Histology: Endocrine

Question 1

Endocrine Glands

Answer 1

Secrete into bloodstream (no ducts)

• Pituitary Gland
• Pineal Gland
• Thyroid Gland
• Parathyroid Gland
• Adrenal Glands
• Testes/Ovaries

Diffuse neuroendocrine system

• Scattered, hormone producing cells within organs (pancreatic islets, enteroendocrine cells in gut)

Question 2

Hypothalamus & Pituitary Gland

Answer 2

“Master regulators of endocrine system”

• Hypothalamus regulates pituitary gland
• Pituitary secretions regulate other endocrine glands

Examples:

• Blood pressure
• Body Temp
• Fluid/Electrolyte Balance
• Body Weight
• Appetite
• Reproductive System

Question 3

Pituitary Gland

Answer 3

below hypothalamus

• Anterior Lobe (adenohypophysis)
  
  • _​_Glandular epithelium
  • Three Regions:
    
    • Pars Distalis
    • Pars Intermedia
    • Pars Tuberalis (wraps around infundibulum, attached to hypothalamus)
• Posterior lobe (neuohypophysis)
  
  • _​_Neurosecretory tissue
  • Two Regions:
    
    • Infundibular Stalk
    • Pars Nervosa

Question 4

Development of Pituitary Gland

Answer 4

Anterior Lobe:

• Derives from ectoderm from oropharynx roof (Rathke’s Pouch)

Posterior Lobe:

• Neuroectoderm from floor 3rd ventricle

Question 5

Regulation of Pituitary Gland (via Hypothalamus)

Answer 5

Neurosecretory Cells

• Produce and secrete releasing and inhibitory hormones and act on cells in pars distalis (anterior lobe)

Paraventricular Nucleus***

• Produce oxytocin

Supraoptic Nucleus***

• Produce anti-diuretic hormone (ADH)

***Axons project through infundibular stalk to posterior lobe (pars nervosa)

• Hormones accumulate here and will be released from posterior pituitary when appropriately stimulated

Question 6

Blood Supply to Pituitary Gland

Answer 6

• Hypophysial arteries from internal carotid artery
  
  • ​Superior Hypohysial a. supples upper portions: pars tuberalis, median eminence (floor hypothalamus), infundibulum
  • Inferior Hypophysial a.
    
    • ​Supples pars nervosa
  • *****No direct arterial supply to pars distalis

Question 7

Hypothalamo-Hypophyseal Portal System

Answer 7

• Carries hypothalamic neuroendocrine secretions (from neurosecretory cells)
  
  • Primary Capillary Plexus picks up inhibitory/releasing hormones
    
    • Fenestrated/Sinusoidal
  • Hypophyseal Portal Veins travel along pars tuberalis from primary capillary plexus to secondary capillary plexus
    
    • _​_Surrounds pars distalis
• Blood leaves pituitary gland via hypophyseal veins

Question 8

Pars Distalis

Answer 8

• Main portion of anterior lobe (75%)
• 3 Cell Types:
  
  • Acidiphils (bright pink)
  • Basophils (purple)
  • Chromophobes
    
    • Either undifferentiated stem cells (haven’t yet formed acidiphils or basophils) or recently degranulated cells

Question 9

Acidophils

Answer 9

Somatotrophs

• 50% Cells in anterior lobe
• Secrete Growth Hormone (GH)
  
  • ​Acts on growth plates/skeletal muscle
• Stimulated by GHRH and Ghrelin (coordinates growth with meal consumption)
• Inhibited by somatostatin

Lactotrophs

• Secrete prolactin
  
  • Acts on mammary glands during pregnancy (stimulates milk production)
• Stimulated by thyrotropin releasing hormone and vasoactive inhibitory peptide
• Inhibited by dopamine

Question 10

Basophils

Answer 10

Corticotrophs

• Produce precursor of adrenocorticotropic hormone (POMC)
  
  • POMC cleaved to form ACTH
• Stimulated by CRH
• Inhibited by Cortisol
• Results in release of cortisol from adrenal cortex

Gondaotrophs

• Secrete FSH and LH
• Stimulated by GnRH

Thyrotrophs (least populous cell type)

• Secrete thyroid stimulating hormone
• Stimulated by TRH
• Inhibited by somatostatin

Question 11

Pars Intermedia

Answer 11

• Between pars distalis and posterior lobe
• Unclear function
• Contains colloid-filled cysts
  
  • Remnants of Rathke’s Pouch
• Contains basophils and chromophobes
  
  • Basophils here secrete POMC (cleaved differently into Melanocye Stimulating Hormone & beta-endorphin)

Question 12

Pars Tuberalis

Answer 12

• Surrounds infundibular stalk
• Contains vessles of hypothalamo-hypophyseal portal system

Question 13

Pituitary Adenoma

Answer 13

• Benign, slow-growing
• Affect on somatotroph:
  
  • ​Growth hormone secreting tumor
  • In children –> Gigantism (longer bones)
  • In adults –> Acromegaly (thicker bones)
    
    • Usually in forehead/jaw, hands/feet
• Affect on corticotroph:
  
  • Cushing’s Disease
    
    • Overproduction of ACTH, too much cortisol
• Affect on Lactotrophs
  
  • Prolactinomas
    
    • ​Milk production in non-lactating females
    • Amenorrhea
    • Erectile dysfunction
    • Infertility

Question 14

Posterior Pituitary

Answer 14

• Not a true endocrine gland
• Contains unmyelinated axons of secretory neurons
  
  • Cell bodies exist in suproptic and paraventricular nuclei of hypothalamus
• Pituicytes
  
  • _​_Glial cells in postioter pituitary
  • Support axons

NB: neurosecreotry bodies/Herring Bodies (dilations of axon terminals)

• Storing hormones released from posterior pituitary
• In close association with fenestrated capillaries
• Two Hormones:
  
  • ​ADH (vasopressin)
    
    • Produced in supraoptic nucleus
    • Released in response to low blood volume, or increased osmolality
    • Acts on aquaporin channels of collecting ducts to promote water reabsorption
    • Can stimulate vascular smooth muscle contraction at high concentrations
  • Oxytocin
    
    • ​Produced in paraventricular nucleus
    • Released in responsed to distenstion cervix/vagina during childbirth or nipple stimulation
    • Stimulates contraction of uterine smooth muscle or myoepithelial cells in mammary gland to faciliate milk ejection

Question 15

Adrenal Gland

Answer 15

Adrenal Cortex

• From intermediate mesoderm
• Produces steroid hormones
• Three regions:
  
  • Zona glomerulosa
  • Zona fasciculata
  • Zona reticularis

Adrenal Medulla

• From neural crest cells
• Produces catecholamines (epinephrine and norepinephrine)

Question 16

Zona Glomerulosa

Answer 16

• Just deep to connective tissue capsule
• Small, densely-packed basophilic cells arranged in glomeruli
• Secrete mineralicorticoids (Aldosterone)
  
  • Stimulates Na+ reabsorption in DCT
  • Stimulated by angiotensin II (part of RAAS)
  • Weekly stimulated by ACTH

Question 17

Zona Fasciulata

Answer 17

• Larger polyhedral cells
  
  • Lots of lipid droplets within cytoplasm
  • Arranged in long cords, 1-2 cells thick separated by fenestrated sinusoidal capillaries
• Secretes glucocorticoids (primarily cortisol)
  
  • ​Released in response to ACTH
  • Cortisol works in negative feedback loop

Cortisol:

• Releases energy (breaks down fat/glycogen)

Question 18

Zona Reticularis

Answer 18

• Small cells in anastomosing cord network
  
  • Separated by capillaries
  • Cells contain lipofuscin (stain darker)
• Secrete weak androgens (DHEA)
  
  • Converted to testosterone in both sexes
  • Secretes small amounts of glucocorticoids
  • Stimulated by ACTH

Question 19

Adrenal Medulla

Answer 19

• Chromaffin Cells (stain with chromium dyes)
  
  • Large, pale-staining
  • Granules that store epinephrine and norepinephrine
  • Modified postganglionic sympathetic neuron
    
    • Innervation from preganglionic sympathetic neuron causes release of granules

Question 20

Blood Supply of Adrenal Gland

Answer 20

• Suprarenal arteries form subcapsular capillary plexus
  
  • Cortical Arterioles
    
    • _​_Form adreno-cortico sinusoid that goes to medulla (venous blood)
  • Medullary Arterioles
    
    • Passing blood straight to medulla (arterial blood)
• Central Adrenomedullar veins collect blood in medulla
  
  • Longitudinal smooth muscle to faciliate efflux of hormones

Question 21

Thyroid Gland

Answer 21

• Surrounded by thin connective tissue capsule, extend septa that separate gland into lobules
• Comprised of epithelial follicles filled with colloid
  
  • Gelatinous storage form of inactive thyroud hormones

Follicular Cells

• Darker-staining; cuboidal when active, squamous when inactive
• Line lumens of follicles
• Secrete thryoglobulin; stimulated by TSH
  
  • This is converted to T3, T4

Parafollicular cells (C-Cells)

• Larger, pale-staining
• At periphery of follicles or between follicles
• Secrete calcitonin in response to elevated blood Ca2+ levels
  
  • Promotes Ca2+ deposition into bones by suppressing osteoclasts

Question 22

Follilcular Cell EM

Answer 22

• Short microvilli on apical surface
• Abundant RER, golgi, lysozomes (break down colloid to secrete thyroid hormones)

Question 23

Parafollicular Cells EM

Answer 23

• Indented nucleus
• Secretory granules (calcitonin) within the cytoplasm

Question 24

T3 and T4

Answer 24

• Produced via iodination of tyrosine residues in thryoglobulin
• Stored as colloid in inactive form bound to thyroglobulin
  
  • Stores product outside cell
• T4 is more abundant; T3 is more potent
  
  • Regulate basal metabolism, heat production, contributes to growth and development

Question 25

Thyroid Disease

Answer 25

* **Goiter** * Enlargement of thyroid gland * Indicates hyper or hypoactive gland * **Hyperthyroidism** (Grave's Disease) * Increased metabolism/weight loss, excessive sweating * Protrusion of eyes due to increased CT deposition behind eyeballs * **Hypothyroidism** (Myxedema) * Mental/physical sluggishness, weight gain, chills * Insufficient dietary iodine, Hashimoto's disease (autoimmune disease that destroys follicular cells)

Question 26

Parathyroid Gland

Answer 26

* Embedded in posterior aspect of thyroid gland * Surrounded by CT capsule * Septa form irregular lobules * Cells in cords: * **Principal Cells** (purple) * Most numerous * Small cells, **secrete Parathyroid Hormone** * **​**Regulates blood calcium, secreted in response to *low* blood calcium levels * Stimulates resorption of bone, increased absorption of Ca2+ in gut * Stimulates Kidneys to convert vitamin D to active form * **Absense leads to decreased blood Ca2+ levels** (tetany can result) * **Oxyphil Cells** * Larger, eosinophilic * No known secretory roll * **Adipose Cells**

Question 27

Pineal Gland

Answer 27

* Regulates circadian rhythms * **Secrete melatonin** in response to darkness * Light inhibits melatonin secretion * Also secretes serotonin, norepinephrine and dopamine * Derived from neuroectoderm **Pinealocytes** * Most of cells * **Secrete melatonin and serotonin** **Interstitial Glial Cells** (less than 5%) * Supporting cells, resemble astrocytes **Corpora Arenacea** * "Brain Sand" * Unknown function * More numerous with age