Wound Healing Flashcards
What are the phases of wound healing?
- haemostasis
- inflammation
- repair
- remodelling
There are defined boundaries between each phase of healing. True or false
False
The wound healing process should be regarded as a series of ________ as opposed to a sequence of events
series of transitions
How does the healing process differ between the placement of resorbable and non-resorbable sutures ?
In non-resorbable sutures:
* the process of hemostasis and inflammation are followed by provisional matrix formation (as occurs in normal tissue)
* however as opposed to scarring, the process ends in a fibrous encapsulation of the implant
In resorbable sutures:
* there is a heightened inflammatory phase (required to break down/tackle the suture) as the degradation products are cleared
* there is no fibrous encapsulation phase as the bulk of the suture degrades
What is the duration of hemostasis?
seconds to hours
What are the earliest signals of tissue injury?
release of molecules such as ATP and the exposure of collagen on the blood vessel wall
What role does platelet activation play in the inflammatory phase?
platelet activation causes the release of a number of signalling molecules such as PDGF, TFG-beta and VEGF from their cytoplasmic granules.
Inflammatory and reparative cells are chemotactically attracted to this resevoir of molecules stored within the clot and thus gives rise to inflammation
TGF - transforming growth factor
VEGF- vascular endothelial GF
What is the aim of inflammation in the healing process?
acts to contain, neutralise or dilute the injury causing agent or lesion
How is inflammation initiated?
by the release of signalling molecules from the wound site during hemostasis
Describe the tissue environment in which inflammation begins
- mixture of injured tissue
- components of the clot (platelets, RBCs and fibrin)
- extravasated serum proteins
- foreign materials introduced at the time of injury e.g. suture
How do components of the complement system contribute to the inflammatory process?
complement fragments such as C5a and less potently C3A and C4a are important inflammatory activators which can induce vascular permeability, recruitment and activation of phagocytes
How can members of the complement system be activated?
They are activated by foreign surfaces such as an implant (suture?) or a microbial cell wall
What type of inflammatory cells are first recruited to the site of injury?
Neutrophils
They are recruited within the first 24 hours
How are neutrophils attracted to the site of injury?
- chemoattractants released by platelets
- chemokines present on endothelial cell membranes
Where does the leukocyte receptor PSGL-1 bind ?
- binds to P-selectin expressed on both platelets and endothelial cells
What is the consequence of low affinity binding of PSGL-1 receptors to P-selectin?
facilitates rolling of flowing neutrophils and brief tethering of neutrophils to endothelial cells
What is the consequence of neutrophil rolling ? Briefly outline the sequence of events
Neutrophil rolling eventually leads to firm attachment to endothelial cells.
- chemoattractants suchs as IL-8 and macrophage inflammatory protein (MIP-1Beta) are released by endothelial cells.
- Neutrophils bind to these chemoattractants leading to Beta-2 integrins are activated as a result.
- beta-2 integrins bind to firmly endothelial ICAM-1
Following firm attachment of neutrophils on endothelial cells, briefly outline what occurs ?
- neutrophil extravasate through the vessel walls to the site of injury
- release of proteolytic enzymes for the digestion of foreign debris and killing bacteria
How are neutrophils able to kill bacteria?
- phagocytosis
- then superoxide and hydrogen peroxide production
What occurs following completion of wound decontamination?
- neutrophils undergo apoptosis within 24-48 hours
- replaced by extravasating monocytes and macrophages
What attracts monocytes and macrophages to the site of injury ?
futher release of MIP-alpha and beta, monocyte chemotractant protein-1 and chemotactic cytokine called RANTES by activated endothelial cells
Inflammation is continued 2-3 days following injury by…
monocytes which are recruited from the blood which differentiate into macrophages
State some pro-inflammatory cytokines released by macrophages once recruited to the site of injury
- IL-1
- TNF- alpha
What is the function of recruited macrophages at the site of injury?
removal of foreign debris
The duration of the presence of macrophages at the injury site is dependent on …
- the extent of injury
- amount of foreign and necrotic debris to be cleared
they can remain present for as long as a few months
Macrophages are believed to be more important than neutrophils for successful inflammation resolution. True or false. Briefly elucidate why this is thought to be the case
True
In studies where neutrophils are depleted, wound repair was not disturbed. However, when macrophages were removed, there was limited clearance of necrotic debris at the injury site
What is the contribution of mast cells in the inflammatory response?
release histamine and serotonin to enhance blood vessel permeability and macrophage migration
How is the inflammatory phase resolved?
By a mixture of anti-inflammatory cytokines
____ released by ____ is thought to be one of the major attentuators of inflammation.
TGF-beta released by macrophages
Briefly elucidate the “mechanism” by which TGF-beta is able to attenutate the inflammatory process
- it is involved in the promotion and recruitment of fibroblasts to push the wound healing response towards the repair phase
Give another example of a pro-wound healing factor produced by macrophages. How does it encourage the repair phase?
VEGF
It initiates angiogenesis within the hypoxic tissue environment
What is the duration of the repair phase?
days to weeks
Why is the overall tissue strength of a wound minimal during inflammation?
the normal functional strentgh of tissues is not regained until inflammation transitions to repair
The transformation from inflammation to repair is mediated by…
macrophages
The repair phase usually occurs ____ following the initial injury
1 week
Why is the repair phase essential to wound healing?
This is because it establishes the scaffold necessary to support and rebuild the damaged tissue
Tissue repair is characterised by…
- cell proliferation
- capillary budding
- synthesis of ECM
What is the purpose of ECM synthesis in wound healing?
it is used to fill in the damaged tissue that has been cleared during inflammation
Initially, the ECM material is usually made up of …
fibrinogen and fibronectin
After early matrix material has been synthesised, ground substance is made. What are the components of ground substance of the ECM ?
- proteoglycans
Proteoglycans are synthesised by…
fibroblasts
What are proteoglycans?
they are large macromolecules with a core protein and one or more covalently attached glycosaminoglycan molecules
Complete repair of injured tissue can only occur in what types of wounds? Why is this?
- it can only occur for shallow skin wounds
- this is because the surface epidermis layer can regenerate
- however, deep tissue wounds in which the dermis is lost will undergo secondary healing which requires excess ECM production. This leads to the formation of fibous scar tissue
What is the first stage of tissue repair?
stabilisation of the discontinuity created by the injury
What kind of tissue injury will undergo primary healing?
tissue that has little or no gap seperating the wound boundaries.
primary healing will occur from the apposed edges of the tissue
What kind of tissue injury will undergo secondary healing ?
- tissue that is unstable with a large gap
- discontinuity injury
What is secondary healing?
this is where excess ECM is produced to secure and fill the lesion
The ECM of secondary healing is referred to as …
granulation tissue
this term has arisen as a result of its appearance
Granulation tissue is vascularised. True or false
true
The amount of granulation tissue formed is proportional to the ____________
eventual level of scarring
What type of repair is absent of ECM production ?
primary repair /primary healing
What is a partial thickness burn?
this is one that involves the epidermis and some of the dermis
What is left intact following a partial thickness burn?
- basement membrane (?? dermis layer is affected in a partial thickness burn)
- hair follicles
- sweat and sebaceous glands
refer to image downloaded on layers of the skin
Why is there no need for ECM production in a partial thickness burn?
- only the epidermal surface needs to be replaced
- epithelial progenitor cells also remain intact below the wound
- synthesis and deposition of collagen is not necessary
Why is there no need for ECM production for a surface lesion such as a paper cut?
this is because the wound can be reepithelialised by the migration of epidermal cells from below the wound and the wound edges
What is the degree of re-epithelialisation of a surface wound dependent on?
- amount of tissue lost
- depth of the wound
- width of the wound
When does re-epithelialzation take place?
- begins within 24-48 hours
Re-epithelialization takes places by a process called…
Lamellopoildial crawling
Briefly describe the process of lamellopoidial crawling
- this is where uninjured epidermal cells bore through or underneath the fibrin clot, crawling into and across the wound
How are epidermal cells able to undergo migration in the lamellopoidial crawling process?
- epidermal cells release their tethers from the basal lamina
- they crawl by attaching to fibronectin and vitronectin contained within the clot
- they also secrete proteases to dissolve the dense fibrin matrix
What is the result of the start of migration of epidermal cells?
they move one by one over the wound site until the wound is covered completely by a layer of cells
What does a proliferative burst with regard to wound healing refer to?
This is where the (newly migrated?) epidermal cells behind the wounds edge proliferate in order to replace the epidermal cells lost during injury
Additional epidermal layers over the basal layers are also formed as a result
When does a proliferative burst occur during wound healing
After migration is complete in the repair phase
What growth factors are thought to drive cell proliferation and wound closure?
- EGF (epidermal)
- TGF-alpha (transforming)
- KGF (keratinocyte)
When does migration and cell proliferation stop in the repair phase?
when epidermal cells receive a stop signal likely contact inhibition
What happens following contact inhibition?
a new basement membrane is made
cell- matrix adhesions are established
Re-epithelialization is only sufficient for healing what kinds of wounds?
surface/superficial/partial thickness wounds
When do ECM producing dermal fibroblasts migrate into the wound bed?
within 3- 4 days
(this is after they have been activated and are proliferating)
Migrating fibroblasts initially synthesize…
fibronectin
Following the release of ____ by macrophages, fibroblasts begin to lay down the ____________.
TGF- beta
collagen matrix which provides structural support for the wound
Re-epithelialisation can occur following formation of the collagen matrix. Briefly describe how this happens
epidermal cells utilize the newly produced ECM to migrate over and epithelialise the site of injury
they are able to now crawl on the newly synthesised fibronectin; they can release protease to dissolve the matrix as well
the newly synthesised matrix essentially provide a pathway for epidermal cells to reach the injury site
Aside from facilitating the migration of epidermal cells. What other functions does the ECM have?
- act as a conduit (channel) where new capillaries are formed as endothelial cells migrate into the wound site after responding to fibroblast GF-2 AND VEGF
Why does the replacement granulation tissue appear pink?
this is because angiogenesis delivers nutrients to the migrating fibroblasts at the site of injury
What is the potential consequence of angiogenic failure?
chronic wounds such as venous ulcers which are unable to heal
fibroblasts are not supplied with nutrients and such they die; they can no longer form the matrix and fill the space left behind by the injury
monocyte/macrophage extravasation can be impaired?
What is the duration of the remodelling phase?
weeks to months
What is involved in secondary healing remodelling ?
- fibre alignment
- contraction to reduce wound size and re-establish tissue strength
Why is complete recovery of the original tissue strength rarely obtained in secondary healing?
- this is because repaired tissue remains less organised than non-injured tissue
- this results in scar formation
Tissue that has undergone primary healing is usually virtually indistinguishable from non-injured tissue; no scar is formed. True or false
True
What is the major aim of secondary wound remodelling?
- reduce the amount of excess ECM
- align the ECM through contraction
When is little or no scarring observed following secondary repair of an injury?
- if the extent of the wound is relatively small
- reorganisation of the ECM is sufficient
- relatively little contracture occurs
What types of injuies result in scarring and why?
- larger injuries
- Larger injuries where more extensive repair is required by ECM production
- there will be more significant remodelling required in these cases
- this results in scarring
How are fibroblasts able to facilitate wound closure (outside of production of the matrix) ?
- tractional forces created as fibroblasts move throgh the wounded tissue generate mechanical tension promoting wound closure
When does the remodelling phase of wound healing officially begin ?
when fibroblasts differentiate into myofibroblasts (the more contractile phenotype)
What factors stimulate the differentiation of fibroblasts into myofibroblasts?
TGF-beta and other cytokines released by platelets and macrophages
Myofibroblasts are characterised by…
- their expression of alpha smooth muscle actin
- production of collagen I
What happens once myofibroblasts are activated?
- increase their cytoskeletal stress fibres and focal adhesions
- this provides constant tension to contract the wound bed
Contractures of ________ micrometers per day are possible without the need to generate large amounts of force. Why is this?
10- 20 micrometres
this is because of the low basal tension level of the skin
What is the collagen pattern observed in uninjured skin?
basket weave pattern
What is the appearance of collagen fibres in injured tissue?
Striated scar tissue
they are orientated parallel to the wound bed along lines of stress
Image on IPAD to illustrate this
Once wound contraction occurs, ____________ causes myofibroblasts to return to a quiescent state
stress relaxation
Myofibroblasts in a quiescent state receive signals for apotosis. Briefly describe the transformation of the wound that occurs following this.
cell rich granulation tissue becomes cell-poor scar tissue with an excess of ECM
the capillary dentisy diminishes, the wound will lose its pink colour and become paler
What is the ultimate end point of the remodelling phase?
the formation of acellular scar tissue that is poorly organised into dense parallel bundles as opposed to the tightly wove meshwork of normal dermal tissue
Dermal tissues that are lost during injury are regenerated following the remodelling process. True or false
Give examples of such dermal tissues
False; they are not regenerated
- hair follicles
- sweat glands
- sebaceous glands
After 3 months, regenerated tissue can have a maximum of ____% of the strength of the unwounded tissue.
80%
this is generally the highest level of strength that healed tissue can achieve
What is the aim of placing sutures?
- closing dead space within the wound
- supporting wounds until their strength is increased through healing
- approximating skin edges for an aesthetically pleasing result
- minimising the risk of bleeding and infection
Why are multifilament sutures more likely to become infected than their monofilament counterparts?
- increased surface area
- tendency to harbour liquids and pathogens
What ways are absorbable sutures usuallly degraded?
- hydrolysis
- enzymes
What kind of foreign body reaction is associated with smooth surface sutures or implants ?
a layer of macrophages one or two cells thick
What kind of foreign body reaction is associated with rough surfaces?
macrophages and multimucleated foreign body giant cells (FBGCs) at the surface can persist at the tissue implant interface for the lifetime of the implant
What characteristic reactions have studies associated with implant with porous coatings?
- significantly thinner capsular tissue than smooth implants
- fibrous capsules with decreased density
Remember non resorbable suture or implants are encapsulated by a stable and concentric fibrous capsule
Rememver non resorbable suture or implants are encapsulated by fibrous tissue
## Footnote
Within minutes of suture implantation, the implant surface is covered in a layer of …
proteins (plasma proteins)
What types of proteins adhere to the implant (suture) first ?
the most concentrated protein in the tissue plasma
initial adherence is done in a non-specific manner
What is the Vroman effect?
this is where proteins that can adhere to the implant the best replace those that initially adhered to the surface.
(proteins that can adhere the best i.e. proteins that have better antigens that offer a stronger bond?)
What is the purpose of protein adhesion to the surface of the implant ?
to initiate the inflammatory response
- they can promote or facilitate leukocyte attachment
What is the effect of albumin on the inflammatory response to implants?
albumin “passivates” the surface of the biomaterial and reduces monocyte adhesion
passivates- to render unreactive
What is the inflammatory reaction around a suture dependent on?
the amount of injury that inflicted by the suturing technique
Macrophages fuse to become ____________ in an attempt to degrade suture material
Foreign body giant cells (FBGCs)
When macrophages reach the suture site, what do they do?
they remove neutrophil apoptotic bodies and other cellular debris. This occurs after 1-2 days of suture being placed
remember neutrophils undergo apoptosis 24-48h if wound decontamination is completed
Macrophages are activated further in an attempt to interrogate the suture further. What facilitates further activation of macrophages?
binding of macrophage complement receptors to complement proteins or IgG or IgM antibodies
What ways are macrophages able to degrade suture material ?
- phagocytosis
- if phagocytosis is not possible then proteilytic enzymes can be released
- macrophages can also fuse to become FBGCs in an attempt to engulf the material- FBGCs have improved degradation ability over macrophages
If FBGCs fail to engulf the sutures, what do they do?
they remain along the implant surface for the lifetime of the non-degradable suture
What induces FBGC formation?
Release of IL-4 and IL-3 by helper T lymphocytes
The loss of absorbable suture breaking strength is proportional to absorption. True or false
False
Poliglecaprone is a ______ suture
Monocryl
Give examples of sutures with negligible tensile strength
Polygalactin (vicryl)
Poliglecaprone (monocryl)
Give an example of a suture that is more slowly absorbed
Polydioxanone
Following complete absorption of the suture, how is the site characterised ?
there are small regions of macrophages containing brown, foamy cytoplasm surrounded by fibroblasts
the remainder of the tissue surrounding the degradable suture regenerates with keratinocytes in the epidermis migrating inward, replacing the fibrous tissue that once surrounded the suture.
Non resorbable sutures generally have a stable chronic reaction after ____ days and contain lower levels of inflammatory cell infiltrate
30 days
Describe the changes observed in the concentric fibrous capsule surrounding non-degradable sutures over time
they become less cellular over time
the thickness of the capsule may also decrease with time
