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THE ENDOCRINE SYSTEM
Ch 17, Human Anatomy
Sonya Schuh-Huerta, Ph.D.
The Endocrine System: An
Overview
• A system of ductless glands
– Secrete messenger molecules called
hormones
• Interacts closely with the nervous system
• Endocrinology
– Study of hormones & endocrine glands
Endocrine Organs
• Scattered throughout the body
• Pure endocrine organs
– Pituitary, pineal, thyroid, parathyroid, & adrenal glands
• Organs containing endocrine cells
– Pancreas, thymus, gonads, & hypothalamus
• Hypothalamus is a neuroendocrine organ
– Produces hormones & also has nervous functions
Location of the Major Endocrine Glands
Pineal gland
Hypothalamus
Pituitary gland
Thyroid gland
Parathyroid glands
(on dorsal aspect
of thyroid gland)
Thymus
Adrenal glands
Pancreas
Ovary (female)
Testis (male)
Hormones
• Classes of hormones
– Amino acid–based hormones
– Steroids  derived from cholesterol
• Basic hormone action
– Circulate throughout the body in the blood
– Influences only specific tissues  target cells
• A hormone can have different effects on different
target cells
Control of Hormone Secretion
• Secretion triggered by 3 major types of
stimuli
– Humoral  simplest of endocrine control
mechanisms
• Secretion in direct response to changing ion or
nutrient levels in the blood
• Parathyroid monitors calcium levels
– Responds to decline by secreting hormone to reverse
decline
Control of Hormone Secretion
• Secretion triggered by 3 major types of
stimuli (cont…)
– Neural
• Nerve fibers stimulate hormone secretion
• Sympathetic nerve fibers stimulate cells in the
adrenal medulla
• Induces release of epinephrine & norepinephrine
– Hormonal
• Certain hormones trigger secretion of other
hormones
– Hypothalamus secretes hormones  stimulates pituitary
to secrete hormones  stimulates other glands to
secrete hormones
3 Major Types of Stimuli
(a)
Humoral stimulus
1 Capillary blood contains
low concentration of Ca2+,
which stimulates…
(b)
Neural stimulus
1 Preganglionic sympathetic fibers
stimulate adrenal medulla cells…
(c)
Hormonal stimulus
1 The hypothalamus secretes
hormones that…
Hypothalamus
CNS (spinal cord)
Capillary (low Ca2+
in blood)
2 …stimulate
the anterior
pituitary gland
to secrete
hormones
that…
Thyroid gland
(posterior view)
Parathyroid
glands
Pituitary
gland
Preganglionic
sympathetic
fibers
Thyroid
gland
Adrenal
cortex
Gonad
(Testis)
Medulla of
adrenal gland
Parathyroid
glands
PTH
Capillary
2 …secretion of parathyroid hormone
2 …to secrete catecholamines
(PTH) by parathyroid glands. PTH acts to (epinephrine and norepinephrine)
increase blood Ca2+.
3 …stimulate other endocrine glands
to secrete hormones
Control of Hormone Secretion
• Always controlled by feedback loops
– Blood concentration declines below a
minimum
• More hormone is secreted
– Blood concentration exceeds maximum
• Hormone production is halted
• Referred to as positive & negative
feedback loops  usually multiple
hormones involved in the pathway
The Pituitary Gland
• Secretes 9 major hormones
• Attached to the hypothalamus
by the infundibulum (= pituitary stalk)
• 2 basic divisions of the pituitary gland:
– Adenohypophysis (= anterior pituitary)
• Has 3 major divisions
– Neurohypophysis (= posterior pituitary)  has
2 major divisions
The Pituitary Gland
Corpus
callosum
Thalamus
Pineal
Hypothalamus
Mammillary
body
Brain stem
(b)
Pituitary (hypophysis)
Acidophil Basophil
(a)
Optic chiasma
Median eminence
of hypothalamus
Anterior lobe
Pars tuberalis
Pars intermedia
Pars distalis
Mammillary
body
Tuber cinereum
Posterior lobe
Infundibulum
Pars nervosa
Capillary with
red blood cells
(c)
Chromophobe cell
(d)
Spherical cluster
of cells
Anterior Pituitary
• The largest division of the anterior lobe:
– Makes & secretes 7 different hormones
– Tropic hormones regulate the hormone
secretion of other glands
• Includes: TSH, ACTH, FSH, LH
Anterior Pituitary
• GH, PRL, & MSH
– Act directly on non-endocrine target tissues
Anterior Pituitary
• Growth hormone/GH (= somatotropic
hormone)
– Produced by somatotropic cells
– Stimulates body growth by stimulating
increased protein production & growth of
epiphyseal plates
– Stimulates growth directly & indirectly by the
liver’s secretion of insulin-like growth factor-1
(IGF1)
Anterior Pituitary
• Thyroid-stimulating hormone (TSH)
– Produced by thyrotropic cells
– Signals thyroid gland to secrete thyroid
hormones
• Adrenocorticotropic hormone (ACTH)
– Stimulates adrenal cortex to secrete
hormones that help cope with stress
Anterior Pituitary
• Melanocyte-stimulating hormone (MSH)
– In humans, MSH functions in appetite
supression (“My Stomach’s not Hungry” hormone)
• Gonadotropins  FSH & LH
– Produced by gonadotropic cells
– they increase sex steroid production by the
gonads & the maturation of gametes
Anterior Pituitary
• Prolactin (PRL)  produced by prolactin
cells
– Targets milk-producing glands in the breast 
stimulates milk production after childbirth &
during lactation
– There are prolactin-secreting tumors that
result in milk production in both women & men
(galactorrhea; can lead to problems with fertility)
– Prolactin central to milk production in all
female mammals (exception – male bats)
Anterior Pituitary
Anterior Pituitary
Hypothalamic Control of Hormone
Secretion from the Anterior Lobe
• The hypothalamus
– Controls secretion of anterior lobe hormones
– Exerts control by secreting:
• Releasing hormones  prompt anterior lobe to
release hormones
• Gonadotropin-releasing hormone (GnRH) 
produced & secreted by specific neurons of
hypothalamus  cause release of FSH & LH
• Inhibiting hormones  turn off secretion of
anterior lobe hormones
Hypothalamic Control of Hormone
Secretion from the Anterior Lobe
• Releasing hormones
– Are secreted like neurotransmitters
– Enter a primary capillary plexus
– Travel in hypophyseal portal veins to a
secondary capillary plexus
• From the secondary capillary plexus, the releasing
hormones trigger the anterior lobe to secrete the
specific hormone, which is dumped into the
secondary capillary plexus, and enters general
circulation & travels to target organs
Hypothalamic Control of Ant. Pituitary
Secretion
Hypothalamic neuron
cell bodies
Superior hypophyseal
artery
Hypophyseal
portal system
Primary capillary
plexus
Hypophyseal
portal veins
Secondary
capillary
plexus
Anterior lobe
of pituitary
TSH, FSH, LH,
ACTH, GH, PRL
Hypothalamus
1 When appropriately
stimulated, hypothalamic
neurons secrete releasing
and inhibiting hormones into
the primary capillary plexus.
2 Hypothalamic hormones
travel through the portal
veins to the anterior pituitary
where they stimulate or
inhibit release of hormones
from the anterior lobe.
3 Anterior pituitary
hormones are secreted into
the secondary capillary
plexus.
(a) Relationship between the anterior pituitary & hypothalamus
Posterior Pituitary
• Is structurally part of the brain
• Its axons make up the hypothalamic–
hypophyseal tract
– Arises from neuronal cell bodies in the
hypothalamus
• Supraoptic nucleus
• Paraventricular nucleus
Relationship Between Posterior
Pituitary & Hypothalamus
1 Hypothalamic neurons
synthesize oxytocin and
ADH.
Paraventricular nucleus
Hypothalamus
Supraoptic nucleus
2 Oxytocin and ADH are
transported along the
hypothalamichypophyseal tract to the
posterior lobe.
Optic chiasma
Infundibulum (connecting stalk)
Hypothalamichypophyseal
tract
Inferior
hypophyseal
artery
Axon
terminals
Posterior
lobe of
pituitary
Oxytocin
ADH
3 Oxytocin and ADH are
stored in axon terminals
in the posterior pituitary.
4 Oxytocin and ADH are
released into the blood
when hypothalamic
neurons fire.
(b) Relationship between the posterior pituitary & hypothalamus
Posterior Pituitary
• Does not make hormones
• Stores & releases hormones made in the
hypothalamus!
• Releases 2 very important peptide
hormones:
– Anti-Diuretic hormone (ADH) –remember this?
– Oxytocin
Posterior Pituitary
• ADH (Anti-Diuretic hormone; =vasopressin)
– Targets kidneys to resorb water; prevents
diuresis
• Oxytocin
– Induces smooth muscle contraction of
reproductive organs, ejects milk during breast
feeding, & signals contraction of the uterus
during childbirth
Hormones Made by Hypothalamus &
Secreted by Posterior Pituitary
Pineal Gland
• Located on the roof of the diencephalon
• Shaped like a pine cone
• Pinealocytes secrete melatonin
– A hormone that regulates circadian rhythms
Thyroid Gland
• Located in anterior neck around trachea
• Largest pure endocrine gland
• Functions in  metabolism & metabolic rate; how
quickly the body uses energy; making proteins, etc.
• Composed of follicles & areolar
connective tissue 
Thyroid Gland
Hyoid bone
Thyroid cartilage
Common carotid
artery
Epiglottis
External carotid
artery
Superior thyroid
artery
Inferior thyroid
artery
Isthmus of
thyroid gland
Right
subclavian
artery
Left subclavian
artery
Trachea
Left lateral
lobe of thyroid
gland
Aorta
(a) Gross anatomy of the thyroid gland, anterior view
Thyroid Gland Hormones
• Produces 2 types of hormones:
– Thyroid hormones (TH = T3 & T4)
– Thyroxine (T4) & triiodothyronine (T3)  tyrosine-based
hormones produced by thyroid
– Involved in regulation of metabolism  act on nearly all
cells of body; increase basal metabolic rate, protein
synthesis; long bone growth; increase body’s sensitivity to
catecholamines; regulate protein, fat & carb metabolism
– Secreted by  follicular cells
– Calcitonin  Secreted by parafollicular cells; lowers
blood Ca2+ levels (inhibits intestinal Ca2+ absorption &
inhibits osteoclast activity in bones; protects against
calcium loss from skeleton during pregnancy & lactation
Microscopic Anatomy of Thyroid Gland
Colloid-filled Follicular cells
follicles
(secrete thyroid hormone)
Parafollicular cell
(secretes calcitonin)
(b) Photomicrograph of thyroid gland follicles (160)
Parathyroid Glands
• Lie on the posterior surface of thyroid gland
• Contain 2 types of endocrine cells
– Chief cells
• Produce Parathyroid Hormone (PTH)
• Functions to increase blood Ca2+
levels by acting on receptors in
bone, kidney & intestine  all
leads to higher plasma calcium
levels! (opposite of calcitonin)
– Oxyphil cells
• Function unknown...
Parathyroid Glands: Gross & Microscopic
Pharynx
(posterior
aspect)
Thyroid
gland
Esophagus
Parathyroid
glands
Parathyroid
cells
(secrete
parathyroid
hormone)
Oxyphil
cells
Trachea
Capillary
(a) Location of parathyroid
glands, posterior view
(b) Photomicrograph of
parathyroid gland
tissue (360)
Adrenal Glands (Suprarenal
Glands)
• Pyramid-shaped glands located on the
superior surface of each kidney
• Supplied by about 60 suprarenal arteries
• Nerve supply is almost exclusively
sympathetic fibers!!!
• Functions in…? -remember?
Adrenal Glands
• 2 endocrine glands in one!
– Adrenal medulla  a cluster
of neurons
• Part of Sympathetic Nervous System!
• Postganglionic nerve fibers
– Adrenal cortex  forms the bulk of the gland
• Functions  All adrenal hormones help
one cope with danger, terror, or stress
Adrenal Medulla
• Chromaffin cells
– Are modified ganglionic sympathetic neurons
– Secrete amine hormones epinephrine &
norepinephrine (= catecholamines)
• Enhance “fight-or-flight” response
• Hormones are stored in secretory vesicles
– Are arranged in spherical clusters & some
branching cords
Adrenal Cortex
• Secretes lipid-based steroid hormones 
glucocorticoids, mineralocorticoids,
androgens, E2
• Cortex is composed of 3 layers (zones):
– Zona glomerulosa  cells arranged in
spherical clusters
– Zona fasciculata  cells arranged in parallel
cords; contains lipid droplets
– Zona reticularis  cells arranged in a
branching network
The Adrenal Gland: Gross & Microscopic
Hormones
secreted
Capsule
Zona
glomerulosa
Aldosterone
Adrenal gland
Medulla
Cortex
Zona
fasciculata
Cortisol
and
androgens
Cortex
Zona
reticularis
Medulla
Kidney
Adrenal
medulla
(a)Drawing of the histology of the
adrenal cortex and a portion of
the adrenal medulla
Epinephrine
and
norepinephrine
(b) Photomicrograph (140X)
Stress & the Adrenal Gland
Short-term stress
More prolonged stress
Stress
Nerve impulses
Hypothalamus
CRH (corticotropinreleasing hormone)
Spinal cord
Corticotroph cells
of anterior pituitary
Preganglionic
sympathetic
fibers
To target in blood
Adrenal medulla
(secretes amino acid–
based hormones)
Catecholamines
(epinephrine and
norepinephrine)
Short-term stress response
1.Increased heart rate
2.Increased blood pressure
3.Liver converts glycogen to glucose and releases
glucose to blood
4.Dilation of bronchioles
5.Changes in blood flow patterns leading to decreased
digestive system activity and reduced urine output
6.Increased metabolic rate
Adrenal cortex
(secretes steroid
hormones)
ACTH
Mineralocorticoids
Glucocorticoids
Long-term stress response
1.Retention of sodium
and water by kidneys
2.Increased blood volume
and blood pressure
1.Proteins and fats converted
to glucose or broken down
for energy
2.Increased blood glucose
3.Suppression of immune
system
Pancreas
• Located in the posterior abdominal
wall
• Functions  regulation of blood sugar, metabolism
& digestion
• Contains endocrine & exocrine cells
– Exocrine cells
• Acinar cells  secrete digestive enzymes (remember?..)
– Endocrine cells
• Islets of Langerhans
• About one million islets  scattered throughout the
pancreas
Islets of
Langerhans
• Main endocrine cell types:
– Alpha cells ( cells)  secrete glucagon
• Signal liver to release glucose from glycogen
• Raises blood sugar
– Beta cells ( cells)  secrete insulin
• Signal most body cells to take up glucose from the
blood
• Promotes storage of glucose as glycogen in liver
• Lowers blood sugar
Pancreas
• Pancreatic islets contain 2 rare cell types
– Delta (∂) cells
• Secrete somatostatin
– Inhibits secretion of insulin & glucagon
– F (PP) cells
• Secrete pancreatic polypeptide (PP)
– May inhibit exocrine activity of the pancreas
Thymus
• Located in the lower neck
& anterior thorax
• Function  Important immune organ;
T-lymphocytes arise from precursor cells here;
decreases in size &
function after puberty
Gonads
• Main sources of sex steroid
hormones 
– Testes & ovaries
• Male
– Interstitial Leydig cells secrete androgens
• Primarily testosterone
– Promotes development of the testes
– Promotes the formation & survival of sperm
– Development & maintenance
of secondary sex characteristics
Gonads
• Female
– Ovaries
• Androgens & estrogens secreted by follicular cells
• Estrogen
– Dev & maintenance of secondary sex
characteristics
– Dev of eggs
– Regulation of menstrual cycle
• Progesterone
– Prepares the uterus for Pregnancy
Sex Steroid Hormones
• Critical for normal reproductive function
• Affect many other tissues/organs of the body
•
•
•
•
•
Bone
Muscle
Fat
Liver & kidney
Brain (sexual behavior, mood, cognition)
Sex Steroid Hormones & Bone
• Promote bone growth (adolescent growth spurt)
2008
2010
Sex Steroid Hormones & Bone
• Also maintain bone mass
• With aging or in diseases that render the gonads
dysfunctional (low T/E2) loss of bone mass & osteoporosis!
Could Skeletal Tissue Exert
Reciprocal Effects on Gonads?
???
sex steroids
gonad
bone
Osteocalcin – A Hormone
Secreted by Bone
• Secreted by osteoblasts (bone-forming cells of skeleton)
• Involved in bone growth & bone remodeling
• Roles in energy metabolism & glucose homeostasis
-Ocn-/- mice have skeletal & metabolic defects  greater bone
density, fat, & insulin resistance
(Lee et al. Cell, 2007; Ducy et al. Nature, 1996)
• Ocn-/- mice – reduced T & reduced fertility (Oury et al. Cell, 2011)
• Osteocalcin – a hormone linking bone with metabolism &
reproduction
Bone & Fertility in Balance
Energy Metabolism
Aging
Fertility Disorders
Metabolic Dysfunction
Genes
Environmental factors
o
Ad

 
Gs
Ocn binding
to receptor
P
GT
Functional gonad
(testis)
Osteocalcin (Ocn)
Testosterone
(& other factors)
Healthy bones
(bone mass)
Aging
Fertility Disorders
Metabolic Dysfunction
Genes
Environmental factors
Germ cell development
(sperm)
Male Fertility
(Female Fertility?)
Other Endocrine Structures…
• Endocrine cells occur within:
– The heart
• Atria contain atrial natriuretic peptide (ANP) 
vasodialator; involved in control of BP
– The GI tract
• Enteroendocrine cells (Ghrelin, CCK, gastrin,
secretin, motilin, enterocrinin, etc.…)
• Ghrelin  the hunger (“belly-growling”) hormone!
– The placenta
• Sustains the fetus & secretes several hormones
(human chorionic gonadotropin, progesterone, E2,
relaxin…)
Other Endocrine Structures
– The kidneys
• Cells of the juxtaglomerular apparatus (JGA)
secrete renin
• Endothelial cells & interstitial connective tissue 
secrete erythropoietin
– The skin
• With sunlight, modified cholesterol molecules
convert to a precursor of vitamin D
– Bone
• Osteocalcin, & others…
Pituitary Disorders
• Gigantism
– Hypersecretion of GH in children
• Pituitary dwarfism
– Hyposecretion of GH
• Diabetes insipidus  excessive thirst &
urination
– Hypothalamus does not make enough ADH
Disorders of the Pancreas:
Diabetes Mellitus
• Caused by
– Insufficient secretion of insulin
– Resistance of body cells to the effects of
insulin
• Type 1 Diabetes
– Develops suddenly, usually before age 15
– T cell–mediated autoimmune response
destroys beta cells of pancreas  no more
insulin
Diabetes Mellitus
• Type 2 Diabetes
– Adult onset
– Usually occurs after age 40
– Associated with higher BMI/obesity
– Body cells have decreased
sensitivity to insulin
– Controlled by dietary
changes & regular
exercise!
Disorders of the Thyroid
• Grave’s disease
– Most common type of hyperthyroidism
– Immune system makes abnormal antibodies
• Stimulates the over-secretion of TH by follicle cells
– Leads to nervousness, weight loss, sweating,
& rapid heart rate; protrusion of the eyeballs
– Too much thyroid action  metabolism too high!
Disorders of the Thyroid
• Myxedema
– Adult hypothyroidism
– Antibodies attack & destroy thyroid tissue
– Low metabolic rate & weight gain are
common symptoms
Disorders of the Thyroid
• Endemic goiter
– Due to lack of iodine in the diet
• Cretinism
– Hypothyroidism in children
– Short, disproportionate body, thick tongue,
& mental retardation
Thyroid Disorders
Disorders of the Adrenal Cortex
• Cushing’s syndrome
– Caused by hypersecretion of glucocorticoid hormones
(cortisol)  usually a pituitary tumor
– Weight gain on trunk, fat pads, “moon face,” thinning
of the skin, striae, etc.
• Addison’s disease
– Hyposecretory disorder of the adrenal cortex
– Deficiencies of both mineralocorticoids &
glucocorticoids
– Fatigue, weakness, weight loss, mood changes,
hyperpigmentation
Adrenal Cortex Disorders
Adrenal Cortex Disorders
Cushing’s
Addison’s
The Endocrine System with Age
• Endocrine organs operate effectively until old age
• Anterior pituitary
– Increase in connective tissue & lipofuscin
– Decrease in vascularization & number of hormonesecreting cells
• Adrenal cortex
– Normal rates of glucocorticoid secretion continue
• Adrenal medulla
– No age-related changes in catecholamine secretion
• Gonads
– Testes: decline in sperm counts, but can function
throughout life
– Ovaries: loss of all eggs/diminished function in mid-life
Congratulations!!!!
You made it to the end
of Anatomy!
What’s Next?
Today’s Lab: Final Lab Exam 5!
Mon, May 20, 5:25 pm: Final Exam!