline are all biologically active and have their own specific receptors.

Figure 4.

Biosynthesis of arachidonic derivatives: (PG), (PS), (TX) and (LT). The key enzymes are cyclooxygenase (towards prostaglandins, thromboxanes and prostacyclins), thromboxane synthetase, prostacyclin oxycyclase and 5‐lipoxygenase (towards leucotrienes).

Figure 5.

Biosynthesis of thyroid hormones from the substrate tyrosine.

Figure 6.

Biosynthesis of melatonin from tryptophan. Serotonin is an important intermediate metabolite that has its own specific receptors.

Figure 7.

The biosynthesis of peptide/protein hormones, from DNA to the hormone. A gene or gene family may be selectively transcribed. There then follow differential processing and differential proteolysis and/or chemical modifications, such as phosphorylation or glycosylation, to generate the final hormone.

Figure 8.

The processing of the preprohormone (POMC). Genes express the full POMC molecule within which sequences for a variety of biologically active fragments are present. These are selectively cleaved to release adrenocorticotrophin (ACTH), β‐endorphin (β‐END), β‐lipotrophin (β‐LPH), corti (CLIP), γ‐LPH, and α‐melanotrophin (α‐MSH). These posttranslational events take place in different regions of the pituitary gland. The amino acid sequences cleaved are shown on the top line.

Figure 9.

Simplified scheme to illustrate the intracellular topographic distributions of corticosteroid‐forming events. Lipoproteins containing cholesterol enter the cell to be first taken up by mitochondria, which produce pregnenolone, which in turn moves into the smooth endoplasmic reticulum where 17‐α‐hydroxyprogesterone or progesterone is produced. These metabolites then re‐enter the mitochondria, which depending on the species and circumstances will generate aldosterone, cortisol or corticosterone for secretion. Full pathways are given in Figure 3. Equivalent intracellular processing and trafficking occurs in the production of gonadal steroids.


Further Reading

Baulieu E‐E and Kelly PA (eds) (1990) Hormones – From Molecules to Disease. New York: Chapman and Hall.

Bentley PJ (1998) Comparative Vertebrate Endocrinology, 3rd edn. Cambridge: Cambridge University Press.

Berne HA (1990) The ‘new’ endocrinology: its scope and its impact. American Zoologist 30: 877–885.

Bole‐Feysot C, Goffin V, Edery M, Binart N and Kelly PA (1998) Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in the PRL receptor knockout mouse. Endocrine Reviews 19: 225–268.

Gorbman A, Dickhoff WW, Vigna SR, Clark NB and Ralph CL (1983) Comparative Endocrinology. New York: Wiley.

Hadley ME (1992) Endocrinology. London: Prentice‐Hall International.

Henderson IW (1987) The expanding endocrine system. Journal of Endocrinology 115: 195–197.

Henderson IW (1997) Endocrinology of the vertebrates. Dantzler WH Handbook of Physiology, Section 13: Comparative Physiology, vol. 2, pp. 623–749. Oxford: Oxford University Press/New York: American Physiological Society

Matsumoto A and Ishii S (eds) (1992) Atlas of Endocrine Organs. London: Springer‐Verlag.

Niall HD (1982) The evolution of peptide hormones. Annual Review of Physiology 44: 615–624.

Norris DO (1996) Vertebrate Endocrinology, 3rd edn. London: Academic Press.

O’Malley B (1989) Did eucaryote steroid receptors evolve from intracrine regulators? Endocrinology 125: 1119–1120.

Wilson JD and Foster DW (eds) (1992) Williams Textbook of Endocrinology. London: WB Saunders.

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Henderson, Ian W(Apr 2001) Endocrine System in Vertebrates. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001845]