C h a p t e r 3 1
Mechanisms of Endocrine Control
761
Nuclear Receptors.
Steroid
hormones, vitamin D, thyroid hor-
mones, and other lipid-soluble
hormones diffuse across the cell
membrane into the cytoplasm of the
target cell. Once inside, they bind to
an intracellular receptor that is acti-
vated by the interaction. The acti-
vated hormone–receptor complex
then moves to the nucleus, where
the hormone binds to a hormone
response element (HRE) in the pro-
moters on a target gene or to another
transcription factor. Attachment to
the HRE results in transcription of
a specific messenger RNA (mRNA).
The mRNA then moves into the
cytoplasm, where the “transcribed
message” is translated and used by
cytoplasmic ribosomes to produce
new cellular proteins or changes in
the production of existing proteins.
These proteins promote a specific
cellular response or, in some cases,
the synthesis of a structural protein
that is exported from the cell.
2
Pituitary Hormones.
The pituitary is a pea-sized
endocrine gland that is located at the base of the brain,
where it lies in a saddle-shaped depression in the sphe-
noid bone called the
sella turcica
. The pituitary gland
has two functional units: an anterior lobe, or
adeno-
hypophysis,
and a posterior lobe, or
neurohypophysis
.
Embryologically, the anterior lobe of the pituitary gland
develops from glandular tissue and the posterior lobe
from neural tissue.
The pituitary gland has been called the
master gland
because its hormones control the functions of many
target glands and cells. The anterior pituitary gland
contains five cell types: (1) thyrotrophs, which pro-
duce thyrotropin, also called
TSH
; (2) corticotrophs,
which produce corticotropin, also called
ACTH
;
(3) gonadotrophs, which produce the gonadotropins,
LH, and FSH; (4) somatotrophs, which produce GH;
and (5) lactotrophs, which produce prolactin. The pos-
terior pituitary gland stores and releases antidiuretic
hormone (ADH) and oxytocin, which are synthesized
in the hypothalamus. Hormones produced by the ante-
rior pituitary control body growth and metabolism
(GH), function of the thyroid gland (TSH), glucocor-
ticoid hormone levels (ACTH), function of the gonads
(FSH and LH), and breast growth and milk production
(prolactin). Melanocyte-stimulating hormone (MSH),
which is involved in the control of pigmentation of the
skin, is produced by the pars intermedia (the region
between the two lobes of the pituitary gland). The func-
tions of many of these hormones are discussed in other
parts of this book (e.g., thyroid hormone, GH, and
the corticosteroids in Chapter 32; the sex hormones in
Chapters 39 and 40; and ADH from the posterior pitu-
itary in Chapter 8).
New proteins
Ribosome
mRNA
mRNA
Translation
Receptor
Receptor
Nucleus
DNA
s
s
s
Transcription
Extracellular fluid
Intracellular fluid
Cell membrane
Hormone
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