Cellular adaptations (atrophy, hypertrophy)

Cellular Adaptations - The Cell's Survival Kit

Cells adapt to stress to achieve a new steady state, avoiding injury. This involves changes in size, number, or phenotype.

• Hypertrophy: Increase in cell size → increased organ size.

  • Mechanism: ↑ synthesis of structural proteins & organelles.
  • Physiologic: Uterine growth in pregnancy; skeletal muscle with exercise.
  • Pathologic: Cardiac hypertrophy from chronic hypertension.

• Atrophy: Decrease in cell size → decreased organ size.

  • Mechanism: ↓ protein synthesis & ↑ protein degradation via ubiquitin-proteasome pathway; autophagy.
  • Causes: Disuse, denervation, ↓ blood supply, poor nutrition, loss of endocrine stimulation.

⭐ In cardiac hypertrophy, heart muscle cells enlarge, but this adaptation can progress to contractile dysfunction and heart failure.

Atrophy - Shrink to Survive

• Definition: A reversible adaptation involving a ↓ in cell size and/or number, leading to reduced organ size or function. It's a retreat to a smaller size to conserve energy and survive.
• Mechanisms: Primarily driven by ↓ protein synthesis and ↑ protein degradation.
  • Ubiquitin-Proteasome Pathway: Key for degrading cytosolic and nuclear proteins.
  • Autophagy: "Self-eating" where the cell digests its own components via lysosomes, often leaving behind residual bodies (lipofuscin granules).
• Common Causes:
  • ↓ Workload (disuse atrophy)
  • ↓ Innervation (denervation atrophy)
  • ↓ Blood Supply (ischemia)
  • ↓ Nutrition (marasmus, cachexia)
  • ↓ Endocrine Stimulation (e.g., post-menopausal endometrium)
  • Aging (senile atrophy)

⭐ High-Yield: In atrophic cells, accumulations of brownish lipofuscin granules represent indigestible lipid residues from autophagolysosomes, a key sign of free radical-induced lipid peroxidation and cellular aging.

Hypertrophy - Bulking Up Under Stress

• Definition: ↑ in cell size, leading to ↑ organ size. Occurs in non-dividing cells (e.g., myocytes, neurons). No new cells, just bigger ones.

• Triggers & Pathways:

  • Mechanical stretch (e.g., chronic pressure/volume load).
  • Trophic signals (e.g., hormones, growth factors).
  • Activates signal transduction (PI3K/Akt, GPCRs) → transcription factors (GATA4, NFAT) → ↑ synthesis of structural proteins.

• Types & Examples:

  • Physiologic: Uterine smooth muscle in pregnancy; skeletal muscle with exercise.
  • Pathologic: Cardiac hypertrophy from hypertension or aortic stenosis.

⭐ In cardiac hypertrophy, there's a switch to fetal/neonatal forms of contractile proteins (e.g., β-myosin heavy chain). This provides slower, more energy-efficient contraction but at the cost of reduced contractile performance, predisposing to heart failure.

High‑Yield Points - ⚡ Biggest Takeaways

• Atrophy is a decrease in cell size and number, commonly from ↓ workload, denervation, or ischemia.
• The primary mechanism for atrophy is the ubiquitin-proteasome pathway.
• Hypertrophy is an increase in cell size, occurring in non-dividing cells like cardiac and skeletal muscle.
• It is triggered by ↑ functional demand or specific hormonal stimulation.
• Pathologic cardiac hypertrophy is a major risk factor for sudden cardiac death.