Cellular Adaptation and Injury

• To function correctly, cells need balanced physiologic demands, cellular structure, and metabolic capacity, called homeostasis
• Cells can adjust their function when faced with mild stress to maintain balance
• Excessive or harmful stress can lead to Adaptation, Reversible Injury, Irreversible Injury, or Cell Death

Adaptation

• Occurs when cells respond to stress from normal or abnormal factors
• Cells change but remain viable via:
• Hyperplasia: Increase in cell number
• Hypertrophy: Increase in cell size or mass
• Atrophy: Decrease in cell size or function
• Metaplasia: Change in cell type or structure for better stress handling

Hyperplasia

• Means an increase in the number of cells in a tissue or organ
• Often occurs with hypertrophy (cells getting bigger)
• Occurs only in cells that can divide such as epithelial, blood-forming, and connective tissue cells
• Nerve, heart, and skeletal muscle cells can't divide and grow by hypertrophy when under stress or hormone stimulation
• Hyperplasia can be physiologic (normal) or pathologic (abnormal)

Physiologic Hyperplasia Types

• Hormonal Hyperplasia such as the growth of breast glands during puberty and pregnancy
• Compensatory Hyperplasia such as liver cells multiplying after partial hepatectomy (removal of part of the liver)

Pathologic Hyperplasia Types

• Caused by excessive hormone stimulation shown in hyperestrinism leading to atypical endometrial hyperplasia
• Caused by local growth factors as is seen in the overgrowth of connective tissue cells during wound healing
• In pathologic hyperplasia, removing the stimulus usually resolves extra cell growth
• However, it can create a high-risk environment for cancer development

Hypertrophy

• Involves an increase in cell size, which leads to a bigger organ or tissue
• Can be physiologic (normal) or pathologic (abnormal)
• Increased functional demand such as muscle growth in bodybuilders, or heart muscle thickening in heart disease
• Hormonal stimulation such as Uterine hypertrophy during pregnancy due to hormonal changes

Atrophy

• Shrinkage in cell size due to loss of cell substance
• Causes of atrophy include:
• Decreased workload
• Loss of innervation
• Diminished blood supply
• Inadequate nutrition
• Loss of endocrine stimulation
• Aging

Metaplasia

• A reversible change where one adult cell type is replaced by another type (either epithelial or mesenchymal) for better stress handling
• Most common example: Columnar to squamous cell change in the respiratory tract, seen in smokers due to chronic irritation (squamous metaplasia)
• While usually benign, continued harmful stimuli can lead to atypical metaplasia, potentially leading to cancer
• Can happen in mesenchymal cells like fibroblasts changing into osteoblasts or chondroblasts
• This can lead to bone or cartilage forming in unusual places

Reversible Injury

• Cell damage can return to normal if the cause is removed or is mild
• The structure and function of the cell can still recover

Irreversible Injury

• Irreversible damage is too severe or prolonged
• The cell can’t recover and will die
• Usually by necrosis or apoptosis
• There are two morphologic patterns of cell death: Necrosis and Apoptosis

Causes of Cell Injury

• Oxygen Deprivation (Hypoxia): Reduces ATP production, leading to cell damage
• Ischemia (Loss of blood supply)
• Poor oxygenation (heart or lung failure)
• Loss of oxygen-carrying capacity (anemia, carbon monoxide (CO) poisoning)
• Physical Agents such as trauma, heat, cold, radiation, and electric shock
• Chemical Agents & Drugs such as poisons, therapeutic drugs, pollutants, alcohol, and narcotics
• Infectious Agents such as viruses, bacteria, fungi, and parasites
• Immunologic Reactions such as autoimmune diseases attacking the body's own cells
• Genetic Derangements such as chromosome abnormalities and gene mutations
• Nutritional Imbalances such as deficiencies (protein, vitamins) or excess (obesity)