Functional Anatomy of the Hypothalamus and Pituitary Gland
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 (click on the image to see a larger, unlabeled image).
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 a separate 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 image to the right shows a frontal view of a pig pituitary gland and hypothalamus. The posterior gland can be seen peeking out behind the anterior gland; pass your mouse cursor over the image for labels (image courtesy of Dr. G. P. Kozlowski).
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. 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 hypothalmic 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 diagramatically below. Note also the hypothalamic-hypophyseal portal vessels in the image of a real pituitary gland seen above.
The utility of this unconventional vascular system is that minute quantities of hypothalamic hormones are carried in a concentrated form directly to their target cells in the anterior pituitary, and are not diluted out in the systemic circulation.
Advanced and Supplemental Topics
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