Notes
Anterior Pituitary Lobe Anatomy & Physiology
Sections
Anterior Lobe of the Pituitary Gland
Key Principles
The hypothalamus regulates the endocrine functions of the pituitary gland via hormonal and neural mechanisms; as we'll see, the hypothalamus can either stimulate or inhibit the the pituitary gland.
The pituitary gland (aka, hypophysis) is structurally and functionally divisible into two lobes.
The anterior lobe aka, adenohypophysis is derived embryologically from the foregut; it receives hypothalamic regulating signals via the hypothalamic-hypophyseal portal veins.
6 tropic hormones are secreted by the anterior pituitary lobe; "tropic" means they act on target tissues to stimulate release of other endocrine products.
The posterior lobe aka, the neurohypophysis, receives hypothalamic signals via neural connections. As we'll learn elsewhere, the posterior lobe is derived from neural tissues.
Anatomy
Hypothalamus is superior to the pituitary gland.
Pituitary gland comprises anterior and posterior lobes.
Infundibulum, aka, pituitary stalk, which connects the hypothalamus to the pituitary gland.
Secretion Pathway:
Cell bodies of hypothalamic neurons send axons inferiorly towards the pituitary gland.
Axons deliver hypothalamic hormones to the portal blood vessels.
Hypothalamic-hypophyseal portal blood vessels deliver blood and hormonal signals from the hypothalamus to the pituitary gland (hypophysis).
The the primary capillary plexus forms at the base of the hypothalamus (specifically, at the median eminence); it arises from the superior hypophyseal artery and drains, via portal vessels, inferiorly to the secondary capillary plexus, which bathes the endocrine cells of the anterior pituitary lobe.
The secondary capillary plexus delivers neurohormones that stimulate or inhibit hormonal secretion by the nearby anterior lobe endocrine cells.
Upon secretion, the anterior lobe hormones drain into systemic venous return to the heart; from here, they circulate within the systemic arterial blood to reach their target tissues.
Anterior Lobe Hormones
FLATPiG
- Follicle-Stimulating hormone
- Lutenizing hormone
- Adrenocorticotropic hormone
- Thyroid-stimulating hormone
- Prolactin
- Growth hormone
CRH (corticotropin-releasing hormone)
Stimulates the corticotrophs of the anterior pituitary lobe to release ACTH (adrenocorticotropic hormone).
ACTH travels in systemic blood to reach the cells of the adrenal gland cortex. As we'll discuss elsewhere, ACTH causes the adrenal cortex to secrete its own endocrine products.
GHRH (growth hormone-releasing hormone) stimulates somatotrophs of the anterior lobe to release growth hormone.
Growth hormone has widespread metabolic effects in the body, particularly in the musculoskeletal system.
Somatostatin (aka, growth hormone-inhibiting hormone), inhibits growth hormone secretion from the anterior lobe endocrine cells.
GnRH (gonadotropin-releasing hormone)
Stimulates gonadotrophs in the anterior pituitary lobe to secrete FSH (follicle-stimulating hormone) and LH (luteinizing hormone), which travel in the bloodstream to act on gonadal cells (aka, ovarian and testicular cells) (details regarding these hormonal pathways are discussed elsewhere).
Thyroid-releasing hormone
Released from the hypothalamus and triggers thyrotrophs to secrete thyroid-stimulating hormone, which stimulates endocrine cells of the thyroid gland.
PRH (prolactin-releasing hormone)
Stimulates lactotrophs (aka, mammotrophic cells) of the anterior lobe to secrete prolactin, which triggers mammary gland growth and milk production in females.
Indicate that PIH (prolactin-inhibiting hormone, aka, dopamine) inhibits the release of prolactin.
Bromocriptine (a dopamine agonist) is used to treat prolactinomas (prolactin-secreting tumors).
As always, be aware that we've simplified the hormonal pathways and their effects, for clarity. Separate tutorials address the regulatory feedback mechanisms that govern the secretion of anterior lobe hormones.
Full-Length Text
- Here we will learn about the hypothalamus and the anterior lobe of the pituitary gland.
- To begin, start a table and denote that:
- The hypothalamus regulates the endocrine functions of the pituitary gland via hormonal and neural mechanisms; as we'll see, the hypothalamus can either stimulate or inhibit the the pituitary gland.
- The pituitary gland (aka, hypophysis) is structurally and functionally divisible into two lobes.
- The anterior lobe, aka, adenohypophysis, is derived embryologically from the foregut; it receives hypothalamic regulating signals via the hypothalamic-hypophyseal portal veins.
In this tutorial, we'll learn the 6 tropic hormones secreted by the anterior pituitary lobe; "tropic" means they act on target tissues to stimulate release of other endocrine products.
- The hormones produced by the anterior lobe of the pituitary gland will be the focus of this tutorial; for completeness, denote that:
- The posterior lobe, aka, the neurohypophysis, receives hypothalamic signals via neural connections.
- As we'll learn elsewhere, the posterior lobe is derived from neural tissues.
Next, let's draw the basic anatomy of the hypothalamus and pituitary gland.
- First, draw the brain in sagittal section;
- Then, draw the enlarged anterior hypothalamus and pituitary gland.
- Label the anterior and posterior lobes of the pituitary gland, and,
- Indicate the infundibulum, aka, pituitary stalk, which connects the hypothalamus to the pituitary gland.
- With this basic anatomy as a background, draw an enlarged view of them, so we can illustrate the hormonal pathways.
- Within the hypothalamus, draw the cell bodies of hypothalamic neurons; show that their axons travel inferiorly towards the pituitary gland.
- Next, show the hypothalamic-hypophyseal portal blood vessels, which, as their name implies, deliver blood and hormonal signals from the hypothalamus to the pituitary gland (hypophysis).
- Indicate that the the primary capillary plexus forms at the base of the hypothalamus (specifically, at the median eminence); it arises from the superior hypophyseal artery and drains, via portal vessels, inferiorly to the:
- Secondary capillary plexus, which bathes the endocrine cells of the anterior pituitary lobe.
- Now, show that hypothalamic axons transport the hypothalamic hormones (aka, neurohormones) from the cell bodies in the hypothalamus and release them into the primary capillary plexus;
- From here, they drain into the secondary capillary plexus and stimulate or inhibit hormonal secretion by the nearby anterior lobe endocrine cells.
- Upon secretion, the anterior lobe hormones drain into systemic venous return to the heart; from here, they circulate within the systemic arterial blood to reach their target tissues.
With the key steps of the endocrine pathways as a backdrop, let's write the hypothalamic hormones, anterior lobe endocrine cells and hormones, and their targets.
Let's start with a simple, easy to recall acronym: FLATPiG, which stands for
- Follicle-Stimulating hormone,
- Lutenizing hormone,
- Adenocorticotropic hormone)
- Thyroid-stimulating hormone
- Prolactin, and,
- Growth hormone
- First, write that CRH (corticotropin-releasing hormone), as its name implies, stimulates the corticotrophs of the anterior pituitary lobe to release ACTH (adrenocorticotropic hormone);
- ACTH travels in systemic blood to reach the cells of the adrenal gland cortex. As we'll discuss elsewhere, ACTH causes the adrenal cortex to secrete its own endocrine products.
- Next, write that GHRH (growth hormone-releasing hormone) stimulates somatotrophs of the anterior lobe to release growth hormone.
- Growth hormone has widespread metabolic effects in the body, particularly in the musculoskeletal system.
- Indicate that another hypothalamic hormone, somatostatin (aka, growth hormone-inhibiting hormone), inhibits growth hormone secretion from the anterior lobe endocrine cells.
- Next, indicate that GnRH (gonadotropin-releasing hormone) stimulates gonadotrophs in the anterior pituitary lobe to secrete FSH (follicle-stimulating hormone) and LH (luteinizing hormone), which travel in the bloodstream to act on gonadal cells (aka, ovarian and testicular cells) (details regarding these hormonal pathways are discussed elsewhere).
- Thyroid-releasing hormone is released from the hypothalamus and triggers thyrotrophs to secrete thyroid-stimulating hormone, which stimulates endocrine cells of the thyroid gland.
- Finally, write that PRH (prolactin-releasing hormone) stimulates lactotrophs (aka, mammotrophic cells) of the anterior lobe to secrete prolactin, which triggers mammary gland growth and milk production in females.
- Indicate that PIH (prolactin-inhibiting hormone, aka, dopamine) inhibits the release of prolactin.
- For this reason, as a clinical/pharmacologic correlation, write that bromocriptine (a dopamine agonist) is used to treat prolactinomas (prolactin-secreting tumors).