Pituitary: Definition, Anatomy, Associated Disorders and Tumors in the Pituitary Gland

Also called the pituitary gland, it is the gland of the endocrine system that secretes hormones directly into the bloodstream.

The term pituitary comes from the Greek hipo (below) and fisis (to grow) refers to the position in which the gland is located with respect to the brain.

The pituitary gland is also called the ” master gland ” because its hormones regulate other important endocrine glands, including the adrenal, thyroid, and reproductive glands (eg, ovaries and testes).

At the same time, the pituitary gland also releases a ton of hormones that circulate through our system and help maintain our internal homeostasis and in some cases have direct regulatory effects on important tissues, such as those of the musculoskeletal system.

Anatomy

The pituitary gland is located approximately in the center of the human skull, it rests under the hypothalamus of the brain and behind the bridge of our noses.

Its configuration really makes sense, in light of the role of the hypothalamus with respect to the activity of the pituitary gland.

This is possible thanks to the nerve fibers that span these two structures and allow easy communication.

Similarly, a thin vascular connection that is forged within the pituitary stalk, or infundibulum, facilitates control of the hypothalamus.

In addition, the pituitary gland itself is supplied by branches outside the internal carotid artery.

Its regulation is tuned by a negative feedback relationship between the pituitary gland and the hypothalamus.

The pituitary gland is linked to the hypothalamus by a stem composed of neuronal axons and the so-called pituitary portal veins.

The pituitary gland is completely of ectodermal origin, but it is composed of 2 functionally distinct structures that differ in embryonic development and anatomy: the anterior pituitary gland (anterior pituitary gland) and the neurohypophysis (posterior pituitary gland).

Between the two lobes there is an intermediate lobe (intermediate pairs).

This intermediate lobe is almost non-existent in adults, although it can increase in size during pregnancy.

The anterior pituitary develops from Rathke’s pouch, which is an ascending intussusception of the oral ectoderm from the roof of the stommode.

In contrast, the neurohypophysis develops from the infundibulum, which is a downward extension of the neural ectoderm of the floor of the diencephalon.

The oral ectoderm and the neuronal ectoderm that form the pituitary installation are in close contact during early embryogenesis, and this connection is critical for the development of the pituitary gland.

The fully developed pituitary gland is the size of a pea and weighs approximately 0.5 g.

The anterior pituitary makes up approximately 80% of the pituitary gland and produces a series of peptide hormones.

The release of these pituitary hormones is mediated by hypothalamic neurohormones secreted by the median eminence (a site where the axon terminals emanate from the hypothalamus) and which reach the adenohypophysis through a portal venous system.

Unlike the anterior pituitary, the neurohypophysis is not glandular and does not synthesize hormones.

Instead, it is a site where axons are projected from neuronal cell bodies in the supra-optic and paraventricular nuclei of the hypothalamus.

These hypothalamic cell bodies produce hormones that undergo axonal transport through the pituitary stalk and terminal axons within the neurohypophysis.

The hormones are stored and released directly into the systemic vasculature.

The pituitary gland is surrounded by the dura mater and is located inside the sella turcica of the sphenoid bone.

The Turkish saddle is a saddle-shaped depression that surrounds the inferior, anterior and posterior aspects of the pituitary gland.

The upper aspect of the pituitary is covered by the diaphragm of the sella turcica, which is a fold of the dura mater that separates the subarachnoid space filled with cerebrospinal fluid from the pituitary gland.

The infundibulum perforates the diaphragm of the chair to connect the pituitary gland to the hypothalamus.

The blood supply to the hypothalamus and pituitary gland is derived from the circle of Willis at the base of the brain.

The most important blood supply comes from the superior pituitary arteries that arise from the internal carotids.

The blood from this artery enters a primary capillary plexus in the middle eminence. The blood from this plexus flows into the long portal vessels to a secondary capillary plexus in the adenohypophysis.

The portal vessels run through the pituitary stalk (infundibulum) to reach the pituitary gland. This system is known as the hypothalamic-pituitary portal system.

Anterior pituitary

The anterior pituitary contains five types of endocrine cells, and they are defined by the hormones they secrete: somatotrophs (GH), lactotropes (PRL), gonadotrophs (LH and FSH), corticotropes (ACTH) and thyrotropes (TSH).

Thyrotropes synthesize and secrete thyrotropin (thyroid stimulating hormone, TSH); gonadotrophs, luteinizing hormone (LH) and follicle-stimulating hormone (FSH); corticotropes, adrenocorticotropic hormone (ACTH, corticotropin); somatotropes, growth hormone (GH, somatotropin) and lactotropin, prolactin.

It also contains follicular non-endocrine cells that are believed to stimulate endocrine cell populations.

The hormones secreted by the anterior pituitary are tropical trophic hormones.

Trophic hormones directly affect growth, whether hyperplasia or hypertrophy in the tissue that stimulates.

Tropical hormones are named for their ability to act directly on target tissues or other endocrine glands that release hormones, which causes numerous physiological cascade responses.

In addition to hypothalamic control of the anterior pituitary it has been shown that other systems in the body regulate the function of the anterior pituitary.

The γ-aminobutyric acid (GABA) can stimulate or inhibit the secretion of luteinizing hormone (LH) and growth hormone (GH) and can stimulate the secretion of thyroid stimulating hormone (TSH).

It is now known that prostaglandins inhibit adrenocorticotropic hormone (ACTH) and also stimulate the release of TSH, GH and LH.

The homeostatic maintenance of the anterior pituitary is crucial for our physiological well-being.

The increase in plasma TSH levels induces hyperthermia through a mechanism that involves increased metabolism and cutaneous vasodilation.

The increase in LH levels also causes hypothermia , but through a decrease in the action of metabolism.

ACTH increases the metabolism and induces cutaneous vasoconstriction, the increase in plasma levels also results in hyperthermia and prolactin decreases with decreasing temperature values.

The follicle-stimulating hormone (FSH) can also cause hypothermia if it increases beyond the homeostatic levels through a greater metabolic mechanism.

The gonadotrophs, mainly luteinizing hormone (LH) secreted by the anterior pituitary gland, stimulate the ovulation cycle in mammalian females, while in males, LH stimulates the synthesis of androgens, which drives the continuous willingness to mate together with a constant production of sperm

The anterior pituitary plays a role in the stress response.

The corticotropin-releasing hormone (CRH) of the hypothalamus stimulates the release of ACTH in a cascade effect that ends with the production of glucocorticoids in the adrenal cortex.

Posterior pituitary:

The posterior pituitary is not glandular, as is the anterior pituitary gland.

Instead, it is largely a collection of axonal projections of the hypothalamus that ends behind the anterior pituitary.

Its function concentrates in secreting hormones.

Two hormones are considered classically related to the posterior pituitary: oxytocin and vasopressin.

These hormones are created in the hypothalamus and are released in the posterior pituitary gland.

After creation, they are stored in neurosecretory vesicles regrouped in bodies of herring before being secreted in the posterior pituitary gland through the bloodstream.

The hypothalamic-neurohypophysial system is composed of the hypothalamus (the paraventricular nucleus and the supraoptic nucleus), the posterior pituitary and these axonal projections.

These axons store and release the neurohypophyseal hormones oxytocin and vasopressin in the neurohypophyseal capillaries, from there they enter the circulatory system (and partly back to the pituitary portal system).

In addition to axons, the posterior pituitary also contains pituicytes, specialized glial cells that resemble astrocytes that help store and release hormones.

The classification of the posterior pituitary gland varies, but most sources include the following two regions:

  • Nervous pair: also called neural lobe or posterior lobe, this region constitutes the majority of the posterior pituitary and is the storage site of oxytocin and vasopressin.
  • Stem infundibular:  also known as infundibulum or pituitary stalk, the infundibular stem joins the hypothalamic and pituitary systems.

Disorders of the pituitary gland

Deficiencies in any of the hormones mentioned above can cause diseases, which vary in severity.

Starting with the posterior lobe, an ADH deficiency will increase our thirst and urination.

The lack of prolactin will inevitably lead to an inability to breastfeed, which to this day can not be treated.

TSH deficiency has symptoms similar to those of a compromised thyroid gland, including fatigue, memory loss, and body weakness.

The lack of LH or FSH will result in a decrease in libido , irregular menstruation, erectile dysfunction and mood swings.

ACTH deficiency will cause nausea, body aches, lack of appetite and even low blood sugar and blood pressure.

Insufficient secretion of vasopressin underlies diabetes insipidus, a condition in which the body loses the ability to concentrate urine.

The affected people excrete up to 20 liters of diluted urine per day.

Finally, poor growth hormone will reduce muscle mass and bone density, which has long-term ramifications in our quality of life.

An overproduction of hormones has its own consequences.

Too much growth hormone can cause gigantism and acromegaly, or an overgrowth of bones and soft tissues that can cause heart problems and sleep apnea .

Hypersecretion of vasopressin causes inappropriate antidiuretic hormone syndrome

Too much TSH will cause tremors, irritability and high blood pressure .

Too much prolactin will cause an inappropriate expression of breast milk that can occur in women or men, and a weakening of the bones.

Excess ACTH will cause weight gain between brittle bones and mood instability.

Finally, excess FSH and LH are related to infertility and irregular menstruation.

However, the most common type of disorder of the pituitary gland are tumors.

The vast majority of pituitary tumors are benign, or simply a noncancerous inflammation in the gland that may not cause any symptoms and may never be symptomatic.

Unlike many types of tumors, most people affected by pituitary tumors have no previous family history of problems with the pituitary gland and are not regularly genetically inherited.

One of these exceptions is multiple endocrine neoplasia (MEN), which is a group of inherited disorders that lead the endocrine glands of the body, including the pituitary gland, to overexpress hormones.

But tumors of the pituitary gland, specifically, still remain benign.

Tumors in the pituitary gland

There are several types of pituitary tumors.

In general, people with a tumor of the pituitary gland will experience a series of telltale symptoms.

Most will have vision problems, headaches, menstrual changes, infertility, mood swings, fatigue and even Cushing’s syndrome , which has its own set of symptoms that include, among others, high blood pressure and weight gain due to the excessive release of ACTH.

The most common type of pituitary tumor is called “nonfunctional” tumor. The name derives from its inability to produce hormones.

These patients will have problems with their vision and headaches.

In addition, pituitary tumors can be divided into three groups according to their problematic actions:

  • Hypersecretion: refers to the manufacture of too much hormone, and this is a problem affected by a secretory pituitary tumor.
  • Hypnosis: is a very low production of hormones and is usually caused by a large pituitary tumor that will physically block the pituitary gland so that it does not produce hormones. It can also be the result of surgical resection of a tumor.
  • The effects of tumor mass: are the problems that arise from a tumor of the growing pituitary gland that presses against the pituitary gland and can cause vision and compromised headaches.

Other pituitary conditions that are worth mentioning include craniopharyngioma .

This is a type of cyst or tumor that is congenital, which means that it is present at birth.

It can swell and fill with fluid, and can cause headaches and vision problems, as well as sleep problems.

The ESS (for its acronym in English), or empty sella syndrome, is a disorder that arises from an affliction in the bone structure that surrounds the brain and surrounds the pituitary gland.

A primary ESS will be a small defect that will result in a high pressure on the bone base that causes the gland to collapse.

This is related to high blood pressure and obesity in women.

On the other hand, secondary ESS will be the result of a surgery or injury that has caused the regression of the pituitary gland.

The symptoms will be related to the loss of pituitary function, such as infertility and fatigue.

Pituitary tumors have not been related to any known external risk factor.

As a result, there is no known way to prevent these tumors at this time.

Even so, for people at high risk of pituitary tumors (due to certain inherited syndromes), there may be ways to find and treat them early, before they cause problems.