Day 4- Lecture 1- Healing And Repair Flashcards
What are the 3 processes involved in wound healing?
- Haemostasis as vessels are open
- Inflammation as there has been tissue injury
- Healing as structures have been injured or destroyed: regeneration (also called resolution) and repair
What is regeneration/resolution?
It is the growth of cells and tissues to replace lost structures - restitution with no or minimal evidence that there was a previous injury (so healing by primary intention of a superficial abrasion)
Give an example of when regeneration takes place
Regeneration of the epithelia of the skin and GI tract after injury (as long as the stem cells of these tissues are not destroyed)
What does regeneration require? And when can it not take place?
Regeneration requires an intact connective tissue scaffold- hence it cannot take place if the damage to the tissue is extensive
Regeneration can only take place in labile or stable tissues
What are the 3 different types of tissue in the body?
- Labile tissues
- Stable tissues
- Permanent tissues
They are divided on the basis of their proliferative activity
What are labile tissues? Give some examples of labile tissues
Labile tissues are continually dividing tissues- they proliferate throughout life replacing cells that are destroyed (contain short lived cells that are replaced by cells derived from stem cells)
Examples:
- Surface epithelia
- Lining mucosa of secretory ducts of the glands of the body
- Columnar epithelia of GI tract and uterus
- Transitional epithelium of urinary tract
- Cells of the bone marrow and haematopoietic tissues
What are stable tissue (quiescent tissues)? Give some examples of stable tissues
Stable tissues normally have a low level of replication but cells in these tissues can undergo rapid division in response to stimuli that can reconstruct the tissue of origin (both stem cells and mature cells proliferate)
Examples:
- Parenchymal cells of the liver, kidney and pancreas
- Mesenchymal cells e.g. Fibroblasts, bone osteoclasts and smooth muscle cells
- Vascular endothelial cells
- Resting lymphocytes (and other WBCs)
What are permanent tissues? Give some examples of permanent tissues
They are non dividing tissues- they contain cells that have left the cell cycle and cannot undergo mitotic division in postnatal life. They have no or only a few stem cells that can be recruited to replace cells.
Examples:
- Neurones
- Skeletal muscle cells
- Cardiac muscle cells
In order for tissues to regenerate, terminally differentiated cells that are lost must be replaced. What normally replaces these cells?
Many terminally differentiated cells cannot divide, so in tissues where regeneration is possible, differentiated cells are very often replaced by cells derived from STEM CELLS
What are stem cells?
Cells with prolonged proliferative activity which show asymmetric replication - they replace lost or damaged cells in tissues
Stem cells show asymmetric replication- what does this mean?
One of the daughter cells remains as a stem cell while the other differentiated into a mature, non-dividing cells
What is the difference between embryonic and adult stem cells?
Embryonic stem cells are totipotent- can give rise to any of the tissues of the human body
Most adult stem cells are unipotent- can only give rise to one type of adult cell, so they are lineage specific e.g. Epithelia
Some adult cells are multipotent as they can produce several types of differentiated cells e.g. Haematopoietic stem cells which can give rise to any constituents of the blood
Explain the nature of cells in labile tissues
- They contain cells that are short lived
- They are continually being replaced by cells derived from stem cells
Explain the nature of cells in permanent tissues
- Contain terminally differentiated cells which cannot replicate
- Stem cells can be present within these tissues but cannot mount an effective proliferation response to significant cell loss
What damage occurs to permanent tissues- what happens?
Tissues heal with a scar
In the CNS- the space where the neurones were is filled with glial cells (which are supporting cells of the CNS)
Explain the nature of cells in stable tissues
- Mature cells and stem cells are involved in proliferation
- Mature cells are usually non-replicating but an be indicted to enter the cell cycle and replicate if necessary (i.e. These cells are in GO but can enter G1)
- Stem cells are present in these tissues and are normally quiescent or proliferate very slowly- they can however proliferate persistently when required
What is fibrous repair (organisation) and when does it take place?
In regeneration, return of tissue to normal state is essential for restoration of full functionality and a normal appearance to the injured tissue
-If collagen framework of a tissue is destroyed (significant tissue loss)
-If there is ongoing chronic inflammation
-If there is necrosis of specialised parenchymal cells that cannot be replaced
-If permanent or complex tissue is damaged
THEN…. fibrovascular connective tissue will grow into the area- this is called fibrous repair (healing with fibrous connective tissue)!
What are the 5 generic processes involved in fibrous repair?
- Phagocytosis of necrotic tissue debris
- Proliferation of endothelial cells which results in small capillaries that grow into the area (angiogenesis)
- Proliferation of fibroblasts and myofibroblasts that synthesise collagen and cause wound contraction- the repair tissue at this stage is called granulation tissue
- Granulation tissue becomes less vascular and matures into a fibrous scar
- The scar matures and shrinks due to contraction of fibrils within myofibroblasts
Collagen provides the extracellular framework for all multicellular organisms- type 1 is the most common fibrillar collagen), give some examples of where this type of collagen is present. Also what does type 4 collagen form?
Both hard ad soft tissues:
- Bones
- Tendons
- Ligaments
- Skin
- Sclera
- Cornea
- Blood vessels
- Hollow organs
Type 4: secreted by epithelial cells and make up the basement membranes
Give a brief overview of the synthesis and structure of collagen-
-Collagen consists of a triple helix of 3 polypeptide alpha chains with gly-x-y repeating sequence
Collagen is synthesised in the ER of fibroblasts and myofibroblasts in the following way…
-There are enzymatic modification steps including vitamin c dependant hydroxylation
-Preprocollagen is produced in the cell
-Modified to form procollagen which takes on the triple helix form and is secreted from the cell (as procollagen is soluble)
-Procollagen is cleaved to produce fibrillar collagen (tropocollagen)
-Tropocollagen polymerises to form microfibrils and then to fibrils
-Bundles of fibrils form fibres
-There is cross-linking between molecules which produces the tensile strength
Name 4 diseases associated with defects in collagen synthesis?
-Scurvy Inherited -Ehlers-Danlos syndrome -Osteogenesis imperfecta -Alport syndrome
What causes scurvy, and what complication arises with regards to wound healing?
- Due to a vitamin C deficiency
- Vitamin C is required for hydroxylation of procollagen alpha chains- so leads to reduced cross-linking and defective helix formation
Symptoms:
- People with scurvy are unable to heal wounds adequately and have a tendency to bleed
- Tooth loss- collagen in periodontal ligament has a short half life so is continually being replaced, normal collagen is replaced by defective collagen
- Old scars break down and open up as fresh wounds- collagen turnover in scars remains high long after healing process appears to be clinically complete
What is Ehlers-Danlos syndrome, what are the symptoms, and what effect does it have on wound healing?
A heterogenous group of 6 inherited disorders where collagen fibres lack adequate tensile strength- there is defective conversion of procollagen to tropocollagen
Symptoms:
- Skin= hyperextensible, thin, fragile, susceptible to injury (skin can recoil as elastic fibres are normal, just missing the tethering effect of normal collagen fibres)
- Joints= hypermobile and predisposition to dislocation
- Wound healing is poor
Collagen of internal organs is also affected which can lead to:
- Rupture of the colon
- Rupture of large arteries
- Corneal rupture and retinal detachment
What is osteogenesis imperfecta? What are the symptoms?
Also called brittle bone disease: patients have too little collagen in bone tissue and hence extreme skeletal fragility- affected people have to try and avoid mechanical stress and some develop severe, progressive deformation of long bones
Symptoms:
- Blue sclerae as there is too little collagen in the sclerae making them translucent
- Hearing impairment
- Dental abnormalities
What is alport syndrome? What are the symptoms and possible complications?
Usually X-linked disease- type 4 collagen (which makes basement membrane) is abnormal and results in dysfunction of:
- The glomerular basement membrane
- The cochlea of the ear
- The lens of the eye
Results in:
- Patients- usually male- present with haematuria (blood in urine) as children or adolescents
- This progresses to chronic renal failure
- They also have neural deafness and eye disorders
In regeneration and repair it is known that cells communicate with one another to produce a fibroproliferative response- how do cells communicate?
Cell to cell communication to initiate a proliferation response
- Via local mediators (e.g. Growth factors)
- By hormones
- By direct cell to cell contact
- By ire the cell to stroma contact
What forms of cell communication use local mediators or hormones? Give a brief overview of these
- Autocrine: cells respond to signalling molecules that they themselves produce
- Paracrine: a cell produces the signalling molecule, this acts on adjacent cells. The responding cells are close to the secreting cell and are often of a different type
- Endocrine: hormones are synthesised by cells in an endocrine organ, they are then conveyed in the blood stream to target cells to effect physiological activity
Which local mediators are most important in regeneration and fibrous repair?
Growth factors
What are growth factors?
- Polypeptides that act on specific cell surface receptors
- They can be considered as ‘local hormones’ as they act over a short distance or even on the secreting cell itself
What codes for growth factors?
Proto-oncogenes
What do growth factors do?
There are a large number of growth factors- some act on many cell types and other have restricted targets-
They:
- Stimulate cell proliferation
- Inhibit cell proliferation
- Can affect cell locomotion
- Can affect cell contractility
- Can affect cell differentiation
- Can affect cell visibility
- Can affect cell activation
- Can affect angiogenesis
Growth factors bind to specific receptors and stimulate transcription of genes that regulate the entry of the cell into the cell cycle and the cell’s passage through it
Are 4 different types of growth factor
- Epidermal growth factor
- Vascular endothelial growth factor
- Platelet derived growth factor
- Tumour necrosis factor
Produced by cells such as platelets, macrophages and endothelial cells
Give an overview of epidermal growth factor
- Binds to epidermal growth factor receptor (EGFR)
- Mitogenic for epithelial cells, hepatocytes and fibroblasts
- They are produced by keratinocytes, macrophages ad inflammatory cells
What are 2 functions of the vascular endothelial growth factor
- Induces blood vessel development (vasculogenesis)
- Has a role in growth of new blood vessels in tumours, chronic inflammation and wound healing (angiogenesis)
Give an overview of platelet-derived growth factor
- Stored in platelet alpha granules
- Released on platelet activation
- Produced by macrophages, endothelial cells, smooth muscle cells and tumour cells
- It causes migration and proliferation of fibroblasts, smooth muscle cells and monocytes
Name 3 functions of the tumour necrosis factor
- Induces fibroblast migration
- Induces fibroblast proliferation
- Leads to collagenase secretion
What is contact inhibition and how does it work?
- Normal cells when isolated from other cells around them will replicate until they have cells touching them and then replication will stop
- I.e. They form a monolayer sheet of cells with no cell overlap
- This occurs in wound healing
- Cells adhere to each other and to the extracellular matrix by adhesion molecules
- Cadherins: adhesion molecules binding cells together
- Intergrins: adhesion molecules binding cells to the ECM
Contain inhibition inhibits proliferation in intact tissues and promotes proliferation in damaged tissues (altered in malignant cells that do not have contact inhibition)
What are the 2 ways in which wound healing can be classified?
Healing by primary or secondary intention depending on the size of the wound and the amount of tissue that as been lost
What are the characteristics of a wound which is healing by primary intention?
- Incisional
- Closed
- Non-infected
- Sutured
I.e. Clean wounds with apposed edges
What is disrupted in wounds which are healed by primary intention?
- There is disruption of the epithelial basement membrane continuity
- Death of only a limited number of epithelial and connective tissue cells
(They heal with less scarring than those that heal with secondary intention)
Describe, in detail, the process of healing with primary intention (include time periods)
HAEMOSTASIS: seconds to minutes
- Severed arteries constrict- reflex to stop bleeding
- Narrow space fills with clotted blood (fibrin, platelets and RBCs)
- Scab formation as surface of the clot dehydrates
- Scab acts as a temporary seal from foreign material from entering, and stops continual bleeding
ACUTE INFLAMMATION: minutes to hours
- Triggered automatically (not by bacteria)
- Neutrophils appear at the margins of the incision and target bacteria
- Sterile wounds have a low number of leukocytes so cannot classify the fluid as pus
CHRONIC INFLAMMATION/MARGINATION OF CELLS: up to 48 hrs
- Macrophages remove dead neutrophils
- Activated macrophages secrete cytokines which attract other cells like fibroblasts and endothelial cells
- Capillary sprouts begin to appear
- Epidermis begins to regenerate: spurs of basal epidermal cells at the edge of the cut creep over denuded cells whilst depositing basement membrane components. They fuse at the midline beneath the scab and due to contact inhibition stop proliferating.
GRANULATION TISSUE FORMATION/REGENERATION: 3 days later
- Macrophages replace neutrophils
- Granulation tissue (fibroblasts and new capillaries invade the space)
- Epithelial cell proliferation thickens the epidermal layer and epidermal cells undermine the scar which then falls off
- Activated fibroblasts produce collagen (lay down ECM)
- Angiogenesis progresses- forms blood vessels which provide oxygen and nutrients
EARLY SCAR: 7-10 days later
- Wound is filled with granulation tissue
- Fibroblasts proliferate and deposit collagen fibres to form a fibrous mass (scar)
- Epidermis normalises and keratinises by skin appendages
- White cell infiltrate, oedema, increased vascularity disappears
- Regression of vascular channels
SCAR MATURATION: one month-two years
- A scar is a mass of fibrous tissue with many collagen fibres
- Very few cells and vessels
- Few elastic fibres- little recoil hence scars stretch
- As capillaries disappear old scars appear white (new scars appear pink)
When is healing by secondary intention seen?
- Excisional wounds
- Wounds with tissue loss or separated edges
- Infected wounds
Examples: infarct, ulcer, abscess
What happens in healing by secondary intention?
- The open wound is filled by abundant granulation tissue which grows in from the wound margins
- As there is a larger clot and more necrotic debris than a wound healing by primary intention, the inflammatory reaction is more intense
- Considerable wound contraction must take place to close the wound
- Initially this occurs as the scab contracts when it dries and shrinks
- After about a week myofibroblasts appear and contract
- An open wound contracts as if its margins were being drawn into the centre- the final shape of the scar depends on the original shape of the wound
What are myofibroblasts?
Fibroblasts that develop contractile properties- they resemble smooth muscle cells and are an intermediate between smooth muscle cells and fibroblasts
What is usually the result of healing by secondary intention?
A substantial scar forms, and the new epidermis is often thinner than usual -> usually takes longer than healing with primary intention
What can delay healing by secondary intention?
As infection
Describe the process of healing of bone fractures?
HAEMATOMA (mass of clotted blood)
-Fills the gap and surround the bone injury
GRANULATION TISSUE FORMATION
- Platelets and inflammatory cells release cytokines
- Cytokines activate osteoprogenitor cells to carry out osteoclastic and osteoblastic activity
SOFT CALLUS FORMATION (fibrocartilaginous/procallus)
- 1 week later
- It consists of fibrous tissue and and cartilage within which woven bone begins to form
- It usually extends beyond the volume occupied by the uninjured bone and forms a bulge around the fracture site
HARD CALLUS FORMATION
- After several weeks
- Woven bone is initially laid down by osteoblasts
- Woven bone is weaker and less organised than lamellae bone
- Woven bone can form quickly
LAMELLAE BONE REPLACES WOVEN BONE
- More organised and stronger lamellae bone replaces woven bone
- Remodelling of the bone occurs in response to mechanical stresses placed on it
- Bone not stressed is resorted and the outline of the bone is reestablished
What local factors influence healing and repair?
1-Size, location and type of wound- indicates if healing is by primary or secondary intention and if regeneration or scarring will occur (e.g. Foot less likely to heal than face)
2-Blood supply e.g. Arteriosclerosis impedes healing, areas with high vascularity (e.g. Face) heal well
3-Denervation- impairs healing
4-Local infection- produces persistent tissue injury and inflammation
5-Foreign bodies- produce persistent inflammation and favour infection
6-Haematoma- if large and persistent can slow healing
7-Necrotic tissue- needs clearing during the process of repair, therefore if a large amount is present healing can take longer
8-Mechanical stress- can pull apart delicate tissues in the early stages of healing (keep moving- can keep splitting the wound apart)
9-Protection (dressing)- help to keep the wound clean and free from infection
10-Surgical techniques- good techniques promote rapid healing and minimise scarring
What systemic factors influence healing and repair?
1-Age- children heal quickly, elderly people more slowly
2-Anaemia, hypoxia and hypovolaemia (e.g. Following trauma)- poorer oxygen deliver to healing tissue
3-Obesity- can cause increased tension on wounds and wound dehiscence
4-Diabetes- microangiopathy impairs blood supple to damaged area. There is also a decreased resistance to infection.
5-Malignancy- due to cachexia (wasting of the body) seen with malignant tumours
6-Genetic disorders e.g. Ehlers-Danlos syndrome
7-Drugs- steroids (inhibit collagen synthesis), cytotoxic (anti-mitogenic and impair cell proliferation and healing), antibiotics (treat bacterial infections, reduce inflammation and can speed up healing)
8-Vitamin deficiency- vitamin c deficiency inhibits collagen synthesis
9-Malnutrition- lack of essential substances such as amino acids for protein synthesis
As well as delayed healing due to local or systemic factors- complications of fibrous repair can result from the fibrosis itself- give 5 examples and explain them
Insufficient fibrosis (not enough scarring) - wound dehiscence, hernia, ulceration with local/systemic factors inhibiting fibrosis
Formation of fibrous adhesions- adhesions are areas of fibrotic tissues formed surrounding or between organs:
(Example: pleural adhesions) compromising organ function or blocking tubes
Loss of function due to replacement of specialised functional parenchymal cells by non-functioning collagenous scar tissue
(Example: necrosis of cardiac muscle.. in healed myocardial infarction where the scar tissue in the heart does not contract)
Disruption of complex tissue relationships within an organ i,e, distortion of architecture interfering with normal function
(Example: as seen in liver cirrhosis)
Overproduction of fibrous scar tissue
(Example: keloid scar- an overgrowth of fibrous tissue, due to an overproduction of collagen, that exceeds the borders of the scar. They don’t regress and excision just creates another one. More common in Afro-caribbeans.
Excessive scar contraction causing obstruction of tubes, disfiguring scars followings burns or joint contractures (fixed flexures). If very severe it can even impair blood circulation.
Describe healing in cardiac muscle-
- Has very limited, if any, regenerative capacity
- Myocardial infarction is followed by scar formation
- Can compromise cardiac function
Describe healing in the liver-
- Has remarkable capacity to regenerate
- If part of the liver is removed compensatory growth of liver tissue occurs and there is restoration of liver mass by enlargement of the lobes that remain
- Almost all hepatocytes replicate during regeneration
- This is followed by replication of non-parenchymal cells
Describe healing of peripheral nerves-
- When a nerve is severed the axons degenerate
- The proximal stumps of the degenerated axons sprout and elongate
- Use Schwann cells vacated by the distal degenerated axons to guide them back to the tissue that the nerve innervates
Describe healing of cartilage-
It does not heal well as it lacks blood supply, lymphatic drainage or innervation
Describe healing of the central nervous system-
- Neural tissue is a permanent (non-proliferative tissue)
- When tissue damage occurs in the CNS the neural tissue is replaced by proliferation of CNS supportive elements (glial cells)
When there is injury and inflammation leading to necrosis of labile or stable tissues, what determines if regeneration occurs of fibrous repair?
If the collagen framework is intact= regeneration
If the collagen framework is destroyed= fibrous repair (scar)
If there is on going chronic inflammation = fibrous repair (scar)
Why are scars pale, stretched and hairless?
Pale: melanocytes do not regenerate
Stretched: elastic fibres do not regenerate in an area of damaged skin and therefore scars can stretch as they mature
Hairless: hair follicles do not regenerate in an area of damaged skin as they are complex structure
What is granulation tissue, what does it consist of and what is its function?
-Has a granular appearance
Consists of:
- Developing capillaries (red colour, forms from delicate little buds from endothelial cell proliferation)
- Fibroblasts and myofibroblasts
- Chronic inflammatory cells
Functions:
- Fills the gap
- Capillaries supple oxygen, nutrients and cells
- Contracts and closes the hole
What are the cells involved in fibrous repair and what do they do?
Inflammatory cells
- Phagocytosis of debris (neutrophils and macrophages)
- Produce chemical mediators (lymphocytes and macrophages)
Endothelial cells
-Proliferation results in angiogenesis (development of new capillaries)
Fibroblasts
-Produce extracellular matrix e.g. Collagen
Myofibroblasts
-Responsible for wound contraction pulling the edges of the wound to generate, this is due to the fibrils within myofibroblasts
