Neurosecretory cells in the hypothalamus of both
males and females secrete pulses of
gonadotropin-
releasing hormone (GnRH)
into the pituitary blood
supply at regular intervals.
GnRH stimulates the anterior pituitary lobe to secrete
similar pulses of follicle-stimulating hormone (FSH)
and luteinizing hormone (LH) into the bloodstream.
In the male, FSH and LH stimulate sperm development
and testosterone production by the testes. Testosterone
in the blood inhibits secretions of GnRH from the
hypothalamus and LH and FSH secretions from the
pituitary gland.
In the female, LH and FSH cause the egg cells to
mature and to produce estrogen:
• As the estrogen level increases and an egg develops,
there comes a point when a massive release of LH
and FSH from the pituitary gland occurs, leading to
ovulation.
• After ovulation, the remnants
of the follicle (corpus luteum)
continue to produce estrogen
and
progesterone,
which
suppress
GnRH
secretion
from the hypothalamus and
LH and FSH secretions from
the pituitary gland.
• Inhibin
produced
by
the
ovaries
helps
to
suppress
FSH
secretion
from
the
pituitary gland and inhibit
the
development
of other
egg cells.
• During
pregnancy,
the
ovaries produce relaxin to increase the flexibility of
uterine and vaginal smooth muscle in preparation
for childbirth.
gonadotropin-
releasing
hormone (GnRH)
A hormone released
by the hypothalamus
that stimulates the
anterior pituitary
gland to secrete
hormones—specifi-
cally luteinizing
hormone (LH) and
follicle stimulating
hormone (FSH)—that
act on the ovaries
and testes.
R egulation o f sex h o rm o n e secre tio n s by th e hyp o th alam u s,
p itu itary g lan d , and g o n a d s • Figure 9.18
The coordination of GnRH, FSH, and LH
secretion, along with the secretion of sex hormones by the gonads, signal the onset of puberty
and gamete production in both males and females. In addition, this process signals ovulation and maintenance of pregnancy in females.
Female
Hypothalamus
The hypothalamus releases pulses
of GnRH at regular intervals.
Male
LH
FSH
2
GnRH
stimulates
the anterior
pituitary to
secrete
LH and FSH
‘ Anterior
at regular
pituitary
intervals.
©
\
©
v, _
--------
Estrogen
Progesterone
Hypothalamus
Anterior
pituitary
Testes
©
- Testosterone
S
FSH and LH act on the ovary and lead to development of an egg and
production of estrogen and progesterone. These hormones influence the
hypothalamus and pituitary via either positive or negative feedback,
depending on the point in the menstrual cycle when secretion occurs.
FSH and LH act on the testes and
lead to testosterone production, which
exerts effects on the hypothalamus
and pituitary via negative feedback.
Endocrine Glands Regulate Other Key Body Functions
273
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