Wound Healing—Aiming for
Perfect Skin Regeneration
Paul Martin
The healing of an adult skin wound is a complex process requiring the collaborative efforts of many different tissues and cell lineages.The behavior of each of the contributing cell types during the phas of proliferation,migration,matrix synthesis,and contraction, as well as the growth factor and matrix signals prent at a wound site,are now roughly understood.Details of how the signals control wound cell activities are beginning to emerge,and studies of healing in embryos have begun to show how the normal adult repair process might be readjusted to make it less like patching up and more like regeneration.
A dult skin consists of two tissue layers:a keratinized stratified epidermis and an un-derlying thick layer of collagen-rich dermal connective tissue providing support and nourishment.Appendages such as hairs and glands are derived from,and linked to,the epidermis but project deep into the dermal layer.Becau the skin rves as a protective barrier against the outside world,any break in it must be rapidly and efficiently mend-ed.A temporary repair is achieved in the form of a clot that plugs the
defect,and over subquent days steps to regenerate the missing parts are initiated.Inflammatory cells and then fibroblasts and capillaries invade the clot to form a contractile gran-ulation tissue that draws the wound margins together;meanwhile,the cut epidermal edges migrate forward to cover the denuded wound surface(1)(Fig.1).Fundamental to our understanding of wound-healing biolo-gy is a knowledge of the signals that trigger relatively dentary cell lineages at the wound margin to proliferate,to become invasive,and then to lay down new matrix in the wound gap.Studies in the last decade have provided a list of the growth factors and matrix components that are available to provide the“start”signals,and one of the tasks now begun is to relate the fac-tors specifically to the starting and stopping of each of the many cell activities by which the wound is healed.
Most skin lesions are healed rapidly and efficiently within a week or two.However, the end product is neither aesthetically nor functionally perfect.Epidermal appendages that have been lost at the site of damage do not regenerate,and when the wound has healed there remains a connective tissue scar where the collagen matrix has been poorly reconstituted,in den parallel bun-
dles,unlike the mechanically efficient bas-
ket-weave meshwork of collagen in un-
wounded dermis.A major goal of wound-
healing biology is to figure out how skin can
be induced to reconstruct the damaged
parts more perfectly.Clues as to how this
might be achieved come from studies of
wound healing in embryos,where repair is
fast and efficient and results in esntially
perfect regeneration of any lost tissue.
The Fibrin Clot健儿药丸说明书
Most wounds to the skin will cau leakage
of blood from damaged blood vesls.The
formation of a clot then rves as a tempo-
rary shield protecting the denuded wound
tissues and provides a provisional matrix
over and through which cells can migrate
during the repair process.The clot consists
of platelets embedded in a mesh of cross-
linked fibrin fibers derived by thrombin
cleavage of fibrinogen,together with small-
er amounts of plasma fibronectin,vitronec-
tin,and thrombospondin(2).Importantly,
the clot also rves as a rervoir of cyto-
kines and growth factors that are relead as
activated platelets degranulate.This early
cocktail of growth factors(Table1)“kick
starts”the wound closure process:It pro-
vides chemotactic cues to recruit circulat-
ing inflammatory cells to the wound site,
initiates the tissue movements of reepithe-
lialization and connective tissue contrac-
tion,and stimulates the characteristic
wound angiogenic respon.
Recruitment of Inflammatory
Cells to the Wound Site
Neutrophils and monocytes are attracted to
wound sites by a huge variety of chemotac-
tic signals.The include not only growth
factors relead by degranulating platelets,
but also cues as diver as formyl methionyl
peptides cleaved from bacterial proteins and
the by-products of proteolysis of fibrin and
other matrix components(3).Both neutro-
phils and monocytes are recruited from the
circulating blood in respon to molecular
changes in the surface of endothelial cells
lining capillaries at the wound site.Initial-
ly,members of the lectin family of adhe-
sion molecules are expresd to allow rapid
but light adhesion so that leukocytes are
slowed and pulled from rapid circulation
in the blood;then tighter adhesions and
arrest,mediated by the2class of inte-
grins,lead to diapedesis,whereby the ac-
tivated leukocytes crawl out between en-
dothelial cells into the extravascular space
(4).Transgenic mou studies are begin-
ning to pinpoint the crucial adhesion in-
teractions in this process;for example,in
the p-lectin knockout mou,leukocyte
rolling and extravasation are verely im-
paired(5).Neutrophils normally begin ar-
riving at the wound site within minutes of
injury;their role has long been considered
to be confined to clearing the initial rush
of contaminating bacteria,but recent
studies have shown that neutrophils are
also a source of pro-inflammatory cyto-
kines that probably rve as some of the
earliest signals to activate local fibroblasts
and keratinocytes(6).Unless a wound is
grossly infected,the neutrophil infiltra-
tion ceas after a few days,and expended
neutrophils are themlves phagocytod
by tissue macrophages.Macrophages con-
tinue to accumulate at the wound site by
recruitment of blood-borne monocytes
and are esntial for effective wound heal-
ing;if macrophage infiltration is prevent-
ed,then healing is verely impaired(7).
Macrophage tasks include phagocytosis of抬腿卷腹
any remaining pathogenic organisms and
other cell and matrix debris.Once activat-
ed,macrophages also relea a battery of
growth factors and cytokines at the wound
site(Table1),thus amplifying the earlier
wound signals relead by degranulating
platelets and neutrophils.
Reepithelialization
In unwounded skin,the basal keratinocyte
layer attaches to a carpet of specialized ma-
trix,the basal lamina.The keratinocyte’s
primary anchoring contacts are hemidesmo-
somes,which bind to laminin in the basal
lamina by way of␣64integrins and have
intracellular links with the keratin cytoskel-
etal network.The hemidesmosome attach-
ments have to be dissolved and leading edge
keratinocytes have to express new integrins,
primarily the␣51and␣v6fibronectin/
tenascin receptors and the␣v5vitronectin
receptor,and relocalize␣21collagen recep-
tors,in order to grasp hold of,and crawl
The author is in the Department of Anatomy and Devel-opmental Biology and Division of Plastic and Reconstruc-tive Surgery,Department of Surgery,University College London,Gower Street,London WC1E6BT,UK.E-mail: paul.martin@ucl.ac.uk
follicle remnant
Reconstituting
epidermal
cells Granulation
to illustrate the key players in the healing of a
temporarily plugged with a fibrin
dergo the standard differentiation program of cells in the outer layers of unwounded epider-mis.We know little about keratinocyte“stop”signals except that they probably include con-tact inhibition arising from mechanical cues. Coincident with the ont of basal lamina synthesis,MMP expression is shut off,and new hemidesmosomal adhesions to the basal lamina reasmble.Biopsies from healed skin established by grafting cultured keratinocytes onto naked wound beds suggest that the last components of the epidermal attachment ma-chinery to reach maturity are the anchoring fibrils that link basal lamina to underlying connective tissue(14).
Growth Factors Regulating
Reepithelialization
For many years the EGF family of growth factors,comprising epidermal growth factor (EGF)itlf,transforming growth factor–␣(TGF-␣),and more recently heparin binding epidermal growth factor(HB-EGF),all acting as ligands for the EGF receptor,were consid-ered the key regulators of keratinocyte prolif-eration at a wound edge.Indeed all three of the factors are relead in abundance at a site of injury(Table1).Moreover,exogenous application of EGF or TGF-␣to burn wounds on the backs of pigs enhances reepithelializa-tion(24).Study of keratinocyte responsive-ness
to EGFs in culture suggests that the growth factors act on the epidermis as moto-gens as well as mitogens to drive wound clo-sure(25).Until recently it was not clear how extracellular signals might affect cell motility, but it is now known that some growth factors, including EGF,are able to activate the small guanosine triphosphata(GTPa)Rac, which mediates lamellipodial extension and the asmbly of focal adhesion complexes as part of the crawling respon of tissue culture fibroblasts and epithelial cells(26).
Recently the EGFs have had to share
their status as chief epidermal wound regu-
lators with keratinocyte growth factor
(KGF),or FGF7,which acts specifically on
keratinocytes through a constitutively ex-
presd splice-variant of FGFR2.KGF is up-
regulated more than100-fold within24
hours by dermal fibroblasts at the wound
margin,possibly in respon to pro-inflam-
matory cytokines(27).In glucocorticoid-
treated mice and genetically diabetic mice
with impaired healing,KGF(but not KGFR)
expression is reduced,suggesting that a de-
fect in KGF regulation might underly various
wound-healing disorders(28).Transgenic
knockout mice lacking KGF em not to
suffer impaired healing(29),but this may
reflect genetic redundancy,becau a domi-
nant-negative mutant form of the FGFR2
expresd in the basal keratinocyte layer
(making the cells unresponsive to KGF)
blocks cell proliferation at the wound margin
and delays reepithelialization(30).Exoge-
nous KGF applied to skin wounds has mito-
genic and motogenic effects on the healing
epidermis(31)and stimulates high plasmin-
ogen activator and MMP-10expression in
the motile keratinocytes,which might speed
up the rate of healing in vivo by enhancing
the capacity of the epithelial edge to cut
through the clot(32).
Other growth factors may also regulate
epidermal repair.For example,TGF-1and
some pro-inflammatory cytokines appear to
stimulate expression of some of the integrin
subunits that facilitate keratinocyte migra-
tion(33).
Reepithelialization of
Embryonic Wounds
Early embryos show a remarkable capacity to
rapidly reepithelialize wounds,but the basal
epidermal cells do not move forward by lamel-
lipodial crawling.Rather,they are drawn for-
ward by contraction of an actin cable that acts
like a pur-string to pull the wound edges
together(34)(Fig.3).Thus,embryonic epi-
dermal cells have no need to alter their inte-
grins and may begin moving promptly,with-
out a lag pha.The actin cable asmbles
within minutes of wounding.It is not yet clear
what signals regulate embryonic reepithelial-
ization,but they are mediated by another of
the small GTPas,Rho.Inactivation of Rho
prevents cable formation and results in a fail-
ure of reepithelialization(35).It will be fasci-
nating to discover whether adult wound kera-
tinocytes can be induced to move by a pur-
string mechanism rather than by crawling.In
particular,one might expect a pur string to
be an effective means of repairing small
wounds,where the high curvature of the
wound margin will allow a pur string to
generate a strong centripetal force.Hints that
this might be the ca come from studies of
gut epithelium in which closure of small
wounds is necessarily rapid and efficient and
may also u pur-string reepithelialization
(36).
A Role for Keratins
Although the actin cytoskeleton is critical
for crawling motility of adult keratinocytes
and pur-string closure of embryonic epi-
dermal wounds,it might be presumed that
the keratin cytoskeletal network would
supply esntial cell and tissue strength
during such strenuous epithelial move-
ments.Indeed,in mice with a deletion of
the gene encoding the bullous pemphigoid
antigen(BPAG1),which mediates link-
age between keratin filaments and the
hemidesmosomal␣64integrins,inci-
sional wounds are unable to reepithelialize
(37).Some keratins may play rather more
subtle roles than simply providing cell
A
Fig.2.Histology of adult skin repair.(A)Resin ction through the leading front of keratinocytes(arrow)as they cut their way through a clot(C).(B) Transmission electron micrograph of a front row cell showing classic lamel-lipodial crawling morphology(arrow).Bars:100m(A)and1m(B).[(A)and (B)courtesy of M.Turmaine]
B
strength.As with integrins(12),keratins that are normally basally restricted appear suprabasally in keratinocytes at the wound margin.New,short-filament keratins6, 16,and17are also induced and appear to help retract other cellular keratins into juxtanuclear aggregates within actively crawling cells(38).Keratins may be less important in the embryo.In mou embry-os lacking keratin8and suppodly miss-ing all keratin filaments,reepithelializa-tion of a wound appears to proceed exactly as in wild-type embryos,suggesting that the embryonic epidermis does not need intermediate filament support during the repair process(39).
Regeneration of Hair and
Sweat Glands
If an adult skin wound is deeper than the level of hair bulbs in the dermis so that no remnants of hair follicles remain,the repair-ing epithelium does not regenerate hairs; the same is also true for sweat glands lost at the site of injury.During embryogenesis, the dermal connective-tissue fibroblasts supply permissive and instructive signals that govern the positions and types of hairs and other cutaneous appendages that will differentiate from the overlying epidermis (40).The timing and nature of the signals remain unclear,but some clues come from
accounts of the expression of patterning
genes—notably lymphoid enhancer fac-
tor–1(LEF-1),sonic hedgehog(Shh),bone
morphogenetic protein–2(BMP-2)and
FGF-4—in the developing hair and feather
buds of mou and chick embryos(41),and
from reports that transgenic knockout mice
null for various FGF and EGF family mem-
bers exhibit a range of defects in hair de-
velopment(29,42).Adult wound epider-
mis fails to regenerate hairs,not becau it
is unable to respond to hair-inducing sig-
nals,but becau it does not receive such
signals from the underlying wound dermis.
Competence to make hairs has been dem-
onstrated by eding the wound site with
inductive dermal papilla cells(43).
Contraction of the Wound
The job of reepithelializing a wound is
made easier by the underlying contractile
connective tissue,which shrinks in size to
bring the wound margins toward one an-
other.As an early respon to injury,res-
ident dermal fibroblasts in the neighbor-
hood of the wound begin to proliferate,
and then3or4days after the wound insult
they begin migration into the provisional
matrix of the wound clot where they lay
down their own collagen-rich matrix(44).
The premigratory lag pha appears to be
largely due to the time required for fibro-
blasts to emerge from quiescence,becau
it does not occur a cond time if the
wound is re-wounded and a new provision-
al matrix laid down(45).Many of the
growth factors prent at a wound site can
act either as mitogens or as chemotactic
factors for wound fibroblasts,and some,
notably isoforms of platelet-derived
growth factor(PDGF)(46)and TGF-
(47),may do both(Table1).TheA and
B isoforms of the TGF-–related growth
factor activin are induced in the prolifer-
ative fibroblasts of a wound margin and in
the adjacent wound-edge keratinocytes,
respectively;it is not yet clear which cells
respond to the activin signals,but al-
most certainly there will be significant
functional overlap with the TGF-signals
(48).Connective-tissue growth factor
(CTGF),which is homologous to the
product of the Drosophila morphogenesis
gene twisted gastrulation,is expresd at
high levels by wound fibroblasts as an
immediate-early gene respon to TGF-
1.In Drosophila,twisted gastrulation may
lie genetically downstream of decapentaple-
gic,a TGF-family member,suggesting
that some of the signaling cascades of
Drosophila embryogenesis have been con-
rved and are reud during vertebrate清明雨上吉他谱
A
B
Fig.3.Healing of the embryonic epidermis.(A)Resin ction at the wound
margin showing a rounded leading edge(arrow)with no lamellipodia.(B)A confocal ction through the basal layer of the healing epidermis(to the left)
with the contractile actin cable(arrows)extending around the circumference
of the wound in the front row cells.Nuclei are stained with7AAD(red),and
actin with fluorescein isothiocyanate phalloidin(yellow).(C)Scanning electron micrograph of the epidermal front sweeping forward over expod wound menchyme.Bars:100m(A),20m(B),and100m(C).[(C)courtesy of
J.McCluskey]
C
tissue repair programs(49).
Just as wound-edge keratinocytes have
to adjust their integrin profile before migra-
tion,dermal fibroblasts,which normally lie
in a collagen-I–rich matrix,must down-
regulate their collagen receptors and up-
regulate integrins that bind fibrin,fibronec-
tin,and vitronectin in order to crawl into
the clot.Fibroblasts read and act according
to dual signals from their matrix surround-
ings and from the growth factor milieu in
which they are bathed.If fibroblasts are
cultured in a fibrin-fibronectin gel,then
exposure to PDGF will trigger up-regulation
of the provisional-matrix integrin subunits,␣3and␣5,whereas in a collagen gel the same growth factor signal instead supports
expression of collagen-specific␣2subunits
and not provisional-matrix receptors(50).
Fibroblasts may u a fibronectin conduit to
lead them into the fibrin clot(51),and in
this regard it is interesting to note that the
predominant splice-variant of fibronectin
expresd by fibroblasts and macrophages at
the wound interface is a form otherwi
unique to sites of embryonic-cell migrations
(52),suggesting that this fibronectin is an
exceptionally good substratum for cell mi-
gration.Little is known about wound-trig-
gered regulation of the actinomyosin cy-
toskeleton,which must be crucial in fibro-
blast migration,but almost certainly of rel-
evance is the obrvation that fibroblasts
from transgenic knockout mice lacking the
actin vering and capping protein gelsolin
have impaired migratory respon in culture
(53).Becau PDGF activates the small
GTPa Rac in fibroblasts(26),and gelso-
lin is a downstream effector of Rac(54),it
ems likely that Rac may be one of the key
molecular switches responsible for the ont
of fibroblast migration into a wound.
By about a week after wounding,the
wound clot will have been fully invaded and
all but replaced by activated fibroblasts that
are stimulated by TGF-1and other growth
factors to synthesize and remodel a new col-
lagen-rich matrix(44);at this stage,a pro-
portion of the wound fibroblasts transform
into myofibroblasts,which express␣-smooth
muscle actin and remble smooth muscle
cells in their capacity for generating strong
contractile forces(55).This conversion is
triggered by growth factors such as TGF-1
(56)and mechanical cues related to the
forces resisting contraction(57).
The various tensile forces acting on and
exerted by wound fibroblasts before,during,
and after contraction have been studied in
collagen-gel model systems.For example,a
number of growth factors at the wound site
are potent stimulators of fibroblast-driven
gel contraction and presumably signal gran-
ulation tissue contraction in vivo(58).Po-
tential“stop”signals for wound contraction are being analyzed by releasing mechanically
stresd anchored gels from their substrate
attatchments to simulate the loss of resis-
tance after a wound has clod.Within min-
utes of relea from resisting forces,fibro-
blasts activate an adenosine3Ј,5Ј-mono-
phosphate(cAMP)signal transduction
pathway,which involves influx of extracel-
lular Ca2ϩions and production of phospha-
approximate
tidic acid by phospholipa D(59).Sub-
quently,PDGF and EGF receptors on the
cell surface become densitized(60)and the
relaxed cells return to a quiescent state sim-
ilar to that existing before the injury.Pro-
grammed cell death occurs in some of the
wound fibroblasts,probably the myofibro-
blasts,after wound contraction has cead
(61).
Wound Angiogenesis and the
Neural Respon
The wound connective tissue is known as
granulation tissue becau of the pink gran-
ular appearance of numerous capillaries that
invade the wound neodermis.FGF2and
vascular endothelial growth factor(VEGF)
relead at the wound site promote angio-
genesis.FGF2,or basic FGF,is relead at
the wound site by damaged endothelial cells
and by macrophages(62);when this growth
factor is experimentally depleted with
monospecific antibodies raid against
FGF2,wound angiogenesis is almost com-
pletely blocked(63).VEGF,also called vas-
cular permeability factor,is induced in
wound-edge keratinocytes and macro-
phages,possibly in respon to KGF and
TGF-␣,and synchronously at least one of
its receptors,flt-1,is up-regulated by endo-
thelial cells at the site of injury(64).Evi-
dence that VEGF may promote healing
comes from a study of genetically diabetic
mice,in which VEGF expression fails at the
wound site and healing is impaired(65).
Endothelial cells must up-regulate␣
v

3
integrins if they are to respond to any
wound angiogenic signal.␣
v

3
is expresd
transiently at the tips of sprouting capillar-
ies in the granulation tissue,and the pres-
ence of blocking peptides or antibodies
against this integrin caus angiogenesis to
fail and results in verely impaired wound
healing(66).Just as with all other cell
migrations at the wound site,capillary mor-
phogenesis is also dependent on tightly reg-
ulated proteolysis of the matrix surrounds
during the invasion pha(67).
As the embryo develops,its skin be-
comes denly innervated by a plexus of
nsory and sympathetic nerves rving the
blood vesls and cutaneous appendages as
well as supplying nsation.The nsory
nerve termini are exquisitely nsitive to
signals relead after injury,resulting in
transient nerve sprouting at the site of an
adult skin lesion and more dramatic,per-
manent hyperinnervation after wounding of
neonatal skin(68).The wound-induced sig-
nal controlling this nerve overgrowth may
be nerve growth factor(NGF)(69),and
becau NGF is up-regulated after exposure
to any of the TGF-isoforms(70),it is
tempting to consider nerves as another in-Table1.Growth factor signals at the wound site.
Growth
factor
Source Primary target cells and effect Refs.
EGF Platelets Keratinocyte motogen and mitogen(88) TGF-␣Macrophages;keratinocytes Keratinocyte motogen and mitogen(88,89) HB-EGF Macrophages Keratinocyte and fibroblast mitogen(90) FGFs1,2,
and4
Macrophages and damaged
endothelial cells
Angiogenic and fibroblast mitogen(27,62) FGF7
(KGF)
Dermal fibroblasts Keratinocyte motogen and mitogen(27,62) PDGF Platelets;macrophages;
keratinocytes
Chemotactic for macrophages,
fibroblasts;macrophage
activation,fibroblast mitogen,and
matrix production
(46)
IGF-1Plasma;platelets Endothelial cell and fibroblast
mitogen
(89,91) VEGF Keratinocytes;macrophages Angiogenesis(64) TGF-1
and-2
Platelets;macrophages Keratinocyte migration;chemotactic
for macrophages and fibroblasts;
fibroblast matrix synthesis and
清蒸狮子头remodeling
(47)
TGF-3Macrophages Antiscarring(47,82) CTGF Fibroblasts;endothelia Fibroblasts;downstream of TGF-1(49) Activin Fibroblasts;keratinocytes Currently unknown(48) IL-1␣and
-
Neutrophils Early activators of growth factor
expression in macrophages,
keratinocytes,and fibroblasts
(6) TNF-␣Neutrophils Similar to the IL-1s(6)
direct target for TGF-at the wound site. Given the importance of nerves in regener-ation of limbs in urodele amphibians(71), it is interesting to wonder whether sprout-ing nerves may play some stimulatory role in the healing process by delivering neu-ropeptides and other factors to the wound site(72).Indeed,sparly innervated re-gions of the body tend to heal poorly,and transgenic mice lacking the low-affinity NGF receptor p75suffer from impaired wound healing(73).形容祖国繁荣昌盛的成语
TGF-and Scarring Connective-tissue contraction clos em-bryonic as well as adult wounds;but in embryos there is no apparent conversion from fibroblast to myofibroblast(34,74), and neither is there a significant angio-genic respon.Amazingly,until late fetal stages there is generally no sign of a con-nective-tissue scar where the wound has healed:The repair is perfect.Numerous studies have compared embryonic and adult healing in a arch for molecular differences that could explain why this is so(75).Trivial explanations such as dif-ferences in exposure to bacterial infection or to the dryness of the atmosphere are ruled out by grafting adult skin to a fetal environment,where it still heals with a scar(76),and by obrvations of healing in marsupials,which are born at develop-mental stages equivalent to young amniote fetus and heal wounds without a scar for the first few days of their postnatal period (77).There is a strong correlation be-tween the the age of ont of scarring and the first stage in development when a noticeable inflammatory respon is raid after wounding(78);TGF-1may again provide the link.In the embryo,TGF-1 is expresd transiently and at low levels after injury(79),but at the adult wound site it is prent at high levels for the duration of healing and beyond(47). TGF-1is implicated in pathogenic fi-brotic conditions in kidney,liver,and lung dia(80),and now in scarring of skin wounds as well.Delivery of antibodies that neutralize TGF-1and-2at the
time of wounding reduces scarring(81),as
does exogenous application of TGF-3,
which down-regulates the other two
TGF-isoforms(82),suggesting that a
balance among the TGF-isoforms may
be critical.A recent understanding of
TGF-activation,in particular the per-
保护环境人人有责missive involvement of the manno-6-
phosphate(M-6-P)–IGFII receptor,has
suggested further ways to block this signal:
M-6-P directly applied to wounds will also
prevent scarring(83).
Future Prospects
Our understanding of wound-healing
mechanisms has progresd considerably
in recent years.What remaining questions
are tractable in the foreeable future,and
what more do we need to know in order to
help clinicians deal with problems of skin
healing?
Part of the difficulty in unraveling tis-
sue repair mechanisms is a conquence of
redundancy and cross-talk in the system:
Most wound signals probably control more
than one cell activity,and most cell ac-
tivities are respons to cocktails of sig-
nals.The redundancy of the multiple sig-
nals is becoming more apparent through
study of transgenic mice.Although only a
trickle of knockout mice have been
wounded so far,there have been some
surprisingly normal healing phenotypes re-
ported(Table2).Other candidate wound-
healing genes turn out to be so important
in normal development that a full gene
knockout is lethal to the embryo.None-
theless,interbreeding of knockout mice
and the careful design of transgenic mice
with gene knockouts or dominant-nega-
tive receptor constructs targeted to partic-
ular skin cell types will provide a wealth of
further insight.
We know little about how the various
wound signals are translated and transduced
into changes in cell activity.Transcription
factors such as c-fos and Egr-1are induced
after wounding in embryos and in tissue
culture monolayers(84),but little is known
about their roles in adult healing.The same
applies to the small GTPa molecular
switches Rho,Rac,and Cdc42,which reg-
ulate actin reorganization in tissue culture
cells(26)and may also govern cell motility
during wound closure.
It is almost certain that growth factor and
matrix signals are not the only relevant in-
fluences.Changes of gap-junctional connec-
tions between keratinocytes at the healing
margin(85)may help to coordinate cell
proliferative and migratory activities at the
wound edge.Mechanical signals in the form
of cell stretching and even ripping of the
plasma membrane at the time of wounding
may prove to be important activators of the
wound respon.Mechanical stress at the
wound site may also play a role in guiding
collagen fibrillogenesis becau altered ten-
sions during wound closure affect the extent
of scarring(86).
A differential display study designed to
find novel genes induced in keratinocytes
after KGF exposure(87)identified a gluta-
thione peroxida that does indeed become
up-regulated soon after skin injury.In hind-
sight,it makes n that cells should syn-
thesize such enzymes to protect themlves
from oxidative damage at the wound site;
nonetheless,the result was a surpri.Inno-
vative studies of this sort will certainly come
up with more surpris,offering new poten-
tial targets for therapeutic intervention.
The next few years in wound-healing
rearch will be exciting as we test whether
we can improve on nature and induce adult
wounds to heal like embryonic wounds—
without delay,without scarring,and with
full regeneration of hairs and glands.
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___________________________
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Table2.Wounding of transgenic knockout mice.A full and regularly updated compendium of wounding studies in knockout mice will be available to Science Online subscribers at sciencemag. Plea e-mail details of your study,published or not,to p.martin@ucl.ac.uk.
Gene knockout Healing phenotype Refs. Plasminogen Reepithelialization blocked(18) Tenascin Skin
healing normal(92) KGF Skin healing normal(29) TGF-␣Skin healing normal{29) Gelsolin Fibroblast migration hindered in vitro(53) BPAG1Reepithelialization fails(37) Keratin8Embryonic healing normal(39)
