Revision sheet: Endocrine System Fundamentals

Endine System Revision Sheet

1. 📌 Essentials

• The endocrine system regulates physiological functions via hormone secretion into the blood.
• Endocrine tissues include glands (thyroid, hypophysis cell clusters (Leydig, Langerhans), and diffuse neuroendocrine cells.
• Hormones are classified as hydrophilic (proteins, peptides, amines) or hydrophobic (steroids, thyroid hormones).
• Peptide hormones are synthesized as preprohormones, processed in ER and Golgi, stored in granules, secreted by exocytosis.
• Steroid hormones derive from cholesterol, are not stored, diffuse freely into target cells.
• Hormone secretion is regulated by hormonal, humoral, neural stimuli, often via negative feedback.
• Hormone secretion exhibits circadian, pulsatile, or complex rhythms.
• Receptors: membrane-bound (G-protein, enzyme-linked, ion channels) or intracellular (cytoplasmic, nuclear).
• Hydrophilic hormones trigger intracellular cascades; lipophilic hormones modulate gene transcription.
• Disorders include hyposecretion and hypersecretion, primary or secondary.

2. 🧩 Key Structures & Components

• Endocrine epithelia — hormone-secreting cells within glands or diffuse tissue.
• Glands — thyroid, hypophysis, adrenal cortex/medulla, gonads.
• Cell clusters — Leydig cells (testes), Langerhans islets (pancreas).
• Neuroendocrine cells — in digestive tract, neurohypophysis.
• Hormone mediators — hydrophilic (proteins, peptides, amines), hydrophobic (steroids).
• Transport proteins — albumin, CBG (cortisol-binding globulin), SHBG (sex hormone-binding globulin), TBG (thyroxine-binding globulin).
• Receptors — membrane-bound (GPCRs, enzyme-linked, ion channels), intracellular (cytoplasmic, nuclear).

3. 🔬 Functions, Mechanisms & Relationships

• Hormone synthesis:
  • Peptides: synthesized as preprohormones, processed in ER and Golgi, stored, secreted via exocytosis.
  • Steroids: synthesized from cholesterol in mitochondria/REL, not stored, diffuse into target cells.
• Transport:
  • Hydrophilic hormones: circulate freely, rapidly degraded.
  • Lipophilic hormones: bound to plasma proteins, serve as a reservoir.
• Receptor binding:
  • Membrane receptors activate second messenger cascades (cAMP, IP3, DAG).
  • Nuclear receptors bind lipophilic hormones, modulate gene transcription.
• Regulation:
  • Negative feedback: high hormone levels inhibit secretion.
  • Positive feedback: hormone stimulates its own release.
• Rhythms:
  • Circadian (e.g., cortisol, melatonin).
  • Pulsatile (e.g., GH, GnRH).
  • Complex (e.g., cortisol with circadian + pulsatile patterns).
• Disorders:
  • Hyposecretion: gland failure (primary), trophic hormone deficiency (secondary).
  • Hypersecretion: gland overactivity (primary), excess stimulation (secondary).

4. 📊 Comparative Table

Item	Key Features	Notes / Differences
Peptide hormones	Synthesized as preprohormones, stored in granules, secreted by exocytosis	Examples: insulin, ACTH, TSH
Steroid hormones	Derived from cholesterol, synthesized in mitochondria/REL, not stored	Examples: cortisol, testosterone, estrogens
Transport	Hydrophilic: circulate freely; Lipophilic: bound to plasma proteins	Only free form is active
Receptor location	Membrane (GPCR, enzyme-linked, ion channels); Intracellular (cytoplasmic, nuclear)	Determines mechanism of action
Mechanism of action	Hydrophilic: second messenger cascades; Lipophilic: gene transcription	Different pathways

5. 🗂️ Hierarchical Diagram (ASCII)

Endocrine System
 ├─ Organization
 │   ├─ Glands (thyroid, hypophysis)
 │   └─ Cell clusters (Leydig, Langerhans)
 ├─ Hormone Mediators
 │   ├─ Hydrophilic (proteins, peptides, amines)
 │   └─ Hydrophobic (steroids)
 ├─ Synthesis & Storage
 │   ├─ Peptides: preprohormones, stored in granules
 │   └─ Steroids: derived from cholesterol, not stored
 ├─ Regulation
 │   ├─ Hormonal (hypothalamic-pituitary axis)
 │   ├─ Humoral (ions/nutrients)
 │   └─ Neural (autonomic)
 ├─ Mechanisms of Action
 │   ├─ Hydrophilic: membrane receptors, cascades
 │   └─ Lipophilic: nuclear receptors, gene transcription
 └─ Disorders
     ├─ Hyposecretion
     └─ Hypersecretion

6. ⚠️ High-Yield Pitfalls & Confusions

• Confusing peptide hormones with steroid hormones regarding synthesis and storage.
• Mistaking plasma transport proteins as active carriers; only free hormones are active.
• Overlooking the difference in receptor location and mechanism between hydrophilic and lipophilic hormones.
• Assuming all hormones are secreted in a pulsatile manner; some are continuous.
• Misidentifying primary vs. secondary endocrine disorders.
• Confusing the role of negative vs. positive feedback loops.
• Overgeneralizing hormone effects without considering receptor subtype specificity.
• Ignoring circadian and pulsatile rhythms in hormone secretion patterns.

7. ✅ Final Exam Checklist

• Know the main endocrine glands and their hormone products.
• Distinguish between hydrophilic and hydrophobic hormones, including synthesis and transport.
• Understand receptor types and mechanisms of hormone action.
• Be able to explain hormone regulation via feedback loops.
• Recognize the hormonal rhythms and their physiological significance.
• Identify common disorders: hypo- and hypersecretion, primary vs. secondary.
• Comprehend the neuroendocrine integration with nervous system responses.
• Recall examples of hormones: insulin, adrenaline, cortisol, TSH, testosterone, T3/T4.
• Understand hormone precursors and processing pathways.
• Know the clinical relevance of hormone transport proteins.
• Be familiar with the hierarchical organization of endocrine tissues.
• Recognize the impact of circadian and pulsatile secretion on physiology.
• Be aware of common pitfalls in hormone classification and mechanism.

End of Revision Sheet