Lecture 7.1 - Cellular adaptations

Cellular Adaptations

• Cells can adapt through:
  • Hyperplasia: increase in cell numbers above normal
  • Hypertrophy: increase in cell size
  • Atrophy: cells become smaller
  • Metaplasia: cells are replaced by cells of a different type

Hyperplasia

• Increase in tissue or organ size due to increased cell numbers
• Occurs in labile or stable tissues
• Caused by increased functional demand or hormonal stimulation
• Remains under physiological control and is reversible
• Can occur secondary to a pathological cause, but the proliferation itself is a normal response

Examples of Physiological and Pathological Hyperplasia

• Physiological: uterine growth during pregnancy, muscle growth in response to exercise
• Pathological: breast tissue growth in response to hormonal stimulation, prostate growth in response to androgens

Hypertrophy

• Increase in tissue or organ size due to increased cell size
• Occurs in labile, stable, and especially permanent tissues
• Caused by increased functional demand or hormonal stimulation
• Cells contain more structural components, sharing workload with a greater mass of cellular components

Examples of Physiological and Pathological Hypertrophy

• Physiological: muscle growth in response to exercise, heart muscle growth in response to increased blood pressure
• Pathological: cardiac hypertrophy in response to chronic hypertension, kidney hypertrophy in response to diabetes

Atrophy

• Shrinkage of a tissue or organ due to an acquired decrease in size and/or number of cells
• Cells shrink to a size at which survival is still possible
• Reduced structural components of the cell and cell function
• May eventually result in cell death

Examples of Physiological and Pathological Atrophy

• Physiological: uterine atrophy after childbirth, muscle atrophy after disuse
• Pathological: muscle atrophy after denervation, skin atrophy due to peripheral vascular disease

Metaplasia

• Reversible change of one differentiated cell type to another
• May represent adaptive substitution of cells that are sensitive to stress by cell types better able to withstand the adverse environment
• Metaplastic cells are fully differentiated and the process is reversible

Examples of Metaplasia

• Barrett's epithelium and oesophageal adenocarcinoma
• Intestinal metaplasia of the stomach and gastric adenocarcinoma

Cell Proliferation

• Controlled by chemical signals from the microenvironment
• Signals stimulate or inhibit cell proliferation
• Receptors are usually in the cell membrane, but can be in the cytoplasm or nucleus

Cell Cycle

• Mitosis: nuclear division
• Cytokinesis: cell division
• Checkpoints: restriction (R) point, most critical checkpoint
• Cell cycle can be controlled by cyclins and cyclin-dependent kinases (CDKs)

Cell Growth and Differentiation

• Proliferation: increase in cell numbers
• Growth: increase in cell size
• Differentiation: acquiring a specific morphology and function

Cell Regeneration

• Regeneration: the ability to replace cells or tissues, destroyed by injury or disease (identical functionality)
• Labile cells: high regenerative ability and turnover
• Occurs only in labile or stable cell types
• Involves expression of a new genetic programme

Aplasia and Hypoplasia

• Aplasia: complete failure of a specific tissue or organ to develop
• Hypoplasia: underdevelopment or incomplete development of tissue or organ at the embryonic stage
• Examples: thymic aplasia, aplasia of a kidney, renal hypoplasia, breast hypoplasia

Involution and Reconstruction

• Involution: normal programmed shrinkage of an organ
• Examples: uterus after childbirth, thymus in early life, pro and mesonephros
• Reconstruction: replacement of a lost part of the body
• Examples: angiogenesis, replantation of a severed limb

Atresia and Dysplasia

• Atresia: no orifice, congenital imperforation of an opening
• Examples: pulmonary valve atresia, anal atresia, vaginal atresia
• Dysplasia: abnormal maturation of cells within a tissue
• Potentially reversible
• Often pre-cancerous condition