Download FOETAL WOUND HEALING

FOETAL WOUND HEALING
DIRK LAZARUS
11 NOVEMBER 1996
SUMMARY
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resembles growth and regeneration rather than healing by scar formation
different in different tissues
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skin / bone - (relatively) scarless
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muscle - scars
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depends on stage of gestation
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late in gestation the healing resembles that of adults
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amniotic fluid is rich in ECM proteins and growth factors
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foetal environment is hypoxic relative to the adult’s
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sterile environment
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increased number of growth factor receptors with an increased affinity
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increased levels of hyaluronic acid, fibronectin and tenascin
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collagen deposition is predominantly types III and V
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foetal skin’s outermost layer is periderm, which may have special
absorptive functions
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minimal inflammatory reaction
INTRODUCTION
 Human Foetal Surgery: Prenatal U/S has led to earlier Dx of various conditions.
With earlier Dx comes the desire to institute earlier Rx. Life-threatening conditions
of the foetus can be treated by maternal laparotomy, hysterotomy and open foetal
surgery.
 Scarring: A macroscopic disturbance of the normal structure and function of the skin
architecture resulting from the end product of a healed wound. It may lead to
cosmetic, functional or growth problems. It may manifest as an alteration in skin
texture, colour, vascularity, nerve supply, refelctance and biomechanical properties.
This results from a histological alteration of the epidermis, dermis or subcutaneous
tissue.
 Scarless cutaneous healing has been demonstrated in the foetuses of many species.
 The observation was made that foetal cutaneous wound healing is scarless, or
nearly so, especially if surgery was done in the mid-gestation period when wounds
seem to heal by mesenchymal proliferation.
 It is not just foetal skin that exhibits remarkable regenerative qualities. Fractures
heal without a trace and large bony defects that would not close in the adult are
able to heal in the foetus.
 Adhesions have been noted, however, in foetal diaphragmatic hernia repair
suggesting that foetal mesothelial wounds heal differently from foetal skin wounds.
Even when diaphragmatic wounds were allowed to heal in contact with the amniotic
fluid, the same results were seen, ie, the muscle healed by fibrosis. Not all foetal
tissues have the ability to heal without scars.
 Late in gestation, cutaneous foetal wounds tend to heal more in the adults fashion.
There is thus a spectrum of wound healing as a function of gestational age: skin
wound healing became adult-like with scar formation late in gestational age.
 Animal experimental work supports these clinical findings - that foetal wound healing
is different from adult wound healing. The question is why?
1. Differences in foetal cells?
2.
 Growth:
Differences in foetal wound healing environment?
The foetus is primed for growth. There is a rich milieu of GF’s and
developmental gene expression. When wounded, the foetus does not respond to
wounding like an adult would, but rather foetal wound repair is a process more
closely resembling regeneration and growth rather than healing by scar formation.
Growth also confounds the issue.
The younger the foetus is at the time of
wounding, the less the scar. Is this because of scarless healing or growth with
dilution of the scarred ECM by new matrix that has grown?
THE FOETAL WOUND HEALING ENVIRONMENT
1.
The foetal environment
a)
Foetal skin wounds are continuously bathed in warm sterile amniotic fluid that is
rich in GF’s and ECM constituents (hyaluronic acid, fibronectin, etc).
 To investigate the effect of the foetal environment on healing, full thickness adult
sheep skin was transplanted on to the backs of foetal lambs in utero. Previous
work had demonstrated that the adult skin would not be rejected. The adult skin
was thus bathed in amniotic fluid and perfused by the foetal lamb’s blood. 40 days
later, once the skin had taken, two incisions were made: one in the adult skin graft
and the other in the adjacent foetal lamb skin. At 2 weeks the foetal wounds had
healed without scarring, but the adult skin wounds had healed with scarring. Thus
neither the amniotic fluid nor the foetal blood prevented scarring in the adult skin.
This suggests that the ability of foetal skin to heal without scarring is not a factor of
the foetal environment.
b)
Hypoxia
 The foetus is in a relatively hypoxic environment.
The foetus depends on
transplacental O2 from the maternal circulation and thus the arterial PO2 in the
foetus is extremely low (20 mmHg). This is only partially compensated for by the
presence of foetal Hb which has a much higher affinity for O2.
 Adult wound healing is enhanced by a high pO2 and oxygentation. It thus seems
paradoxical that foetal wounds heal so well and this does not seem to be a factor in
healing without scars.
 The difference may lie in the foetal fibroblast which may prefer hypoxia.
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2.
The Foetal Skin
 Phases of development of the skin
i. stratified (9-14 weeks gestation)
ii. follicular keratinization (14-24 weeks gestation)
iii. interfollicular keratinization (after 24 weeks)
 Between 4 and 24 weeks, human skin has an outermost layer = periderm, which
appears as multiple microvilli projecting from the surface. Periderm may have an
absorptive function as an amniotic fluid dialysate membrane.
3.
The Foetal Immune System
 Foetal immune system is different from the adult’s.
i. The foetal external environment is sterile.
ii. The foetus has not been exposed to Ags.
iii. The foetus is slightly neutropenic and the neutrophils are immature with poor
chemotactic ability. In the foetus, wound healing occurs without a neutrophil infiltrate.
iv. Self-nonself immune identity has not developed.
v. Foetuses have a minimal inflammatory response associated with wound healing.

 Neutrophils: When injected with irritants, the primary response in foetuses is
mononuclear cells with minimal or absent neutrophil response, fibrin or oedema. Although it was thought that the absence of neutrophils allowed healing to be scarless,
subsequent experiments in rabbit foetuses showed the presence of neutophils, yet
healing remained scarless. Neutropenic adults heal with scars.
 Macrophages are the crucial inflammatory cell orchestrating adult wound healing.
They are essential regulatory cells that co-ordinate initial debridement and secrete
mediators
of
inflammation,
fibrogenensis,
angiogenesis
and
cell
growth.
Macrophages have been seen to be present at the foetal wound site and have been
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noted to secrete GFs. Whether their role is different in the foetal wound remains to
be seen.
 Lymphocytes are present, but in small numbers.
 GFs play a role in the healing of foetal wounds. The addition of TGFand PDGF
can create an adult-like fibro-proliferative response in the foetus. This response can
be blocked by TGF Abs and further implicates TGF in scar formation. In addition,
blocking these GFs in adults, reduces scarring.
 Relative to adults, foetal fibroblasts have an ed number of receptors for TGF, as
well as an ed binding affinity.
 TGF has been found to be absent in foetal mouse lip wounds that heal without
scarring. In the neonate and adult, by contrast, lip wounds healed with scarring and
showed abundant TGF.
Foetal wound fluid, however, is abundant in TGF.
Perhaps a difference in isoforms is important.
 EGF also seems to have a greater effect in the foetus than in the adult, stimulating
more rapid epithelialisation.
 GFs are as important in foetal wound healing as in adult wound healing.
 The foetus has high corticosteroid levels from an enlarged adrenal.
This may
attenuate the inflammatory response.
 C’: Although many of the components have been identified in the foetus, the levels
are less than in the adult.
 Introduction of live bacteria into foetal wounds results in an intense inflammatory response with resultant ed collagen deposition and fibrosis, and angiogenesis. The
foetus is thus capable of an intense inflammatory response if provoked.
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4. The Foetal Wound Matrix
 Rapid healing of foetal lamb incisional wounds within 2 weeks, suggests that foetal
wound matrix turnover and repair are rapid and efficient. Foetal wound matrix is
rich in GAGS and HA, more so than adult wounds.
a)
Hyaluronic Acid (HA) (now called Hyaloronan)
 The key structural and functional component of the ECM.
 It is found whenever there is rapid tissue proliferation, regeneration and repair.
 HA fosters an environment that promotes cell proliferation and mobility.
 Although HA is laid done early in both adult and foetal wounds, this process is
sustained in foetuses (and only transient in adults). This may signal healing by
regeneration rather than repair.
 Two reasons for sustained level of HA in the foetus are:
1) HASA
degradation
 HASA: Foetal serum, wound fluid and amniotic fluid has a HA stimulating activity
(HASA). This glycoprotein has levels that peak in mid gestation for foetal serum
and wound fluid and at term for amniotic fluid (d/t foetal urine being rich in HASA).
In adults, HA is only briefly deposited in the early stages of wound healing.
Following deposition of HA in the adult wound, hyaluronidase is produced, HA is
removed, and the wound matrix is replaced by collagen. In the adult wound, the
local deposition of HA by the platelet plug and the fibrin clot obviates the need for
an exogenous mechanism for stimulating local HA production.
 HA degradation: In the foetus, sustained levels of HA are also maintained by ed
degradation. If hyaluronidase is added to foetal wounds, there is ed inflammation,
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ed collagen deposition and fibrosis, and ed angiogenesis: scarring occurs as in
the adult. If HA is digested by hyaluronidase and the fragments added to foetal
wounds, the same effect is seen, ie, the breakdown products have biological activity.
Degradation requires internalisation of the HA by the fibroblast. Adult fibroblasts
have greater hyaluronidase activity than does foetal fibroblasts.
 Hyaluronan was first isolated by Meyer and Palmer in 1934. It is a ubiquitous
macromolecule found in connective tissue. Normally very little is present in the
blood. The liver has the ability to remove large quantities from the blood, indicating
that high blood levels may be dangerous or undesirable. The source of HA that is
seen in early in the process of wound healing is unknown and the mechanism of
biosnthesis is not completely understood. It seems to arise from fibroblasts. The
chemical structure is known (repeating disaccharide units of hexosamine and uronic
acid). Variations in its structure are possible. It is one of the GAGS, but unlike the
others, does not seem to contain a protein residue. It is important in growth and
development as well as wound healing. Its facilitates movement of cells in the
ECM.
 HA levels are ed early in wound healing and it plays an important role:
a) facilitates cell detachment, proliferation and movement
b) inhibits cell differentiation
c) inhibits angiogenesis (although its degradation products induce it)
d) seems to bind to fibrin creating a matrix that attracts inflammatory cells.
These cells then secrete hyaluronidase and plasminogen activator to degrade
the HA-fibrin matrix. The degradation products of the matrix are important in
mediating further inflammation and repair.
 Addition of HA to adult wounds, however, has not produced healing without scars.
 HA associated proteins can inhibit fibroblast proliferation.
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b)
Fibronectin is more abundant in foetal skin than in adult skin. In addition, it is
deposited earlier in foetal than in adult wounds which provides a favourable
environment for cell proliferation and migration.
Amniotic fluid, which contains
fibronectin, coats foetal skin wounds continuously and may be an important source
of fibronectin deposition.
a)
Tenascin, an ECM glycoprotein implicated in epithelial-mesenchymal interactions
during foetal development, antagonises the adhesive properties of fibronectin and
permits cells to detach from the wound matrix and to migrate. It also appears
rapidly in foetal wounds and together with fibronectin, may facilitate the rapid
epithelialisation of foetal wounds.
5.
Collagen Formation and Deposition in the Foetal Wound
 Collagen deposition occurs in the foetus.
In fact, foetal fibroblasts are more
proficient at collagen synthesis than their adult counterparts and collagen appears
sooner. They produce more type III and V collagen than adult fibroblasts (which
produce type I). The collagen that is laid down, is done so in an organised way,
unlike in the adult.
Scarless healing is therefore not d/t a lack of collagen
deposition in the wound.
 The foetus therefore lays down abundant, HA rich, ECM and lots of collagen.
 How come healing is then scarless? It seems as if the abundant HA-rich matrix
provides a more ‘permissive’ environment for organisation of the collagen fibres.
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Neither healing by secondary intention nor wound contraction are observed in foetal
wounds. In fact, foetal wounds are seen to gape open and to actually  in size.
Myofibroblasts have not been seen in foetal wounds. Amniotic fluid seems to contain a
substance that inhibits wound contraction both in the foetal and the adult wound. Other
studies have shown the opposite: myofibroblasts were detected and wound contraction
observed. Human amniotic fluid has been noted to inhibit wound contraction, a feature
which may show benefit in treating pathological contraction.
CONCLUSIONS
Foetal wound healing is a complex phenomenon, at this stage still poorly understood.
Healing in the foetus is scarless as a result of alterations of all phases and
components of the wound healing process: inflammation and repair. Both the foetal
cells and the foetal environment are different from the adult’s. Amniotic fluid and its
components as well as hypoxia and sterility of the foetal environment affect wound
healing.
Periderm is the outermost layer of foetal skin that may make it more
susceptible to the positive influences of this environment on wound healing. The foetal
immune system is immature and wound healing is associated with a minimal
inflammatory response. The foetus is primed for growth, and wound healing more
closely resembles regeneration and growth than wound healing by scar formation. The
ECM contains abundant HA, fibronectin and tenascin, all of which aid wound healing.
The foetal fibroblasts are more proficient at laying down collagen, and the collagen is
laid down in a bed of HA which allows reorientation of fibres more easily than in the
adult. GF’s seem to work better in the foetus: they have an ed number of receptors
with an ed affinity for the GF’s.
Open foetal surgery is still a risky procedure both for the mother and the foetus and
the risks, currently, do not outweigh the benefits except possibly in life-threatening
defects.
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Unravelling the secrets of foetal wound healing may provide the blueprint for ideal
tissue repair. With greater knowledge, we may be able to modulate scarring and
fibrosis, but much more work needs to be done before we find the keys.
REFERENCES
1. Longaker PRS 87(4), 788, April 1991.
2.
Longaker (Mastery of Surgery Series)