326
UNIT 3
Organ Systems
PTH indirectly promotes osteoclasts by stimulating
osteoblasts to produce
osteoclast differentiation factor
, also
known as
RANKL
, which stimulates precursors (monocytes)
to differentiate and fuse to become multinuclear osteoclasts.
Increased numbers of osteoclasts cause active bone resorption,
which results in more Ca
++
being released into the blood. PTH
also affects the distal tubules of the kidney to increase blood cal-
cium levels by enhancing the reabsorption of calcium from distal
tubules.
Adrenal Glands
The
adrenal glands
lie on the superior tips of the kidneys, in the
posterior portion of the abdominal cavity. The adrenal glands can
be divided into the
cortex
and the
medulla
(Fig. 17-10A). The
cor-
tex
has three zones: (moving from external to internal) the
zona
glomerulosa
,
zona fasciculata
, and
zona reticularis
(Fig. 17-10B).
Cells in the cortex produce various corticosteroid hormones
including
mineralocorticoids
,
glucocorticoids
, and
weak andro-
gens
(see Table 17-1). The
medulla
contains
ganglion neurons
and
chromafF n cells.
The chromaf± n cells produce
adrenaline (epi-
nephrine)
and
noradrenaline
(
norepinephrine
)
.
These hormones
are known as
sympathomimetic hormones
(Fig. 17-12A,B).
Pineal Gland
The
pineal gland
is located inside the skull and lies above the
superior colliculi of the midbrain. It is considered part of the
epithalamus of the brain. It contains
pinealocytes
,
neuroglial
cells
, and calci±
ed structures called
brain sand
(
corpora
arenacea
). The corpora arenacea are derived from the organic
matter in the pineal gland and are rich in calcium and phos-
phate. The pineal gland has a rich blood supply. Pinealocytes,
modi±
ed neurons that produce
melatonin
, are the predominant
cells in the pineal gland. Melatonin is an important hormone
in the regulation of the day and night cycle called the
circadian
rhythm
(Fig. 17-13A,B).
Endocrine Pancreas (Islets of Langerhans)
There are many
islets of Langerhans
interspersed in the exocrine
portion of the pancreas (Figs. 17-14A to 17-15B). The islets of
Langerhans contain
alpha cells
,
beta cells
,
delta cells
, and
pancre-
atic polypeptide (PP) cells
. These hormone secretory cells produce
glucagon
,
insulin
,
somatostatin
, and
PP
, important hormones in
regulating blood glucose levels.
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