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Endocrine System
What we’ll talk about…
• Hypothalamus and Pituitary Gland
• Adrenal Gland
• Thyroid Gland
• Parathyroid Gland
• Endocrine Pancreas
Cerebralcortex
Hypothalamus
Anterior Pituitary
Adrenalgland Thyroid
Target tissues
Releasing Hormones
ACTH TSH
Cortisol T3/T4
Low Blood Pressure
Aldosterone
Pancreas
Glucose
Insulin
Parathyroid
Low Ca2+
PTH
Pituitary Gland
The pituitary contains distinct anterior and posterior regions and interfaces with the hypothalamus.
Hypothalamus
Anterior Pituitary Posterior Pituitary
Pituitary Stalk Hypothalamic Axons
Cluster of hormome-secreting cells
Axon termini release hormones
The anterior pituitary contains different cells that produce different unique hormones.
• Adrenocorticotrophic Hormone (ACTH) - Corticotrophs
• Thyroid-Stimulating Hormone (TSH) - Thyrotrophs
• Growth Hormone (GH) - Somatotrophs
• Prolactin - Mammotrophs
• Leutenizing Hormone (LH) - Gonadotrophs
• Follicle-Stimulating Hormone (FSH) - Gonadotrophs
Hypothalamic neurons release oxytocin or vasopressin in the posterior pituitary.
• Oxytocin - induce uterine contraction and ejection of milk from mammary glands
• Vasopressin - increase water absorption in collecting ducts
The anterior pituitary is highly cellular while the posterior pituitary connects to the hypothalamus.
Anterior Pituitary
Posterior Pituitary
Hypothalamus
Pituitary Stalk
Optic Chiasm
H&E stain reveals a mix of acidophils, basophils, and chromophobes in the anterior pituitary.
Acidophil
Basophil
Chromophobe
Reticulin fibers organize cells into clusters in the anterior pituitary.
Reticulin Fibers
Capillary
Releasing hormones trigger secretion of specific hormones by cells in the anterior pituitary.
• Corticotropin Releasing Hormone (CRH) -> ACTH
• Thryotropin Releasing Hormone (TRH) -> TSH
• Growth Hormone Releasing Hormone (GHRH) -> GH
• Gonadotropin Releasing Hormone -> FSH and LH
The hypothalamus produces releasing hormones which move through the portal vein to the anterior pituitary.
Hypothalamus
Anterior Pituitary
Portal Vein
Cells in the anterior pituitary store hormones in granules and
Fenestrated Endothelium
SomatotrophSecretory Granule
Interstitium
Capillary Lumen
The posterior pituitary contains the axons of neurons whose cell bodies reside in the hypothalamus.
Hypothalamus
Anterior Pituitary Posterior Pituitary
Pituitary Stalk Hypothalamic Axons
Cluster of hormome-secreting cells
Axon termini release hormones
Pituicytes are support cells in the posterior pituitary.
Herring Body
Pituicyte
Blood Vessel
Hormones released in the posterior pituitary diffuse across a fenestrated endothelium.
Capillary Lumen
Fenestrated Endothelium
Axon Terminus
Adrenal Glands
The adrenal gland is composed of glomerulosa, fasciculata, retirularis and medulla.
GlomerulosaFasciculataReticularis
CortexMedulla
Cells in the glomerulosa secrete mineralocorticoids and are clustered by trabeculae.
Glomerulosa Capsule
Fasciculata
Trabecula
Cells in the fasciculata secrete glucocorticoids and stain more lightly than glomerulosa.
Fasciculata
Glomerulosa
Cortisol generates negative feedback by inhibiting secretion of CRH and ACTH.
CRH
ACTH
AdrenalGlands
Cortisol
Corticotroph
Hypothalamus
Pituitary Cell
Target Cell
Hormone
Throphic Hormone Nega
tive
Feed
back
Cortisol Pathway
Cortisol and aldosterone are structurally similar and produced by the same biochemical pathway.
O
CH3
HOCH3
C O
CH2OH
O
CH3
HOC
C O
CH2OH
O
H
O
CH3
HOCH3
C O
CH2OH
OH
O
CH3
OCH3
C O
CH2OH
OH
Aldosterone
Corticosterone Cortisol Cortisone
Glomerulosa Cells
17 α-Hydroxylase 11 β-Hydroxysteroid dehydrogenase (HSD)
Cholesterol
The reticular is the inner most layer of the adrenal cortex and produces androgens.
Reticularis
Fasciculata
Cells in the medulla secrete epinephrine and norepinephrine in response to external stress.
Medulla
Reticularis
Blood can follow two different pathways through the adrenal gland.
Central Vein
Fasciculata
Glomerulosa
Reticularis
Medulla
Capsule
Plexus
Long Cortical Artery
Short Cortical Artery
Thyroid
The thyroid secretes and stores inactive hormone in large pools called colloid.
C-cell
Colloid
Principle Cell
Na+ I-
I-I0
CH2
OH
CH2
OH
CH2
OH
CH2
OH
CH2
OH
CH2
OH
CH2
OH
CH2
OHCH2
O
OH
CH2
O
OH
CH2
O
OH
CH2
O
OH
Thyroglobulin secretion
Iodination
EndocyticVesicle
Lysosome
Conjugation
Cl-
T3 T4
TPO
T4T3
Colloid
Principle Cell
Interstitium
T3 and T4 exert negative feedback on the release of TRH and TSH.
CRH TRH
TSHACTH
AdrenalGlands
Cortisol T4/T3
Corticotroph
Hypothalamus
Pituitary Cell
Target Gland
Hormone
Throphic Hormone Nega
tive
Feed
back
Cortisol Pathway T4/T3 Pathway
Thyroid
Nega
tive
Feed
back
Thyrotroph
Parathyroid
Parathryoid contains chief cells that release parathyroid hormone when serum calcium is low.
Chief Cells
Oxyphil Cell
Endocrine Pancreas
Cells in the endocrine pancreas secrete glucagon (α), insulin (β) or somatostatin (δ).
Exocrine Pancreas
Islet of Langerhans
Take home messages…
• Hypothalamus regulates secretions from anterior pituitary.
• Hormones from anterior pituitary regulate cells in other organs.
• The adrenal gland generates mineralcorticoids, glucocorticoids, androgens and epinephrine from different regions.
• The thyroid gland produces T3 and T4 hormones.
• The hypothalamic-pituitary-adrenal axis produces cortisol in response to stress.