Repair by connective tissue

Muhammad Shoaib m.phil biochemistery

Overview

• Severe or persistent tissue injury with damage both to parenchymal cells and to the stromal framework leads to a situation in which repair cannot be accomplished by parenchymal regeneration alone.

• Repair occurs by replacement of the non regenerated parenchymal cells with connective tissue.

INTRODUCTION

• Scar - formed as part of healing process following damage to skin as body lays down collagen fibres

• If epithelial layer alone is damaged, there is often little or no scarring - as it heals by regeneration.

• If dermal layer is damaged - healing is by repair.• Scar revision - plastic surgery performed to improve

condition /appearance of scar anywhere on body. • Scar evaluation& revision techniques are chief among

most important skills in facial plastic and reconstructive surgeon's armamentarium.

General components of this process are:

Formation of new blood vessels (Angiogenesis) Migration and proliferation of fibroblast Deposition of ECM Maturation and reorganization of the fibrous

tissue (Remodeling)

WOUND HEALING

CELLULAR ACTIVITY

Angiogenesis from endothelial precursors

Common precursor: hemangioblast that gives rise to angioblast and hematopoietic stem cells.

Angioblasts then proliferate and differentiate into endothelial cells that form arteries,veins and lymphatics.

ABCs (angioblast like cells) in the bone marrow can be recruited into tissues to initiate angiogenesis.

What is VASCULOGENESIS?

Angiogenesis from preexisting vessel• Vasodilation in response to nitric acid and VEGF.

• Increased permeability of pre-existing vessel.

• Proteolytic degradation of the parent vessel BM by metalloproteinases and disruption of cell to cell contact btw endothelial cells of the vessel by plasminogen activator.

• Migration of endothelial cells from the original capillary toward an angiogenic stimulus.

• Proliferation of the endothelial cells behind the leading edge of migrating cells.

• Maturation of endothelial cells with inhibition of growth and organization into capillary tubes.

• Recruitment and proliferation of pericytes (for capillaries) and smooth muscle cells (for larger vessel) to support the endothelial tube.

Growth factors and receptors involved in angiogenesis

VEGF secreted by mesenchymal cells and stromal cells.

Angiopoietins(Ang1 and Ang2)FGF-2

PDGF

TGF-B

Angiopoietins ,PDGF,TGF-B participate in the stabilization process,

PDGF participates in the recruitmenmt of the smooth muscle cells,

TGF-b stabilizes newly formed vessel by

enhancing the production Of ECM production.

ECM proteins as REGULATORS OF ANGIOGENESIS

• Integrins , • Thrombospondin 1 and SPARC, tenascin

C,destabilize cell matrix interactions and promote angiogenesis.

• Proteinases , plasminogen activators and matrix metalloproteinases : important role in tissue remodeling during endothelial invasion.

Scar formation

• Emigration and proliferation of fibroblasts in the site of injury

• Deposition of ECM • Tissue remodeling

Fibroblast Migration and Proliferation

• Exudation and deposition of plasma protein, fibrinogen and plasma fibronectin in the ECM of granulation tissue provides a provisional stroma for fibroblasts and endothelial cell ingrowth.

• Migration of fibroblasts to the site of injury and subsequent proliferation are triggered by multiple growth factors TGF-B, PDGF, EGF, FGF and the cytokines IL-1 and TNF

ECM Deposition and Scar Formation

• TGF-B appears to be the most important because of the multitude of effects that favors fibrous tissue deposition.

• It is produced by most of the cells in granulation tissue and causes fibroblast migration and proliferation, increased synthesis of collagen and fibronectin and decreased degradation of ECM by metalloproteinases.

• Fibrillar collagens form a major portion of the connective tissue in repair sites and development of strength in healing wounds

• As the scar matures, vascular regression continues, eventually transforming the richly vascularized granulation tissue into a pale, avascular scar

Scar Classification• Mature scar• Immature scar• Linear hypertrophic scar• Widespread hypertrophic• Minor keloid• Major keloid• Contractures

• Superficial macular scars• Ice pick scar• Rolling scars• Boxcar scars

• Mature scar - light-colored, flat scar.

• Immature scar - red, sometimes itchy or painful and slightly elevated scar in the process of remodeling. Many of these will mature normally over time and become flat

• Linear hypertrophic (e.g surgical/traumatic) scar- red, raised, sometimes itchy scar confined to the border of original surgical incision occurring within weeks following surgery & can regress of its own

• Widespread hypertrophic (e.g., burn) scar—A widespread red, raised, sometimes itchy scar that remains within the borders of the burn injury.

• Minor keloid - focally raised, itchy scar extending over normal tissue & may develop up to 1 year after injury and does not regress on its own

• Major keloid - large, raised (0.5 cm) scar, possibly painful / pruritic extending over normal tissue often resulting from minor trauma and can continue to spread over years.

• Contractures - restrict movement due to skin & underlying tissue that pull together during healing and can occur when there is a large amount of tissue loss or where a wound crosses a joint.

• Superficial macular scars: occur only if epidermis & superficial dermis are involved.

appear as erythematous/pigmented macules

• Ice pick scars (cone-shaped): narrow, deep & sharply marginated epithelial tracts extend vertically to deep dermis or subcutaneous tissue.

• Rolling scars (wavy): occur from dermal tethering of otherwise relatively normal appearing skin.

Abnormal fibrous anchoring of dermis to subcutis lead to superficial shadowing & rolling or undulating appearance t0 the overlying skin

• Boxcar scars (chicken pox scar like): round to oval depressions with sharply demarcated vertical edges, similar to varicela scars.

Are clinically wider at the surface than ice pick scars and do taper to a point at the base

Tissue Remodeling

• The balance between ECM synthesis and degradation results in remodeling of the CT framework.

• Degradation of collagen and other ECM proteinS is achieved by a family of matrix metalloproteinases which are dependant on zinc ions for their activity.

• MMPs include interstitial collagenases (MMP1, 2, 3) which cleave the fibrillar collagen type 1, 2 and 3

• Gelatinases (MMP2 and 9) degrade amorphous collagens and fibronectin.

• Stromelysin (MMP-3,10,11) act on proteoglycan , laminin,fibronectin,amorphous collagen.

• MMPS are synthesized as propeptides that require proteolytic cleavage for activation.

• Their secretion is induced by certain stimuli such as growth factors,cytokines,phagocytosis,physical stress and is inhibited by TGF-b and steroids

• Collagenases are synthesized in as a latent precursor (procollagenase) that is activated by chemicals such as free radicals produced in oxidative burst of leukocytes and proteinases

• Once formed activated collagenases are rapidly inhibited by a family of specific tissue inhibitor of metalloproteinases (TIMP).

WOUND HEALING

• Cutaneous wound healing is divided into three phases

• Inflammation (early and late)• Granulation tissue formation and

reepithilialization• Wound contraction ,ECM deposition and

remodeling.

• Wound healing is a fibroproliferative response that is mediated through growth factors and cytokines.

• Skin wounds are classically described to heal by primary or secondary intention.

Healing by first intention (wounds with opposed edges)

• Common example of wound repair is healing of a clean, uninfected surgical wound approximated by sutures. such healing is called as primary union or healing by first intention.

• Incision causes,• Death of limited number of epithelial cells and

connective tissue.• Disruption of epithelial BM.

Process

• Within 24 hours;• Neutrophils appear at the margin of the

incision moving towards the fibrin clot.• In 24 to 48 hours;• Movement of the epithelial cells from wound

edges from dermis cut margins depositing BM components as they move

• Day 3;• Neutrophils are replaced by macrophages.• Invasion of granulaton tissue invading the

incision space.• Collagen fibers are present in the margin of

the incision but does not bridge the incision.• Epithelial cell proliferation thickens the

epidermal layer

• Day5;• Incisional space filled with granulation tissue.• Neovascularization is maximal• Abundant collagen fibers ,bridging the

incision.• Mature epidermal architecture with surface

kertinization.

• Second week;• Continued accumulation of collagen and proliferation

of fibroblast .• Dissapperance of edema ,leukocyte infiltrate, and

increased vascularity.• End of first month;• Scar is made of cellular C.T inflammatory cell

infilterate , covered now by intact epidermis.• Dermal appendages that have been destroyed in the

line of incision are permanently lost.

Healing by second intention (wound with separate edges)

extensive loss of cells and tissue.Large defects.Abundant granulation tissue grows in from the

margin to complete the repair

Difference

• Larger tissue defect• Larger fibrin clot• More necrotic debris and exudate• Inflammatory reaction is more intense.• Abundant granulation tissue is formed

Wound contraction

• Formation of a network of actin containing fibroblasts at the edge of the wound.

• Permanent wound contraction requires the action of myofibroblasts.

• Contraction of these cells at the wound site decreases the gap between the dermal edges of the wound.

Wound strength

• When sutures are removed at the end of the first week ,strength is approximately 10% that of unwounded skin, but strength increases rapidly over the next 4 wks.

• At the end of third month it the rate decreases and the wound strength is about 70 to80% of the unwounded skin.

Factors that influence wound healing

• Systemic factors;• Nutrition (protein def,Zn, vit.C def) inhibit

collagen synthesis• Metabolic status, diabetes mellitis ,delayed

wound healing, ( microangiopathy)• Inadequate B.S (arteriosclerosis, varicose

veins)• Hormones. ( glucocorticoids)

Local factors

• Infection (persistent tissue injury and inflammation)

• Mechanical factors ,(mobility ,compression of B.V)

• Foreign bodies, (sutures ,fragments of bone, steel or glass)

Localization and size of wound (face, foot)

Complications

Inadequate formation of granulation tissue.

Wound dehiscence• Rupture of a wound is most common after

abdominal surgery and is due to increased abdominal pressure

• Vomiting, coughing , ileus

• Exuberant granulation is another deviation in wound healing consisting of the formation of excessive amounts of granulation tissue, which protrudes above the level of the surrounding skin and blocks re-epithelialization called proud flesh.

• Ulceration of the wounds because of inadequate vascularization during healing. (peripheral vascular diseases)

• Non healing wounds are also found in areas of denervation. (diabetic peripheral neuropathy)

Excessive formation of the components of the repair

• Excessive amount of collagen lead to hypertrophic scar.

• Is a raised scar • Keloid ,it is a scar tissue which goes beyond

the boundaries of the original wound and does not regress.

• Incisional scars or traumatic injuries may be followed by exuberant proliferation of fibroblasts and other connective tissue elements that may recur after excision. Called desmoids, or aggressive fibromatoses, these lie in the interface between benign proliferations and malignant (though low-grade) tumors.

• Contracture• Exaggeration in the process of contraction in

the wound is called contracture.• Severe burns• Palms,soles.thorax

• Healing wounds may also generate excessive granulation tissue that protrudes above the level of the surrounding skin and in fact hinders re-epithelialization. This is called exuberant granulation, or proud flesh, and restoration of epithelial continuity requires cautery or surgical resection of the granulation tissue.

Thank you,