The adrenal (suprarenal) glands are small endocrine organs located on the superior poles of the kidneys.
They consist of two functionally and embryologically distinct regions:
- Adrenal Cortex → derived from mesoderm
- Adrenal Medulla → derived from neural crest cells
Anatomy Overview
| Structure | Embryologic Origin | Main Secretion | Primary Regulation |
|---|---|---|---|
| Adrenal Cortex | Mesoderm | Steroid hormones | ACTH, CRH, Angiotensin II |
| Adrenal Medulla | Neural crest | Catecholamines (Epinephrine, Norepinephrine) | Sympathetic preganglionic fibers (ACh) |
Adrenal Cortex
The adrenal cortex has three layers, each responsible for secreting a different class of steroid hormones.
- G → Zona Glomerulosa → Mineralocorticoids (Salt)
- F → Zona Fasciculata → Glucocorticoids (Sweet)
- R → Zona Reticularis → Androgens (Sex)
Layers of the Adrenal Cortex
| Layer | Main Hormone Produced | Hormone Class | Primary Regulation | Function |
|---|---|---|---|---|
| Zona Glomerulosa | Aldosterone | Mineralocorticoid | Angiotensin II (RAAS system) | Regulates Na⁺ reabsorption, K⁺ excretion, and BP |
| Zona Fasciculata | Cortisol | Glucocorticoid | ACTH, CRH | ↑ Gluconeogenesis, ↓ immune response, ↑ BP |
| Zona Reticularis | DHEA, Androstenedione | Androgens | ACTH, CRH | Secondary sexual characteristics, precursors of sex hormones |
Mnemonic
GFR (from outer → inner cortex):
- G = Glomerulosa → Salt (Aldosterone)
- F = Fasciculata → Sugar (Cortisol)
- R = Reticularis → Sex (Androgens)
Mnemonic for Regulation:
- Aldosterone → Angiotensin II
- Cortisol → ACTH
- Androgens → ACTH
Adrenal Medulla
- Origin & Function
- Derived from neural crest cells
- Acts as a modified sympathetic ganglion
- Contains chromaffin cells, which secrete:
- Epinephrine (80%)
- Norepinephrine (20%)
- Regulation
- Stimulated by preganglionic sympathetic fibers releasing acetylcholine (ACh).
- Stress → ↑ sympathetic discharge → ↑ catecholamine release.
- Key Enzyme
- Phenylethanolamine-N-methyltransferase (PNMT) converts norepinephrine → epinephrine.
- Stimulated by cortisol from the adrenal cortex.
Functional Summary Table
| Region | Main Cells | Hormones | Regulated By | Primary Role |
|---|---|---|---|---|
| Zona Glomerulosa | Columnar cells | Aldosterone | Angiotensin II, K⁺ | BP & electrolyte balance |
| Zona Fasciculata | Foamy cells | Cortisol | ACTH | Metabolism, stress response |
| Zona Reticularis | Irregular cells | DHEA | ACTH | Androgen production |
| Medulla | Chromaffin cells | Epinephrine, Norepinephrine | Sympathetic ACh | Fight-or-flight response |
Histology Tip
- Cortex: Organized in GFR pattern from capsule inward.
- Medulla: Basophilic chromaffin cells with granular cytoplasm containing catecholamines.
Key Points to Remember
- Cortex = Steroid hormones (slow-acting)
- Medulla = Catecholamines (fast-acting)
- Cortex = Mesoderm, Medulla = Neural crest
- PNMT activity depends on cortisol from the cortex.
- GFR mnemonic helps recall order, function, and hormones.
Learning Objective
By the end of this topic, you should be able to:
- Identify the zones and hormones of the adrenal cortex.
- Explain the embryologic origin and regulation of the adrenal cortex and medulla.
- Correlate hormone functions with physiologic responses.
- Recognize clinical implications of adrenal hormone imbalance (e.g., Addison’s, Cushing’s, Conn’s).
