Lecture Notes on Cell Adaptations

Introduction

• Cell adaptations are reversible changes in size, number, phenotype, metabolic activity, or functions of cells due to environmental changes.
• Four main types of adaptations:
  • Hypertrophy
  • Hyperplasia
  • Atrophy
  • Metaplasia

Hypertrophy

• Definition: Increase in cell size resulting in an increase in organ size.
• Characteristics:
  • No new cells, just larger cells.
  • Due to synthesis of more structural components.
• Stimuli:
  • Increased functional demand.
  • Hormonal or growth factor stimulation.
• Types:
  • Physiological (e.g., muscle growth in bodybuilders).
  • Pathological (e.g., cardiac hypertrophy due to hypertension).
• Mechanisms:
  • Mechanical sensors, growth factors, and vasoactive agents activate pathways (PI3K/Akt and GPCR pathways).
  • Possible switch from adult to fetal protein forms.
  • Example: Barbiturate-treated patients showing SER hypertrophy in hepatocytes.

Hyperplasia

• Definition: Increase in the number of cells in an organ or tissue.
• Types:
  • Physiological Hyperplasia:
    • Hormonal (e.g., breast tissue in pregnancy).
    • Compensatory (e.g., liver regeneration post-hepatectomy).
  • Pathological Hyperplasia:
    • Due to excess hormones or growth factors (e.g., endometrial hyperplasia).
• Characteristics:
  • Controlled process, distinct from cancer.

Atrophy

• Definition: Reduced size of an organ or tissue due to decreased cell size and number.
• Types:
  • Physiological (e.g., atrophy of the uterus post-parturition).
  • Pathological (e.g., muscle atrophy due to immobilization).
• Causes:
  • Starvation, aging, loss of endocrine stimulation, etc.
• Mechanisms:
  • Increased protein degradation (ubiquitin-proteasome pathway).
  • Increased autophagy (cells digest own components).

Metaplasia

• Definition: Replacement of one differentiated cell type by another.
• Types:
  • Epithelial Metaplasia:
    • Common: Squamous metaplasia (e.g., due to smoking).
    • Barrett's esophagus: Squamous to columnar transformation.
  • Connective Tissue Metaplasia: Formation of bone or cartilage in unusual places.
• Mechanisms:
  • Reprogramming of stem cells.
  • Mediated by cytokines and growth factors.

Conclusion

• Cell adaptations help in adjusting to environmental changes but may have disadvantages (e.g., loss of function or risk of malignancy in metaplasia).