Robbins - Le basi patologiche delle malattie Vol.1-1 (1)

Tissue Renewal, Regeneration, and Repair

• Key Concepts:

  • Normal Homeostasis: Balance between proliferation and apoptosis.

  • Tissue Renewal: Involves various tissue types including stable tissues and the impact of post-injury healing processes.

Types of Healing

• Complete Healing:

  • Involves compensatory growth and healing of lesions.

  • Examples include epidermis, epithelial tissues of liver, kidneys, and hematopoietic system.

• Pathological Healing:

  • Healing from acute pathology may occur via regeneration (normal tissue structure restored) or repair (scar formation).

  • Chronic pathology often leads to scar formation and fibrosis.

Role of ECM in Healing

• The extracellular matrix (ECM) assists in cell migration and maintains cell polarization necessary for reconstructing stratified structures.

• ECM components (fibroblasts, macrophages) produce growth factors, cytokines, and chemokines essential for regeneration and repair.

Impacts of Tissue Repair

• Tissue repair allows for healing of injuries but can also lead to dysfunctions, like in later sclerosis.

• Understanding regenerative and reparative processes requires knowledge of:

  • Control mechanisms of proliferation.

  • Signal transduction pathways.

  • Functions of ECM components.

Cell Cycle and Proliferative Potential

• In adult tissues, cellular population sizes are determined by proliferation, differentiation, and apoptotic death rates.

• Cell growth may be due to increased proliferation or decreased apoptosis.

• Apoptosis is a physiological process vital for tissue homeostasis but can also be induced by pathological stimuli.

• Terminally differentiated cells cannot replicate but replace lost cells through stem cells in continuously renewing tissues.

Examples of Physiological Proliferation

• Endometrial proliferation stimulated by estrogens during the menstrual cycle.

• Thyroid cell replication induced by TSH during pregnancy leads to an increase in gland size.

• Physiological stimuli can lead to pathological states if excessive, e.g., nodular prostatic hyperplasia from dihydrotestosterone.

Control of Proliferation

• Proliferation is primarily regulated by soluble or contact-dependent signals in the microenvironment.

• Excessive activators or insufficient inhibitors result in net cellular growth, which can lead to uncontrolled growth in cancer.

Classification of Tissues Based on Proliferative Potential

• Proliferating Tissues: Labile tissues that renew constantly.

• Quiescent Tissues: Stable tissues that have limited regeneration capabilities.

• Non-Proliferating Tissues: Permanent cells that do not regenerate.