In this chapter, the endocrine system is introduced by describing mechanisms of hormone action and the types of hormones. Six groups of hormones and/or endocrine systems will then be discussed, which include: (1) hypothalamus, pituitary and pineal glands; (2) reproduction (puberty, menstrual cycle, pregnancy, lactation and menopause); (3) growth; (4) metabolism and the pancreas; (5) thyroid; (6) adrenal.
Mechanisms of Hormone Action and Second Messenger Systems
Cell Surface Receptors
Hormones may act in an autocrine (acting upon the cells that produced them), paracrine (acting on neighbouring cells) or endocrine manner (acting on cells at a distant site having been transported to that site in the blood or lymphatic system). In the circulation, some hormones such as steroids, insulin-related growth factors (IGFs) and thyroid hormones are bound to carrier proteins. Only the free hormone, the fraction of the total hormone level that is unbound, is active and available to bind to specific receptors to induce its effects. These receptors may be on the cell surface and have associated secondary messenger systems or may be nuclear with effects directly on the deoxyribonucleic acid (DNA) to alter messenger RNA (mRNA) levels (i.e. gene expression). At each receptor, a hormone may function as an agonist, a partial agonist or an antagonist. Furthermore, there is some ‘promiscuity’, with various hormones acting at multiple receptor types, as well as the complexity of hormone receptor crosstalk, which is beyond the scope of this chapter.
Peptide hormones and neurotransmitters act predominantly through cell surface receptors. These are divided into four main groups: (1) seven-transmembrane domain (luteinising hormone (LH), follicle stimulating hormone (FSH), thyroid-stimulating hormone (TSH), β-adrenergic, typically linked to G-protein second messenger system); (2) single transmembrane domain growth factor receptors (insulin, IGFs, linked to tyrosine kinase second messenger system); (3) cytokine receptors (cytokines, growth hormone (GH), prolactin); (4) guanylyl cyclase-linked receptors (natriuretic peptides related to guanyl cyclase second messenger system).
The seven-transmembrane receptors, as their name implies, loop in and out of the cytoplasm ( Fig. 11.1 ). The amino (-NH 3 ) terminus has the hormone binding domain and the carboxy terminus (-COOH), the G-protein transducer. There are multiple types of G-protein, which are heterotrimers made up of an α-, β- and γ-subunit ( Fig. 11.2 ). Each type (determined by the α-subunit) may relate to different receptors and be linked to different second messenger systems. For example, the β-adrenergic system is linked to the α s G-protein, which in turn is linked to adenylyl cyclase. β-adrenergic activation is thus associated with an increase in intracellular cyclic adenosine monophosphate (cAMP) (see Fig. 11.2 ). Each G-protein is made up of a guanosine diphosphate–guanosine triphosphate (GDP–GTP) binding domain (the α-subunit), and a β- and γ-subunit. In the absence of stimulation, the G-protein is bound to GDP. With receptor activation, the α-subunit binds to the receptor and dissociates from the GDP and the β- and γ-subunit. GTP then binds to the receptor-linked α-subunit, initiating its dissociation from the hormone–receptor complex. The activated G-protein then activates its second messenger system (e.g. adenylyl cyclase). The deactivated GDP–α-subunit complex re-associates with the β- and γ-subunit (see Fig. 11.2 ). The G-protein system can be manipulated experimentally by using agents such as cholera toxin, which prolongs activity of the α-subunit–GTP complex, or using the pertussis toxin, which uncouples the G-protein system and inhibits its activity.
As described earlier, adenylyl cyclase activation generates cAMP, which activates protein kinase A (PKA), leading to phosphorylation and activation of other intracellular proteins, such as the cyclic AMP response element binding protein (CREB). This protein mediates many of the transcriptional effects of cAMP. The Gα q is linked to phospholipase Cβ, an initiator of another second messenger system that, when activated, cleaves phosphoinositol 4,5-bisphosphonate, generating inositol 1,4,5-triphosphate (IP 3 ) and diacylglycerol (DAG). IP 3 acts via specific receptors to increase intracellular calcium and DAG activates protein kinase C. Activation of phospholipase A releases arachidonic acid, which is a precursor molecule for prostaglandins and leukotrienes.
Growth factor receptors span the cellular membrane once and are linked to tyrosine kinase. For instance, binding to the GH receptor initiates phosphorylation of the receptor itself and of tyrosines in other molecules, triggering a cascade of intracellular responses. The cytokine receptors, like the growth factor receptors, cross the cell membrane once; they are linked to the Janus Kinase/Signal Transducers and Activators of Transcription (JAK/STAT) pathway. Binding of a cytokine to its receptor activates JAK, which in turn activates docking sites for STAT that is activated and translocates to the nucleus where it alters gene expression. The pathway is controlled by a negative feedback loop involving Supressors of Cytokine Signalling (SOCs).
The guanylyl cyclase-linked receptors can be activated in three ways:
They may be activated by nitric oxide (NO) produced by nitric oxide synthase (NOS). NOS exists in either constitutive (endothelial (eNOS) or neuronal (nNOS)) or inducible (iNOS) forms. In the vasculature, agents such as acetylcholine or bradykinin bind to endothelial cell surface receptors and increase intracellular calcium, which enhances eNOS activity and increases NO production. Increased NO levels diffuse into the smooth muscle cell and activate soluble guanylyl cyclase; this in turn produces cyclic guanosine monophosphate (cGMP), which stimulates relaxation of the smooth muscle.
iNOS is present predominantly in immune cells but is also found in vascular smooth muscle cells. As its name implies, it can be induced by various hormones, leading to an increase in NO production and cGMP levels.
Guanylyl cyclase is also linked to peptide receptors and the ligand–receptor interaction activates it directly.
Peptide hormones also affect the transcription of genes through the activation of c-jun and c-fos (via kinases and phosphatases). These are nuclear transcription factors that bind to specific sites on the DNA to alter gene expression. cAMP activates PKA, which, as mentioned above, phosphorylates a number of proteins including the transcription factor CREB, which alters gene expression.
The nuclear receptor superfamily has a critical role in development, general physiology, fertility and disease. Several hormones act through nuclear receptors. These include steroid hormones (progesterone, oestrogen and androgens), thyroid hormones, retinoic acid and vitamin D. Some of the receptors exist principally in the cytoplasm (the ‘steroid’ family includes glucocorticoid, mineralocorticoid, androgen and progesterone receptors) or nucleus (the ‘thyroid’ family that includes oestrogen, retinoic acid and vitamin D receptors). However, independent of their location, when these hormones bind to their receptors, most will act in the nucleus to alter gene expression, though in recent years non-genomic effects of activation of nuclear receptors have been recognised. While this is worth noting, it is beyond the scope of this chapter. The different receptor groups are shown as an example in Fig. 11.3 .
Regarding the steroid family of NRs, these tend to exist in the cytoplasm as a complex with multiple protein coregulators, as well as heat shock protein (HSP). When the ligand binds with the receptor, HSP dissociates, revealing a nuclear translocation signal that initiates the transport of the hormone–receptor complex to the nucleus where it binds to the hormone response element to exert its effect (see Fig. 11.3 ). The DNA binding region has two zinc ‘fingers’; between the two zinc molecules lies the amino acid sequence which binds to the DNA. The thyroid family of nuclear receptors exists in the nucleus and, except for the oestrogen receptor, does not associate with HSP. They bind to DNA as dimers; for oestrogen this is a homodimer (i.e. two oestrogen receptor molecules) and for the other members of the family, as heterodimers formed between the receptor molecule and a retinoid X receptor.
Most hormones are peptides, and a peptide is made up of a chain of a variable number of amino acids. The precise sequence is determined by the DNA encoding the peptide. Therefore, peptide synthesis is initiated by the transcription of DNA into a specific mRNA. This passes from the nucleus into the cytoplasm, where it binds to ribosomes in the rough endoplasmic reticulum (RER) and is translated into the peptide sequence (illustrated by insulin in Fig. 11.4 ). This is usually in the form of a pre-pro-hormone, which is then cleaved to first form a pro-hormone and then the hormone itself. Some peptides are secreted immediately, while others are stored in secretory granules.
In terms of reproduction, this is the most important group of hormones. Steroid hormones are synthesised from cholesterol and all have the same basic ring structure consisting of 17 carbon atoms with different numbers of carbon atoms added. Glucocorticoids (stress and metabolism), aldosterone (fluid balance) and progesterone (reproduction) have 21 carbon atoms; testosterone and other androgens have 19 carbon atoms, while oestrogens have 18 ( Fig. 11.5 ). The synthetic pathways are the same in the ovary, testis and adrenal, but the dominant product varies from tissue to tissue ( Fig. 11.6 ). The pathway always starts from cholesterol, which is derived either from circulating LDL or from intracellular cholesterol esters.
Ovarian steroid production varies during the cycle. Overall, the ovary is the main source of circulating oestrogens, although peripheral conversion of androgens also makes a significant contribution in some situations. During the follicular phase of the cycle, the ovary produces oestrogens predominantly, and both oestrogen and progesterone in the luteal phase.
The adrenal cortex is divided into three zones, which can be remembered using the acronym GFR: (1) the outer zona glomerulosa (zG); (2) the middle zona fasciculata (zF), which consists of cells full of cholesterol and (3) the inner zona reticularis (zR). The zG is the site of aldosterone secretion. It is regulated by the control of the renin–angiotensin pathway, whereas last two are controlled by adrenocorticotrophic hormone (ACTH) and are concerned primarily with the secretion of cortisol and, to a lesser extent, adrenal androgens. More details will be given about each in their relevant sections.
The Leydig cells of the testis produce testosterone in response to LH. This circulates predominantly bound (97%) to sex hormone binding globulin (SHBG) and, to a lesser extent, to albumin. In some tissues, testosterone is active, but in others it has to be converted to dihydrotestosterone (DHT) by the enzyme 5α-reductase. Both testosterone and DHT bind to a cytoplasmic receptor before passing into the cell nucleus to bind to specific areas of DNA to produce their effect.
During pregnancy, the placenta synthesises and releases large amounts of progesterone into the maternal circulation. Pregnenolone is also released into the fetal circulation to be converted by the fetal adrenal into androgens, which pass back to the placenta to be aromatised to oestrogens and released into the maternal circulation ( Fig. 11.7 ).
Steroid Binding and Metabolism
In the circulation, all steroid hormones circulate bound to various proteins ( Table 11.1 ). Steroid hormone metabolism occurs in the liver. For example, oestradiol is converted to oestrone, which may re-enter the circulation, be further metabolised to oestrogens or conjugated to form oestrone sulphate, and excreted. Progesterone is converted to pregnanediol and conjugated to glucuronic acid, after which it is excreted as pregnanediol glucuronide. Androgens are metabolised and excreted predominantly as 17-oxosteroids (which used to be measured to assess androgen synthesis). Cortisol is mainly conjugated to glucuronide and excreted. Its metabolites can be measured in the urine in the form of 17-oxogenic steroids (not to be confused with the androgen metabolites, 17-oxosteroids), but this is rarely measured now, as cortisol can be measured in the urine directly.
|Plasma Conc. (nmol/L)
Amino Acid Hormones
Several hormones, thyroid (tyrosine), catecholamines (tyrosine) and melatonin (tryptophan) are derived from amino acids, and all are stored in granules. Their activities are regulated by their release and by the expression of the enzymes necessary for their synthesis.
Prostaglandins and Leukotrienes
Collectively known as the eicosanoids, these hormones are derived from arachidonic acid. Synthesis occurs in the cell wall and the hormones pass either into the cell cytoplasm or out of the cell ( Fig. 11.8 ).
Hypothalamus and Pituitary
The hypothalamus is at the centre of the different endocrine, autonomic and homeostatic mechanisms that maintain the body and allow it to reproduce. It directly controls the pituitary, which in turn controls the reproductive axis, lactation, growth, the thyroid, and adrenal glands.
The thalamus and the hypothalamus develop from the diencephalon, which with the telencephalon (which forms the cerebral hemispheres) forms the prosencephalon. Both the thalamus and the hypothalamus develop in the lateral walls of the diencephalon, the cavity that becomes the third ventricle ( Fig. 11.9 ).
The pituitary develops in close association with the hypothalamus and is made up of two parts: (1) the anterior or adenohypophysis and (2) the posterior or neurohypophysis. The anterior pituitary is formed from the ventral ridges of the primitive neural tube, which are pushed forward by the developing Rathke’s pouch ( Fig. 11.10A–C ). By 7 weeks, the sella floor has formed and the pituitary starts to develop under the influence of the hypothalamus. The posterior pituitary is formed by a downward evagination of the diencephalon called the infundibulum. Thus, the neurohypophysis is in direct contact with the hypothalamus, while the anterior pituitary is connected to the hypothalamus via the richly vascular portal system. The portal system carries all the hypothalamic hormones that regulate the function of the anterior pituitary ( Fig. 11.11 ). A small part of the anterior pituitary immediately opposed to the neurohypophysis becomes the intermediate lobe ( Fig. 11.12 ).
The thalamus lies superior to the hypothalamus, separated from it by the hypothalamic sulcus. Medially the third ventricle, superiorly the thalamus and inferiorly the pituitary stalk provides anatomical limits for the hypothalamus; the hypothalamus is without distinct boundaries anteriorly, posteriorly and laterally.
The pituitary lies within the sella turcica; the sphenoid sinus lies anteriorly and inferiorly, the cavernous sinus laterally (containing internal carotid arteries, and sixth cranial nerve), the clinoid processes posteriorly of the sphenoid bone (often eroded on skull x-rays in the presence of a pituitary tumour), and the pituitary stalk superiorly, which merges into the hypothalamus (see Fig. 11.12 ). The optic chiasma lies anterior to the pituitary stalk, which may be compressed by an expanding pituitary tumour, giving the typical presentation of bi-temporal hemianopia.
The hypothalamus, pituitary stalk and the pituitary are supplied by the carotid arteries via the superior and inferior hypophyseal arteries (see Fig. 11.11 ). The superior hypophyseal arteries form a primary plexus in the base of the hypothalamus in a region called the median eminence. The plexus forms into the portal vessels that pass on either side of the pituitary stalk to the anterior pituitary, where they form a secondary plexus. The nerves from the hypothalamic nuclei, which regulate anterior pituitary function, end close to primary plexus and release their regulatory hormones, which are taken up and carried via the portal vessels to the anterior pituitary. The posterior pituitary is supplied by the inferior hypophyseal artery.
The hypothalamus is made up of a series of nuclei arranged around the third ventricle. The nuclei consist of the cell bodies of neurones. In the case of the nuclei, which regulate the anterior pituitary, the axons pass to the area of the median eminence (see earlier). The axons of the paraventricular (situated in the lateral wall of the third ventricle) and the supraoptic nuclei (situated above the optic tract) pass down the pituitary stalk to the posterior pituitary. Both synthesise and release oxytocin and vasopressin ( Fig. 11.13 ).
The pituitary gland develops from two parts. The anterior pituitary is made up of a mixture of cells with different secretory properties divided into three groups, based on their staining with Haematoxylin and Eosin. The chromophobes (which do not stain) are thought to be resting cells, but chromophobe adenomas have been shown to secrete gonadotrophin subunits. The acidophils synthesise prolactin, GH and the basophils, which secrete the gonadotrophins, TSH and ACTH. The posterior pituitary is pale and consists of the nerve terminals of the paraventricular and the supraoptic nuclei. The axons are surrounded by glial cells called pituicytes, which regulate the rate of transmission and the crosstalk between neurones.
Table 11.2 summarises the hormones produced by the hypothalamus that regulate anterior pituitary function. More details are provided in the relevant sections further in this chapter.
|Stimulates LH and FSH release
|Anterior paraventricular nucleus
|Stimulates ACTH release
|Stimulates GH release
|Inhibits GH release
|Medial paraventricular nucleus
|Stimulates TSH release
|Inhibits prolactin release
Pituitary Gland Products
|Stimulates ovarian hormone synthesis and oocyte release
|Stimulates follicle maturation
|Stimulates thyroid hormone release
|Stimulates cortisol synthesis in the adrenal
|Stimulates hepatic IGF-II synthesis and release
|Lateral and superior paraventricular and supraoptic nuclei
|Stimulates contraction of the myoepithelial cells of the breast causing milk let-down, and of the uterine myocytes in labour
|Lateral and superior paraventricular and supraoptic nuclei
|Retains water by altering the permeability of the collecting ducts in the kidney; cardiovascular regulation; enhances CRH-stimulated ACTH release
The pineal gland lies in the roof of the third ventricle at the posterior end and it produces melatonin, with roles in the regulation of the ‘body clock’ and puberty. Pineal gland tumours are associated with symptoms and signs of a space-occupying lesion with deficiency of hypothalamic hormones or occasionally with precocious puberty. With age, the pineal gland calcifies and may be visible on an x-ray of the skull.
The reproductive axis is made up of the hypothalamus, pituitary and gonads. The embryology and anatomy of the reproductive tract are discussed elsewhere. The endocrine aspects are considered here.
Gonadotrophin-releasing hormone (GnRH) is synthesised in the pre-optic area of the hypothalamus and passes via the median eminence and the portal vessels to the anterior pituitary, where it stimulates the gonadotrophs to synthesise and release LH and FSH. It is released in a pulsatile manner. In females, the pulsatile release of GnRH varies with the phase of the cycle; it is released every 60 minutes during the follicular phase and every 90 minutes during the luteal phase. GnRH is synthesised from a 92-amino-acid pro-hormone, which is split into GnRH and a 56-amino-acid GnRH-associated peptide (GAP). The physiological role of GAP is unknown, but it has been shown to inhibit prolactin secretion.
The release of GnRH is modulated by opioid and catecholamine inputs. In recent years, the critical importance of the kisspeptin system has been shown to be key for normal GnRH secretion, and acts as an salient gatekeeper in the initiation of puberty, as well as the regulation of roles in reproductive function. The KISS1 gene, which encodes Kisspeptin, is highly expressed in the brain and other organs, including the placenta and kisspeptin signals via the G protein coupled receptor GPR54, which is expressed in multiple sites of the body ( Fig. 11.14 ).