The pituitary gland secretes nine nonsteroid hor-
mones, many of which control other endocrine glands.
However, the pituitary gland itself is controlled by the
brain, specifically the hypothalamus. The
p o sterio r p i -
tu ita r y
is essentially an extension of the brain—axons
and terminals of specialized nerve cells from the hypo-
thalamus called
neurosecretory
cells
make up the posterior pitu-
itary lobe. The cells dump their
contents directly into the blood
vessels
supplying
the
posterior
pituitary lobe. The
a n terio r p i t u -
ita ry
shares a blood supply with
the hypothalamus—some neuro-
secretory cells within the hypo-
thalamus synapse on blood vessels that are connected
to blood vessels of the anterior pituitary lobe. Secre-
tions from these neurosecretory cells influence the cells
of the anterior pituitary lobe. The hypothalamic secre-
tions can either stimulate the pituitary gland (releas-
ing hormones) or inhibit the pituitary gland (inhibiting
hormones).
Hormones of the anterior pituitary regulate growth,
metabolism, sexual maturation and reproduction, milk
production, glucocorticoid production, and melanocyte ac-
tivity (
Table 9.1
). Four of the hormones secreted by the an-
terior pituitary (TSH, FSH, LH, and ACTH; see Table 9.1)
alter body functions indirectly by influencing the hormone
secretion of other endocrine glands (the thyroid gland,
ovaries, testes, and adrenal gland). The secretions of the
anterior pituitary are controlled by hypothalamic releas-
ing hormones (GHRH, TRH, GnRH, CRH, and PRH; see
Table 9.1) and hypothalamic inhibiting hormones (GHIH
and PIH; see Table 9.1). Remnant cells from the
p a rs in -
term edia
secrete melanocyte-stimulating hormone (MSH),
but its role in humans is not known.
neurosecretory
cells
A specialized
form of nerve cells
that secrete a neu-
rotransmitter into the
bloodstream rather
than into a synaptic
cleft.
Hypothalamic hormones and corresponding anterior pituitary hormones
Table 9.1
Hypothalamic hormone
Anterior pituitary hormone
Target cell
Action
Grow th-horm one releasing hor-
m one (G H RH) prom otes growth
horm one secretion.
Grow th-horm one inhibit-
ing horm one (GHIH) inhibits
growth horm one secretion*
Hum an grow th horm one (hGH)
Various tissu es
(e.g., liver, m uscle,
bones, cartilage)
M akes insulinlike grow th factor (IGF) in tar-
get cells to control grow th (stim ulate cell
divisio n in m uscle, bone, and cartilage)
Thyrotropin-releasing horm one
(TRH )t
Thyroid-stim ulating horm one
(TSH )
Thyroid follicle
cells
Secretes thyroid horm ones (T3, T 4) to con-
trol m etabolism
G onadotropin-releasing hor-
m one (GnRH)
Follicle-stim ulating horm one (FSH )
O varies, testes
Prom otes egg and sperm developm ent
Luteinizing horm one (LH)
O varies, testes
Controls ovulation and production of e s-
trogen, progesterone, and testosterone
Prolactin-releasing horm one
(PRH) stim ulates prolactin
release.
Prolactin (PRL)
Breast cells
Produces m ilk
Prolactin-inhibiting horm one
(PIH) inhibits prolactin release.
Corticotropin-releasing hor-
m one (CRH)
Adrenocorticotropic horm one
(ACTH )
Adrenal cortex
cells
Produces glucocorticoid to break down
proteins and fats, form glu cose, and re-
duce inflam m ation
M elanocyte stim ulating horm one
(M SH); e xce ssive CRH can stim u -
late M SH release
M elanocytes
Have an unknow n function in hum ans, but
it m ay influence brain activity
Notes
*
Hypothalamic-inhibitory hormones are denoted in blue and have opposite effects of the releasing hormones.
t
The terms (or endings)
tropic
(
T R O -pik
)
hormones, trophic
(
TR O -fik
)
hormones,
and
tropins
refer to hormones that act on
other endocrine glands.
Endocrine Glands Regulate Key Body Functions
259
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