Healing and repair

Introduction

Repair is the fundamental process of all living things. It begins soon after injury and attempt to restore the anatomic and functional integrity of the tissue.

Healing is the replacement of the destroyed tissue by either cells of the same types (regeneration) or fibrous connective tissue (substitution) or both. The ideal results of healing is to restore the tissue to its normal state, a process called resolution. If the irritant causes little necrosis and after removal of the necrotic cells and cellular debri, the necrotic cells replaced by new cells of same type in process called regeneration. In case of the damaged cells replaced by fibrous connective tissue, the process called repair by scar formation.

Resolution

The injurious tissue is said to resolve when no structural cells have been lost after the inflammatory process is completed and phagocytosis has cleaned up the area. It occurs in uncomplicated cases after removal of the cause.

Regeneration

Regeneration is a characteristic feature of lower animals. Hydra can be cut into small pieces and each will develop into a completely new organism. In mammals, most tissues have a very limited ability to regenerate. Epithelial and connective tissue regenerate extensively.

Repair by scar formation substitution.

Repair by scar formation occurs when the injured tissue is replaced by fibrous connective tissue 

Classification of cells by their proliferative potential

The pattern of cell division occurs at different rats. Some cells have the ability to divide every 12 hours. Other adult cells do not divide at all. The cells can divide into

1. Labile cells

The labile cells have the ability to heal by same kind of cells (regeneration). It observed in tissue when 1.5% of normal adult cells are in mitosis. Normally, the labile cells rapidly divide throughout life to replace the lost cells. Labile cells have short resting (G₀) phase. It includes epithelium beside hemopoietic stem cells in bone marrow.

2. Stable cells

Normally, the stable cells have long life span and stay for long period in resting phase besides they have low division rate. The tissue is composed from stable cells when less than 1.5% of the cells in normal adult tissue are in mitosis. Such tissue is healed either by regeneration or substitution according to the persistence or destruction of the reticulin framework. Liver, endocrine gland and endothelium are example for stable cells.

3. Permanent cells

Permanent cells have no capacity to divide. A tissue, in which no cells are in mitosis, is composed of permanent cells. Neuron, cardiac myocytes and skin appendage are permanent cells. If permanent cells were lost heal occurs by substitution.

Healing of skin wounds

Traditionally, the wound healing is classified into healing by first intention (apposed edges wound) and healing by secondary intention (separated edges wound). Although the end results are clearly different (minimal and prominent scars respectively) the mechanism are the same for both. Moreover, it means that the different between two types is quantitative not qualitative. Wound healing is a complex cascade of biochemical and cellular events in response to tissue injury. Because of healing by primary and second intention have the same mechanism and also for simplification we can divide the wound repair into (Figs 1 ,2, 3, 4 & 5)   .

1. Inflammatory phase

Due to effect of insult on the affected area, The vascular integrity is affected leading to extravasation of blood into the surrounding tissue. The injured blood vessels constrict.

Platelet adhesion activates the coagulation cascade leading to thrombus formation. The functions of thrombus are hemostasis and provide a preliminary matrix for further repair process.

Moreover, the fibrinogen from plasma escaping from the damaged vasculature and fibrinogen, fibronectin and thrombospondin from activated platelets are polymerize by intrinsic or extrinsic coagulation cascade to form a extracellular gel matrix that fill the cavity created by the initial wound.

After clot formation chemotactic substances (kinin system, fibrinolytic system and complement cascade) attract neutrophils (within 24 hours) and macrophages (3-5days) to the injured area using the extracellular matrix as substrate for leukocytes migration.

The phagocytic cells kill any bacteria in the injured area. The neutrophilic infiltration resolve after the first few days.

The macrophages play a critical role in the induction of repair mechanism. It assists neutrophils in phagocytosis of injures agent and tissue debri.

Moreover, it release a number of growth factors and cytokines, which is very important in maintenance of inflammatory reaction, initiation, maturation and control of the healing process.

2. Fibroblastic phase

This phase include fibroplasia of fibroblasts, neovascularization or angiogenesis (reach its peak by day 5), wound contraction and reepithelialization.

Fibroplasia results from migration of fibroblasts from the edge of wound under the effect of growth factors (secrete by macrophages) extracellular matrix (give a medium for adhesion, guidance and migration of the cells) and the anatomic nature of the wound it self provide a stimulus for cell migration (free-edge effect).

Neovascularization (angiogenesis) occurs in the same time with fibroplasia to form the granulation tissue. The endothelium, at the intact capillaries at the edge of wound, secrete collagenase and plasminogen resulted in break down the basement membrane. The endothelial cells then migrate toward the injured area using the wound extracellular matrix and forming new capillaries. After that the extracellular matrix are manufactured from the newly formed fibroblasts. In early stage, the extracellular matrix is composed of fibronectin and hyaluronic acid. Late on, the extracellular matrix is composed mainly from collagen. At the same time collagen is being produced, some fibroblasts transformed into myofibroblasts which responsible for wound contraction.

Within hours after cutaneous injury, reepithelialization begins by migration of the epithelial cells from the wound edge.

3. Maturation and remodelling

In this phase maturation and remodelling of the extracellular matrix occurs which take several months to years. This process involves continuos production of collagen, digestion, aggregation and orientation of collagen.

Healing by second intention (secondary union)

It occurs when the necrosis is extensive, presence of foreign body or when infection occurs. Secondary healing differs from primary one in the following respects:

It contain large necrotic debri, exudate and fibrin that must be removed. So the inflammation is more severe.

Much large amount of granulation tissue is formed.

Large amount of scar formation.

Factors affect wound healing

Local factors

Type, size and location of wound: Clean surgical wound heals faster than infected one. Also, small one with good vacularization heals faster than large one with poor blood supply.

Movement: Early motions before maturation of collagen fiber retard healing.

Ionizing radiation: Radiation led to vascular lesion and lead to slow healing. Acute radiation leads to stop cell proliferation and retard healing

Ultraviolet rays: Exposure to ultraviolet rays accelerate healing.

Systemic factors

Circulatory status: Cardiovascular diseases impaired healing

Infection: Systemic infection delayed wound healing

Malnutrition: Vitamin C, Zinc and protein are important for healing

Hormones: Corticosteroids impair healing

Diseases: Diabetes mellitus, tumors and hematological diseases lead to delaying wound healing.

Figure 1: Skin of dog showing necrosis.

Figure 2: Skin showing healing with scar formation

Figure3: Skin showing healing by scar formation with damage of dermis. H&E

Figure 4: Skin showing ulcer (left) and healing with scar (right).

Figure 5: Skin showing ulcer represented by complete loss of epidermis and the ulcer base lied on dermis. H&E

Healing of bone

The discontinuity of bone (fracture) is the most common lesions in bone. Damage and hemorrhage of adjacent soft tissues usually accompany tacks a fracture. The fracture may be simple, compound or complicated.    Bones takes a longer time to heal than wounds. Healing of bone is accomplished by osteoblasts. They originate from the periosteum, endosteum and capillary endosteum.The healing of the fracture is divided into three phases (Figs 6 & 7) .

Inflammatory phase

The fracture leads to extensive damage of the periosteum and hemorrhage into the adjacent tissues and medullary cavity of the bone. Later on, the hemorrhage forms a blood clot between the broken ends and extends into the soft tissue and medullary cavity of the bone. Within 5 days neo-vacularization begins to occurs and presence of inflammatory cells. By the end of first week, the clot completely replaced by granulation tissue containing osteoblasts.

Reparative phase

It starts by the end of the first week and extends for months. The inflammatory cells disappear and the granulation tissue is replaced by soft callus (provisional callus). This is accomplished by the rearrangement of osteoblasts to form irregular latticework of bone matrix without Haversian system. The soft callus is composed from external, intermediate and internal callus.

Remodeling phase

Hard one replaces the soft callus, where the bone reorganized to Haversian system and ossification occurs. Finally the external and internal callus are removed by the osteoclasts.

Factors which interfere the healing of bone:

1. Old age.

2. Wound infection.

3. Chronic debilitating disease or malnutrition.

4. Necrotic tissue and masses of bone (sequester).

5. Widely separated ends of bone as by muscles.

6. Continued movement leads to pseudoothrosis or the formation of false

joint.

Figure 6: Healing of bone showing soft callus. H&E.

Figure 7: Healing of bone of ribs .H&E

3. Healing of liver

The liver healing may occurs through regeneration, substitution or a combination of both. The outcome depends on chronicity, and extends of tissue damage. Recovery in focal necrosis occurs by regeneration (Fig. 8) .

The normal architecture is restored and no fibrosis occurs because the extracellular matrix framework (EMF) is not affected. In massive hepatic necrosis with destroyed EMF, the remaining hepatocytes regenerated, forming hepatic nodules separated by fibrous tissue (Figs 9 & 10) . Repeated hepatic injury with destroyed EMF results in cirrhosis.

Figure 8: Liver cell regeneration expressed by the presence of binucleate cells obviously due to increase rate of liver cell death. Mitoses are rare in the liver.

Figure 9: Liver healing by substitution. H&E.

Figure 10: Liver healing by substitution. Mason's Trichrome.

4. Healing of kidney

The kidney has a limited regeneration capacity. The tubular epithelium regenerate when the damage is not extensive and EMF is not destroyed. Meanwhile, in case of destruction of the EMF regeneration is not completed and repairs by scar formation (Fig. 11) . Glomeruli do not regenerate and heal by scarring.

Figure 11: Kidney showing healing of coagulative necrosis by granulation tissue. H&E

5. Healing of Nervous system

A. Central nervous system

The damage tissue in brain or spinal cord replaced by gliosis equivalent of scar formation in other tissue. Axonal regeneration in injured spinal cord can be seen up to 2 weeks after injury

B. Peripheral nervous system

Neuron in the peripheral nervous system can regenerate their axon with complete functional recovery. In some cases, granulation tissue grows between the cut ends resulting in a traumatic neuroma.

6. Heart

Myocardial cells have no regenerative capacity. Healing of injured heart occurs by granulation tissue and scarring.

7. Lung

The epithelial lining of the respiratory tract has the ability to regenerate. Extensive damage of the alveolar basement membrane results in scaring and fibrosis (carnification).