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Endocrine System
A. Introduction
1. endocrine organs hypophysis, thyroid gland, adrenal gland,
parathyroid gland, pineal gland
3. Typical secretions: hormones which affect distant
targets and/or many tissues
a. peptide hormones - proteins, glycoproteins, or
peptides which bind to receptors on cell surfaces; work
via second messengers, water soluble; e.g. insulin,
glucagon, follicle stimulating hormone
b. steroid hormones - lipid soluble and bind to
intracellular targets, e.g. to DNA or hormone receptors;
derived from cholesterol, e.g. progesterone, estradiol,
testosterone
c. amino acid derived hormones - water soluble,
catecholamines, thyroxin and epinephrine
B. Hypophysis, aka pituitary gland
1. Basic structure and origin:
• direct connection between nervous system and
the other endocrine organs
• lies in the sella turcica, a depression in the
sphenoid bone (hypophyseal fossa), with two
ridges cranial and caudal to it (supposed to look
like a Turkish saddle....)
Canine, dorsal
view, calvarium
removed.
Ventral view, canine brain
Hypophysis
B. Hypophysis, aka pituitary gland
1. Basic structure and origin:
•divided into two main regions, neurohypophysis
and adenohypophysis
Sagittal section
Brain, diencephalon
Cranial
Caudal
Adenohypophysis
Neurohypophysis
Arcuate n
Mammillary
Body
Superior
Hypophyseal a
Adenohypophysis
Median eminence
with primary capillary
bed
Neurohypophysis
These factors are
transported to the
adenohypophysis
by small portal veins
where they leave
the capillaries to act
on the secretory
cells there.
Portal v
Adenohypophysis
Releasing or inhibiting
factors bind to receptors
in or on the secretory cells
in the adenohypophysis to
affect their secretion rates
of a DIFFERENT
hormone. E.g.
Corticotropin releasing
hormone stimulates the
release of corticotropin
(ACTH)
Secondary
hormone
Releasing hormone
C. Adrenal Glands
1. General morphology
• Embedded in the adipose tissue on top of
kidneys.
•Triangular shape
(sometimes)
Capsule
Cortex
Medulla
3. Cortex
Divided into 3 visibly
different zones, all of
which are steroidsecreting and all of
which synthesize
hormones from
cholesterol, but in
different regions
different types of
hormones are
produced.
3. Cortex:
a. zona
glomerulosa
outermost layer,
cells look
clustered
Secretes mineralocorticoids (e.g. aldosterone, which is
involved in salt balance via the kidney), and
deoxycorticosterone.
Secretion is stimulated by angiotensin II from
juxtaglomerular apparatus and ACTH from hypophysis.
3. Cortex
b. zona fasciculata
• middle layer, longitudinal cords of cells
• secretes glucocorticoids (e.g. cortisol,
corticosterone) and adrenal androgens
• secretion stimulated by ACTH
3. Cortex
c. zona reticularis
• innermost layer, much thinner than other layers,
network of cell cords
• cells secrete androgens and small amounts of
glucocorticoids
• secretion stimulated by ACTH
4. Medulla
• Secretion is triggered
separately by preganglionic
axons (norepinephrine release
is associated with fear,
epinephrine with pain), the fight
or flight responses.
• Portal system glucocorticoids
stimulate conversion/synthesis
of norepinephrine to
epinephrine. The cell
distribution is related to
capillary distribution, so that the
epinephrine secreting cells are
nearest to capillaries from
cortex.
Adrenal gland, medulla
Chromaffin
cells
(virtually
every cell)
Ganglion
cell