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OBJECTIVES

OBJECTIVES

After studying this chapter, you should be able to:

  • Describe the structure of the pituitary gland and how it relates to function.

  • Define the cell types present in the anterior pituitary and understand how their numbers are controlled.

  • Understand the function of hormones derived from proopiomelanocortin, and how they are involved in regulating pigmentation.

  • Define the effects of the growth hormone on growth and metabolic function, and how insulin-like growth factor I (IGF-I) may mediate some of its actions in the periphery.

  • List the stimuli that regulate growth hormone secretion and define their underlying mechanisms.

  • Understand the relevance of pituitary secretion of gonadotropins and prolactin, and how this is regulated.

  • Understand the basis of conditions where pituitary function and growth hormone secretion and function are abnormal, and how they can be treated.

INTRODUCTION

The pituitary gland lies in a pocket of the sphenoid bone at the base of the brain. It is a coordinating center for control of many downstream endocrine glands. In many ways, it can be considered to consist of at least two separate endocrine organs that contain a plethora of hormonally active substances. The anterior pituitary secretes thyroid-stimulating hormone (TSH, thyrotropin), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and growth hormone, and receives almost all of its blood supply from the portal hypophysial vessels. This vascular arrangement positions the anterior pituitary to respond efficiently to regulatory factors released from the hypothalamus. Of the listed hormones, prolactin acts on the breast. The remaining five are tropic hormones; that is, they stimulate secretion of hormonally active substances. The tropic hormones for some endocrine glands are discussed in the chapter on that gland: TSH in Chapter 19; and ACTH in Chapter 20. However, the gonadotropins FSH and LH, along with prolactin, are covered here. To avoid redundancy, this chapter focuses predominantly on growth hormone and its role in growth and facilitating the activity of other hormones, along with general considerations about the pituitary.

The posterior pituitary in mammals consists predominantly of nerves that have their cell bodies in the hypothalamus, and stores oxytocin and vasopressin in their termini (see Chapter 17). In some species, there is also a well-developed intermediate lobe of the pituitary, whereas in humans it is rudimentary. Nevertheless, the intermediate lobe, as well as the anterior pituitary, contains hormonally active derivatives of the proopiomelanocortin (POMC) molecule that regulate skin pigmentation.

MORPHOLOGY

GROSS ANATOMY

The anatomy of the pituitary gland was discussed in detail in Chapter 17. The posterior pituitary is made up largely of the endings of axons from the hypothalamus and arises initially as an extension of this structure. The anterior pituitary contains endocrine cells that store its characteristic hormones and arises embryologically as an invagination of the pharynx (Rathke pouch). In species where it is ...

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