Technical Discussion

The hypothalamus is a region of the brain that controls an immense number of bodily functions. It is located in the middle of the base of the brain, and encapsulates the ventral portion of the third ventricle.

The pituitary gland, also known as the hypophysis, is a roundish organ that lies immediately beneath the hypothalamus, resting in a depression of the base of the skull called the sella turcica ("Turkish saddle"). In an adult human or sheep, the pituitary is roughly the size and shape of a garbonzo bean.

The image to the right, from the Visible Human Project, shows these anatomical relationships in the Visible Woman.

Lay Interpretation

Human Growth Hormone (and many others) are produced within the pituitary gland. 

The pituitary gland is attached to the hypothalamus near the base of the skull over the rear roof of the nasal cavity.

The pituitary gland, the reason we are here (on this site) is about the size of a large garden pea and is considered the "master gland" of the body.

Careful examination of the pituitary gland reveals that it composed of two distinctive parts:

• The anterior pituitary or adenohypophysis is a classical gland composed predominantly of cells that secrete protein hormones.

• The posterior pituitary or neurohypophysis is not really an organ, but an extension of the hypothalamus. It is composed largely of the axons of hypothalamic neurons which extend downward as a large bundle behind the anterior pituitary. It also forms the so-called pituitary stalk, which appears to suspend the anterior gland from the hypothalamus.

The Pituitary gland is divided into three distinct sections during foetal development, a rear, middle and front lobe.

The middle (intermediate) lobe in humans is incorporated into the anterior lobe very early.

The anterior (front) pituitary gland produces primarily protein hormones (peptides).

The posterior (rear) pituitary gland is actually an extension of the hypothalamus.

The image below shows a frontal view of a sheep pituitary gland and hypothalamus. The posterior gland can be seen peeking out behind the anterior gland; (image courtesy of Dr. Terry Nett).

The anterior and posterior pituitary have separate embryological origins. In many mammals, there is also an intermediate lobe (pars intermedia) between the anterior and posterior pituitary.

A key to understanding the endocrine relationship between hypothalamus and anterior pituitary is to appreciate the vascular connections between these organs.

here is an image of a sheep's pituitary gland showing the distinct differences between the tissues of the hypothalamus and pituitary.

As will be emphasized in later sections, secretion of hormones from the anterior pituitary is under strict control by hypothalamic hormones. These hypothalamic hormones reach the anterior pituitary through the following route:

• A branch of the hypophyseal artery ramifies into a capillary bed in the lower hypothalamus, and hypothalamic hormones destined for the anterior pituitary are secreted into that capillary blood.

• Blood from those capillaries drains into hypothalamic-hypophyseal portal veins. Portal veins are defined as veins between two capillary beds; the hypothalamic-hypophyseal portal veins branch again into another series of capillaries within the anterior pituitary.

• Capillaries within the anterior pituitary, which carry hormones secreted by that gland, coalesce into veins that drain into the systemic venous blood. Those veins also collect capillary blood from the posterior pituitary gland.

This pattern of vascular connections is presented diagrammatically below. Note also the hypothalamic-hypophyseal portal vessels in the image of a real pituitary gland seen above.

The hypothalamus is in strict control of the function of the anterior pituitary gland's functions.

The way it controls the anterior pituitary gland, where hGH is produced, is interesting at least.

Hypothalamic hormones go into a common blood supply for the anterior pituitary. These then make a leap across a very specific capillary bed in the pituitary, entering the anterior pituitary structure.  The anterior pituitary is then stimulated to produce its own hormones which are released into that same venous blood to be channelled back to the heart.

This is interesting mainly because it is a blood borne hormone that is directed immediately into the receptor tissues without going for a few trips around the body first.