Learning Objective

By the end of this section, learners will be able to explain the major types of cellular adaptations, distinguish between physiologic and pathologic responses, and relate these adaptations to the progression toward cell injury and malignancy.

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Cellular Adaptations

Cells undergo reversible changes in response to stress or altered conditions. These changes may be:

• Physiologic
  Example: uterine enlargement during pregnancy
• Pathologic
  Example: myocardial hypertrophy secondary to hypertension

If stress is severe or persistent, these adaptations may progress to cell injury, organ dysfunction, and even malignancy.

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Types of Cellular Adaptations

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Hypertrophy

Definition: Increase in the size of individual cells due to increased synthesis of structural proteins and organelles.
Outcome: Increased organ size.

Example:

• Cardiac hypertrophy resulting from chronic systemic hypertension
  Prolonged hypertrophy → myocardial injury → heart failure.

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Hyperplasia

Definition: Controlled increase in the number of cells due to increased proliferation of stem cells or mature cells.

Examples:

• Physiologic: breast tissue growth during puberty and pregnancy
• Pathologic: benign prostatic hyperplasia
• Excessive stimulation → endometrial hyperplasia, which may progress to dysplasia and cancer.

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Activity

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Atrophy

Definition: Reduction in tissue mass caused by decreased cell size and/or decreased cell number.

Mechanisms:

• Decreased synthesis of proteins
• Increased degradation of cytoskeletal elements (via ubiquitin–proteasome pathway)
• Autophagy of damaged organelles
• Apoptosis (reducing cell number)

Causes include:

• Disuse (immobilization, casting)
• Denervation
• Reduced blood supply
• Loss of endocrine support
• Malnutrition or cachexia

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Metaplasia

Definition: Reversible reprogramming of stem cells leading to the replacement of one differentiated cell type with another that can better withstand stress.

Common examples:

• Barrett’s esophagus:
  Squamous → intestinal-type epithelium due to acid exposure
• Respiratory tract in smokers:
  Ciliated columnar → stratified squamous epithelium

Clinical relevance:

Persistence of the harmful stimulus can lead to dysplasia → cancer.

Other type:

• Metaplasia of connective tissue
  Example: myositis ossificans (bone formation in muscle after trauma)

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Dysplasia

Definition: Disordered, precancerous epithelial cell growth; not a true adaptive process.

Key features:

• Loss of uniform cell size and shape (pleomorphism)
• Disorganized tissue architecture
• Enlarged nucleus-to-cytoplasm ratio
• Hyperchromatic, clumped chromatin

Reversibility:

• Mild/moderate dysplasia → may regress if stimulus is removed
• Severe dysplasia → often progresses to carcinoma in situ

Often preceded by:

• Persistent metaplasia
• Pathologic hyperplasia

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Activity

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Summary Concept