Download Pituitary Gland

CHAPTER 5 – PITUITARY HORMONES
CHAPTER 7 – NEUROHYPOPHYSIAL
HORMONES
Last 2 chapters before the test – Sept. 18th , Test 1
Pituitary

Generally considered to have up to 3 parts
 Anterior
pituitary (lobe) – adenohypophysis
 Posterior pituitary (lobe) – neurohypophysis
 Pars intermedia
 Not

in all species – sort-of in humans
Once considered the most important of endocrine
organs
 Associated
with several important physiological
processes in the body
 Secretes a number of hormones
Pituitary hormones have wideranging effects on endocrine and
non-endocrine target tissues
Pituitary – general information


Once considered the
collector of brain mucous
Later considered ‘the master
gland’


Position within several important
physiological responses
However, highly regulated


Important “upper-level
manager”
Important connection between
hypothalamus and body
Pituitary – general information


0.5-1g in adult human male, larger in pregnant
female
Located within the sella turcica
 Most

inaccessible gland
Situated below hypothalamus, near optic chiasm
 Connected


via an infundibular stem or stalk
Richly vascularized tissue
2 main parts, different embryological origin
Pituitary – general information

Size of the pituitary and depends on
species and physiological condition
 Color-changing
animals, pars intermedia large
 Will
secrete large amounts of hormones that
influence color changes
 Posterior
pituitary larger in land vertebrates
than aquatic vertebrates
 Cell population of anterior pituitary can
change in reproductive individuals
Sella turcica – cuplike structure where
pituitary rests
Posterior Pituitary

Derived from neural ectoderm of floor of forebrain
Collection of nerve endings
 Sometimes considered part of hypothalamus

Nerve bodies located in hypothalamus
 Nerve endings collected in posterior pituitary (neurohemal organ)

 Pass
through median eminence and axons make up much of
infundibular stem

Secretes vasopressin (AVP, ADH) and oxytocin (OT)
These 2 hormones similar in amino acid sequence
 Genes both on chromosome 20


May represent a duplication and inversion
Posterior pituitary

Two hypothalamic nuclei
have extensions into the
posterior pituitary



Large neurosecretory
granules found in nerve
endings


Supraoptic nuclei (SON)
Paraventricular nuclei (PVN)
Contain hormones for quick
release
Numerous pituicytes
interspersed among nerves

Supporting function
Posterior pituitary

Blood supply from the inferior
hypophysial artery





Note, neurons end on the
capillary network
Quick release of hormone
products into circulation
PVN neurons secrete oxytocin
(OT)
SON neurons secrete
vasopressin (AVP)
Two hormones are similar, nuclei
of production are close
together, genes for hormones
close together

Different stimuli for release
Oxytocin (OT)


Induces milk let-down (in mammals) and uterine
contractions in labor (major actions)
Milk release (let-down)

Nerve stimulation of mammary tissue induces release
Neural pathways conduct stimulii
 Although auditory and optic sensory can be influential


Uterine contractions

Induces myometrium contractions


May work in concert with rising estrogen levels to induce OTreceptor upregulation in uterus
These effects clearly sex-specific. OT in males does
have actions
Oxytocin (OT)

Contractile actions on smooth muscle
Can induce contraction in vascular smooth muscle, unsure
how widespread
 Potential influences on smooth muscle contraction in
reproductive organs (during copulation)


Effects on maternal behavior


Mating behavior


May work in concert with estrogen
May work in concert with estrogen and progesterone
(ovulation) to make females more receptive
Social behavior – esp. mate affiliation
Vasopressin (AVP)


Two major functions – smooth muscle contraction or
relaxation, movement of water across epithelial
tissue
Osmoregulation
When osmolality low, AVP decreased and water lost
 When osmolality high, AVP released and water
retained


Blood volume regulation

Changes in volume/pressure



Loss of volume or pressure (hemorrhage and blood loss) AVP
released and water retained
Baroreceptors in heart monitor pressure
Influences on social behavior
Suppresses
vasopressin (ADH),
makes you urinate
(contributes to your
hangover, which is
mostly dehydration)
Adenohypophysial and other functions

Both posterior pituitary hormones influence the anterior
pituitary

OT appears to influence stress response

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
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
Inhibitory
OT may also influence prolactin secretion
AVP may be stimulatory to the stress response
AVP may also stimulate release of hormones that influence thyroid gland
Vasotocin (AVT) – a version of vasopressin in non-mammalian
vertebrates

Strong impacts on reproductive behavior
Anterior pituitary

Invaginates from oral ectoderm – Rathke’s Pouch


Pars intermedia – next to the posterior pituitary


Pars distalis – anterior pituitary itself
Considerable size in some species
Pars tuberalis - forms from dorsal extensions of anterior
pituitary,
Surrounds infundibular stalk
 Facilitates connection to hypothalamus


Not actually innervated
 Blood
link with hypothalamus
Anterior pituitary

Vascularized by the hypophysial
portal system


Portal systems connect organs, blood
flows through one then another in
quick succession
Blood enters high in median
eminence



Hypothalamic nerve cells connected
to capillary bed
Blood moves through anterior
pituitary
Hypothalamic hormones transferred
to anterior pituitary
Anterior pituitary

Variety of cell types, named after type of stain
accepted

Staining storage vesicles (granules)


Cells types further divided by what type of hormone
they release



Acidophils, basophils, or chromophobes
Somatotrophs, lactotrophs, corticotrophs, thyrotrophs,
gonadotrophs
Pituicytes interspersed among these cell types, can
isolate off populations of different types
Populations are not static
Pituitary-hypothalamus association
This is showing both the blood flow through the pituitary
(anterior pituitary and hypothalamus have blood link) and
neurosecretory nerve cell orientation
Anterior pituitary – hormones


Hormones from anterior pituitary influence mammary
glands, gonads, adrenal glands, and thyroid glands
Can be grouped into families
Growth hormone & prolactin – similar amino acid sequence
 Thyroid stimulating hormone (TSH), follicle stimulating
hormone (FSH), luteinizing hormone (LH) – all glycoproteins,
structurally similar
 Adrenocorticotrophic hormone (ACTH) & alpha-melanocyte
stimulating hormone (αMSH) – similar amino acid sequences,
Proopiomelanocortin (POMC) precursor

Growth hormone (GH)


Secreted by somatotrophs (acidophils)
No specific target tissue

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Induces chondrocytes (cartilagenous growth plates) to
produce more extracellular matrix
Enhances amino acid incorporation into muscle


Influences general somatic growth
Source of abuse as performance enhancer
Effects on protein metabolism & electrolytes

Mediated by somatomedins (IGFs)
Lengthening of cartilagenous growth plates induces
elongation of long bones and increases in height
Growth hormone (GH)


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Accounts for 4-10% wet weight of anterior pituitary
Circulates with a binding protein - protection
Levels stable through childhood

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Decline with age in adults

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Peak during periods of growth
GH is new anti-aging drug
Daily secretion fluctuates
GHRH and somatostatin control release
Similar to prolactin – 161 amino acids similar (both
191 total), 2 sulfide bonds each, in same place
Prolactin (PRL)


Produced by lactotrophs
(acidophils)
Mammary growth &
development

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Levels increase at puberty

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Promotes lactogenesis
Lactotroph population changes
during lactation
Estrogen influence
Levels higher in females,
but both sexes have daily
variation in levels
Prolactin (PRL)


An important role in mammals is lactation (hence the
name) – but not the only role (and in non-mammals)
Role in male reproduction
 PRL
may help maintain expression of luteinizing hormone
receptors in testes

Evolutionarily old – multiple functions other spp.
 Mostly
on integument structures
 Some influences on reproduction, many not related to
reproduction
Pituitary hormones – glycoprotein hormones
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
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Luteinizing hormone (LH), follicle-stimulating hormone
(FSH), thyroid-stimulating hormone (TSH)
All have covalently bound carbohydrate moieties
All comprised of 2 chains – α and β
The α chain is identical to all three
 The β chains are specific to each hormone
 Chains made separately and then joined
 More α chain molecules produced than β chains for each
hormone

Thyroid stimulating hormone (thyrotropin)


Secreted by thyrotrophs (basophils)
Acts on thyroid gland
 Induces


release of thyroxine and triiodothyronine
Important to temperature regulation in endotherms
Has impacts on metamorphosis in ectotherms
Luteinizing hormone (LH)


Secreted by gonadotrophs (basophils)
Has actions on the gonads
 Ovaries,

Females – will induce ovulation
 Works

testes
in concert with estrogen
Males – will induce testosterone release (Leydig
cells)
 FSH
important to receptor regulation
 Spermatogenesis
Follicle stimulating hormone (FSH)


Secreted by gonadotrophs (basophils)
Note, both FSH and LH come from same cell type
 Pulses
of GnRH from hypothalamus important to what
hormone released
 Populations of gonadotrophs that only release one type



Target tissue is the gonad
Females – influences early follicle development
Males – potentiates LH receptors, induces testicular
androgen-binding protein
Adrenocorticotropic hormone (corticotropin)



Produced in corticotrophs (chromophobes, but these
are basophils)
Smallest hormone from pituitary (39 amino acids)
Acts on the adrenal glands


Initiates the stress response – IMPORTANT
Derived from proopimelanocortin (POMC)

Large precursor that yields other hormones (based on
where it’s split
Melanocyte-stimulating hormone
 A. Roulin – coloration and CORT connection


Melanocyte-stimulating hormone

From pars intermedia, but what if you don’t have this?
POMC a large molecule
that can produce two
hormones ACTH and
MSH depending on where
it is split
Control of pituitary hormone secretion

Long-loop and short-loop feedback
 The

number of steps to regulation
Autoinhibition
 Allows

organ to ‘fine tune’ hormone production
Negative and positive feed back systems
Positive feedback in ovulation; LH & estradiol increase
until egg released
 Negative feedback for the thyroid; T3 & T4 suppress TSH
production

Pathophysiology

Can see under- and over-secretion of pituitary
hormones

Hyposecretion


Hypersecretion


Due to destruction of pituitary (tumor) or loss of vascular
connections, disease to the hypothalamus
Due to tumors, disease to the hypothalamus, inhibition of feedback
controls
Examples
GH excess and deficiency
 TSH excess and deficiency
 ACTH excess and deficiency
