Video: Pituitary gland

Ladies and gentlemen, welcome to a night of the ultimate endocrine symphony. The star of tonight's show is none other than the lead conductor, acclaimed far and wide, as the master gland of the endocrine orchestra – the pituitary gland. Get ready for a spectacular performance where the pituitary gland guides his symphony of hormones, creating a 'hormone-ous' melody that governs the rhythm of our bodily functions.

All right, let's find out more about how the pituitary gland does this 'symphony of hormones' as we unravel the secrets of the pituitary gland.

The pituitary gland is a small, ovoid-shaped structure, found on the underside or ventral aspect of the cerebrum and is slightly larger than the size of a pea. It is suspended by what's known as the infundibulum, or stalk of the pituitary gland, which aptly matches the other name for the pituitary gland, the hypophysis. Hypophysis literally means undergrowth or outgrowth on the underside.

The pituitary gland is lodged within a bony depression known as the hypophyseal fossa, which is surrounded by a structure known as the sella turcica, a bony eminence located on the body of the sphenoid bone. The name sella turcica literally means 'Turkish saddle', which aptly describes its shape.

In terms of its function, the pituitary gland is often referred to as the master gland of the endocrine system due to the fact that it serves the primary endocrine interface between the central nervous system and the rest of the body. This means it controls many vital functions in the body related to metabolism, growth, sexual maturation, reproduction, blood pressure, and many other physical functions and processes.

The pituitary gland is made up of two functional lobes – the anterior lobe, also known as the adenohypophysis, and the posterior lobe, also known as the neurohypophysis. Now, as much as these two lobes sit together like two peas in a pod, they have different embryological origins which result in them having different functions. Let's take a look at these lobes in a bit more detail, starting off with the anterior lobe or adenohypophysis.

The embryological origin of this adenohypophysis is from an outpouching of the roof of the oral cavity known as the adenohypophyseal pouch, or Rathke's pouch. It is derived from oral ectoderm, meaning it has a different origin from most of the rest of the nervous system.

The adenohypophysis comprises three parts. The pars distalis, also known as the anterior or glandular part, is the largest component and is responsible for most of the secretory activity of the adenohypophysis. The pars tuberalis, or infundibular part, is a posterior extension of the anterior lobe that extends into the infundibular stalk that we saw earlier. Lastly is the pars intermedia, a thin epithelial layer and avascular zone which borders the posterior lobe.

The adenohypophysis produces and secrete several different endocrine hormones which it releases into the bloodstream to act on target cells and perform various functions around the body and this glandular function is why it has the name adenohypophysis.

Taking a look at the hormones of the adenohypophysis, first up we have growth hormone, or somatotropin, and its target is tissues like your liver, bones, and muscles which are in turn induced to release growth factors which as their name suggests stimulates growth and increases metabolism rate. Next, we have prolactin which targets the mammary glands and promotes the production of breast milk.

The pituitary gland also releases hormones targeted towards the reproductive system. Follicle-stimulating hormone, commonly known as FSH, targets the gonads and regulates the production of sperm cells in males and the maturation of ovarian follicles in females. Luteinizing hormone similarly leads to the release of sex hormones like estrogen and testosterone by the gonads and it triggers the release of an egg from the ovary. Thyroid-stimulating hormone targets the thyroid gland and leads to the release of thyroid hormone.

Adrenocorticotropic hormone, also known as corticotropin, targets the suprarenal glands and leads to the production of glucocorticoids which regulate various metabolic, immunologic, and homeostatic processes.

Now as we can see, the pituitary gland has mastery in orchestrating a multitude of functions within the body. Yet, you might wonder how does this petite gland, no larger than two peas in a pod, commands such control? How does it know when to release each hormone or when to stop? Well, not to expose the pituitary gland, but it turns out, it actually has a higher authority telling it what to do. Its boss is the hypothalamus.

The hypothalamus is the main regulator of the adenohypophyseal or anterior pituitary activity. It monitors and responds to changes in things like body temperature, energy needs, or electrolyte balance. This monitoring and control happens through two main pathways between the hypothalamus and the pituitary gland. One is a vascular pathway and the other is a neural one. Let's take a closer look at how the hypothalamus and pituitary gland connect.

The hypothalamus contains neurosecretory neurons which produce neurohormones in their cell bodies. These cell bodies are grouped together in clusters called nuclei. There are two primary nuclei that relate to the adenohypophysis – the paraventricular nucleus and the arcuate nucleus.

These nuclei are involved in producing several neurohormones – thyrotropin-releasing hormone, corticotropin-releasing hormone, gonadotropin-releasing hormone; growth hormone-releasing hormone, also known as somatocrinin; growth hormone inhibiting hormone, also known as somatostatin; and finally, dopamine which, in this context, is sometimes referred to as prolactin-inhibiting hormone, or prolactostatin.

So once the hormones are released from the cell body, they travel down the axons of these neurons into the median eminence, which is located at the base of the hypothalamus. It is the site into which the hypothalamic-releasing hormones are released into the primary capillary network of the hypophyseal portal system.

Now this capillary bed is formed by branches of the superior hypophyseal artery which arises from the internal carotid artery. It comprises many tiny fenestrated capillaries which merge to form larger blood vessels known as the long and short hypophyseal portal veins which then travel through the infundibulum to the secondary capillary network of the hypophyseal portal system in the anterior pituitary.

When we have a system in which veins drain blood from one capillary bed to another, we call this a venous portal system. So this pathway between the hypothalamus and anterior pituitary is known as the vascular hypothalamo-hypophyseal pathway, or the hypophyseal portal system. Remember that this vascular pathway holds significant importance as it serves the direct link between the release site of hypothalamic hormones, specifically the median eminence, and the glandular cells of the adenohypophysis.

When hypothalamic hormones are released from the secondary capillary network, they act on the glandular cells of the adenohypophysis to either stimulate or inhibit the release hormones. These are then carried via hypophyseal veins to the cavernous sinus and onwards through the systemic circulation.

Now let's take a look at the other pea in the pod – the posterior lobe of the pituitary gland, or the neurohypophysis.

The embryological origin of this neurohypophysis, or posterior lobe, is neural ectoderm, which also gives rise to the rest of the central nervous system. The neurohypophysis is a diencephalic structure and is essentially an extension of neural tissue of the hypothalamus, which is why it gets its name neurohypophysis.

It can be divided into two main parts – the superior portion, also known as the infundibular stalk, and the inferior portion, known as the pars nervosa. Just like the adenohypophysis, the neurohypophysis also shares a connection with the hypothalamus.

The infundibular stalk is continuous with the median eminence of the hypothalamus. It's composed of thousands of unmyelinated axons of neurons whose cell bodies are located mainly in the supraoptic and paraventricular nuclei of the hypothalamus. These axons collectively form the hypothalamo-hypophyseal tract which serve to connect the hypothalamus to the pituitary gland, this time as a neural hypothalamo-hypophyseal pathway. Vesicles containing neurohormones are transported directly from the hypothalamus to the neurohypophysis.

The pars nervosa, or the neural lobe, is the larger portion of the neurohypophysis. It mainly consists of the terminal endings of the hypothalamo-hypophyseal neurons which travel from the hypothalamus via the infundibular stalk. Unlike the anterior lobe, the neurohypophysis does not contain any glandular tissue. This means that it does not actively produce hormones itself. Instead, its main function is to store and release two hormones – antidiuretic hormone, also known as vasopressin, and oxytocin – which are both produced in neuronal cell bodies in the hypothalamus.

These hormones travel to the neurohypophysis in vesicles via the axons of these neurons. The hormones are then stored in the terminal endings of the neurons and are released into the bloodstream when triggered by action potentials traveling down the axons. The capillary bed, which receives these hormones, is formed by the branches of the inferior hypophyseal artery, which are drained by the hypophyseal veins to the cavernous sinus and onwards through the systemic circulation.

Antidiuretic hormone is released to target your kidneys and helps regulate water balance in the body by increasing water reabsorption and thereby reducing urine output. It also functions to maintain adequate blood pressure by means of constriction of the blood vessels, also known as vasoconstriction.

Oxytocin targets social and reproductive behavior and is best known for its role in the rhythmic contraction of the uterus during childbirth as well as the letdown or milk ejection reflex in breastfeeding. It's also often referred to as the love hormone due to its role in the promotion of emotions of related bonding and attachment and other behaviors related to nurturing and caregiving.

We're done for today but before you dash off, consider this: Much like a captivating performance, our exploration into the realm of the pituitary gland is just the opening act. To delve even deeper into the symphony of hormones orchestrated by this master gland, make sure to check out our study units and articles. Then, for the grand finale, put your understanding to the test with our quizzes.

Until next time. Let the endocrine symphony play on.