Tissue Repair: Regeneration, Healing, and Fibrosis PDF

Tissue Repair: Regeneration, Healing, and Fibrosis General Pathology Dr Osariemen Okhuaihesuyi Modification of lecture notes originally created by Dr. Adepitan Owosho Copyright © 2021, A.T. Still...

Tissue Repair: Regeneration, Healing, and Fibrosis General Pathology Dr Osariemen Okhuaihesuyi Modification of lecture notes originally created by Dr. Adepitan Owosho Copyright © 2021, A.T. Still University/Missouri School of Dentistry and Oral Health. This presentation is intended for ATSU/MOSDOH use only. No part of this presentation may be distributed, reproduced or uploaded/posted on any Internet web sites without the expressed written consent from the author Learning Objectives 1. Describe the mechanisms of tissue repair: regeneration, Scar formation, fibrosis, Organization 2. Describe the control of cellular proliferation 3. Understand the concept of the cell cycle 4. Describe the cell signaling mechanisms 5. Describe the process of wound healing by primary and secondary repair Tissue Repair As injury is occurring, processes are set into motion that: Contain or restrict injury Prepare surviving cells to replicate Regeneration, healing and fibrosis Repair also called healing: Restoration of tissue architecture and function after an injury. Tissue repair occurs by 2 processes Regeneration: Ability to replace damaged components with identical tissue and return to a normal state. Scar formation: Repair by laying down of connective (fibrous) tissue. Regeneration, healing and fibrosis Fibrosis: extensive deposition of collagen as a consequence of chronic inflammation (e.g., lungs, liver, kidney) or myocardial ischemic necrosis. Organization: Fibrosis in a tissue space occupied by an inflammatory exudate (e.g., organizing pneumonia, organizing clot) Regeneration Scarring/fibrosis Figure 3-1. In this example, In this example, injury to the liver injury to the liver is repaired by is repaired by regeneration if only regeneration if only the the hepatocytes are damaged. hepatocytes are damaged. Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier  blue_09 Myocardial infarction (fibrosis) Control of cell proliferation The normal size of a cell population depends on a balance of: 1. Cell proliferation 2. Cell death by apoptosis 3. Emergence of new cells from differentiating stem cells 4. Differentiation of cells from the baseline population  Figure 1.20 Mechanisms regulating cell populations. Cell numbers can be altered by increased or decreased rates of stem cell input, cell death resulting from apoptosis, or changes in the rates of proliferation or differentiation. (Modified from McCarthy NJ, et al: Apoptosis in the development of the immune system: growth factors, clonal selection and bcl-2, Cancer Metastasis Rev 11:157, 1992.) Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Cell Proliferation Regulated by factors in the cell micro-environment These factors enhance or inhibit cell proliferation. ↑ stimulators or ↓ inhibitors = Net cell growth Neoplasia and repair both require proliferation of cells. Gap Phases Exist in the cell cycle between synthesis and mitosis G0 – resting G1 – pre-synthesis G2 – pre-mitotic These gap phases are altered (shortened or lengthened) by growth factors. Figure 3-3 Cell populations and cycle landmarks. Note the cell cycle stages (G0, G1, S, G2 and M), the G1 restriction point, and the G1/S and G2/M checkpoints. Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Proliferation Continuously dividing tissues (labile tissues). Some cell populations continuously cycle and proliferate. They multiply throughout life; (and are always in the cell cycle). Examples bone marrow epithelium (epidermis, GI mucosa)  Proliferation Stable tissues. These are quiescent (in G0) but can enter the cell cycle Proliferate in response to injury or loss of tissue mass. Parenchymal tissues, liver, kidney, pancreas Endothelium Fibroblasts Proliferation Permanent tissues Terminally-differentiated, non-proliferative Neurons and cardiac myocytes Skeletal muscle has a very limited ability to proliferate. Permanent tissue heals mostly by fibrosis/scarring Stem Cells As cells die, new differentiated cells arise from stem cells. Stem cells Capacity for self-renewal Asymmetric replication Some cells differentiate into mature cells, while others retain stem cell characteristics. Embryonic stem cells: pluripotent—can generate multiple kinds of adult cells. Tissue stem cells. These are present in adult tissues, such as bone marrow Laboratory techniques have led to stem cell therapy/ therapeutic cloning. Figure 1.21Embryonal stem (ES) cells. The zygote, formed by the union of sperm and egg, divides to form blastocysts, and the inner cell mass of the blastocyst generates the embryo. The pluripotent cells of the inner cell mass, known as ES cells, can be induced to differentiate into cells of multiple lineages. In the embryo, pluripotent stem cells can asymmetrically divide to yield a residual stable pool of ES cells in addition to generating populations that have progressively more restricted developmental capacity, eventually generating stem cells that are committed to just specific lineages. ES cells can be cultured in vitro and induced to differentiate into cells characteristic of all three germ layers. Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier  Tissue stem cells Stem cell niches in various tissues. (A) Skin stem cells are located in the bulge area of the hair follicle, in sebaceous glands, and in the lower layer of the epidermis. (B) Small intestinal crypt base columnar (CBC) stem cells are located at the base of the crypt interspersed between Paneth cells. (C) Liver stem cells (oval cells) are located in the canals of Hering (thick arrow), structures that connect bile ductules (thin arrow) to parenchymal hepatocytes. Bile duct cells and canals of Hering are highlighted here by an immunohistochemical stain for cytokeratin 7. (C, Courtesy Tania Roskams, MD, University of Leuven, Belgium.) Figure 1.23Induced pluripotent stem (iPS) cells. Genes that confer stem cell properties are introduced into a patient's differentiated cells, giving rise to stem cells that can be induced to differentiate into various lineages. (Modified from Hochedlinger K, Jaenisch R: Nuclear transplantation, embryonic stem cells, and the potential for cell therapy, N Engl J Med 349:275, 2003.) Signaling mechanisms Autocrine: Cell primarily acts on itself Immune response Liver regeneration Paracrine: Substance acts in immediate vicinity Recruitment of inflammatory cells to site of infection Wound healing Endocrine: Regulatory substance released into the blood stream and acts at a distance Insulin, glucagon, TSH, LH, prolactin, etc. Figure 3-5 Patterns of extracellular signaling, demonstrating autocrine, paracrine, and endocrine signaling (see text). (Modified from Lodish, et al [eds]: Molecular Cell Biology, 3rd ed. New York, WH Freeman, 1995.) Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Major types of cell surface receptors and signal transduction pathways Figure 3-7 The major components of the extracellular matrix (ECM), including collagens, proteoglycans, and adhesive glycoproteins. Note that although there are some overlaps in their constituents, basement membrane and interstitial ECM have different general compositions and architecture. Both epithelial and mesenchymal cells (e.g., fibroblasts) interact with ECM via integrins. For the sake of simplification, many ECM components have been left out (e.g., elastin, fibrillin, hyaluronan, syndecan). Figure 1.13 Interactions of extracellular matrix (ECM) and growth factors to mediate cell signaling. Signals from both ECM interactions and growth factors can be integrated by the cells to produce specific responses, including changes in proliferation, locomotion, or differentiation. Mechanisms of Tissue Regeneration E.g. Liver Regeneration Apte, U. (2015). Liver Regeneration: An Introduction. Liver Regeneration, 2-11. https://doi.org/10.1016/B978-0-12-420128-6.00001-4 Mechanisms of Tissue Regeneration E.g. Liver Regeneration Liver regeneration occurs by two major mechanisms: Proliferation of hepatocytes following partial hepatectomy: Resection of up to 90% of the liver can be corrected by residual hepatocyte Liver regeneration from progenitor cells: liver progenitor cells contribute to repopulation when hepatocyte proliferative ability is impaired Priming, followed by growth factor-induced proliferation. Once the mass of the liver is restored, the proliferation is terminated (not shown). Figure 3-10 Regeneration of human liver. Computed tomography scans of the donor liver in living-donor liver transplantation. A, The liver of the donor before the operation. Note the right lobe (outline), which will be resected and used as a transplant. B, Scan of the same liver 1 week after resection of the right lobe; note the enlargement of the left lobe (outline) without regrowth of the right lobe. (Courtesy of R. Troisi, MD, Ghent University, Flanders, Belgium.) Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Repair by Connective Tissue Deposition If repair cannot be accomplished by regeneration alone, it occurs by replacement of the injured cells with connective tissue deposition, “patches” rather than restores the tissue. The term scar cells in any tissue by collagen. Repair by Connective Tissue Deposition: Steps in scar formation Hemostatic plug (immediate) Inflammation Cell proliferation Epithelial cells Endothelial cells and pericyte (angiogenesis) Fibroblasts Formation of granulation tissue Deposition of connective tissue (A) Inflammation. (B) Proliferation of epithelial cells; formation of granulation tissue by vessel growth and proliferating fibroblasts. (C) Remodeling to produce the fibrous scar. Repair by connective tissue deposition Granulation tissue Grossly, pink, soft, “granular” tissue beneath a scab. Histologically, a proliferation of fibroblasts and thin-walled capillaries in a loose ECM and leukocytes. granulation tissue vs granulomas?  Granulation tissue Mature scar (A) Granulation tissue showing numerous blood vessels, edema, and a loose extracellular matrix containing occasional inflammatory cells. (B) Trichrome stain of mature scar, showing dense collagen, with only scattered vascular channels.  Formation of granulation tissue Macrophages: Clearing offending agents and dead tissue; M2 Provide growth factors: cell proliferation type Secrete cytokine: stimulate fibroblast proliferation and CT synthesis Repair begins within 24 hours of injury; by 3 to 5 days, granulation tissue is apparent Angiogenesis is the formation of new blood vessels Deposition of connective tissue Connective tissue formation and remodeling. The amount of connective tissue increases, forming a scar that can remodel over time. Angiogenesis Vasodilation and increased permeability in response to NO and VEGF Separation of pericytes Migration of endothelial cells Proliferation of endothelial cells Remodeling into capillary tubes Recruitment of periendothelial cells Suppression of endothelial proliferation and migration, and deposition of basement membrane Dow nloaded from: StudentConsult (on 30 April 2010 07:58 PM) © 2005 Elsevier Deposition of Connective Tissue Deposition of connective tissue occurs through fibroblast migration and proliferation Fibroblast migration and proliferation, Increased synthesis of collagen and fibronectin, and decreased degradation of ECM by inhibiting Matrix metalloproteinase (MMP) production. As healing progresses, fibroblasts become less proliferative and more synthetic, depositing of ECM (collagen is particularly critical to wound strength). Granulation tissue eventually becomes scar composed of fibroblasts, dense collagen, and other ECM components. Vascular regression → largely avascular scar Myofibroblasts: fibroblasts with contractile features contribute to scar contraction Remodeling of Connective Tissue After its deposition, the connective tissue in the scar continues to be modified and remodeled. Collagens and other ECM components are degraded by a family of matrix metalloproteinases (MMPs), During scar formation, MMPs are activated to remodel the deposited ECM, and then their activity is shut down by the TIMPs. Figure 3-14 Phases of wound healing. Wound contraction occurs only in healing by second intention (see text). (Data from Clark RAF: Cutaneous wound repair. I. Basic biologic considerations. J Am Acad Dermatol 13:702, 1985.) Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier  Healing of Skin Wounds 2 types of healing Healing by primary intention Healing by secondary intention Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Healing of Skin Wounds Primary intention—24 h Least-complicated The wound is a clean, uninfected surgical excision Death and loss of a few cells Narrow defect fills with clotted blood Neutrophils migrate into the area within 24 hrs  Primary intention--days Neutrophils are present Basal epithelial cells undergo mitotic activity at edge, causing it to thicken Spurs or fingers of epithelium extend to opposing surface over bed of granulation tissue Basal cells contact to reform basement membrane zone At about day 3 Neutrophils have been mostly replaced by macrophages, At about day 5 Neovascularization reaches its peak with ongoing migration of fibroblasts and ECM production.  Primary intention--weeks Second week: Granulation tissue fibroblasts secrete collagen Decreases leukocyte infiltrate, edema, and vascularity 1 month: the scar comprises a cellular connective tissue covered by normal epidermis. The tensile strength of the wound increases over time Secondary Intention Healing Occurs when epithelial margins & connective tissue margins can’t be closely adapted. Tissue has been excised, avulsed, or destroyed. Examples: infarction, ulceration, abscess formation, surface wounds that create large defects Repair of mucosa or skin requires CT restitution & re-epithelialization. Secondary Intention Healing In wounds causing large tissue deficits, Larger clot the fibrin clot is larger More exudate and necrotic debris in the 24 hours wounded area. More intense inflammation to remove necrotic debris, exudate, and fibrin Larger amounts of granulation tissue formed results in a greater mass of scar tissue. 3-7 days Secondary Intention Healing At first a provisional matrix is formed, but in about 2 weeks initial matrix (containing fibrin, plasma fibronectin, and type III collagen) is replaced by a matrix composed primarily of type I collagen. Granulation tissue scaffold eventually becomes an avascular scar. Permanent loss of dermal appendages Weeks 1 month, the scar is made up of acellular connective tissue devoid of inflammatory Wound contracture, myofibroblasts infiltrate, covered by intact epidermis. reduce wound volume as much as 50%, depending on site and depth (A–D) External appearance of skin ulcers. (A) Venous leg ulcer; (B) arterial ulcer, (C) diabetic ulcer; (D) pressure sore. (. (E ) Ulcer crater; (F) chronic inflammation and granulation tissue. (A–D) From Eming SA, Margin P, Tomic-Canic M: Wound repair and regeneration: mechanisms, signaling, and translation, Sci Transl Med 6:265, 2014.) © 2005 Elsevier  Figure 3-15 Steps in wound healing by first intention (left) and second intention (right). In the latter, note the large amount of granulation tissue and wound contraction. Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Factors that can affect repair Systemic factors Local factors Diabetes mellitus Infection Nutrition Mechanical factors e.g vit c and protein deficiency Poor perfusion Steroids (glucocorticoids) Eg varicose vein, arteriosclerosis Anemia Foreign bodies Immunocompromise Eg glass, steel; bone Type and volume of wound Eg Surgical vs avulsed injury Complications of wound healing Defects in Healing Excessive scaring Chronic wound- non-healing Hypertrophic scars wound Keloid Venous and arterial ulcers Diabetic ulcers Exuberant granulation – proud Pressure ulcers flesh Non-union of fracture Contracture Wound dehiscence Pathologic Aspects of Repair Wound dehiscence(splitting open or gaping): Too little granulation tissue formed leads to an ulcer – not enough tissue to cover over the defect Excessive formation of repair elements Too much collagen (type III)– keloid or hypertrophic scar Exuberant granulation – “proud flesh” Figure 3-17 Keloid. A, Excess collagen deposition in the skin forming a raised scar known as a keloid. B, Thick connective tissue deposition in the dermis. (A, From Murphy GF, Herzberg AJ: Atlas of Dermatology. Philadelphia, WB Saunders, 1996. B, Courtesy of Z. Argenyi, MD, University of Washington, Seattle.) Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier Contracture Wound contraction is a part of the normal process When excessive: deforms wound and surrounding tissues – restricts mobility, perfusion  Figure 3-18 Overview of repair responses. Repair after injury can occur by regeneration of cells or tissues that restores normal tissue structure, or by healing, which leads to the formation of a scar. Chronic inflammation may cause massive fibrosis. Downloaded from: S tud entCon sult (on 30 April 201 0 0 7:5 8 P M) © 2005 Elsevier  Thank you Questions???