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.