Introduction

Even as cells and tissues are injured, events that contain the

damage and prepare the surviving cells to replicate are set into motion.

Repair consists of the replacement of dead cells by viable cells.

These new cells may be either derived from the parenchyma or from

the connective tissue stroma of the injured tissue. During the

evolutionary process, mammals lost the capacity to regenerate total

structures, such as limbs, as many of the simpler aquatic and

amphibious animals can. Indeed, the restorative capacity of humans is

quite limited. Only some of their cells are capable of regeneration.

Repair of destroyed cells because usually involves some connective

tissue proliferation with the formation of a fibrous scar. Although, the

anatomic continuity of the tissue may be restored thereby, such impair

is obviously imperfect, since it replaces functioning parenchymal cells

with non-specialized connective tissue.

Repair begins very early in the process of inflammation and

involves two processes:

I] Regeneration of the injured tissue.

II] Replacement by connective tissue.

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I] Regeneration

- Some parenchymal cells are short-lived while others have a

longer life span. In order to maintain the proper structure of

tissues, these cells are under the constant regulatory control of

their cell cycle.

which is defined as the period between 2 successive cell

divisions and is divided into 4 unequal phases.

(a) (b) (c)

M (mitosis) phase

G1 (gap 1) phase

the daughter cell enters G1 phase after

mitosis

S (synthesis phase)

synthesis of nuclear DNA takes

place

(d)

G2 (gap 2) phase

after completion of nuclear DNA the cell enters G2 phase

(e)

G0 (gap 0) phase

is the quiescent or the resting phase of the cell after an M

phase

Regeneration can be divided into:

Movement of the surviving cells

into the vacant space made

available by loss due to wound /

necrosis

Proliferation of the surviving cells

to replace the loss

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The cells of the body have been divided into 3 groups on the basis of

their regenerative capacity and their relationship to the cell-cycle.

(A) Continuously dividing (labile cells)

These cells continue to multiply throughout life under normal

physiologic conditions. These include:

- Surface epithelial cells of epidermis.

- Alimentary tract.

- Respiratory tract.

- Urinary tract.

- Vagina.

- Cervix.

- Uterine endometrium.

- Haemopoietic cells of bone marrow.

- Cells of lymph nodes and spleen.

(B)Quiescent (stable cells)

- These cells decrease or lose their ability to proliferate after

adolescence but retain the capacity to multiply in response to

stimuli throughout life.

The growth capacity of stable cells is best exemplified by the

ability of the liver to regenerate after hepatectomy or following toxic /

viral / chemical injury. These include:

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- Parenchymal cells of organic like liver, pancreas, kidneys,

adrenal and thyroid.

- Mesenchymal cells like smooth muscle cells

- Fibroblasts.

- Vascular endothelium

- Bone

- Cartilage cells.

(C)Non-dividing (Permanent cells)

- Exited the cell cycle at some point in intra-uterine development

and cannot undergo further mitotic division in post-natal life.

These include:

o Neurons of nervous system.

o Skeletal muscle.

o Cardiac muscle cells.

i.e. why destruction of a neuron whether in central nervous

system / ganglia, represents a permanent loss. The perfection of

parenchymal repair of an injury depends on more than the ability of the

cells to regenerate. Preservation of the stromal architecture of the

injured tissue is also necessary. Thus, the perfection of repair depends

to a considerable extent on the survival of the basic framework of the

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tissue when this is lost, regeneration may restore mass but not

complete function.

Relationship of parenchymal cells to the cell cycle

a. Labile cells which are continuously dividing cells remain

in the cell cycle from one mitosis to the next.

b. Stable cells are in the resting phase but can be stimulated

to enter the cell cycle.

c. Permanent cells leave the cell cycle and die after injury.

II] Repair

- Repair is the replacement of injured tissue by fibrous tissue.

Two processes are involved.

A. Granulation tissue formation.

B. Contraction of wounds.

A. Granulation tissue formation

Refers / derived from the pink, soft, granular gross

appearance such as that seen beneath the scab of a skin wound.

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Histologically, each granule corresponds to the proliferation of new

small blood vessels which are slightly lifted on the surface by a thin

covering of fibroblasts and young collagen. There are 3 components

to granulation tissue formation.

i) PHASE OF ACUTE INFLAMMATION

- Following trauma / injury, blood clots at the site of injury.

There is acute inflammatory response with exudation of plasma,

neutrophils and some monocytes within 24 hours.

ii) PHASE OF CLEARANCE

- Combination of proteolytic enzymes liberated from neutrophils,

autolytic enzymes from dead tissue cells, and phagocytic activity

of macrophages clear off the necrotic tissue, debris and red

blood cells.

iii) PHASE OF INGROWTH OF GRANULATION TISSUE

Angiogenesis / Neovascularization Formation of fibrous tissue

Formation of new blood vessels at

the site of injury takes place by

proliferation of endothelial cells

from the margins of severed blood

vessels. Initially, the proliferated

endothelial cells are solid buds but

within a few hours develop a

Formation of fibrous tissue

- As the granulation tissue

natures, inflammatory cells

decrease in number, fibroblasts

lay down collagen and the

capillaries become much less

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lumen and start carrying blood.

The newly formed blood vessels

are more leaky, accounting for the

oedematous appearance of new

granulation tissue.

Summarizing:

- Proteolytic – degeneration

of the parent vessel basement

membrane allowing formation

of a capillary sprout.

- Migration of the endothelial

cells towards the angiogenic

stimulus.

- Proliferation of the

endothelial cells.

- Maturation of the

endothelial cells with

organization.

prominent what emerges is an

avascular, relatively acellular scar

with inactive spindle-shaped

fibroblasts tucked in between

collagen fibres. This is known as

Cicartrisation.

B. Contraction of wounds

The wound starts contracting after 2-3 days and the process is

completed by the 14 th day. During this time, the wound is reduced

to approximately 80% of its original size. Contracted wound results

in rapid healing since lesser surface area of the injured tissue has to

be replaced.

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In order to explain the mechanism of wound healing, a number

of factors have been proposed. These are:

a) Dehydration as a result of fluid removal by the

drying of the wound but this was not substantiated.

b) Contraction of collagen Was thought to be

responsible for contraction but wound contraction proceeds at

a stage when the collagen content of the granulation tissue is

very small.

c) Discovery of myofibroblasts appearing in active

granulation tissue has resolved the controversy surrounding

the mechanism of wound contraction. These cells have

features intermediate between those of fibroblasts and

smooth muscle cells. Their migration into the wound area and

their active contraction decreases the size of the defect.

With all this background on granulation tissue, we can now

discuss the two forms of repair.

Healing by first intention Healing by second intention

(primary union) (secondary union)

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The sequence of events are:

1. Initial haemorrhage

Within the first post-operative day, after the wound has been

approximated by surgical sutures, the line of incision promptly fills

with blood clots which seals the wound against dehydration and

infection.

2. Acute inflammatory response

This occurs within 24 hours by appearance of polymorphs from

the margins of the incision. By 3 rd day, polymorphs are replaced by

macrophages.

3. Epithelial changes

The basal cells of epidermis from both the cut margins start

proliferating and migrating towards incisional space in the form of

epithelial spurs. A well approximated wound is covered by a layer of

epithelium in 48 hours. The migrated epidermal cells separate the

underlying viable dermis from the overlying necrotic material and

blood clot, forming a scab which is cast off.

By 5th day a multilayered new epidermis is formed which is

differentiated into superficial and deeper layer.

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4. Organisation

By 3rd day, fibroblasts also invade the wound area. By 5 th day,

the acute inflammatory response begins to subside and the neutrophils

are replaced by macrophages which debride the wound margins of

destroyed cells and bits and pieces of fibrin.

In 4 weeks, the scar tissue with scanty cellular and vascular

elements, new inflammatory cells and epithelialized surface is formed.

I] Primary union

This is defined as healing of a wound which has the following

characteristics.

- Clean and uninfected.

- Surgically incised.

- Without much loss of cells and tissue, and

- Edges of the wound are approximated by surgical sutures.

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The incised wound as well as sutures track

on either side are filled with blood clot and

there is inflammatory response from the

margins.

Spurs of epidermal cells migrate along the

incised margin on either side as well as

around the suture track. Formation of

granulation tissue also begins from below.

Removal of suture at around 7 th day results

in scar tissue at the site of incision and

suture track.

Suture tacks

Each suture track is a separate wound and incites the same

phenomenon as in healing of the primary wound.

When sutures are removed around 7 th day, much of the

epithelialized suture tack is avulsed and the remaining epithelial tissue

in the track is absorbed. However, sometimes the suture track gets

infected (Stitch absecess), or the epithelial cells may persist in the

track (implantation / epidermal cysts).

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II] Secondary union

This is defined as the healing of the wound having, the following

characteristics:

- Open with a large tissue defect; at times infected.

- Having extensive loss of cells and tissue and

- The wound is not approximated by surgical sutures but is left

open.

- The basic events in secondary union are similar to primary union

but differ in having a larger defect which has to be bridged.

Hence, healing takes place from the base upwards, as well as

from the margins inwards. The healing by second intention is

slow and results in a large at times, ugly, scar as compared to

rapid healing and neat scar of primary union.

The open wound is filled with blood clot

and there is inflammatory response at the

junction of viable tissue.

Epithelial spurs from the margins of wound

meet in the middle to cover the gap and

separate the underlying viable tissue from

necrotic tissue at the surface forming.

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After contraction of the wound, a scar

smaller than the original wound is left.

The sequence of events are:

1. Initial haemorrhage

As a result of injury, the wound space is filled with blood clot

and fibrin clot which dries.

2. Inflammatory phase

There is an initial acute inflammatory phase followed by

appearance of macrophages which clear off the debris as in primary

union.

3. Epithelial changes

As in primary healing, the epidermal cells from both the margins

of wound proliferate and migrate into the wound in the form of

epithelial spurs till they meet in the middle and re-epithelialise the gap

completely. However, the proliferating epithelial cells do not cover the

surface fully till granulation tissue from base has started filling the

wound space. In this way, preexisting viable connective tissue is

separated from necrotic material and clot on the surface, forming seal

which is cast off.

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4. Granulation tissue

The main bulk of secondary healing is by granulations. The

newly-formed granulation tissue is deep red, granular and very fragile.

With time, this scar on maturation becomes pale and white due to

increase in collagen and decrease in vascularity.

5. Wound contraction

Due to the action of myofibroblasts present in granulation tissue,

the wound contracts to 1/3rd – ¼th of its original size.

6. Presence of infection

Bacterial contamination of an open wound delays the process of

healing due to release of bacterial toxins that provoke necrosis,

suppuration and thrombosis. Surgical removal of dead and necrosed

tissue helps in preventing the bacterial infection of open wounds.

Complications of wound healing:

- Infection (due to entry of bacteria).

- Implantation cyst.

- Pigmentation (healed wounds may at times have rust like colour

due to staining with haemoglobin).

- Deficient scar formation (due to inadequate G.T. formation).

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- Incisional hermia (a weak scar may be site of bursting open of a

wound) (Wound dehiscence).

Two main aberrations occur in wound healing whether the

process of 1st/2nd intention.

a) The accumulation of excessive amounts of collagen may give

rise to a protruding, tumorous scar known as a keloid thick

interlacing bundles of collagen causing marked swelling at the

site of the wound.

b) The other deviation is the formation of excessive amounts of

granulation tissue which protrudes above the level of the

surrounding skin and in fact blocks re-epithelization. This has

been called as ‘exuberant granulation / pround flesh’.

PATHOLOGIC ASPECTS OF REPAIR / Factors Influencing Repair

In wound healing, normal cell growth and fibrosis may be

altered by a variety of influences, frequently reducing the quality and

adequacy of the reparative process.

These factors may be either extrinsic / intrinsic.

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Systemic factors: Systemic / Local

1. Hypoxia

Healing is often imperfect in regions with a poor blood

supply e.g., various ulcers in a lower limb made hypoxic by

varicose veins, often fails to heal.

In contrast to this, mild hypoxia helps healing by increasing

the differentiation of fibrocytes, collagen formation and the

cross-linkage of collagen-fibrils.

2. Scurvy

Is a classic cause of defective wound healing though now

rare. Scar that results is weak and in the granulation tissue, new

capillaries are few and ill-formed.

3. Severe and prolonged protein deficiency

Inhibits wound healing. Deficiency of sulfur containing

amino acids like cystine and methionine is particularly

important.

4. Glucocorticoids

Of the adrenal gland interfere with wound healing when

given in large doses.

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5. Age

Wound healing is rapid in young and slow in aged and

debilitated patients due to poor blood supply to the injured area.

6. Uncontrolled diabetic – are more prone to develop infections

and hence delay in healing.

7. Local factors

a) Movement of the injured part.

b) Fixation of the injured tissue to an unyielding object such

as bone, makes apposition of the wound edges difficult

and hampers healing.

c) Wounds made along a line of mechanical stress in the

tissue tend to fall together and heal easily but wound

across a line of stress tend to gape and heal more slowly.

d) Infection of the wound.

e) Any foreign bodies present.

f) Type, size and location of injury determines whether

healing takes place by resolution / organization.

g) Exposure to u-v light facilitates healing.

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h) Exposure to ionizing radiation delays granulation tissue

formation.

MECHANISMS INVOLVED IN REPAIR3 processes

Cell-cell interactions.

Cell-matrix interactions.

Stimulatory hormones / growth factors.

1. Cell-cell interactions

As mentioned earlier, re-epithelialization of wound begins

within 24 hours of injury and the gap is usually covered within 48

hours. During regeneration of liver, after partial hepatectomy, the liver

cells burst into mitoses but cease to divide when normal liver substance

is restored. What signals these cells to stop dividing? When certain

normal cells are lightly seeded in Petri dishes to investigate the growth

behaviour of cells, they proliferate migrate and eventually form

confluent monolayers. At this point cells cease to divide – a process

called contact inhibition. It is proposed that cells are inhibited from

proliferation by interchange of signals or substances at contact point.

2. Cell-matrix interactions

Much evidence has accumulated to indicate that the orderly

movement and proliferation of the cells within a healing wound is

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influenced not only by signals derived from other cells but also from

the extra-cellular matrix, which is an organized complex of collagen,

glycosaminoglycans, proteoglycans and glycoproteins. Of these much

attention is focused on fibronectins which are a family of adhesive

high molecular weight glycoproteins. Experimental evidence suggests

that migration of endothelial cells and their organization into

capillaries is also facilitated by fibronectin.

3. Growth Factors

Thus far, we have discussed the regeneration migration and

organization of cells in the wound. But what triggers these cells to

divide.

Either loss of factors that or release of growth

normally inhibit cell division stimulating factors.

At one time, the concept that proliferation occurred because of

reduced levels of growth inhibition substance called Chalones, was

popular.

But nowadays, the list of growth factors is ever increasing. E.g.,

- Epidermal G.F. (E.G.F.).

- Nerve G.F. (N.G.F.).

- Platelet derived G.F. (P.D.G.F.).

- Fibroblast G.F. (F.G.F.).

All these are polypeptides with hormone like structure.

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HEALING IN SPECIALIZED TISSUES

a) Internal surfaces – Endothelium

The regeneration of the covering epithelium is very similar to

that of the skin.

b) Bone

Repair of a bone injury is essentially another instance of

connective tissue healing. It differs from soft tissue repair in so far

as formation of the specialized calcified tissues of bone involves the

activity of osteoblasts and osteoclasts.

Repair of a fracture may be taken as a model for the process of

bone healing. However, basic events in healing of any type # are

similar and resemble healing of skin wound to some extent.

Primary Union of fractures occurs occasionally in a few special

situations when the ends of fractures are approximated because bony

union takes place with formation of medullary callus.

Secondary union is the more common process of # healing. It is

described under 3 headings:

- Procallus formation.

- Osseous callus formation.

- Remodelling.

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1) Procallus formation

a. Haematoma

Forms due to bleeding, filling the area surrounding the #.

b. Local inflammatory response

Occurs at the site of injury with exudation of fibrin, polymorphs

and macrophages. Fragments of necrosed bone are scavenged by

macrophages and osteoclasts.

c. Ingrowth of granulation tissue

Begins with neovascularization. A soft tissue callus is that

formed which joins the ends of # bone without much strength.

d. Callus composed of woven bone and

cartilage

The cells of the inner layer of periosteum have osteogenic

potential and lay down the collagen as well as the osteoid matrix in the

granulation tissue. The osteoid undergoes calcification and is called

woven bone callus. At times, callus is composed of woven bone as well

as cartilage, temporarily immobilizing the bone ends.

This stage is called provisional callus / procallus formation and

is arbitrarily divided into:

- External

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- Intermediate.

- Internal procallus.

2) Osseous Callus

The procallus acts as scaffolding on which osseous callus

composed of lamellar bone is formed.

3) Remodelling

During the formation of lamellar bone, osteoblastic laying and

osteoclastic removal are taking place, remodeling the united bone ends,

which after sometime, are indistinguishable from normal bone. The

external callus is cleared away, compact bone (cortex) is formed in

place of intermediate callus and the bone marrow cavity develops in

internal callus.

Haematoma formation and local inflammatory

response at the # site.

Ingrowth of granulation tissue with formation of

soft tissue callus.

Formation of procallus composed of woven bone

and cartilage with its characteristic furiform

appearance and having 3 arbitary components

- External.

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- Intermediate.

- Internal callus

Formation of osseous callus composed of lamellar

bone following clearance of woven bone and

cartilage.

Remodelled bone end; the external callus cleared

away intermediate callus converted into lamellar

bone and internal callus developing bone marrow

cavity.

4) Nervous tissue

Central Nervous System Peripheral Nervous System

Regeneration does not occur. In cases of acute damage, the

initial functional loss often exceeds the loss of actual nerve tissue

because of the reactive changes in the surrounding tissue. As these

changes diminish, functional restoration commences.

Peripheral Nervous System (PNS)

3 types of degenerative processes in the PNS are:

a) Wallerian degeneration

Occurs after transection of the axon which may be as a result

of knife wounds, compression, ischaemia. Then, there is initially

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accumulation of organelles in the proximal and distal end of

transection sites. Subsequently, the axon and myelin sheath

distal to the transection site undergoes disintegration upto the

next node of Ranvier followed by phagocytosis. The process of

regeneration occurs by sprouting of axons and proliferation of

Schwann cells from the distal end.

b) Axonal degeneration

Degeneration of the axon begins at the peripheral terminal

and proceeds backwards towards the nerve cell body. Changes

similar to those seen in Wallerian degeneration are present but

regenerative reaction is limited / absent.

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c) Segmental demyelination

Is demyelination of the segment between two consecutive

nodes of Ranvier, leaving a denuded axon segment. The axon,

however remains intact. Schwann cells proliferation also occurs.

This results in re-myelination of the affected axon. Repeated

episodes of demyelination and remyelination are associated with

concentric proliferation of Schwann cells around axons

producing ‘onion bulbs’ found in hypertrophic neuropathy.

Traumatic Neuroma

Normally, the injured axon of a peripheral nerve regenerates

at the rate of approximately 1mm/day. However, if the process

of regeneration is hampered due to an interposed haematoma or

fibrous scar, the axonal sprouts together with Schwann cells and

fibroblasts form a peripheral mass called Traumatic / Stump

Neuroma.

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d) Muscles

Muscle fibres of all 3 types – Skeletal, Caridac and Visceral

have only limited capacity to regenerate.

- When a mass of muscle tissue is damaged, repair by scarring

occurs.

- If the damage affects individual muscle fibres diffusely and with

varying severity, then regeneration of the specialized fibres is

possible.

Books referred

- Harsh Mohan

- Stanley (Basic Pathology)

- Boyd’s Pathology

- Govan, MacFarlane and Callendar

- Cottran, Robin and Williams

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REPAIR (IN GENERAL)

CONTENTS

Introduction

Process Involved in Repair Regeneration

Replacement

Wound Healing

Pathologic Aspects of Repair

Mechanisms Involved in Repair

Healing in Specialized tissues

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