© 2016 S. Karger AG, Basel
Invited Review
Eur Surg Res 2017;58:81–94
Skin Wound Healing: An Update on the
Current Knowledge and Concepts
Heiko Sorg
a
Daniel J. Tilkorn
a
Stephan Hager
a
Jörg Hauser
a
Ursula Mirastschijski
b, c
a
Department of Plastic, Reconstructive and Aesthetic Surgery, Hand Surgery, Alfried Krupp
Krankenhaus Essen, Essen ,
b
Department of Plastic, Reconstructive and Aesthetic Surgery,
Klinikum Bremen-Mitte, and
c
CBIB, University of Bremen, Bremen , Germany
Keywords
Inflammation · Proliferation · Angiogenesis · Nonthermal plasma · Scarring
Abstract
Background: The integrity of healthy skin plays a crucial role in maintaining physiological
homeostasis of the human body. The skin is the largest organ system of the body. As such, it
plays pivotal roles in the protection against mechanical forces and infections, fluid imbalance,
and thermal dysregulation. At the same time, it allows for flexibility to enable joint function
in some areas of the body and more rigid fixation to hinder shifting of the palm or foot sole.
Many instances lead to inadequate wound healing which necessitates medical intervention.
Chronic conditions such as diabetes mellitus or peripheral vascular disease can lead to im-
paired wound healing. Acute trauma such as degloving or large-scale thermal injuries are fol-
lowed by a loss of skin organ function rendering the organism vulnerable to infections, ther-
mal dysregulation, and fluid loss. Methods: For this update article, we have reviewed the
actual literature on skin wound healing purposes focusing on the main phases of wound heal-
ing, i.e., inflammation, proliferation, epithelialization, angiogenesis, remodeling, and scarring.
Results: The reader will get briefed on new insights and up-to-date concepts in skin wound
healing. The macrophage as a key player in the inflammatory phase will be highlighted. Dur-
ing the epithelialization process, we will present the different concepts of how the wound will
get closed, e.g., leapfrogging, lamellipodial crawling, shuffling, and the stem cell niche. The
neovascularization represents an essential component in wound healing due to its fundamen-
tal impact from the very beginning after skin injury until the end of the wound remodeling.
Here, the distinct pattern of the neovascularization process and the special new functions of
the pericyte will be underscored. At the end, this update will present 3 topics of high interest
in skin wound healing issues, dealing with scarring, tissue engineering, and plasma applica-
tion. Co nc l u s i o n: Although wound healing mechanisms and specific cell functions in wound
Received: December 3, 2016
Accept after revision: December 5, 2016
Published online: December 15, 2016
Heiko Sorg, MD, PhD, MHBA
Department of Plastic, Reconstructive and Aesthetic Surgery, Hand Surgery
Alfried Krupp Krankenhaus Essen, Hellweg 100
DE–45273 Essen (Germany)
E-Mail heiko.sorg
@ t-online.de
www.karger.com/esr
DOI: 10.1159/000454919
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Eur Surg Res 2017;58:81–94
DOI: 10.1159/000454919
Sorg et al.: Skin Wound Healing: An Update on the Current Knowledge and Concepts
www.karger.com/esr
© 2016 S. Karger AG, Basel
repair have been delineated in part, many underlying pathophysiological processes are still
unknown. The purpose of the following update on skin wound healing is to focus on the dif-
ferent phases and to brief the reader on the current knowledge and new insights. Skin wound
healing is a complex process, which is dependent on many cell types and mediators interact-
ing in a highly sophisticated temporal sequence. Although some interactions during the heal-
ing process are crucial, redundancy is high and other cells or mediators can adopt functions
or signaling without major complications.
© 2016 S. Karger AG, Basel
Introduction
Skin wound healing is a fascinating mechanism and represents an evolutionary advantage
not only for mammals. Due to its vital functions as a physical, chemical and bacterial barrier,
skin wound healing is an important step for survival finalizing in wound closure. Despite a
great body of literature with regard to wound healing mechanisms, there are still many ques-
tions. Physiological regulation of skin wound healing is a complex process, which is dependent
on many cell types and mediators interacting in a highly sophisticated temporal sequence.
Although some interactions during the healing process are crucial, redundancy is high and
other cells or mediators can adopt functions or signaling without major complications. The
purpose of the following update on skin wound healing is to focus on the different phases
briefing the reader on actual knowledge and new insights. At the end, this update will briefly
focus on 3 topics of high interest, i.e., scarring, tissue engineering in skin wound repair, and
plasma application in skin wound healing.
From Inflammation to Proliferation
One of the main reasons for skin wound healing seems to be the restoration of the barrier
function in order to prevent further damage or infection. This requires the distinct interplay
and crosstalk of a multitude of cells and mediators from the very onset. However, prolonged
wound healing phases or excessive responses of the organism to the injury impede normal
wound healing and might be associated with scarring. In this context, the transition from the
inflammatory to the proliferative stage of wound repair is a topic of intensive current research
[1] . First of all, skin cells are exposed to acute phase signals such as damage-associated
molecular patterns or pathogen-specific molecular patterns, which are recognized on their
parts by toll-like receptors initiating and perpetuating inflammation
[2, 3] . Leukocytes, espe-
cially neutrophil granulocytes, transmigrate alongside an increasing gradient of chemokines
until arrival at the site of injury
[4, 5] . In addition, neutrophils secrete many pro-inflam-
matory cytokines and thereby amplify the inflammatory response
[6] . The influence of cyto-
kines and chemokines in wound repair has been extensively reviewed elsewhere
[7, 8] . Acti-
vated regulatory T cells are part of the adaptive immune system. Aside from leukocytes, regu-
latory T cells are able to regulate tissue inflammation via the attenuation of the interferon-γ
production and the accumulation of pro-inflammatory macrophages. It is assumed that this
effect is mediated by the epidermal growth factor receptor pathway, which is coopted for the
facilitation of skin wound repair
[9] .
One of the key players in the transition from inflammation to proliferation is, however,
the macrophage
[1, 10] . Depletion studies showed that the absence of macrophages in the
inflammatory or the proliferation phase of wound healing resulted in reduced tissue formation
or hemorrhage. Furthermore, the progression into the next scheduled phase failed
[11] . Skin-
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DOI: 10.1159/000454919
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www.karger.com/esr
© 2016 S. Karger AG, Basel
resident macrophages as well as those differentiated from infiltrating monocytes get acti-
vated by pathogen-specific molecular patterns and damage-associated molecular patterns
[2] . In early stages of wound repair, this results in the differentiation into the M1 subset of
macrophages. M1 macrophages are associated with phagocytic activity, scavenging as well as
the production of pro-inflammatory mediators
[10, 12, 13] . Later on, M1 transform into the
M2 subset, revealing a reparative phenotype of macrophages. M2 macrophages are involved
in the synthesis of anti-inflammatory mediators and the production of extracellular matrix
(ECM), in the initiation of fibroblast proliferation as well as in angiogenic processes
[10, 14] .
M2 macrophages constitute a kind of cleanup crew as they phagocytose neutrophils (i.e., effe-
rocytosis), bacteria, and cell debris in order to prevent further damage to the wound site in
later healing phases. This supports the current paradigm of the M1-M2 switch
[1, 15] . If the
M1-M2 transition does not occur, nonhealing or chronic wounds such as venous ulcers and
diabetic wounds are the result
[16–18] . These observations underpin the intimate and
important role of macrophages throughout the process of skin wound healing. In contrast to
the above-mentioned, however, many cellular or cytokine actions might get adopted by other
cells as Martin et al.
[19] could demonstrate that even macrophage-deficient PU.1 null mice
were able to repair skin wounds with a similar time course to wild-type mice. Furthermore,
these PU.1 null mice showed almost scar-free healing, questioning the impact of the inflam-
matory response for the skin wound healing process
[19] .
Epithelialization in Skin Wound Healing
Cutaneous wounds close by epithelial resurfacing and wound contraction. Dependent on
the species, one or the other process dominates the progress of wound repair. For example,
rodents heal mainly by contraction, whereas in humans, reepithelialization accounts for up
to 80% of wound closure
[20] . Skin wound epithelialization is reliant on the wound specifics
such as the location, the depth, the size, microbial contamination as well as patient-related
health conditions, genetics and epigenetics.
Partial thickness wounds that involve the epidermis and partially the dermis usually heal
by primary intention with intact skin appendages, i.e., hair, nails, and sebaceous and sweat
glands. In contrast, full-thickness wounds are characterized by complete destruction of the
epidermis and dermis as well as deeper structures. Repair of tissue loss is initiated by the
formation of granulation tissue that replaces the defect before epithelial covering can occur.
This form of wound repair is called healing by secondary intention.
Healing by third intention is related to complex cases, e.g., septic conditions when wounds
are left intentionally but temporarily open in order to be closed after regression of the highly
inflammatory and often life-threatening situation. When the patient is stable and wounds are
well-conditioned, wound closure is accomplished by sutures or by plastic surgical recon-
struction
[21] . This comprehensible classification of wound healing gives an estimate on the
duration and course of wound healing phases and, thus, a prediction of later outcomes, e.g.,
complete skin regeneration or defective tissue repair by scarring.
Superficial, small and clean wounds are usually associated with a short duration of hemo-
static and inflammatory phase because blood clot formation is limited to seal the wound with
clearing of minor amounts of cell debris. Deep, large and contaminated/infected wounds,
however, will need more time to heal as the initial phases of wound healing include longer
time for hemostasis and removal of cell debris and necrotic tissue before the start of granu-
lation tissue formation. Reepithelialization already starts some hours after injury by
conversion of cobblestone-shaped stationary keratinocytes into flat migratory keratinocytes
[22] . In pigs, the epidermis regenerates from hair follicles, apocrine gland ducts, and the
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Eur Surg Res 2017;58:81–94
DOI: 10.1159/000454919
Sorg et al.: Skin Wound Healing: An Update on the Current Knowledge and Concepts
www.karger.com/esr
© 2016 S. Karger AG, Basel
wound margin, while in humans, this process seems to originate from pilosebaceous units,
eccrine sweat glands
[22, 23] , and the outer root sheath of the hair follicle [24] . Interestingly,
the anatomical positioning of skin adnexa seems to be specifically configured for the purpose
of highly efficient wound repair. Rittié
[22] described this phenomenon by the fact that “no
outgrowth has to migrate farther than half the distance that separates two adnexal structures
before meeting another outgrowth moving in the opposite direction.” With regard to human
partial thickness wounds, cells have to cover approximately 500 μm of distance and complete
epithelialization normally within 8–10 days
[23] . The resurfacing of an epidermal wound by
migrating keratinocytes was initially described by the term of leapfrogging cells that progres-
sively fall over each other and onto the wound bed without certain migrational activity
[25,
26]
. Other authors depicted leader cells or even entire cell rows that drag others with them
to crawl over the wound
[27–30] . Additionally, 3 other mechanisms might also be involved
such as extension membrane or epidermal tongue, lamellipodial crawling and shuffling
[31,
32]
. The epidermal tongue is formed by the front row of keratinocytes adjacent to the wound
site. Activated keratinocytes reorganize their cytoskeleton. This is followed by a succeeding
advance over the tongue to spread across the wound (leapfrog-like)
[22] . The leading row of
activated keratinocytes drags them out of blood clot-derived fibrin, fibronectin, and vitro-
nectin (lamellipodial crawling) and forward over the wound matrix. Interestingly, the leading
row cells do not migrate centripetally into the wound center but change their shape, loosen
their cell-cell contacts, rearrange themselves and leave the front edge (shuffling)
[31] . Arrived
in the middle of the wound, contact inhibition stops the migratory process of keratinocytes
and the wound covering is finished
[33] . Firm cell-cell contacts are reestablished and kerati-
nocytes acquire their quiescent cobblestone-shaped phenotype followed by epidermal strat-
ification. Of note, this repair process is performed from top to bottom with the purpose of fast
and sufficient wound closure and to prevent further fluid loss or infection. The prerequisite
for effective epithelialization is an appropriate ECM that facilitates keratinocyte migration.
While adipose tissue, even in thin layers, counteracts wound coverage, tissues such as dermis,
fascia or muscle represent optimal wound beds. Except for dermis as underlying substrate,
other connective tissues require the formation of granulation tissue for unimpaired epithelial
migration. Granulation tissue is constituted of macrophages, fibroblasts, blood vessels and a
loose matrix out of type I collagen, glycoprotein, fibronectin, and hyaluronic acid.
After skin injury, the reconstitution of the resulting cellular defect is usually achieved by
invading adult stem cells. In the context of epidermal regeneration, stem cells deriving from
the hair follicle bulge and the interfollicular epidermis niche replace missing cells
[34–36] . A
deregulation of the epidermal stem cell niche is present in chronic wounds, i.e., nonhealing
ulcers
[37] , where the cell pool is limited caused by continuous inflammation due to infection,
hypoxia, ischemia and/or excessive exudates
[38] . The use of stem cells, however, is propa-
gated to overcome the problem of nonhealing wounds with extensive on-going research.
Stem cells play an important role in many wound healing phases enabling the resolution of
inflammation, cell migration, proliferation and differentiation, although their intriguing role
is not yet fully understood
[38–40] .
Angiogenesis in Skin Wound Healing
Neovascularization represents an essential component in uncompromised wound
healing due to its fundamental impact from the very beginning after skin injury until the end
of the wound remodeling
[41, 42] . The (micro)vasculature contributes to the initial hemo-
stasis, reduces blood loss and establishes a provisional wound matrix. Blood clot-derived
cytokines and growth factors drive the recruitment of pivotal cells that are crucial for the
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Eur Surg Res 2017;58:81–94
DOI: 10.1159/000454919
Sorg et al.: Skin Wound Healing: An Update on the Current Knowledge and Concepts
www.karger.com/esr
© 2016 S. Karger AG, Basel
healing process. This provisional wound microenvironment depicts the starting point for
new vessel formation and regeneration thereby ensuring the nutritive perfusion of the wound
and the delivery of immune cells that remove the cell debris. At first sight, the neovascular-
ization process seems very disordered as the healing wound generates a high density of func-
tional as well as dysfunctional new capillaries. Nonfunctional vessels will regress by time via
maturation or apoptotic processes. However, a distinct pattern of the neovascularization
process ( Fig. 1 ) can be described forming a circle with an inner ring of circularly organized
vessels directly at the wound border followed by radially shaped vessels supplying the inner
ones and connecting to the normal, uninjured skin
[43] . Disruption in the neovascularization
process consecutively leads to wound healing disturbances or chronic ulcers, typically seen
in venous insufficiency, arteriosclerotic disease or diabetic foot sores. This pathophysio-
logical phenomenon deserves further attention. Recent research projects focus on blood
vessel neoformation and/or delivery to the injury site in order to restore the perfusion and
support the healing process. A prerequisite for these approaches is a profound understanding
and acknowledgement of the underlying pathophysiological processes that lead to disturbed
wound repair.
With regard to chronic, nonhealing wounds, a plethora of causes are present that fuel and
feed the unfavorable microenvironment that impedes cutaneous repair. Amongst others,
hyperglycemia, persistent inflammation, and growth factor and cytokine deficiencies lead to
impaired stem cell recruitment for sufficient angiogenesis
[41] . In this context, the beneficial
impact of stem cells on skin wound healing is evident, especially for the regeneration of blood
vessels
[41] . Stem cells or progenitor cells seem to support this process by multiple paracrine
effects especially by high levels of pro-angiogenic molecules (i.e., VEGF, HGF, bFGF, EGF,
TGF-β, IGF-1)
[44–47] . These effects could be demonstrated in rodent diabetic wounding
models, further underscoring the significant activity of stem cells and their potential in repair-
resistant chronic wounds
[45, 46, 48, 49] .
Recently, the pericyte received more attention in wound healing issues
[50] . First of all,
the pericyte is well characterized for its function in vascular development and stabilization
of the endothelium in newly formed blood vessels. The pericyte provides blood barriers and
regulates capillary flow-through. Furthermore, it acts in a paracrine way and regulates
immune responses as well as processes that are associated with scarring or fibrosis. Pericytes
provide adhesive substrates, i.e., VCAM-1 and E-selectin but mainly ICAM-1, to initiate
neutrophil crawling at the endothelium in search for gates to migrate into the extravascular
Fig. 1. Schematic cartoon of new-
ly formed microvascular net-
works of a regenerating skin
wound. It depicts the typical ar-
rangement of neovascularization
as given by circular vessels (or-
ange) around the wound margins,
radial vascular networks (green)
building the bridge between the
physiological vascular network
and the newly formed microvas-
culature, and the physiological
microcirculation running like a
net around the hair follicles
(blue).