Cell Adaptation and Homeostasis

Homeostasis

• Normal cells are in a state of balance with surrounding cells.

Adaptation

• Adaptations are reversible changes in response to stress and some pathologic stimuli, allowing the cell to survive and function.
• The cell can recover to its original state when the stress is eliminated.

Adaptation Responses

• Adaptations can be hypertrophy, hyperplasia, atrophy, and metaplasia.

Atrophy

• Atrophy refers to a decrease in cell size due to loss of cell substance causing a decrease in organ or tissue size.
• Atrophy can be caused by decreased workload, denervation, ischemia, undernutrition, and loss of endocrine stimulation.
• The aim of atrophy is to conserve energy and resources to survive with reduced blood supply and nutrition.
• Atrophy leads to a decrease in cell organelles and protein synthesis, an increase in protein degradation due to the ubiquitin-proteasome pathway, and increased autophagy.

Hypertrophy

• Hypertrophy refers to an increase in cell size leading to an increase in organ or tissue size.
• Hypertrophy is caused by the synthesis of more structural components within the cell.
• Hypertrophy can be physiological or pathological.
• Examples include uterine hypertrophy in pregnancy, skeletal muscles in athletes, and left ventricle hypertrophy in systemic hypertension.
• The stimulus for hypertrophy is usually chronic hemodynamic overload from hypertension or faulty valves.

Hyperplasia

• Hyperplasia refers to an increase in the number of cells in an organ or tissue leading to an increase in size.
• Hyperplasia can be physiological or pathological.
• Physiological hyperplasia occurs due to hormonal stimulation or compensatory mechanisms.
• Pathological hyperplasia occurs due to excessive hormonal stimulation or growth factors affecting target cells.
• Hyperplasia is the result of growth factor-driven proliferation of mature cells and increased output of new cells from tissue stem cells.

Metaplasia

• Metaplasia refers to the replacement of one mature cell type with another mature cell type.
• It is an adaptation to a stressful environment.
• Examples include squamous metaplasia of the laryngeal and bronchial epithelium due to smoking, squamous metaplasia of the urothelium in the urinary bladder due to Bilharzia, and columnar metaplasia of esophageal squamous epithelium due to acid reflux.

Dysplasia

• Dysplasia refers to disturbed proliferation of cells associated with atypical changes in cell size, shape, and organization.
• Most commonly encountered in lining epithelia, especially squamous epithelium.
• Characterized by increased proliferative activity, hyperchromatic nuclei, and increased mitotic activity without abnormal mitoses.
• Often found adjacent to foci of cancer, but it doesn't always progress to malignancy.

Hypoplasia

• Hypoplasia refers to incomplete development of an organ due to reduced cell numbers.

Degenerative Changes

• Hyaline change: Homogeneous, pinkish alteration in sections stained with H&E.
  • Intracellular examples include hyaline droplets in renal tubular epithelium, Russell bodies in plasma cells, viral inclusions, and alcoholic hyaline in liver cells (Mallory bodies).
  • Extracellular examples include hyalinization of collagen in old scars, arteriolar walls in hypertension and diabetes, and amyloid deposition.

This quiz covers key concepts of cell adaptation and homeostasis, including definitions, mechanisms, and examples of atrophy and hypertrophy. Understand how cells adjust to stress and the implications of these changes on overall cellular function.