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CHAPTER

24

The Pituitary Gland

OBJECTIVES

After studying this chapter, you should be able to:

■

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

■

Define the cell types present in the anterior pituitary and understand how their

numbers are controlled in response to physiologic demands.

■

Understand the function of hormones derived from proopiomelanocortin in hu-

mans, and how they are involved in regulating pigmentation in humans, other

mammals, and lower vertebrates.

■

Define the effects of the growth hormone in 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 underly-

ing mechanisms.

■

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, or hypophysis, lies in a pocket of the sphe-

noid bone at the base of the brain. It is a coordinating center

for control of many downstream endocrine glands, some of

which are discussed in other chapters. In many ways, it can be

considered to consist of at least two (and in some species,

three) separate endocrine organs that contain a plethora of

hormonally active substances. The anterior pituitary secretes

thyroid-stimulating hormone (TSH, thyrotropin), adreno-

corticotropic hormone (ACTH), luteinizing hormone (LH),

follicle-stimulating hormone (FSH), prolactin,
and
growth

hormone
(see Figure 18–9), and receives almost all of its

blood supply from the portal hypophysial vessels that pass ini-

tially through the median eminence, a structure immediately

below the hypothalamus. This vascular arrangement positions

the cells of the anterior pituitary to respond efficiently to regu-

latory factors released from the hypothalamus. Of the listed

hormones, prolactin acts on the breast. The remaining five are,

at least in part,
tropic hormones;
that is, they stimulate secre-

tion of hormonally active substances by other endocrine

glands or, in the case of growth hormone, the liver and other

tissues (see below). The hormones tropic for a particular endo-

crine gland are discussed in the chapter on that gland: TSH in

Chapter 20; ACTH in Chapter 22; and the gonadotropins FSH

and LH in Chapter 25, along with prolactin.

The posterior pituitary in mammals consists predominantly

of nerves that have their cell bodies in the hypothalamus, and

stores

oxytocin

and

vasopressin

in the termini of these neurons,

to be released into the bloodstream

.

The secretion of these hor-

mones, as well as a discussion of the overall role of the hypothal-

amus and median eminence in regulating both the anterior and

posterior pituitary, were covered in Chapter 18. Finally, in some

species there is a well-developed intermediate lobe of the pitu-

itary, whereas in humans it is rudimentary. Nevertheless, the

intermediate lobe, as well as the anterior pituitary, contain hor-

monally active derivatives of the proopiomelanocortin molecule

that regulate skin pigmentation, among other functions (see