CHAPTER 17
Endocrine System
325
nuclei, and the arcuate nuclei. The hypothalamus itself receives
signals from many areas of the brain, including the amygdala,
hippocampus, brainstem tegmentum, and the infralimbic and
cingulate cortices. The
hypothalamus maintains body homoeo-
stasis by regulating production of the hypothalamic hormones,
which, in turn, control the secretion of the pituitary hormones
from the pituitary gland.
THE ADENOHYPOPHYSIS,
also called the
anterior pitu-
itary
, is the anterior division of the gland and is derived
from the ectoderm of the roof of the developing oral cav-
ity (
Rathke pouch
). It is composed of glandular tissue. The
adenohypophysis can be divided into three regions based on
their anatomic positions: the
pars distalis
,
pars tuberalis
, and
pars intermedia
.
1.
The
pars distalis
is the main body of the adenohypophysis,
containing blood vessels, a capillary network, and two main
types of secretory cells supported by a network of reticu-
lar connective tissues. These secretory cells are classiF ed as
chromophobes
and
chromophils
. The chromophobes do not
effectively take a stain, so they appear clear in the Mallory
trichrome stain. These cells are undifferentiated cells but are
capable of differentiating into chromophils. The chromophils
include
basophils
and
acidophils
(±ig. 17-4A).
Basophils
appear blue in Mallory stain and include three
subtypes of hormone secretory cells:
corticotrophs
,
thyrotrophs
,
and
gonadotrophs
. Various hormones are produced by these
cells, including
ACTH
,
TSH
,
FSH
, and
luteinizing hormone
(
LH
). These hormones stimulate various target organs includ-
ing the cortex of the adrenal glands, the thyroid, the testes, and
the ovaries (see ±ig. 17-2 for details). The secretion of hormones
by cells in the adenohypophysis is controlled by hypothalamic
releasing hormones and
inhibitory hormones.
Corticotrophs
are stimulated by corticotropin-releasing hormone (CRH)
from the hypothalamus.
Thyrotrophs
are stimulated by thy-
rotropin-releasing hormone.
Gonadotrophs
are stimulated by
gonadotropin-releasing hormone.
Acidophils
appear red in Mallory stain and contain two sub-
types of hormone secretory cells:
somatotrophs
and
mammotro-
phs
.
Somatotrophs
secrete somatotropin (growth hormone),
which stimulates the liver to produce the
insulin-like growth
factor
(
IGF-1
) that promotes cartilage and bone growth, pro-
tein deposition, and cell reproduction.
Mammotrophs
secrete
prolactin
, which increases mammary gland size and promotes
milk production.
2.
The
pars tuberalis
is the neck of the adenohypophysis; it wraps
around the infundibular stalk of the pituitary gland (±ig. 17-3A).
It contains a rich capillary network and some low columnar
basophilic cells that are commonly arranged in cords.
3.
The
pars intermedia
is located between the pars distalis and
pars nervosa (±igs. 17-3A and 17-5A). It contains cuboidal
follicular cells and colloid cysts called
Rathke cysts
, which
are lined by follicular cells. Rathke cysts are derived from
the ectoderm of the dorsal portion of the
Rathke pouch
;
these cysts are
considered to be the remnants of the Rathke
pouch that was present during development. The secretory
cells may be involved in producing
melanocyte-stimulating
hormone (MSH)
. These cells are usually lightly stained by
basophilic dye.
THE NEUROHYPOPHYSIS
is derived from the inferior sur-
face of the developing diencephalon. It is considered to be ner-
vous
tissue. It can be divided into the
infundibular stalk
, the
median
eminence
, and the
pars
nervosa
.
1.
The
infundibular stalk
connects the median eminence to the
pars nervosa (±ig. 17-3A,B).
2.
The
median eminence
connects the inferior portion of the hypo-
thalamus to the infundibular stalk of the
neurohypophysis
(±ig. 17-3B). It contains long axons that carry
antidiuretic
hormone (ADH)
and
oxytocin hormone
produced by nuclei
in the hypothalamus. These axons pass through the median
eminence and terminate in the pars nervosa. The median emi-
nence also contains short axons and axon terminal
endings
from the hypothalamus that release neurosecretory hormones
(hypothalamic releasing and inhibiting hormones). These
hormones are transported through the hypophyseal portal
system from the primary capillary plexus to the secondary
capillary plexus, thereby regulating the secretion of the secre-
tory cells in the adenohypophysis.
3.
The
pars nervosa
is the main body of the neurohypophysis
(±igs. 17-3A and 17-6A,B). It contains a fenestrated capil-
lary plexus, pituicytes (glial cells), and axons and axon ter-
minal endings from neuron cell bodies in the hypothalamus.
Pituicytes
provide support and nutrition to the axons of
the neurons. The enlarged axon terminal endings are F
lled
with neurosecretory granules that are called
Herring bod-
ies
. The neurosecretory hormones released in the pars ner-
vosa include
ADH
or
vasopressin
,
oxytocin hormone
, and
neurophysins
.
Thyroid Gland
The
thyroid gland
has two lobes that are located inferior to the
thyroid cartilage and anterior to the trachea. It contains thyroid
follicles that produce
T
3
and
T
4
, which regulate body metabo-
lism (±ig. 17-8A,B). The
parafollicular cells
located between the
follicles are known as
clear cells (C cells)
and produce
calcitonin
hormone. This hormone is released in response to high blood
calcium and inhibits the activity of the osteoclasts. Calcitonin
is involved in calcium and phosphorus metabolism. It decreases
blood calcium levels and has opposing effects to the parathor-
mone or PTH.
Parathyroid Glands
There are typically four small
parathyroid glands
, most com-
monly lying posterior to the thyroid gland. They consist of
chief
cells
and
oxyphil cells
(±ig. 17-9A,B). Chief cells are hormone-
producing cells that secrete
parathormone
, also called
PTH
.
PTH is released in response to low blood calcium levels and
indirectly promotes the proliferation and activity of osteoclasts,
which remove bone. PTH also inhibits the activity of osteo-
blasts, which help to build up new bone.
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