What is a hormone?

A hormone is a substance released by a cell, gland or organ into the blood, allowing it to exert its signalling effects on tissues elsewhere in the body.

What types of hormone exist?

Hormones are classified on the basis of their chemical structure:

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  Peptide hormones, the most common type, may be subclassified as:

  
  
  
  
  1. 
     

     – Short peptide chains; for example, thyrotropin-releasing hormone (TRH), antidiuretic hormone (ADH), adrenocorticotrophic hormone (ACTH) and insulin;

     
     
  2. 
     

     – Longer protein chains; for example, growth hormone (GH) and prolactin (PRL);

     
     
  3. 
     

     – Glycopeptides: a protein chain with carbohydrate groups attached; for example, luteinising hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH).

  
  
  
  

  In general, peptide hormones are stored in granules and are released into the circulation by exocytosis. Once they reach their target tissue, peptide hormones exert their effects by binding to cell surface receptors.

  
  
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  Lipid- and phospholipid-derived hormones, of which there are two main subtypes:

  
  
  
  
  1. 
     

     – Steroid hormones; for example, aldosterone, testosterone, oestrogen and cortisol;

     
     
  2. 
     

     – Eicosanoids; for example, prostaglandins, thromboxanes and leukotrienes.

  
  
  
  

  The steroid hormones are all derived from cholesterol, whilst the eicosanoids are derived from the phospholipid bilayer of cell membranes. Both types of lipid-derived hormone are synthesised as required and immediately released into the circulation; neither is stored. The high lipid solubility of steroid hormones allows them to diffuse across target cell membranes, where they exert their effects by binding to cytosolic receptors. The steroid hormone–receptor complex then travels to the cell nucleus, where it influences gene transcription. The eicosanoids have a wide range of functions in the body and their mechanisms of action are complex.

  
  
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  Monoamine derivatives; that is, hormones derived from a single amino acid. For example:

  
  
  
  
  1. 
     

     – Catecholamines are synthesised from phenylalanine or tyrosine.

     
     
  2. 
     

     – Serotonin is derived from tryptophan.

     
     
  3. 
     

     – Thyroxine is derived from tyrosine.

  
  
  
  

  The monoamine-derived hormones behave very differently:

  
  
  
  
  1. 
     

     – Catecholamines and serotonin are stored in granules prior to release, whilst thyroxine is incorporated within thyroglobulin (see Chapter 81).

     
     
  2. 
     

     – Catecholamines and serotonin exert their effects at the target tissue through cell membrane receptors, whilst thyroxine binds to receptors at the cell nucleus.

  
  

What are the functions of the hypothalamus?

The hypothalamus is located below the thalamus, making up the ventral part of the diencephalon. Though relatively small, the hypothalamus exerts control over a large number of body functions, acting as the link between brain, autonomic nervous system and endocrine system. The functions of the hypothalamus may be classified as:

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  Autonomic. The hypothalamus receives inputs from the limbic system and relays them to the medulla oblongata. Thus, emotional stress (e.g. fear) triggers a sympathetic nervous system response.

  
  
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  Thermoregulation. The hypothalamus integrates signals from peripheral and central (hypothalamic) thermoreceptors and controls the balance of activities of the two hypothalamic centres: the heat loss centre and the heat gain centre (see Chapter 89).

  
  
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  Regulation of hunger. Food intake is controlled through the relative activities of the hypothalamic feeding and satiety centres. These centres are influenced by hypothalamic glucose concentration, gastrointestinal hormones (cholecystokinin and glucagon) and leptin (a hormone released from adipose tissue).

  
  
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  Regulation of body water. As discussed in Chapter 69, the hypothalamus regulates body water through two mechanisms:

  
  
  
  
  1. 
     

     – The thirst centre controls water intake.

     
     
  2. 
     

     – Osmoreceptors control renal water excretion, in conjunction with ADH secretion by the pituitary gland.

  
  
  
  
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  Control of sleep–wake cycles. Stimulation of the anterior hypothalamus leads to sleep, whilst stimulation of the posterior hypothalamus causes wakefulness. Circadian rhythms are thought to originate in the hypothalamus.

  
  
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  Control of pituitary function. The hypothalamus exerts control over the pituitary gland through two mechanisms:

  
  
  
  
  1. 
     

     – The anterior lobe is controlled by the secretion of hypothalamic hormones into the long portal vein.

     
     
  2. 
     

     – The posterior lobe is controlled by direct neural connections from the hypothalamus.

  
  
  
  
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  Behaviour. The hypothalamus contains ‘punishment’ and ‘reward’ centres, which moderate behaviour.

  
  
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  Regulation of sexual function. The hypothalamus controls the pulsatile release of gonadotropins and the surge of gonadotropins that leads to ovulation.

Describe the anatomy of the pituitary gland

The pituitary gland is a pea-sized gland located in the sella turcica, a depression in the sphenoid bone at the base of the skull. The pituitary gland is situated directly below the hypothalamus, to which it is connected by the pituitary stalk. The pituitary gland is almost entirely covered superiorly by a fold of dura mater called the diaphragma sella; a gap allows the pituitary stalk to pass through.

The pituitary gland lies close to some key structures:

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  Superiorly, the pituitary stalk, optic chiasm and third ventricle;

  
  
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  Laterally, the cavernous sinus, which contains cranial nerves III, IV, VI, V₁ and V₂, and the internal carotid artery (ICA).

The pituitary gland itself comprises two main lobes, anterior (larger) and posterior, which are separated by a small pars intermedia. These lobes have different embryological origins:

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  The anterior lobe, or adenohypophysis, develops from a depression of oral ectoderm in the embryo’s pharynx, known as Rathke’s pouch.

  
  
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  The posterior lobe, or neurohypophysis, develops from a downgrowth of neural ectoderm from the hypothalamus. The posterior lobe never separates from the hypothalamus; the downgrowth persists as the pituitary stalk.

  
  
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  The pars intermedia is a very thin layer of cells located between the anterior and posterior lobes that also develops from Rathke’s pouch. It is often considered to be part of the anterior lobe and is not well developed in humans.