Healing of Wounds

Healing of Wounds

Basic principles of healing

Healing /repair of tissue is a dynamic process which starts at the time of tissue injury and continues over a variable time period.

Healing involves inflammatory cells, neovessels, fibroblasts and myofibroblasts, regenerating nerves and other cells specific for the site of injury, Keratinocytes, osteocytes , hepatocytes etc.

Aim of healing: for damaged tissue to achieve its original structure and function – resolution/ (restitution). This is the ideal outcome of healing. However, resolution is not always possible.

Definition of some words

Healing can be by regeneration or repair

Regeneration:  residual cells regenerate and achieve its original architecture and function. This outcome is also called resolution and restitution. This is possible when the tissue involved is composed of labile or stable cells and the connective tissue frame work is intact.

Repair: occurs when resolution is not possible. E.g. extensive injury, injury to permanent cells etc.

(Organization: the process of collagen formation in healing.)

In repair, if the tissue involved has regenerating capability – repair by regeneration and organization. If the tissue involved is composed of permanent cells – repair by organization only. Chronic inflammation and damage to permanent cells comtaining tissue always lead to healing by repair.

Hence, the process of healing depends on:

Involved tissue: 

o Tissue with regenerative ability (labile and stable cells) can regenerate and replace the damaged tissue.


o Permanent cells cannot regenerate and healing is usually by formation of collagen (scar tissue) – organization ( except in the brain, see below)

Extent of tissue damage: 

o Minor injury in tissues with regenerative ability results in restoration of tissue to its normal status – resolution.
Eg. erosions on the mucosal surfaces.


o In extensive injuries, even in tissues with regenerative ability, regeneration alone may not be sufficient to replace the damaged cells. Collapse of normal connective tissue framework in extensive tissue damage too prevents resolution. In such instances both tissue regeneration and organization (scar tissue formation) occur.
Eg. In liver injury


 In minor to moderate liver cell damage resolution is achieved by cell    regeneration alone.

 In massive liver cell necrosis healing occur by both regeneration and fibrous tissue formation, due to extensive cell necrosis and collapse of reticulin frame work, eventually leading to cirrhosis (post necrotic cirrhosis).

Nature of the injury: 

o Low grade persistent injuries are associated with chronic inflammation. When associated with chronic inflammation both cell regeneration and organization (scar tissue formation) occur.Steps in healing

1. Hemostasis

2. Inflammation – An early  response to issue injury is inflammation. Inflammation is an important initial step in the healing process.

o Degradation and removal of necrotic tissue and debris by inflammatory cells like neutrophils and macrophages. This is an essential step prior to successful healing. Persistence of necrotic debris will delay the healing process and may lead to chronic inflammation resulting in excessive scar tissue formation.


o Secretion of chemical mediators and growth factors by inflammatory cells (especially macrophages), platelets and stromal cells. These are needed for cell regeneration, granulation tissue formation and collagen formation.

Proliferation phase

Cell regeneration – regeneration of native cells in the damaged tissue. Mediated by growth factors secreted by inflammatory cells especially macrophages and also by the adjacent undamaged parenchmal cells.

 Organization:
This occurs when healing by resolution is not possible as discussed above.

Steps in organization:
Granulation tissue formation
Collagen synthesis and laying

Remodeling of the scar

      Granulation tissue formation:

Granulation tissue is composed of proliferation of capillaries (angiogenesis), fibroblasts and residual inflammatory cells. Formation of granulation tissue is controlled by various chemical mediators secreted by inflammatory cells, particularly by macrophages, and stromal cells.

Angiogenesis: New capillary formation is by: Sprouting from pre existing vessels or formation of capillary network in the extracellular matrix from endothelial precursor cells (EPCs) from bone marrow.Usually both these processes take place in angiogenesis.

Important growth factors involved in angiogenesis are:

o Vascular endothelial growth factor (VEGF) – secreted by mesenchymal and stromal cells. VEGF increases sprouting of new capillaries by inducing endothelial cells to proliferate, induces mobilization of EPCs from bone marrow and increase proliferation and maturation of these cells.


o Angiopoietin – stabilization of newly formed vessels.

On gross examination, granulation tissue appears pinkish/reddish, fleshy and granular (healthy granulation tissue). With time the entire area to be repaired gets filled by granulation tissue growing from the edges of the tissue defect towards the centre. Unhealthy granulation appears dark (deoxygenated) and can have features of infection like purulent exudates on the surface. These wounds can easily become necrotic.

Synthesis of Collagen:

Collagen is synthesized and laid down by fibroblasts. Collagen formation starts within 3 ‐ 5 days of onset of injury and continues for weeks depending on the size of the injury. With time, with increasing collagen mass, cellularity and vascularity of the area decrease and finally the area get filled with a hypocellular mass of collagen. Collagen is an insoluble, inelastic material with tensile strength. However, other extracellular matrix proteins (ECM) are also synthesized by fibroblast e.g. glycosaminoglycans and proteoglycans..
(Collagen = fibrous tissue = scar tissue)

Fibroblast migration and proliferation is facilitated by various cytokines secreted by macrophages, platelets and activated endothelium. Eg. TGF - increases fibroblast migration and proliferation, increases collagen synthesis and reduces ECM break down by metaloproteinases.Others: PDGF, FGF, IL‐1 and TNF. Collagen synthesis also facilitated by these same chemicals – esp,  TGF ‐ PDGF and IL‐ 1.

Scar tissue tends to contract and therefore, the size of the scar is usually smaller than the area of injury. Scar contraction is mainly due to presence of myofibroblasts in it. Myofibroblasts are specialized fibroblasts with contractile ability.

Read: different types of collagen and their distribution.

Remodeling:

Formation of correct amount of collagen and ECM proteins are achieved by remodeling process. Collagen and ECM protein synthesis and degradation occur as parallel processes. Degradation is done mainly by a class of proteins called matrix metalloproteinases (MMPs). Some of the growth factors that stimulate collagen synthesis also stimulate MMPs. In the early scar the predominant collagen type is type III and during the remodeling process most of these are replaced by type I collagen.

 Role of macrophages in tissue repair


Experimental studies on animal models have identified macrophages (wound macrophages) as an essential key factor in regulating the tissue repair process. The macrophages at the site of tissue healing are called wound macropahges. These are derived mostly from the circulatory monocytes recruited to the site of injury and also by proliferation of tissue macropahges. The other type of macrophage in the site of injury is resident macrophages (eg. Alveolar macrophage, microglia, osteoclasts and tissue histiocytes).

Actions:
o Secretion of inflammatory mediators and growth factors.
Macrophages that are recruited initially (day 1) to the site of injury secrete mainly proinflammatory cytokines (e.g. TNF ‐ , IL‐6). The ones recruited later secrete more TGF ‐ and VEGF and therefore, involved in the healing process. The two most important growth factors in tissue healing are TGF ‐ (regulate fibrogenesis) and VEGF (regulate neovascularisation).

Wound macrophages disappear from wounds with the resolution of the inflammatory phase of the repair. Some die at the wound and others are likely to migrate to draining lymph nodes.

Healing of Skin Wounds

Skin ‐  Epidermis, dermis and dermal appendages.

Epidermis – stratified squamous epithelium. The basal cell layer is the germinal cell layer
labile cells).Epidermal appendages have limited regenerative capacity.

Depending of the nature of the skin wound healing can be described as:


 Healing by first intention (primary union) – occur in clean incised wounds with opposing edges. E.g, sutured clean surgical incisions.

 Healing by second intention (secondary union) – When edges are not opposed together. E.g, in large wounds, presence of foreign material and infection.

Healing by first intention (primary union)

 The narrow gap is sealed by a blood clot, which forms the scab (dehydrated superficial part of the blood clot).


 The epidermis starts regenerating rapidly within 4 hours. Epidermis grows under the scab from edges and establishes the continuity within 48 hours as a one cell thick membrane. This process occurs by basal cell proliferation and epithelial cell migration from the free edge. Regeneration is stimulated by many growth factors. E.g., epidermal growth factor/transforming growth factor a, and keratinocyte growth factor.

 Maturation of the squamous epithelium with cell stratification and shedding of most mature superficial squames. This leads to separation of the scab towards the end of the first week.


 In the dermis healing occur by scar tissue formation.

o Within 24 hours – acute inflammation and appearance of neutrophils. These liquefy the small amount of debris present and blood clot. Later,this work is taken over by macrophages which also clear the area by phagocytosis.

o Within 48 hours – granulation tissue formation starts from the wound edges.
o Within 72 hours – collagen synthesis starts.

o Day 5 – the gap is filled with collagenizing granulationtissue. o Over the next 4 – 6 weeks – amount of collagen increases
Tensile strength of the scar: The maximum tensile strength collagen can achieve (after months) is 70-80% of the normal skin.

Healing by second intention (secondary union)

Healing by second intention occurs in wounds with separated edges.The process of healing by second intention is similar to that of first intension except that the former takes longer time to heal and results in more fibrous tissue formation due to extensive tissue damage.


 Blood clot seals the edges. Necrotic debris may be present.

 Epidermis grows between the blood clot and necrotic debris and viable connective tissue. In large wounds reepithelialization can takes weeks and in such instances skin grafting can speed up the healing process.
 Granulation tissue formation at the wound base and the edges

 Collagen is laid down by granulation tissue and eventual scar tissue formation in weeks.

 Contraction of the scar due to action of myofibroblasts. So that the final size of the scar is smaller than the initial tissue defect.

To speed up the healing process and minimize the scar tissue formation the following measures are taken:


 Control of infection if infected

 Removal of necrotic debris. This process is greatly enhanced by surgical wound debridement.

 Skin grafting

Factors that retard wound healing

Local causes:

 Infection – cause persistent inflammation and tissue injury

 Presence of necrotic tissue or foreign bodies – e.g, metal, glass, bone fragments. This can be prevented by wound debridement.

 Ischaemia – could be due to peripheral vascular disease or venous diseases like varicose veins.

 Tissue irradiation delay wound healing. Therefore, in cancer patients timing of surgery and radiation therapy is important.

Systemic causes:

 Nutrition – protein deficiency and vitamin C deficiency can retard collagen sysnthesis


 Diabetes mellitus– mainly defective microcirculation (diabetic microangiopathy). Associated peripheral vascular disease and defective local immune response in diabetics can be contributory.
 Administration of glucocorticoids – it has anti inflammatory effect and can also retard collagen systhesis.

Complications of wound healing

1. Deficient scar formation
2. Excessive formation of components of repair process

3. Formation of contractures

1. Deficient scar formation


 Can be due to deficient collagen synthesis due to nutritional deficiencies. Ehlers‐Danlos syndrome is also a cause of defective collagen synthesis.

 Due to defective granulaton tissue formation. E.g., Wound dehiscence/rupture – especially in abdominal wounds, especially following abdominal sugery. This is usually associated with increase intrabdominal pressure.Ulceration – due to defective circulation.


2.  Excessive formation of components of repair process

 Excessive scar formation- Excessive collagen formation in wound healing can result in a hypertrophic scar and keloid.
Hypertrophc scar – more fibrous tissue is formed, limited to the boundaries of the original wound. If the scar tissue formed is massive and extends beyond the boundaries of the wound it is called a keloid. Tendancy to form Keloid is usually genetically determined.


 Excessive granulation tissue formation/exuberant granulation tissueformation –This can protrude above the wound surface and interfere with reepithelialization. Cautery or surgical excision is needed to ensure reepithelialization.


3.  Formation of contractures

Excessive contraction of the scar can result in deformities in the wound and the surrounding tissues.Commonly seen in healed extensive burns.