التكيفات الخلوية

Cellular Adaptations

• Cells adapt to changing environmental factors to survive. These adaptations can involve changes in cell size, number, or type.

Atrophy

• Atrophy is a decrease in cell or tissue size.
• Reduced oxygen consumption and metabolism, while sometimes improving cell efficiency.
• Usually reversible, except for atrophy due to nerve loss.
• Causes include prolonged bed rest, disuse of tissues, malnutrition, and decreased blood supply.

Hypertrophy

• Hypertrophy is an increase in cell size and tissue mass.
• Occurs when a cell or tissue is subjected to an increased workload.
• Cannot increase the number of cells to adapt.
• Can be a normal physiological response (like muscle growth with exercise) or pathological (like heart hypertrophy with prolonged high blood pressure).
• Can also be a compensatory mechanism (e.g., remaining kidneys enlarge to compensate for removal of a kidney).

Hyperplasia

• Hyperplasia is an increase in the number of cells in an organ or tissue.
• Only possible in cells capable of division (not muscles or nerve cells).
• Can be normal (like breast or uterine growth during pregnancy) or abnormal (like gingival hyperplasia—excessive gum tissue growth).
• Can be a compensatory mechanism (e.g., remaining liver cells increase in number to maintain liver function after a portion of the liver is surgically removed).

Metaplasia

• Metaplasia is the transformation of one cell type to another type, potentially better suited for existing conditions.
• Usually occurs in response to chronic irritation or inflammation.
• Example: In chronic smokers, the columnar ciliated epithelium lining the airways gradually transforms into stratified squamous epithelium. While stratified squamous epithelium is better at handling continuous irritation, it lacks the necessary cilia for optimal function.

Dysplasia

• Dysplasia is abnormal cell growth, resulting in tissues composed of cells with varying shapes and sizes.
• Typically arises from chronic irritation and inflammation.
• Potentially a significant precursor to cancer in certain tissues, especially the cervix and respiratory tract.

Cellular Injury

• Cellular injury can occur in various ways, often related to the severity and duration of exposure to damaging agents or substances.
• Injury can be reversible (cells recover), or irreversible (leading to cell death).

Free Radical Injury

• Free radicals are highly reactive chemical species with one or more unpaired electrons in their outer shell.
• Examples include superoxide, hydroxyl radicals, and hydrogen peroxide.
• Generated as byproducts of normal cellular metabolism, but also formed by external sources.
• Body has protective enzymes capable of neutralizing them, such as catalase and glutathione peroxidase.
• Excessive free radicals can cause significant cellular damage via mechanisms like lipid peroxidation, protein damage, and DNA mutations.
• External sources include cigarette smoke, organic solvents, pollutants, radiation, and pesticides.
• Implicated in aging and various diseases, such as diabetes, cancer, atherosclerosis, Alzheimer's disease, and rheumatoid arthritis.

Ischemia/Hypoxia Injury

• Ischemia is oxygen deprivation in cells and tissues, often stemming from reduced blood supply.
• During ischemia, cellular aerobic metabolism fails leading to a dramatic decrease in ATP production.
• Affected cells swell due to failure of energy-dependent processes (e.g., ion pumps reliant on ATP).
• pH of the extracellular environment decreases, and anaerobic metabolism byproducts (like lactic acid) accumulate.
• Imbalance of ionic concentrations within the cell can lead to calcium accumulation, and its surplus activates enzymes that break down vital cellular components.
• Injury can be reversible if oxygen is restored rapidly, or irreversible leading to cell death.
• Brain tissue is highly susceptible to oxygen deprivation, and irreversible cell death can happen within minutes.

Cellular Accumulations

• In the instance of cellular injury, cells might gather various substances, including fats, proteins, glycogen, calcium, uric acid, and pigments (like melanin).
• These accumulations are generally reversible but can signal greater cell damage.
• Accumulation can lead to tissue or organ enlargement (e.g., fatty liver).

Cell Death

• Cell death comprises two main types: programmed cell death (apoptosis) and necrosis.

Apoptosis (Programmed Cell Death)

• A natural part of aging, involving regulated cell dismantling and removal.
• May eliminate damaged or genetically abnormal cells.
• Viruses (like Epstein-Barr virus) can induce apoptosis, killing both host cells and the virus.
• Involves activation of specific suicidal genes in response to chemical signals, resulting in cell fragmentation and subsequent engulfment by other cells.
• Disruption of natural apoptosis can contribute to cancer development.

Necrosis

• Uncontrolled enzymatic digestion of the cell and its components.
• Results from irreversible cellular damage.
• Tissues subjected to necrosis can vary in response; commonly recognized types include gangrene (extensive tissue death).
• Gangrene can be dry (shriveled, blackened surrounding skin) or wet (warm, moist, and oozing material). Gas gangrene occurs if bacteria produce gases as a byproduct.

Tissue Repair

• Tissue repair involves two main methods: regeneration (repair with the same tissue) and replacement by scar tissue (repair with connective tissue).
• The method depends on the type of cells damaged.
• Some cells are fully or partially capable of regeneration, while others, such as nerve cells, only regenerate with scar tissue.

Regeneration

• Tissues are repaired with identical replacement tissue.
• There is typically a complete restoration of function, with minimal or no evidence of injury remaining.
• Possible in labile cells (constantly dividing throughout life) or stable cells (stop dividing but can re-enter division under certain conditions).

Scar Tissue Repair (Replacement by Connective Tissue)

• Function for the tissue is lessens or is entirely lost, with the formation of a scar.
• Typically involves connective tissue (like collagen).
• Scar tissue develops as a result of the repair process.
• The process is important for healing following initial injury, but complete restoration of pre-injury strength and function aren't always achieved.

Wound Healing Stages

• Inflammation phase: blood clot formation to stop bleeding; inflammatory cells arrive to clear debris.
• Proliferation phase: collagen formation by fibroblasts, epithelial cell proliferation to close the wound, and new blood vessel formation.
• Remodeling phase: collagen remodeling and strengthening of the scar. Keloids (abnormal) scars result from excessive collagen formation with reduced breakdown.