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 humans, 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 underlying mechanisms.
- Understand the relevance of pituitary secretion of gonadotropins and prolactin, and how these are regulated.
- Understand the basis of conditions where pituitary function and growth hormone secretion and function are abnormal, and how they can be treated.
The pituitary gland, or hypophysis, 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, some of which are discussed in subsequent 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), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, and growth hormone (see Figure 17–9), and receives almost all of its blood supply from the portal hypophysial vessels that pass initially through the median eminence, a structure immediately below the hypothalamus. This vascular arrangement positions the cells of 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, at least in part, tropic hormones; that is, they stimulate secretion of hormonally active substances by other endocrine glands or, in the case of growth hormone, the liver and other tissues (see below). 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.
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 hormones, as well as a discussion of the overall role of the hypothalamus and median eminence in regulating both the anterior and posterior pituitary, was covered in 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, among other functions (see ...