1. Module 2

1.14. Page 3

Lesson 2

Module 2—The Endocrine System

Anterior Pituitary

 

anterior pituitary: an endocrine gland consisting of secretory cells that synthesize and secrete several hormones directly into the bloodstream

Compared to the posterior pituitary, which only stores and releases hormones produced in the hypothalamus, the anterior pituitary synthesizes and releases six major hormones. It is connected to the hypothalamus by blood vessels that run through the stalk. Three of the hormones (FSH, LH, and prolactin) are involved in the reproductive process, and will be studied in detail in Unit B. In this unit you will learn their general functions. The other three hormones, adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), and human growth hormone (hGH), will be introduced in this lesson.

 

The hypothalamus and pituitary control the production and release of these hormones through negative feedback loops. The hypothalamus is stimulated to secrete releasing hormones by conditions in the internal environment of the body. Moving through the blood vessels in the stalk, the releasing hormones stimulate target cells in the anterior pituitary, which produces ACTH, TSH, and hGH. As indicated by its name, ACTH stimulates the outside layer of the adrenal gland, called the adrenal cortex, which releases cortisol, a major stress hormone, and aldosterone. TSH stimulates the thyroid gland and its production of thyroxine.

 

Emily’s anterior pituitary gland overproduces ACTH and undersecretes TSH. You will study the effects of inappropriate amounts of these hormones on the body in the next two lessons.

 

Read

 

releasing hormones: hormones produced by neurosecretory cells in the hypothalamus that stimulate or inhibit the secretion of hormones by the anterior pituitary; sometimes called releasing factors

Releasing hormones, which are produced by the hypothalamus, and ACTH, TSH, and hGH, which are produced by the anterior pituitary, are called tropic hormones. In Lesson 1 you learned that tropic hormones are one group of hormones. Do you remember their characteristics? Explore this group of hormones in more depth by studying “Figure 13.10” on page 441 of your textbook. Read and summarize the section about tropic hormones, “Regulating the Regulators,” on pages 441 and 442 of your textook. Store your notes in your course folder.

 

Watch and Listen

 

You can review some of the concepts on the anterior pituitary by watching the following segment of “The Hypothalamus and Pituitary Gland: The Master Complex.” You may be required to enter a username and password to access this video. Contact your teacher for this information.

  • “Anterior Pituitary Gland”
Self-Check

 

SC 5. Practice matching the structures of the hypothalamus-pituitary complex with their hormones by completing a drag-and-drop activity.

 

To practise applying your knowledge about specific tropic hormones and their regulation by the hypothalamus and anterior pituitary gland, complete the following questions.

 

SC 6. Using ACTH as an example, explain a tropic hormone. Besides ACTH, identify three other tropic hormones.

 

Use the following flow chart, which illustrates regulation by tropic hormones, to answer questions SC 7 through SC 12.

 

The feedback loop illustrates the regulatory pathway of tropic hormones secreted by the hypothalamus, targeting the pituitary and causing the secretion of stimulating hormone. The stimulating hormone then affects its target gland, which responds by secreting the third hormone in the process. The third hormone can inhibit the release of hormone 1 from the hypothalamus, or it can inhibit the release of hormone 2 from the pituitary gland.

 

SC 7. How is the secretion of tropic hormones from the pituitary gland regulated?

 

SC 8. If hormone 2 in the tropic hormone pathway is TSH, what is the target gland?

 

SC 9. If hormone 2 is TSH, what is hormone 3?

 

SC 10. Explain how hormone 3 can inhibit or stimulate the release of hormone 1 and inhibit release of hormone 2?

 

SC 11. If the target gland is the adrenal cortex, identify the specific tropic hormone at work.

 

SC 12. Identify hormone 3 produced by the adrenal cortex.

 

SC 13. Explain how the anterior and posterior pituitaries differ with respect to their relationship to the hypothalamus.

 

SC 14. Explain how the differences between the two regions of the pituitary relate to the nature of their hormonal secretions.

 

SC 15. Suppose a scientist has discovered a new hormone. It is not clear what gland produces the hormone, but people who produce above-average amounts of this hormone also produce very high levels of insulin. Based on your knowledge of how tropic hormones function, provide a possible explanation for the observation.

 

Check your work.
Self-Check Answers

 

SC 6. ACTH is an example of a tropic hormone because it is released by the anterior pituitary and travels through the blood to the adrenal cortex of the adrenal gland (another endocrine gland), which it stimulates to release hormones such as cortisol.

 

Other tropic hormones include releasing hormones from the hypothalamus, thyroid stimulating hormone (TSH), human growth hormone (hGH), LH, and FSH.

 

SC 7. The secretion of tropic hormones from the pituitary gland is regulated by releasing hormones from the hypothalamus, which either stimulate or inhibit the anterior pituitary gland.

 

SC 8. If hormone 2 in the tropic hormone pathway is TSH, the target gland is the thyroid.

 

SC 9. If hormone 2 is TSH, hormone 3 is thyroxine.

 

SC 10. Excessive amounts of thyroxine (hormone 3) inhibit the anterior pituitary gland from releasing TSH and the hypothalamus from secreting releasing hormones. The thyroid gland is inhibited and does not release thyroxine, thus lowering the amount circulating in the blood. Inadequate amounts of thyroxine stimulate the hypothalamus to secrete releasing hormones, which stimulates the anterior pituitary to release TSH, which stimulates the thyroid gland to release more thyroxine.

 

SC 11. If the target gland is the adrenal cortex, then the tropic hormone is ACTH.

 

SC 12. The adrenal cortex is stimulated to produce hormones such as cortisol.

 

SC 13. The posterior pituitary is an extension of the hypothalamus and is composed of specialized neurons, while the anterior pituitary is a true endocrine gland and is composed of several types of secretory cells that produce and release hormones. Both lobes of the pituitary gland are connected to the hypothalamus by a stalk, but the posterior pituitary is connected to the hypothalamus by neurons, while the anterior pituitary is connected to the hypothalamus by blood vessels.

 

SC 14. The neurosecretory cells of the hypothalamus secrete the hormones and move them along the axons to the posterior pituitary, where they are stored and released into the bloodstream as needed. Neurons from the hypothalamus stimulate the secretory cells of the anterior pituitary to synthesize the hormones, and releasing hormones circulating in the bloodstream stimulate the release of the anterior pituitary hormones.

 

SC 15. Since tropic hormones stimulate another gland to produce a hormone, the above-average amount of this new hormone could be what is stimulating the pancreas to secrete high levels of insulin. This new hormone might be produced by the hypothalamus or perhaps by the anterior pituitary.