The Pituitary Gland and Its Function

The pituitary gland is the major gland with an internal secretion that governs the work of most other glands. Those are:

• Thyroid gland
• The adrenal glands
• Ovaries and testicles.

Without its stimulation, these glands would not secrete their hormones at all. In addition, the pituitary gland secretes some hormones that do not control the function of other glands but act directly on distant tissues and organs.

In this article, we will elaborate
on its anatomy, position, function, and disorders.

Anatomy and Position of the Pituitary Gland

The pituitary gland is a small gland, the size of a pea grain, located at the base of the brain, in the projection of the root of the nose. It consists of a larger anterior lobe (adenohypophysis) and a thinner posterior lobe (neurohypophysis) between which is a thin zone (the intermediate lobe) (1).

In general, the anatomy of the pituitary gland is not very complicated. The pituitary gland is an endocrine gland located in the eponymous pit of the so-called Turkish saddle orseat of the sphenoid bone. The entire surface of the gland is enveloped by a dura mater. In the upper part, it forms the pituitary diaphragm.

Just below the floor of the Turkish saddle is the sphenoid sinus, which provides a transphenoidal surgical approach that is performed when the gland tumor is removed. 

Lateral on either side of the pituitary pit is the cavernous sinus. The dorsal (upstream) of the gland is provided by the petiole, which connects the pituitary gland with the hypothalamus (1).

The pituitary gland and other sellar and suprasellar structures are located behind and above the visible intersection. Tumors in this region can press the optic chiasm, causing visible outbreaks including bitemporal hemminopsia.

It is connected to the hypothalamus
via a small “handle” or link. The pituitary and hypothalamus form unique areas
of the nervous system. They communicate via conventional synaptic transmission.
Both structures use neurotransmitters to transmit afferent and efferent
information. The pituitary gland and hypothalamus form the link between the
nervous and endocrine systems.

Hypothalamus is an evolutionarily old, basal part of the brain. The hypothalamus contains nuclei that synthesize pituitary hormone stimulators or inhibitors, which travel through the local bloodstream to the pituitary gland and in the pituitary gland stimulate or inhibit specific cells that secrete their hormones into the circulation.

At the hypothalamus level, many signals are integrated from other parts of the central nervous system and the autonomic nervous system, as well as from the periphery, and are then reversed by hormones of the peripheral glands with internal secretion (2).

Negative feedback at the hypothalamus and pituitary gland levels aligns incoming and outgoing signals and maintains an orderly endocrine function of the hypothalamic-pituitary-subordinate gland axis.

The pituitary gland is divided into
adenohypophysis and neurohypophysis, respectively, anterior and posterior lobe.
This division arises from the different embryological origin of the lobes.
Adenohypophysis comes from the ectoderm. Neurohypophysis originates from the
nerve tissue of the hypothalamus.

Pituitary Anatomy - Adenohypophysis

The anterior lobe forms the greater part of the gland. It is divided into anterior part (pars tuberalis), middle part (pars intermedia) and posterior part (pars distalis). When cells are treated with histological dyes, some of them are stained (chromatophilic) while others are not (chromatophobic).

Chromatophobic cells make up the majority of cells, and they represent the nonsecretory phase. Chromatophilic cells are divided into acidophilic (somatotropic, mammotropic) and basophilic cells (thyrotropic, gonadotropic, luteotropic) (1).

The intermediate part secretes the melanostimulating hormone. The release of the adenohypophysis hormone is controlled by the hypothalamus, via the portal bloodstream, releasing the so-called “releasing factors”. The negative feedback mechanism controls the secretion of hormones and maintains homeostasis.

Adenohypophysis hormones include the
growth hormone, thyrotropic hormone, adenocorticotropic hormone, gonadotropic
hormones (luteinizing hormone - LH and follicle stimulating hormone - FSH),
prolactin, and melanostimulating hormone.

Pituitary anatomy - Neurohypophysis

The neurohypophysis is the last lobe of the pituitary gland. It consists of eminence of the median, infundibulum, and pars posterior pituitary (1). It does not contain glandular cells. Instead, it contains axons originating from the hypothalamus.

The axonal terminals in neurohypophysis secrete two hormones, oxytocin and vasopressin (an antidiuretic hormone) into the circulation (3).

Pituitary gland diseases

Pituitary gland diseases are very rare. There are different causes of pituitary disease. Also, there are several different diseases in this part of the brain. Those are:

• Pituitary tumors (adenomas)
• Empty sella
• Genetic disorders
• Pituitary injury or pituitary handle
  (usually causes Diabetes insipidus)
• Bleeding in the pituitary gland
• Inflammatory (infiltrative)
  processes
• Autoimmune hypophysitis.

As the pituitary tumors or adenomas
are the most common, we will elaborate on these diseases in the following
section.

Pituitary gland tumors

Pituitary tumors are benign. Metastatic tumors are extremely rare. Typically, these are adenomas, benign tumors resulting from the cloning of a genetically modified pituitary cell. In the case of a secretory-active cell, such clonal growth may cause enhanced autonomic secretion of the hormone from the adenoma.

There is usually an increase in the secretion of only one hormone, although sometimes the secretion of two hormones is also possible. These cells are not exactly equal to the healthy ones. They do not respond to physiological stimuli, which is why they are autonomous in their function. In addition to secretory adenomas, some tumors are not secretory active.

Symptoms can be remote (systemic) caused by increased hormone secretion and their action on distant organs or local, caused by tumor compression on surrounding structures.

As the crossing of the optic nerve (chiasma nerve optics) takes place over the pituitary gland, in front of the pituitary grip, larger pituitary tumors protruding upwards can squeeze the optic nerves and cause visual field outages.

Lateral pressure on the surrounding blood vessels or nerves that control the function of the optic nerve is also possible, which may cause venous thrombosis of the cavernous sinus or a loss of function of some eye muscles.

Tumor pressure on the surrounding endocrine cells in the pituitary gland may cause a decrease in their function and a deficiency of certain pituitary hormones. In a state of compression, gonadotropin-secreting cells first fail to function, followed by ACTH and finally TSH.

It is the most common sequence of dysfunction and subordinate glands: first, the function of the sex glands, then the adrenal glands, and finally the thyroid gland function is reduced.

We distinguish between small (microroadenomas) and large (macroadenomas) pituitary tumors. Small adenomas are smaller than 1.0 cm and large ones are larger than 1.0 cm.

Adenomas can be functional (which potently and autonomously secrete pituitary hormones) and afunctional (which do not secrete pituitary hormones).

Functional pituitary adenomas are:

• Prolactin
• Gonadotropin
• Thyrotropin
• Somatotropin (Acromegaly)
• Corticotropin (Cushing's disease).

Treatment of Pituitary Gland Hormone Deficiency

The pituitary gland can secrete hormones in a decreased intensity in different conditions. All pituitary diseases, pituitary surgeries, and radiation can cause hormone deficiency. Regardless of the cause, we treat the pituitary hormone deficiency with a supplementary therapy that contains the missing hormones.

Replacement therapy exists for all hormones that can be missing. If multiple (several) hormones need to be introduced into the replacement therapy, then hydrocortisone (hormone of the adrenal cortex) is first introduced into therapy, followed by l-thyroxine (thyroid hormone) and finally, the sex hormones and growth hormone are introduced.

Thus, even in situations where there is no pituitary function at all, it is possible to achieve an orderly concentration of the missing hormones in the blood and to ensure a high quality of life and the same life expectancy as in healthy individuals.

Injury to the neurohypophysis leads to a lack of the antidiuretic hormone. The therapy uses desmopressin in the form of spray, tablets or injections (in hospital conditions), which can be successfully controlled by daily diuresis.

When it comes to suspected pituitary
diseases, it is important to carry out an adequate diagnostic procedure as soon
as possible, that is, to contact the endocrinologist who has experience in the
diagnosis and treatment of pituitary diseases in a timely manner.

Conclusion

The pituitary gland is a pea-sized gland located in bone tissue (Turkish saddle) at the base of the brain. The Turkish saddle protects the pituitary gland but allows very little room for expansion. If the pituitary gland enlarges, it pushes forward, often pressing on areas of the brain that carry signals from the eyes, which can result in headaches or impaired vision.

The pituitary gland controls the function of the most endocrine glands and is controlled by the hypothalamus, the area of the brain that lies just above the pituitary gland. The pituitary gland has two distinct parts: the anterior (frontal) and posterior lobes.

The hypothalamus controls the anterior lobe (adenohypophysis) by releasing factors or hormone-like substances through the blood vessels that directly connect them; the posterior lobe (neurohypophysis) is controlled by nerve impulses.

Finally, the pituitary gland produces a number of hormones, each affecting a specific part of the body (target organ). Due to the fact that the pituitary gland controls the function of most other endocrine glands, it is often referred to as the superior gland.

References

1. Daniel PM. Anatomy of the hypothalamus and pituitary gland. J Clin Pathol Suppl (Assoc Clin Pathol). 1976;7:1-7. doi: 10.1136/jcp.s1-7.1.1. PMID: 1073162; PMCID: PMC1436118. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1436118/
2. El Sayed SA, Fahmy MW, Schwartz J. Physiology, Pituitary Gland. [Updated 2019 Jul 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Found online at: https://www.ncbi.nlm.nih.gov/books/NBK459247/
3. Nussey S, Whitehead S. Endocrinology: An Integrated Approach. Oxford: BIOS Scientific Publishers; 2001. Chapter 7, The pituitary gland.  Found online at: https://www.ncbi.nlm.nih.gov/books/NBK27/