Notes on Tissue Regeneration

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An Overview:

• Repair has two forms: regeneration and healing.

• Regeneration: partial compensatory growth rather than true regeneration. Seen in liver, kidneys. Requires an intact tissue scaffold.

• Healing is recovering from injury: consists of regeneration and scar formation. Tissue is replaced by collagen. Gives rise to inflammation. Occurs with scar formation when extra-cellular matrix (ECM) network is damaged, causing alternation in tissue structure.

• Cell populations are controlled by rates of cell proliferation, differentiation, and death by apoptosis. These processes are controlled by cellular signals within the body.

• Pluripotent embryonic stem cells can give rise to any tissue of the human body.

• The liver regenerates due to the presence of stem cells in the canals of Hering.

• The brain also regenerates in some areas due to the presence of neural stem cells.

Mechanisms of Tissue Regeneration:

• Done by two processes: reentry of quiescent cells into the cell cycle, and efficient differentiation of stem cells at the site of injury.

• Human liver has the most dramatic regeneration capacity. After liver lobe resection, new lobes don't grow; instead, the remaining ones overgrow to fulfill operational needs. This is called compensatory growth. All quiescent hepatocytes replicate. Liver regeneration is done by cytokines and polypeptide growth factors.

Extra-cellular Matrix (ECM):

• Secreted locally and assembles in the space surrounding cells.

• ECM serves several functions during wound healing: transport of minerals, etc.

• Consists of these proteins: collagens, elastins, adhesive glycoproteins, proteoglycans, and hyaluronic acid. These structural macromolecules may assemble into interstitial matrix and basement membrane.

• Cell adhesion molecules (CAMs) also play an important role. For example, integrin binds to both fibronectin and laminin.

• Laminin is the most abundant protein in the basement membrane.

Healing, Scar Formation, and Fibrosis:

• Healing patches rather than restores original tissue.

• Healing is: inflammation, formation of granulation tissue, scar formation, tissue remodeling.

Cutaneous repair:

• The end product may not be functionally perfect.

• Phases: inflammation, granulation tissue formation, and wound contraction, ECM deposition, remodeling.

Healing by first intention:

• After an incision is made: Neutrophils appear; Neutrophils are then displaced by macrophages and there is formation of granulation tissue; a scar forms in the end without the inflammatory infiltrate.

Healing by second intention:

• Intense inflammatory reaction due to larger tissue loss; larger amounts of granulation tissue with more dramatic wound contraction; substantial amount of scarring.

Additional Reading:

Basic Pathology

1. Cell Injury
2. Inflammation and Repair
3. Immunopathology
4. Water, Electrolyte, Acid-Base, Hemodynamic Disorders
5. Genetic and Developmental Disorders
6. Environmental Pathology
7. Nutritional Disorders
8. Neoplasia
9. Vascular Disorders
10. Heart Disorders
11. Red Blood Cell Disorders
12. White Blood Cell Disorders
13. Lymphoid Tissue Disorders
14. Hemostasis Disorders
15. Blood Banking and Transfusion Disorders
16. Upper and Lower Respiratory Disorders
17. Gastrointestinal Disorders
18. Hepatobiliary and Pancreatic Disorders
19. Kidney Disorders
20. Lower Urinary Tract and Male Reproductive Disorders
21. Female Reproductive and Breast Disorders
22. Endocrine Disorders
23. Musculoskeletal Disorders
24. Skin Disorders
25. Nervous System Disorders
26. Notes on Tissue Regeneration
27. A Table of Bleeding Disorders
28. FAQ on Structure and Function of Red Blood Cells
29. FAQ on Components of Blood
30. Notes on Hemostatic Mechanisms
31. What is Fever?
32. What is Edema?
33. FAQ on Blood Pressure
34. FAQ on principles of fluid and flow dynamics of Blood
35. Causes of Thrombocytopenia
36. Squamous cell carcinoma of head and neck mucosa
37. Four tumors which never metastasize to the brain
38. What is caustic injury?
39. What causes Peripheral Edema?

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