The Pituitary Gland: The Conductor of The Endocrine Orchestra

 

The Pituitary Gland: The Conductor of the Endocrine Orchestra

I have studied physiology, medicine and music, among many other disciplines of sciences with music, my odd combination as a hobby. I have played the violin in an orchestra. One of the most important leader in an orchestra, besides the first and lead violinist sitting in the first row on the right of a symphony orchestra, is the conductor who leads the orchestra into harmony.

Suddenly I thought I should write an article in medicine or in physiology on the pituitary gland as the Conductor of the Endocrine Orchestra where I can combine music and medicine (endocrinology) together in harmony.

Before that, let us imagine ourselves sitting in a very huge music hall where all sorts of people in the audience make up all the cells, tissues, organs and systems of the body and the orchestra in front is the master organ that drives the audience into a harmonious silence as the conductor leads the music of life in the body.

Medicine, nevertheless, is a very huge and a very complex subject and is divided into so many branches, sub-branches and sub-sub-branches where information on medicine would occupy entire national medical libraries in this world. Naturally it is well outside the scope of my blog here to discuss all the information we know in medicine.

Even at the medical library of the Royal Society of Medicine in London where I have been admitted as a Fellow since 1993, a medical library there is the biggest in Europe,  storing untold millions of documents on medicine dating back 5,000 years, and we have to retrieve information electronically, as it is sheer impossible to search published papers and scientific journal manually.  

So we shall not attempt to write even a few short reviews on other aspects of medicine and their discoveries but we shall only concentrate on just a pea-size organ called the pituitary gland that sits at the base of the brain that is able to coordinate all the hormones working together in sympathy with each other.

This area in medicine is called neuroendocrinology. We shall limit ourselves only to briefly discuss its functions that fascinated me since student’s time when I first started to learn physiology as part of medicine till I was involved in medical research at MIT in the US, and at the Institute for Medical Research in my home country in Malaysia.  

Let’s very briefly discuss only this fascinating little organ.

Function and Anatomy of the Pituitary Gland:

 

The posterior pituitary technically called neurohypophysis consists of the core of the pituitary stalk, the infundibular stalk which is an extension of the median eminence, and the posterior (neural) lobe. Large (magnocellular) neuroendocrine calls located in the bilaterally paired supraoptic and paraventricular nuclei of the hypothalamus send their axons (nerve fibres) through the median eminence and infundibulum to terminate in the posterior lobe. The terminals have large secretory vesicles which release the peptide hormones oxytocin and vasopressin which is the antidiuretic hormone (ADH) into fenestrated capillaries. The axons of these neuroendocrine cells do not contain ribosomes; the peptide hormones are synthesized in their cell bodies and move by axonal transport into the terminals. Hence, the posterior pituitary is the site of the hormone secretion, but not of synthesis.

Anterior Pituitary

The anterior pituitary or adenohypophysis is largely the anterior lobe or alternatively called pars distalis. The anterior lobe contains several populations of polygonal-shaped secretary cells. The blood supply of the anterior pituitary, the hypothalamic-pituitary portal system, is remarkable. The superior hypophyseal arteries form a primary capillary bed in the median eminence. This drains into the long portal vessels which run through the pituitary stalk, giving rise to a secondary capillary bed in the anterior lob. Small parvocellular neuroendocrine cells in several hypothalamic nuclei send axons to the median eminence where they secrete hypophysiotropic hormones. These are conveyed  by the hypothalamic-pituitary portal system into the anterior lobe to regulate its secretion. Cells in the anterior pituitary are specialized to synthesize and secrete particular hormones, and can be distinguished by difference in histological staining.

 

Hypophysiotropic and Anterior Pituitary Hormones:

hypophysiotropic hormones produced by the hypothalamus control five separate endocrine axes by either stimulating or inhibiting the synthesis and secretion of hormones by the anterior pituitary. All bar one are peptides. The exception is dopamine, which is a catecholamine. Anterior pituitary hormones in turn regulate the synthesis and release of hormones from a variety of other, more peripheral target organs. The five hypothalamic-pituitary axes are:

1.       Hypothalamic-pituitary-gonadal axis: gonadotropin-releasing hormone (GnRH) from the anterior pituitary regulates the synthesis and release of follicle-stimulating hormone (FSH) and the luteinising hormone (LH). Together, FSH and LH are termed gonadotropins. FSH and LH act on the gonads to regulate the secretion of testosterone in males, and oestrogens and progesterone in females.

2.       Growth hormone axis: GHRH stimulates the synthesis and release of growth hormone by the anterior pituitary. Growth hormone has a tropic effect on the liver, which produces insulin-like growth factor (IGF), but it also has a direct action on tissues, such as bones and muscles to stimulate growth. Somatostatin inhibits the synthesis of growth hormone.

3.       Prolactin axis: prolactin, secreted by the anterior pituitary gland, has a direct effect on breast tissue, stimulating lactation (milk production). This hormone is mainly under inhibitory control by dopamine from the hypothalamus.

4.       Hypothalamic-pituitary-thyroid axis: thyrotropin-releasing hormone (TRH) stimulates the synthesis and release  of thyroid-stimulating hormone (TSH) by the anterior pituitary gland. TSH stimulates  its target organ, the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Somatostatin, secreted by the pancreas, inhibits TSH secretion.

5.       Hypothalamic-pituitary-adrenal axis: corticotropin-releasing hormone (CRH) and vasopressin stimulates the biosynthesis and release of adrenocorticotropic hormone (ACTH), which targets the adrenal cortex to secrete the glucocorticoid cortisol

 

Posterior Pituitary Hormone:

The hormones of the posterior pituitary gland are oxytocin and vasopressin. The paraventricular and supra-optic nuclei of the hypothalamus synthesise and package these hormones, which are then carried by the axons of the neuro-secretory neurons to the posterior pituitary. The hormones are secreted into the circulation by the posterior pituitary. Oxytocin and vasopressin are both peptides.

1.       Oxytocin acts on the smooth muscles of the uterus to maintain labour (parturition) and the lactating breast to eject milk.

2.       Vasopressin acts on vascular smooth muscle and on renal collecting tubules where it functions to promote water reabsorption and so is critical for water homeostasis.

Disorders of Hypothalamic-Pituitary Axes:

Defects in endocrine systems controlled by the hypothalamus and pituitary are classified by the locus of the fault:

1.       Primary endocrine disorders are when the problem occurs in the target organ, such as gonads, adrenal or thyroid gland.

2.       Secondary endocrine disorders result from a defect of the pituitary.

3.       Tertiary disorders are due to defects in the hypothalamus.

Disorders of hypothalamic-pituitary axes may be selective or multiple and result in excess or deficiency of hormone production. Excess pituitary hormone secretion can be due to enhanced hypophysiotropic hormone secretion (tertiary) or to the pituitary tumour. Prolactin-secreting microadenomas are the commonest pituitary tumours. Pituitary hormones can also be secreted by tumours outside the pituitary, for example, some bronchial carcinoma produce prolactin. This is termed as ectopic secretion. Selective deficiencies are typically genetic or due to autoimmune disease. For instance, one type of dwarfism is caused by a defect in expression of the growth hormone receptor.

Multiple loss of anterior pituitary hormones (panhypopituitarism) occur as a result of any agent that destroys the pituitary, including tumours, infections, trauma, infarction, radiation, and the condition reflect the combined loss of secretion by gonads, adrenals and thyroid, and of growth hormones .

 Here is the list of disorders associated with hormonal deficiencies or hormonal excesses:

Hormones Deficiencies / Excess and their Disorders:

 PRL: None / infertility and galactorrhoea in both sexes

TSH: hypothyroidism: child, cretinism, adult, myxoedema / hyperthyroidism (Graves’ disease)

GH: Child, dwarfism / Child, gigantism / Adult, acromegaly

FSH / LH: hypogonadism, infertility in both sexes / None

ACTH: Addison’s disease / Cushing’s disease

Vasopressin: Diabetes insipidus / hypertension

Oxytocin: none reported / none reported

All hormones: Nil reported for deficiency / Panhypopituitarism (empty sella syndrome for excesses)