Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 24

The Pituitary Gland 379

plus a small amount of
β
-endorphin, and these substances are
secreted. In the intermediate lobe cells, POMC is hydrolyzed
to corticotropin-like intermediate-lobe peptide (CLIP),
γ

• LPH, and appreciable quantities of
  β
  -endorphin. The func-
  tions, if any, of CLIP and
  γ
  -LPH are unknown, whereas
  β
  -en-
  dorphin is an opioid peptide (see Chapter 7) that has the five
  amino acid residues of met-enkephalin at its amino terminal
  end. The
  melanotropins
  α


• and
  β
  -MSH are also formed.
  However, the intermediate lobe in humans is rudimentary,
  and it appears that neither
  α
  -MSH nor
  β
  -MSH is secreted in
  adults. In some species, however, the melanotropins have im-
  portant physiological functions, as discussed below.

CONTROL OF SKIN COLORATION

& PIGMENT ABNORMALITIES

Fish, reptiles, and amphibia change the color of their skin for

thermoregulation, camouflage, and behavioral displays. They

do this in part by moving black or brown granules into or out

of the periphery of pigment cells called

melanophores.

The

granules are made up of

melanins,

which are synthesized

from dopamine (see Chapter 7) and dopaquinone. The move-

ment of these granules is controlled by a variety of hormones

and neurotransmitters, including

α

• and
  β
  -MSH, melanin-
  concentrating hormone, melatonin, and catecholamines.
  Mammals have no melanophores containing pigment gran-
  ules that disperse and aggregate, but they do have
  melanocytes,
  which have multiple processes containing melanin granules.
  Melanocytes express
  melanotropin-1
  receptors. Treatment
  with MSHs accelerates melanin synthesis, causing readily
  detectable darkening of the skin in humans in 24 h. As noted

TABLE 24–1
Hormone-secreting cells of the human anterior pituitary gland.

Cell Type Hormones Secreted % of Total Secretory Cells Stain Affinity Diameter of Secretory Granules (nm)

Somatotrope Growth hormone 50 Acidophilic 300–400

Lactotrope Prolactin 10–30 Acidophilic 200

Corticotrope ACTH 10 Basophilic 400–550

Thyrotrope TSH 5 Basophilic 120–200

Gonadotrope FSH, LH 20 Basophilic 250–400

FIGURE 24–2
Schematic representation of the preproopiomelanocortin molecule formed in pituitary cells, neurons, and other
tissues.
The numbers in parentheses identify the amino acid sequences in each of the polypeptide fragments. For convenience, the amino acid
sequences are numbered from the amino terminal of ACTH and read toward the carboxyl terminal portion of the parent molecule, whereas the
amino acid sequences in the other portion of the molecule read to the left to –131, the amino terminal of the parent molecule. The locations of
Lys–Arg and other pairs of basic amino acids residues are also indicated; these are the sites of proteolytic cleavage in the formation of the smaller
fragments of the parent molecule. AL, anterior lobe; IL, intermediate lobe.

Signal

peptide

(–131)

γ−MSH

(–55 to –44)

β-LPH (42–134)

β-LPH

ACTH (1–39)

AL and IL

γ−MSH α-MSH

(1–13)

β-MSH

(84–101)

Met-enkephalin

(104–108)

γ-LPH

(42–101)

β-Endorphin

(104–134)

CLIP

(18–39)

Arg-LysArg-ArgLys-ArgLys-ArgLys-LysArg-ArgLys-Arg Lys-LysLys-Arg Lys-Lys

Fast in IL

Slow in AL

ACTH

AL and IL IL only

Amino terminal fragment