Cellular Adaptations

Cells can undergo reversible changes in size, number, phenotype, metabolic activity, or functional state in response to environmental changes. These adaptations commonly include:

• Hypertrophy
• Hyperplasia
• Atrophy
• Metaplasia

Hypertrophy

• Definition: Increase in cell size leading to an increase in organ size
• Characteristics:
  • No new cells, just larger cells
  • Caused by the synthesis of more structural components
  • Occurs in cells with increased functional demands or hormone stimulation
• Types:
  • Physiological (e.g., muscle growth in bodybuilders)
  • Pathological (e.g., cardiac hypertrophy from hypertension or valve incompetence)
• Mechanisms:
  • Mechanical sensors activated by workload
  • Growth factors (e.g., TGF-beta, IGF-1)
  • Vasoactive agents (e.g., endothelin-1, angiotensin II)
  • Signal transduction pathways: PI3K/Akt (physiological) & GPCR (pathological)
  • Switchover to fetal/neonatal proteins in some cases

Hyperplasia

• Definition: Increase in the number of cells, increasing organ or tissue size
• Characteristics:
  • Occurs in dividing cells
  • Frequently coexists with hypertrophy
• Types:
  • Physiological: Hormonal (e.g., breast tissue in pregnancy), Compensatory (e.g., liver regeneration)
  • Pathological: Often due to excess hormones/growth factors (e.g., endometrial hyperplasia, benign prostatic hyperplasia)
• Mechanism: Doesn't involve mutations in genes regulating cell division, distinct from cancer

Atrophy

• Definition: Decrease in cell size and/or number, reducing organ/tissue size
• Types:
  • Physiological: Common in early development (e.g., ductus arteriosus), later life (e.g., thymus involution)
  • Pathological: Can be generalized (e.g., starvation, senile atrophy) or localized (e.g., disuse, denervation, pressure atrophy)
• Mechanisms:
  • Decreased protein synthesis & increased degradation (ubiquitin-proteasome pathway)
  • Increased autophagy (cell self-digestion for survival)

Metaplasia

• Definition: Replacement of one differentiated cell type by another
• Types:
  • Epithelial Metaplasia: Common is squamous metaplasia (e.g., respiratory tract from cigarette smoking)
  • Detective Tissue Metaplasia: Formation of cartilage, bone in unusual locations (e.g., bone in muscle)
• Mechanism: Reprogramming of stem cells via cytokines, growth factors, and extracellular matrix components, not transdifferentiation
• Consequences: Loss of original cell function, potential for malignant transformation if stimulus persists