Tissue Renewal and Repair Regeneration,Healing & Fibrosis Prof.Dr.Ferda ÖZKAN
The body's ability to replace injured or dead cells and to repair tissues after inflammation is critical to survival. • The repair of tissue can be broadly separated into two processes, regeneration and healing .
Regeneration refers to growth of cells and tissues to replace lost structures. Regeneration requires an intact connective tissue scaffold.
Healing is a response to tissue injury, and represents an attempt to restore integrity to an injured tissue. It is a tissue response: (1) to a wound (commonly in the skin), (2) to inflammatory processes in internal organs, (3) to cell necrosis in organs incapable of regeneration.
Healing consists of variable proportions of two distinct processes-regeneration, and the laying down of fibrous tissue, or scar formation. • Healing with scar formation occurs if the extracellular matrix (ECM) framework is damaged, causing alterations of the tissue architecture.
Extracellular Matrix • Cells grow, move, and differentiate in intimate contact with macromolecules outside the cell that constitute the ECM. • The ECM is secreted locally and assembles into a network in the spaces surrounding cells
Three groups of macromolecules, which are often physically associated, constitute the ECM: • (1) fibrous structural proteins, such as the collagens and elastins; • (2) a diverse group of adhesive glycoproteins; • (3) proteoglycans and hyaluronic acid.
These macromolecules are present in intercellular junctions and cell surfaces and form two general organizations: • interstitial matrix The interstitial matrix is present in spaces between epithelial, endothelial, and smooth muscle cells and in connective tissue. It consists of fibrillar and nonfibrillar collagen, elastin, fibronectin, proteoglycans, hyaluronate, and other components. • basement membrane (BM). BMs are produced by epithelial and mesenchymal cells and are closely associated with the cell surface. They consist of a network of amorphous nonfibrillar collagen (mostly type IV), laminin, heparan sulfate, proteoglycan, and other glycoproteins.
The Extracellular Matrix • Major components • Collagens • Basement membranes • Elastic fibers • Fibronectin • Proteoglycans
The Collagens • Type I: found in skin, bone, and mature scars • Type II: major component of cartilage • Type III: abundant in embryonic and pliable tissues First collagen deposited in wound healing • Type IV: does not form fibers, but associates with laminin and other matrix components, and is exclusively found in basement membranes
Collagen catabolism and collagenase • Mature collagen is slowly remodeled via the action of collagenases • Fibroblasts are the main source of collagenase, although it is also produced by macrophages, epithelial, and endothelial cells • The remodeling process is controlled by the binding of fibronectin and ubiquitous collagenase inhibitors
Basement membranes • All epithelia are separated from the stroma by basement membrane • All vascular endothelial cells are separated from the underlying stroma by a basement membrane, except for the sinusoidal endothelium of the bone marrow, lymphoid organs, and liver • Basement membranes are synthesized by the cells resting on them (epithelia, endothelium) • Contain collagen type IV, laminin, entactin, and heparin sulfate proteoglycan (perlecan).
Elastic fibers • Whereas tensile strength is provided by collagen, elastic fibers provide recoil, and thus tend to be found in the aorta, smaller arteries, skin, lung, and uterus.
Structural glycoproteins • Fibronectin (nectere, to bind) • Binds to collagens, proteoglycans, fibrinogen, fibrin, cell surfaces, bacteria and DNA • One of the first structural molecules deposited during embryonic development, and, by extension, in the early phases of wound healing • Found in tissue and plasma • Synthesized by hepatocytes
Structural glycoproteins • Proteoglycans • Used to be called mucopolysaccharides, but are now properly referred to as glycosaminoglycans • Highly hydrophylic, and form hydrated gels, even at low concentration • Deposited early in embryologic life, and by extension, early in wound healing.
Cell Proliferation • In adult tissues, the size of cell populations is determined by the rates of cell proliferation, differentiation, and death by apoptosis.
Cell Proliferation • Classification of cells by their proliferative potential • Labile • Stable (quiescent) • Permanent
Classification of Cells Labile cells • This sub-population of cells is constantly turned over: • epithelium of skin • mucous membranes (GIS) • oviducts • urothelium • endometrium • seminiferous tubules • bone marrow • lymphoid tissue. • These cells have a short, finite life span, die via apoptosis, and are rapidly replaced.
Stable cellsStable cells are a sub-population of cells that are normally replaced very slowly, but are capable of rapid renewal after tissue loss. • Hepatocytes and the proximal convoluted tubule cells of the kidney are good examples. • all glandular parenchymal cells • fibroblasts, • endothelial cells • smooth muscle cells • osteoblasts • chondroblasts.
Permanent cells • contain cells that have left the cell cycle and cannot undergo mitotic division in postnatal life • Permanent cells are found in the central nervous system and heart. • Once they are destroyed, they cannot regenerate. • neurons • cardiac muscle cells • skeletal muscle cells.
STEM CELLS • Stem cell research is one of the most exciting topics in modern-day biomedical investigation and stands at the core of a new field called regenerative medicine
STEM CELLS • Stem cells are characterized by their prolonged self-renewal capacity and by their asymmetric replication. • Asymmetric replication describes a special property of stem cells; that is, in every cell division, one of the cells retains its self-renewing capacity while the other enters a differentiation pathway and is converted to a mature, nondividing population
Embryonic Stem Cells (ES) • Embryos contain pluripotent ES cells, which can give rise to all the tissues of the human body. • Such cells can be isolated from normal blastocysts, the structures formed at about the 32-cell stage during embryonic development
Adult Stem Cells • Many tissues in adult animals have been shown to contain reservoirs of stem cells, which are called adult stem cells. • Compared to ES cells, which are pluripotent, adult stem cells have a more restricted differentiation capacity and are usually lineage-specific.
Stem cells are located in sites called niches, which differ among various tissues . • in the gastrointestinal tract, they are located at the isthmus of stomach glands and at the base of the crypts of the colon (each colonic crypt is the clonal product of a single stem cell). • Niches have been identified in other tissues, such as the bulge area of hair follicles and the limbus of the cornea. • We first consider bone marrow stem cells and then discuss stem cells located in other tissues (tissue stem cells).
In addition to bone marrow cells that may migrate to various tissues after injury, adult stem cells reside permanently in most organs. • These cells (known as tissue stem cells) can generate the mature cells of the organs in which they reside. • However, their differentiation commitment can change when they are transplanted into a different tissue.
Cell - Matrix Interactions • Margination and diapedesis: • soluble factors attract cells to the wound site (chemotaxis), • whereas the extracellular matrix helps align these cells for proper wound healing. • Integrins: • Integrins are cell-surface receptors. • By interacting with integrins, the extracellular matrix can directly modify cell behavior, via cytoskeletal proteins.
Cytokines and Growth Factors • Macrophage-derived growth factor (MDGF) • Platelet-derived growth factor (PDGF) • Epidermal growth factor (EGF) • Fibroblast growth factor (FGF) • Transforming growth factor-beta (TGF-b)
Cell- Cell Interactions • Cytokines and Growth Factors • A single experimental dose of a cytokine at the time of injury changed the rate of healing, the type of matrix deposited, and the cell types present in the wound. • PDGF accelerates deposition of provisional matrix • TGF-b hastens deposition of collagen • FGF induces a florid angiogenic response
Wound Healing • Induction of an acute inflammatory response • Regeneration of parenchymal cells • Migration and proliferation of both parenchymal and connective tissue cells • Synthesis of extracellular matrix proteins • Remodeling • Collagenization and maturation of wound
Stages of Healing • Inflammation • Granulation tissue (soft callus) • Scar – Fibrosis (hard callus) • Remodeling • Wound strength
Wound Healing by Primary Intention • Contraction • Accounts for a reduction in size of the defect primarily by the action of myofibroblasts • This process produces faster healing, • Myofibroblasts account for contraction, and represent an intermediate type of cell, between a fibroblast and a myocyte
Primary intention: the usual case with a surgical wound, in which there is a clean wound with well-apposed edges, and minimal clot formation. • No/minimal scar. • Secondary intention: when wound edges cannot be apposed, (e.g., following wound infection), then the wound slowly fills with granulation tissue from the bottom up. • A large scar usually results.
Stages of Wound Healing • Immediate :Blood clot • 2-3hours :Early Inflammation • 2-3 days :Macrophages, clearing • 10-14 days :Soft Granulation tissue • Weeks :Fibrous hard scar • M’nths -Yrs :Remodeling, minimal scar
Skin Wound Healing By First intention • Clean cut wound • Less injury • Less dead tissue • Less infection • Faster healing • Minimal scar • Minimal complications By Second intention • Large rough wound • More injury • More dead tissue • More infection • Delayed healing • More scar • More complications
minutes: Fibrinogen forms a meshwork of fibrin and platelets stops the bleeding. 24 hours: Polys have entered the fibrin meshwork Epithelial cells are regenerating from the edges of the wound surface. 3 days: The fibrin meshwork is extensively invaded by macrophages. Granulation tissue is appearing at the edges of the incisions. A thin layer of epithelial cells now covers the wound surface. 5 days: Granulation tissue fills the entire wound, and there is abundant collagen. 2 weeks: Fibroblasts continue to multiply, and collagen continues to accumulate. 4 weeks: The overlying epidermis is now normal, though it will not re-grow adnexal structures. Capillary involution and scar contraction is well underway, and the red scar is turning white. Healing by primary union or first intention
There is a larger fibrin meshwork a scab, (rich in red cells, brown because of methemoglobin), more inflammation, possibly infection, more granulation tissue, and more spectacular wound contraction (up to 90-95% of the original surface area) When epidermis grows underneath some of the fibrin meshwork, the edges of the scab loosen. When re-epithelialization is complete, the scab falls off. As surface epithelium grows into crevices it excites excessive fibroblastic activity. Scarring by secondary intention always produces some deformity. Healing by secondary union or secondary intention
Repair • The orderly process by which a wound is eventually replaced by a scar • Destruction of epithelium only is termed an erosion, and heals exclusively by regeneration • If destruction of the basement membrane occurs (extracellular matrix), then a scar will form
Phases of Wound Healing Inflammation Granulation Wound Contraction Collagen accumulation and remodeling Activity 1 3 10 30 DAYS
Repair • Granulation tissue is the initial event in the repair of an injury, and consists of • richly vascular connective tissue which contains capillaries, • young fibroblasts, • a variable infiltrate of inflammatory cells. • Do not confuse with GRANULOMA
Repair • As the repair of the wound progresses, collagen synthesis exceeds degradation and remodeling, and a scar is formed. • The tensile strength of the wound continues to increase over many months, becoming maximal at about a year.
Regeneration • In skin, cytokine and growth factor signals for regeneration are probably related to the loss of contact inhibition. • Proliferation of epithelia continues, until contact is re-established.
Factors affecting Healing Systemic • Age • Nutrition • Vitamin def. • Immune status • Chronic diseases • DM, TB, etc. Local • Necrosis • Infection • Apposition • Blood supply • Mobility • Foreign body
Factors that influence wound healing • Type, size, and location of the wound • Vascular supply • diabetics heal poorly • Infection • delays wound healing and leads to more granulation tissue and scarring • Movement • wounds over joints do not heal well due to traction • Radiation • ionizing radiation is bad, UV is good.