Adrenals

Anatomic relationships

• Both adrenals are located at the level of the 11th or 12th ribs
  • Right gland located more superiorly than left
    • Recall that right kidney is located more superioly than left
• The adrenals are enclosed within the perirenal (Gerota) fascia and are completely surrounded by perirenal adipose tissue.
• Each gland is separated from the upper pole of the ipsilateral kidney by a thin layer of connective tissue
• The weight of each gland is ≈5g (range 2-6g)
  • No variation between genders
    • Recall that male kidneys are heavier than female
• Adrenal rests are found in proximity to the celiac axis and along the path of gonadal descent.
• Insert figure

Surgical landmarks

• Both adrenal glands are in close proximity to the crus of the diaphragm

Right gland

• Triangular
• Located nearly directly cranial to the upper pole of the right kidney
• Adjacent structures include the
  • Underside of the liver anterolaterally
  • Duodenum anteromedially
  • Lateral margin of the inferior vena cava (IVC) medially
  • Psoas muscle posteriorly

Left gland

• More crecenteric in shape
• Lateral surface is in contact with the medial aspect of the upper pole of the left kidney
• The adjacent structures include the
  • Splenic vessels and body of the pancreas anteriorly
  • Aorta medially
  • Psoas muscle posteriorly

Vasculature

• Unique because the arterial and venous anatomy is highly variable
  • In laparoscopic adrenalectomy, an adrenal artery is identified in only 1% of cases§
• Blood supply is redundant

Arterial Supply

• Sources (3):
  1. Superior adrenal artery, typically arises from the inferior phrenic artery, and rarely from the aorta, celiac axis, or intercostal arteries.
  2. Middle adrenal artery, typically arises from the lateral aspect of the aorta and rarely from the inferior phrenic artery or renal artery.
  3. Inferior adrenal artery typically arises from the superior aspect of the ipsilateral renal artery
  • The superior arterial supply from the phrenic artery is constant; the middle and inferior arteries are variable
• Blood distribution within the adrenal gland
  • Capsular arteries supply only the adrenal capsule and do not penetrate more deeply into the tissue.
  • The medulla has two blood supplies
    1. Arterial blood from the medullary arterioles
       1. Medullary arterioles travel within the trabeculae of the adrenal gland to deliver blood to the medullary capillary sinusoids.
    2. Venous blood from the cortical sinusoid capillaries that have already supplied the adrenal cortex with arterial blood
       • Fenestrated cortical sinusoidal capillaries supply the cortex and then drain into fenestrated medullary capillary sinusoids.
    • This dual vascular supply is important for the medullary production of catecholamines
      • As venous blood from the adrenal cortex reaches the medullary tissue, it contains a high concentration of glucocorticoids, and this situation plays a role in epinephrine synthesis

Venous Drainage

• Both adrenal glands are drained by a single central vein that exits the adrenal anteromedially
  • The right adrenal vein is short and enters the posterior aspect of the IVC.
  • The left adrenal vein is longer and joins with the inferior phrenic vein and enters the cranial aspect of the left renal vein

Lymphatic Drainage

• Right: paracaval lymph nodes
• Left: para-aortic lymph nodes

Nerve Supply

• Sympathetic innervation of the adrenal gland causes release of catecholamines from the chromaffin cells of the medulla.
  • Pre-ganglionic sympathetic nerve fibers from T11-L2 (lower thoracic and lumbar spinal cord) travel through the sympathetic chain to reach a nerve plexus at the adrenal capsule which then traverse the cortex to directly innervate the chromaffin cells of the medulla; there is no post-ganglionic innervation of the medulla
• Post-ganglionic fibers originating from the [parasymp vs. symp?] splanchnic ganglia provide innervation to the adrenal cortex

Histology of Adrenal Glands

• The gland is surrounded by a capsule
• Composed of 2 embryologically and functionally distinct components:
  1. Outer cortex
     • Makes up ≈90% of the adrenal mass
     • Endocrine component
     • Derived from intermediate mesoderm
     • Composed of 3 layers (from outer to inner):
       1. Zona Glomerulosa
          • Comprises ≈15% of the cortex
          • Produces aldosterone as a result of unique zonal expression of aldosterone synthase (CYP11B2)
       2. Zona Fasciculata
          • Comprises ≈80% of the cortex
          • Produces glucocorticoids as a result of zonal expression of 17α-hydroxylase, 21-hydroxylase, and 11β-hydroxylase enzymes
            • Cortisol is the main glucocorticoid
       3. Zona Reticularis
          • Comprises ≈5-7% of the cortex
          • Produces the sex hormones as a result of zonal expression of 17α-hydroxylase and 17,20-lyase (3):
            1. Dehydroepiandrosterone (DHEA)
            2. Sulfated DHEA (DHEA-S)
            3. Androstenedione
          • The sex hormones (DHEA, DHEA-S, and androstenedione) comprise the greatest portion of steroid hormone that is produced by the adrenals (>20 mg/day), but appear to be the least important for adult physiologic homeostasis
            • Only 100-150 mcg/day of aldosterone and approximately 10-20 mg/day of cortisol are produced by the glands.
  2. Inner medulla
     • Neurocrine component
     • Derived from neural crest cells that later give rise to chromaffin cells
     • Chromaffin cells of the medulla secrete the catecholamines (3):
       1. Epinephrine (80%)
       2. Norepinephrine (19%)
       3. Dopamine (1%)
       • These catecholamines are produced from the amino acid tyrosine
       • The adrenal gland is the primary source of systemic epinephrine
         • Despite the presence of similar chromaffin cells elsewhere in the sympathetic nervous system, the enzyme phenylethanolamine-N-methyltransferase (PNMT), which catalyzes the conversion of norepinephrine to epinephrine, is relatively unique to the adrenal medulla (the brain and organ of Zuckerkandl also express this enzyme)

Adrenal gland anatomy and histology. Source: Wikipedia

Radiology

• US
  • More commonly used for differentiating between solid and cystic masses of the adrenal gland.
    • The retroperitoneal adipose tissue can make it difficult to differentiate normal adrenal tissue from the surrounding structures
• CT
  • Most widely used imaging modality for imaging the adrenal glands
  • Normal adrenal tissue (including adenoma) has a density of ≤ 10 Hounsfield units (HU) on non-contrast scan
• MRI
  • Contrast resolution with T1-weighted and T2-weighted images is superior to that of CT

Questions

1. What is the typical arterial blood supply to the adrenal glands?
2. Where do the adrenal veins drain to?
3. What are the 3 zones of the adrenal cortex and what do the secrete?
4. Describe the autonomic innervation of the adrenal medulla
5. Which enzyme catalyzes the conversion of norepinephrine to epinephrine?
6. What are the catecholamines produced by the medulla? From which amino acid are they made?

Answers

1. What is the typical arterial blood supply to the adrenal glands?
   1. Superior adrenal artery arising from the inferior phrenic artery
   2. Middle adrenal artery arising from the aorta
   3. Inferior adrenal artery arising from the adrenal artery
2. Where do the adrenal veins drain to?
   • Right adrenal: IVC
   • Left adrenal: renal vein
3. What are the 3 zones of the adrenal cortex and what do the secrete?
   1. Glomerulosa: aldosterone
   2. Fasiculata: cortisol
   3. Reticularis: sex hormons
4. Describe the autonomic innervation of the adrenal medulla
   • Preganglionic sympathetic nerve fibers act directly on the medulla, there are no postganglionic fibers
5. Which enzyme catalyzes the conversion of norepinephrine to epinephrine?
   • PNMT (phenylethanolamine-N-methyltransferase)
6. What are the catecholamines produced by the medulla? From which amino acid are they made?
   • Dopamine, noepinephrine, epinephrine
   • Tyrosine

References

• Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 64