This document discusses tight junctions in the stratifying epidermis of skin. It finds that tight junctions play a crucial role in regulating the skin barrier function, forming a barrier in the stratum granulosum layer. Cadherins and other proteins like aPKC regulate tight junction formation and function in the epidermis. Loss of E-cadherin specifically impairs the tight junction barrier function in the stratum granulosum through effects on proteins like claudins and ZO-1. Tight junctions and stratum corneum likely cooperate to form the skin barrier through coordinated regulation.
describes various clear cell lesions of head and neck region, its classification, origin, their immunohistochemistry profiles, various clear cell types, physiological and pathological clear cells, their causes.with histopathological images.
The Biology of the Basement Membrane Zone Ibrahim Farag
It is a critical interface between the epidermis and dermis and is a highly specialized structure that allows for communication between different cell types.
Examination of BMZ/Structure of BMZ/Origin of BMZ/Function of BMZ/Examples of Some diseases affecting BMZ
DIFFUSION BASED AND VASCULAR CONSTRUCTS, TRANSPORT OF NUTRIENTS AND METABOLITES Vijay Raj Yanamala
he biggest challenge in the field of tissue engineering remains mass transfer
limitations. This is the limiting factor in the size of any tissue construct grown in vitro.
Within the body, most cells are found no more than 100–200mm from the nearest
capillary, with this spacing providing sufficient diffusion of oxygen, nutrients, and waste
products to support and maintain viable tissue. Likewise, when tissues grown in the
laboratory are implanted into the body, this diffusion limitation allows only cells within
100–200mm from the nearest capillary to survive.
Thus, it is critical that a tissue be pre-vascularized before implantation with proper
consideration given to the cell and tissue type, oxygen and nutrient diffusion rates, overall
construct size, and integration with host vasculature. In the laboratory, limited diffusion
of oxygen is the primary reason that construction of tissues greater than a few hundred
microns in thickness is currently not practicable.
Approaches to address this problem generally fall into six major categories:
scaffold functionalization,
cell-based techniques,
bioreactor designs,
(d)microelectromechanical systems(MEMS)–related approaches,
modular assembly,
in vivo systems
Join live classes, download study aids, sell your documents, join or host your own classes online, get tutoring, tutor students, take practices tests and more at Examville.com
The Extracellular Matrix
Living tissues are not just accumulations of tightly packed cells. Much of a tissue's volume is made up of extracellular space ('extra-' meaning 'outside' or 'beyond,' as in 'extraterrestrial'). This void is filled with a complex meshwork called the extracellular matrix.
Rather than being inert filler material, like the Styrofoam packing around a shipment of glassware, the extracellular matrix is a dynamic, physiologically active component of all living tissues. In addition to providing structural support for the cells embedded within a tissue, the extracellular matrix guides their division, growth, and development. In other words, the extracellular matrix largely determines how a tissue looks and functions.
The extracellular matrix is made up of proteoglycans, water, minerals, and fibrous proteins. A proteoglycan is composed of a protein core surrounded by long chains of starch-like molecules called glycosaminoglycans.
Fibrous Proteins
Several types of fibrous proteins, including collagen, elastin, fibronectin, and laminin, are found in varying amounts within the extracellular matrix of different tissues. These proteins are produced by fibroblasts, but they aren't secreted in their finished form. Rather, they're released as 'precursor' molecules; their subsequent incorporation into the extracellular matrix is guided by the fibroblasts in accordance with the functional needs of a particular tissue.
Collagen is a strong, stretch-resistant fiber that provides tensile strength to your tissues. It's the most abundant protein in the human body. Collagen is the principle constituent of your tendons and ligaments and provides support for your skin. When you sustain an injury to your skin, collagen is the stuff that heals the wound and forms the scar. There are at least a dozen different types of collagen in your body, all adapted to the specific needs of the tissues where they're found.
Elastin is a stretchy and resilient protein. Much like a rubber band, elastin permits tissues to return to their original shape after they've been stretched. Ultraviolet light damages elastin fibers and interferes with their reconstruction, which accounts for the sagging and wrinkling seen in skin that has been chronically exposed to sunlight.
Fibronectin is secreted from fibroblasts in a water-soluble form but is quickly assembled into an insoluble meshwork, which serves several functions. Other cells use the fibronectin matrix to migrate through a tissue, which is particularly important during embryonic development; fibronectin helps position cells within the extracellular matrix; and fibronectin is necessary for cellular division and specialization in many tissues.
Laminin forms sheet-like networks that serve as the 'glue' between dissimilar tissues. It is the principle protein in basement membranes, which are present wherever connective tissue contacts muscle, nervous, or epithelial tissue
A presentation on the topic of microscopic section of gingiva. This topic is mostly looked on by periodontists. A very important chapter in the speciality in dentistry of periodontology and implantology department. Basic understanding of microscopic features and clinical features of gingiva is an important topic for post graduate as well as undergraduate students in the dental field.
describes various clear cell lesions of head and neck region, its classification, origin, their immunohistochemistry profiles, various clear cell types, physiological and pathological clear cells, their causes.with histopathological images.
The Biology of the Basement Membrane Zone Ibrahim Farag
It is a critical interface between the epidermis and dermis and is a highly specialized structure that allows for communication between different cell types.
Examination of BMZ/Structure of BMZ/Origin of BMZ/Function of BMZ/Examples of Some diseases affecting BMZ
DIFFUSION BASED AND VASCULAR CONSTRUCTS, TRANSPORT OF NUTRIENTS AND METABOLITES Vijay Raj Yanamala
he biggest challenge in the field of tissue engineering remains mass transfer
limitations. This is the limiting factor in the size of any tissue construct grown in vitro.
Within the body, most cells are found no more than 100–200mm from the nearest
capillary, with this spacing providing sufficient diffusion of oxygen, nutrients, and waste
products to support and maintain viable tissue. Likewise, when tissues grown in the
laboratory are implanted into the body, this diffusion limitation allows only cells within
100–200mm from the nearest capillary to survive.
Thus, it is critical that a tissue be pre-vascularized before implantation with proper
consideration given to the cell and tissue type, oxygen and nutrient diffusion rates, overall
construct size, and integration with host vasculature. In the laboratory, limited diffusion
of oxygen is the primary reason that construction of tissues greater than a few hundred
microns in thickness is currently not practicable.
Approaches to address this problem generally fall into six major categories:
scaffold functionalization,
cell-based techniques,
bioreactor designs,
(d)microelectromechanical systems(MEMS)–related approaches,
modular assembly,
in vivo systems
Join live classes, download study aids, sell your documents, join or host your own classes online, get tutoring, tutor students, take practices tests and more at Examville.com
The Extracellular Matrix
Living tissues are not just accumulations of tightly packed cells. Much of a tissue's volume is made up of extracellular space ('extra-' meaning 'outside' or 'beyond,' as in 'extraterrestrial'). This void is filled with a complex meshwork called the extracellular matrix.
Rather than being inert filler material, like the Styrofoam packing around a shipment of glassware, the extracellular matrix is a dynamic, physiologically active component of all living tissues. In addition to providing structural support for the cells embedded within a tissue, the extracellular matrix guides their division, growth, and development. In other words, the extracellular matrix largely determines how a tissue looks and functions.
The extracellular matrix is made up of proteoglycans, water, minerals, and fibrous proteins. A proteoglycan is composed of a protein core surrounded by long chains of starch-like molecules called glycosaminoglycans.
Fibrous Proteins
Several types of fibrous proteins, including collagen, elastin, fibronectin, and laminin, are found in varying amounts within the extracellular matrix of different tissues. These proteins are produced by fibroblasts, but they aren't secreted in their finished form. Rather, they're released as 'precursor' molecules; their subsequent incorporation into the extracellular matrix is guided by the fibroblasts in accordance with the functional needs of a particular tissue.
Collagen is a strong, stretch-resistant fiber that provides tensile strength to your tissues. It's the most abundant protein in the human body. Collagen is the principle constituent of your tendons and ligaments and provides support for your skin. When you sustain an injury to your skin, collagen is the stuff that heals the wound and forms the scar. There are at least a dozen different types of collagen in your body, all adapted to the specific needs of the tissues where they're found.
Elastin is a stretchy and resilient protein. Much like a rubber band, elastin permits tissues to return to their original shape after they've been stretched. Ultraviolet light damages elastin fibers and interferes with their reconstruction, which accounts for the sagging and wrinkling seen in skin that has been chronically exposed to sunlight.
Fibronectin is secreted from fibroblasts in a water-soluble form but is quickly assembled into an insoluble meshwork, which serves several functions. Other cells use the fibronectin matrix to migrate through a tissue, which is particularly important during embryonic development; fibronectin helps position cells within the extracellular matrix; and fibronectin is necessary for cellular division and specialization in many tissues.
Laminin forms sheet-like networks that serve as the 'glue' between dissimilar tissues. It is the principle protein in basement membranes, which are present wherever connective tissue contacts muscle, nervous, or epithelial tissue
A presentation on the topic of microscopic section of gingiva. This topic is mostly looked on by periodontists. A very important chapter in the speciality in dentistry of periodontology and implantology department. Basic understanding of microscopic features and clinical features of gingiva is an important topic for post graduate as well as undergraduate students in the dental field.
Cell adhesion, both cell-cell and cell-ECM, can influence whether a c.pdfahntagencies
Cell adhesion, both cell-cell and cell-ECM, can influence whether a cell will proceed through
cell division or die by apoptosis. Compare/contrast the influence on cell survival and cell
division between cell-cell and cell-ECM adhesion. What signal in behavior is an integrin sending
to a cell when the integrin is binding its ligand? Explain your answer. What signal in behavior is
a cadherin sending to a cell when the cadherin is binding to its ligand? Explain your answer.
Compare and contrast how normal cells and cancer cells interpret cell adhesion (either cell-cell
and cell-ECM) to a change in cell behavior.
Solution
Answer B
Integrins and cadherins are adhesive molecules that are also called as cell adhesion molecules or
CAM. Cell matrix adhesion is brought over by integrins. Integrins are the receptors that bring
about ECM adhesion. Integrins activate signal transduction pathways, once they are activated by
ligand binding. These signal transduction pathways mediate cell signals for cell cycle
regulations, intracellular cytoskeleton organisation and new receptor movements to the cell
membranes.
AnswerC
Integrins and cadherins are adhesive molecules that are also called as cell adhesion molecules or
CAM. Junction adherin and cell cell interactions are generally carried out by cadherins. The
cadherins release a beta catenin that plays a role in cell cell adhesion and carries out cadherins
mediated cell division at the plasma membrane. Their name cadherin comes from their
dependency on Ca2+ ions required to function. There are different classes of cadherins. It is
generally seen that that cells containing a specific cadherin subtype tend to cluster together to the
exclusion of other types, both in cell culture and during development. When E cadherin is
expressed there is a release signal for the epithelial cadherin, followed by the expression of N
cadherin which is present for neural development. When the N cadherin is expressed E cadherin
is reduced , finally when there is development of notochord and so mites, E-P and N , cadherins
are expressed.
Answer D
In cancer cells , many of the stem cells show loosened attachment and higher motility. This is
generally seen in the cases where the cancer is highly invasive or malignant, affecting more than
one part of the body. The molecular adhesion fingerprint is altered in cases where the tumour
calls show high mobility showing a high need to dissociate.
Note- could not answer the first part due to lack of appropriate descriptive evidences. Thank you..
Alejo Rodriguez-Fraticelli, from Dr Fernando Martin-Belmonte’s laboratory in the CBMSO in Spain, has recently published with co-workers a paper in the Journal of Cell Biology in which they describe their approach to epithelial cells morphogenesis and uncover the role of cell confinement on epithelial polarity via peripheral actin contractility. The team sheds light on a phenomenon that is crucial to understand the physiology of the organs, but also the mechanisms underlying the development and progression of agressive epithelial cancers.
This presentation intends to explore the communication of the cell within and others for sustainability along the regulation mechanisms by the cellular neural networks and others to sing the song of the life.
Cell, in biology, the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed. A single cell is often a complete organism in itself, such as a bacterium or yeast. Other cells acquire specialized functions as they mature. These cells cooperate with other specialized cells and become the building blocks of large multicellular organisms, such as humans and other animals. Although cells are much larger than atoms, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas; some of these single-celled organisms are spheres as small as 0.2 μm in diameter (1μm = about 0.000039 inch), with a total mass of 10−14 gram—equal to that of 8,000,000,000 hydrogen atoms. Cells of humans typically have a mass 400,000 times larger than the mass of a single mycoplasma bacterium, but even human cells are only about 20 μm across. It would require a sheet of about 10,000 human cells to cover the head of a pin, and each human organism is composed of more than 30,000,000,000,000 cells.
similarities and differences between cells
similarities and differences between cells
Basic similarities between cells and ways cells may vary depending on their function.
See all videos for this article
This article discusses the cell both as an individual unit and as a contributing part of a larger organism. As an individual unit, the cell is capable of metabolizing its own nutrients, synthesizing many types of molecules, providing its own energy, and replicating itself in order to produce succeeding generations. It can be viewed as an enclosed vessel, within which innumerable chemical reactions take place simultaneously. These reactions are under very precise control so that they contribute to the life and procreation of the cell. In a multicellular organism, cells become specialized to perform different functions through the process of differentiation. In order to do this, each cell keeps in constant communication with its neighbours. As it receives nutrients from and expels wastes into its surroundings, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells form tissues, and a cooperation between tissues in turn forms organs, which carry out the functions necessary to sustain the life of an organism.
1. MOLECULAR STRUCTURE AND FUNCTION OF THE TIGHT JUNCTION
Cadherin-Mediated Regulation of Tight
Junctions in Stratifying Epithelia
Christian Michels,a,b
Saeed Yadranji Aghdam,a,b,c
and Carien M. Niessena,b,d
a
Department of Dermatology
b
Center for Molecular Medicine Cologne
c
International Graduate School for Genetics and Functional Genomics
d
Cologne Excellence Cluster on Cellular Stress Responses in Aging-associated Diseases,
University of Cologne, Cologne, Germany
In recent years several seminal breakthroughs have revealed that tight junctions not
only regulate barrier properties of simple epithelial cells but also play crucial functions
in the regulation of the largest barrier of the organism, the stratifying epidermis of the
skin. Here we will address the importance of tight junctions for the skin barrier function
and discuss data from our studies and from others that indicate how cadherins, polarity,
and other pathways may regulate these junctions in stratifying epithelia.
Key words: tight junctions; skin barrier; cadherin; polarity
Introduction
The stratifying epidermis of the skin sepa-
rates the organism from its environment and
serves as its first-line structural and functional
defense against dehydration, chemical sub-
stances, and microorganisms. An important as-
pect in the formation and maintenance of the
barrier is the tight intercellular adhesion be-
tween keratinocytes, which is mediated by in-
tercellular junctions, such as adherens junctions
and desmosomes. This results in the forma-
tion of strong cohesive cell sheets that, through
a complex differentiation process, ultimately
form the cornified layer, the outermost protec-
tive layer of the skin. For keratinizing epithe-
lia, it was originally thought that the secretion
and deposition of this cross-linked protein–lipid
barrier obviated the need for a tight junction
Address for correspondence: Carien M. Niessen, Department of
Dermatology, Center for Molecular Medicine Cologne, University
of Cologne, Joseph Stelzmannstrasse 9, 50931 Cologne, Germany.
carien.niessen@uni-koeln.de
barrier in such tissues. However, ultrastructural
analysis combined with electron dense tracer
studies revealed restricted diffusion to the up-
permost viable layer, the stratum granulosum,
suggesting the existence of a tight junction bar-
rier.1,2
Although tight junction strands were ob-
served by freeze fracture, they appeared incom-
plete, and thus barrier function in the stratum
granulosum was mainly attributed to the in-
tercellular deposition of lamellar bodies.3
This
view remained unchallenged despite the con-
tinued localization/identification of tight junc-
tion components in stratifying epithelia.4,5
A
seminal breakthrough came with the observa-
tion that, in the absence of claudin-1, mice die
of massive epidermal water loss due to impaired
barrier function of the stratum granulosum.6
This provided the first functional evidence that
epidermal barrier function required a tight
junction component. Subsequently, ultrastruc-
tural analysis revealed the presence of a dense
network of continuous strands resembling tight
junctions in the stratum granulosum of hu-
man epidermis,7
thus indicating the formation
of a seal. The importance of tight junctions
Molecular Structure and Function of the Tight Junction: Ann. N.Y. Acad. Sci. 1165: 163–168 (2009).
doi: 10.1111/j.1749-6632.2009.04443.x c 2009 New York Academy of Sciences.
163
2. 164 Annals of the New York Academy of Sciences
in human skin barrier function was recently
underscored by the identification of claudin-
1 mutations in an ichthyosis syndrome addi-
tionally characterized by sclerosing cholangitis
and other features.8
It is now widely accepted
that tight junctions form a critical part of the
barrier in the upper viable layer of stratifying
epithelia.
Classical Cadherins
and the Epidermis
The cadherin adhesion family of Ca2+
-dependent intercellular adhesion molecules is
a key determinant of morphogenesis and tissue
architecture.31
Cadherins do not only mediate
intercellular adhesion but also regulate a wide
spectrum of other cellular functions, such as
polarity, cytoskeleton, signaling, and growth.9
Both the desmosomal cadherins and the classi-
cal cadherins contribute to intercellular adhe-
sion of the epidermis. Unlike desmosomal cad-
herins, which link to the intermediate filament
system, classical cadherins regulate the actin cy-
toskeleton via linker molecules called catenins.
P120ctn regulates cadherin cell surface stability
and potentially connects adhesion to regulation
of Rho GTpases, whereas β-catenin links the
cadherin to the actin regulator α-catenin.10
Two types of classical cadherins are ex-
pressed in the epidermis: P-cadherin, expressed
in the basal layer mainly around and in hair fol-
licles, and E-cadherin, found in all layers of the
epidermis. Next to mediating cell–cell adhe-
sion, cadherins can affect a wide range of cellu-
lar functions that include activation of cell sig-
naling pathways, regulation of the cytoskeleton,
and control of cell polarity.9
Epidermal-specific
deletion of E-cadherin resulted in hair loss11,12
and disturbed barrier function from abnormal
tight junction function13
(see below). It was re-
cently reported that E-cadherin is a target of
autoantibodies in pemphigus, a skin blistering
disease.14
Mutations in human P-cadherin are
associated with a hair disorder “hypotrichosis
with juvenile macular dystrophy,”15
and with
ectodermal dysplasia associated with extro-
dactyly and macular dystrophy.16
In contrast,
loss of P-cadherin in mice did not reveal any
obvious phenotypes,17
suggesting that E- and
P-cadherin either serve partially differential
functions or have a different functional over-
lap in human and mouse.
Epidermal inactivation in mice revealed
overlapping and specific functions for the
cadherin-associated catenins. Loss of β-catenin
in the epidermis confirmed its importance in
the Wnt signaling pathway and its role in hair
follicle morphogenesis and stem cell regulation.
However, no obvious defects in intercellular ad-
hesion and junction formation were observed,
most likely because plakoglobin substituted for
β-catenin in the cadherin complex. Inactiva-
tion of p120ctn
in the epidermis resulted in
reduced adherens junctions and skin inflam-
mation associated with activation of nuclear
factor kappa β (NF-κβ).18
An almost complete
loss of adherens junctions, reduced desmo-
somes, and subsequent skin blistering were ob-
served when α-catenin was deleted from the
epidermis.19
Because both P-cadherin and E-
cadherin bind to the catenins, loss of a sin-
gle cadherin may be insufficient to pheno-
copy deletion of one of the catenins. This
is in agreement with studies in human ker-
atinocytes showing that antibodies to both
E- and P-cadherin inhibit adherens junctions
and desmosomes and interfere with stratifi-
cation. Studies by the Fuchs laboratory and
our group using knockout/knockdown strate-
gies confirmed that classical cadherins are cru-
cial and cooperate in the regulation of junctions
and barrier function20
(Michels et al., submitted
manuscript).
E-Cadherin Regulates
Epidermal Tight Junctions
To examine the role of E-cadherin in strat-
ifying epithelia, such as the epidermis, and to
address if loss of E-cadherin contributes to the
phenotypes observed in the absence of one of
3. Michels et al.: Tight Junctions in Skin 165
the catenins, we used Cre-Lox-P technology
to delete E-cadherin in all layers of the epi-
dermis. Surprisingly, no blistering or any ob-
vious defects in intercellular contacts were ob-
served, as was perhaps to be expected for an
adhesion molecule such as E-cadherin. Mice
with epidermal loss of E-cadherin did have a
red parchment paper-like appearance of the
skin and died from enhanced epidermal wa-
ter loss. The most obvious explanation for
the observed water loss was a disturbed stra-
tum corneum barrier function. However, lu-
cifer yellow penetration assays failed to detect
breaches in the “outside-in” stratum corneum
barrier. In addition, cornified envelopes had
a regular shape and size, and toluidine blue
dye penetration assays revealed no difference
in the temporal development of the stratum
corneum barrier.13
These results resembled ob-
servations for the claudin-1 knockout mice,
which showed a normal outside-in barrier
function but disturbed inside-out barrier func-
tion.6
This prompted us to test the inside-
out barrier function. Whereas control mice
showed restricted flow of dermally injected bi-
otin, this dye diffused past the stratum granulo-
sum in the E-cadherinepi−/−
, indicating impair-
ment of the tight junction inside-out barrier.
This was confirmed by impedance measure-
ments21
and coincided with alterations in local-
ization of key tight junction components, such
as zonula occludens (ZO)-1, claudin-1, and
claudin-4.13
The Polarity Protein
Atypical-Protein Kinase C
Regulates Tight Junctions in
Stratifying Epithelia
In simple epithelia, a close relationship exists
between the establishment of polarity and junc-
tion formation. Moreover, several polarity com-
plex proteins, such as the Par3/Par6/atypical-
protein kinase C (aPKC) complex, localize to
tight junctions where their activity contributes
to tight junction barrier function.9,22
Interest-
ingly, loss of E-cadherin in the epidermis al-
tered the localization of aPKC and its up-
stream regulator, the small GTPase Rac.13
E-cadherin might directly recruit aPKC to the
membrane as we recently obtained evidence for
an interaction between classical cadherins and
aPKC.30
To test the functional involvement of if aPKC
in epidermal tight junction barrier function,
we followed barrier formation over time, us-
ing transepithelial resistance (TER) measure-
ments. Primary keratinocytes are switched to
high calcium medium (1.8 mmol/L Ca2+
) to
induce intercellular junction and thus barrier
formation. Blocking aPKC function, either by
pharmacological inhibition or by overexpres-
sion of a dominant negative aPKCζ mutant,
inhibited the formation of the barrier, whereas
overexpression of a wild-type aPKCζ enhanced
and accelerated barrier formation, indicating
that aPKC is indeed a regulator for epidermal
tight junctions, as was also found by others.23
Interestingly, unlike what is observed in sim-
ple epithelial cell cultures, inhibition of aPKC
function did not result in altered localization
of claudin-1, occludin, and ZO-1.24
Thus, un-
like most systems, our system allows us to sep-
arate initial tight junction formation from its
function and suggests that aPKCs regulate a
late step in the biogenesis of tight junctions.
In mammalians two isoforms of aPKC exist,
aPKCι/λ and aPKCζ, which are encoded by
different genes but share 76% sequence iden-
tity. Although their overlapping and separate
functions are less clear, in vitro studies suggest
that both isoforms can regulate tight junc-
tions. Both isoforms are expressed in the epi-
dermis, albeit with apparently different local-
ization.24
Whereas aPKCζ is confined to the
basal layer, aPKCι/λ appears more promi-
nently enriched at tight junctional contacts,
suggesting that aPKCι/λ may regulate skin
barrier function. If aPKC and Rac are indeed
intermediates in E-cadherin mediated regula-
tion of tight junction function remains to be
answered.
4. 166 Annals of the New York Academy of Sciences
Figure 1. Tight junctions (TJ) in simple and stratifying epithelia.
Phosphoinositide 3-Kinase
Activation Is not Important for
Epidermal Tight Junctions
E-cadherin associates with and activates
phosphoinositide 3 (PI3)-kinase, which itself
can activate Rac. Since epidermal loss of
E-cadherin was associated with a loss of Rac
from cell–cell contacts in vivo13
and an im-
paired activation in vitro (unpublished data),
E-cadherin may thus regulate Rac activity
in the epidermis via PI3-kinase. To test the
possible influence of PI3-kinase, primary ker-
atinocytes were allowed to form intercellular
junctions and barrier function, using the Ca2+
-
switch/TER assay in the presence of different
concentrations of the PI3-kinase wortmannin.
Surprisingly, no difference in resistance was ob-
served in the presence of even high wortmannin
concentration (Fig. 2), suggesting that, at least
in stratifying epithelia, PI3-kinase activity does
not regulate the formation of a functional tight
junctional barrier in the stratum granulosum of
the epidermis.
Cooperation in Skin Barrier
Function by Tight Junctions and
Stratum Corneum
The epidermis is not a classically polar-
ized epithelium like intestine where basolateral
and apical membrane domains are separated
by tight junctions. Instead the epidermis es-
Figure 2. In vitro barrier formation in ker-
atinocytes is not disturbed by activates phospho-
inositide 3 (PI3)-kinase inhibition. Primary mouse
keratinocytes were plated on collagen-coated filters
(0.4 μm pore size) in low-Ca2+
medium (50 μmol/L).
Junction formation was induced by switching Ca2+
concentration to 1.8 mmol/L in the absence or pres-
ence of wortmannin at the indicated concentrations.
Transepithelial resistance (TER) was measured at the
indicated time points, using the Millipore Millicell-ERS
device (Millipore, Billerica, MA, USA).
tablishes a form of junctional polarity along
the apical to basal axis of the tissue, with the
stratum granulosum forming the viable api-
cal boundary (Fig. 1). Because formation of
the stratum corneum depends on the fusion of
lamellar bodies and keratohyalin granules with
plasma membranes at the transition between
stratum granulosum and stratum corneum lay-
ers, it is tempting to speculate that the spe-
cific occurrence of tight junctions in the stra-
tum granulosum relates to the “fence” function
of tight junctions and thereby may regulate the
targeted secretion of “apical” protein and lipid
vesicles directly toward the stratum corneum.
This would imply that changes in tight
5. Michels et al.: Tight Junctions in Skin 167
junctions affect the stratum corneum barrier.
Forced expression of claudin-6 in the upper lay-
ers of the epidermis results in defects not only in
the tight junction but also stratum corneum.25
Tight junctions and stratum corneum may co-
operate in barrier function at other levels be-
cause inactivation of the membrane-anchored
serine protease (CAP)1/Prss8 in the epidermis
also disturbs both barriers.26
Although the un-
derlying mechanisms are unknown, they may
involve the coordinated regulation of both bar-
riers by signal molecules, such as I-κ-β-kinase 1
(IKK1). Inactivation of IKK1 in the epider-
mis severely impairs barrier function associated
with improper epidermal lipid processing and
changes in tight junction component expres-
sion. Epidermal IKK1 function is independent
of NF-κβ signaling but regulates the expression
of retinoic acid receptor target genes, many
of which are involved in epidermal barrier
function.27
Why Are Functional Tight Junctions
only Found in the Granular Layer
of the Epidermis?
The mechanisms that restrict the assembly
of a tight junction barrier to the uppermost
layers of stratifying epithelia are unclear. Be-
cause many tight junction components are ex-
pressed throughout the epidermal layers, it is
possible that a local signal in the granular layer
triggers tight junction formation. We speculate
that the dead cell/keratin layer may initiate this
signal, similar to other simple epithelia that se-
crete and are polarized by an apical matrix
(e.g., follicular epithelia in flies that secrete an
apical cuticle). Alternatively, tight junction for-
mation in the lower layers may be actively in-
hibited by the presence of an overlying viable
cell layer. Either mechanism suggests that the
restriction of tight junctions to the apical-most
layer in stratifying epidermis or apical region
of simple epithelia may be conserved. Indeed,
our data discussed here and other data support
this argument: E-cadherin is required for tight
junction formation in both simple and strati-
fying epithelia. Blocking E-cadherin in vitro in-
hibits tight junctions in simple epithelia,28
as
does genetic loss of epidermal E-cadherin. Sim-
ilarly, blocking αPKC inhibition interferes with
tight junctions in both simple and stratifying
epithelia.23,24,29
Thus, junctional and polarity
proteins required in simple epithelia are also
turning out to be critical for epidermal bar-
rier function, suggesting these processes may
be mechanistically related.
Concluding Remarks
Research in the last decade has provided ex-
citing new insights into how tight junctions con-
tribute to skin barrier function. Because of its
easy accessibility, the visible barrier, and the op-
portunity to isolate primary cells, the skin as a
model system may also provide a unique exper-
imental model system that may provide us with
insights into how barrier function regulates ep-
ithelial tissue homeostasis.
Conflicts of Interest
The authors declare no conflicts of interest.
References
1. Elias, P.M. & S.D. Friend. 1975. The permeability
barrier in mammalian epidermis. J. Cell Biol. 65:
180–191.
2. Logan, K.R., D. Hopwood & G. Milne. 1978. Cel-
lular junctions in human oesophageal epithelium. J.
Pathol. 126: 157–163.
3. Elias, P.M., N.S. McNutt & S.D. Friend. 1977. Mem-
brane alterations during cornification of mammalian
squamous epithelia: a freeze-fracture, tracer, and
thin-section study. Anat. Rec. 189: 577–594.
4. Morita, K., M. Itoh, M. Saitou, et al. 1998. Subcellu-
lar distribution of tight junction-associated proteins
(occludin, ZO-1, ZO-2) in rodent skin. J. Invest. Der-
matol. 110: 862–866.
5. Brandner, J.M., S. Kief, C. Grund, et al. 2002. Orga-
nization and formation of the tight junction system
in human epidermis and cultured keratinocytes. Eur.
J. Cell Biol. 81: 253–263.
6. 168 Annals of the New York Academy of Sciences
6. Furuse, M., M. Hata, K. Furuse, et al. 2002. Claudin-
based tight junctions are crucial for the mammalian
epidermal barrier: a lesson from claudin-1-deficient
mice. J. Cell Biol. 156: 1099–1111.
7. Schluter, H., R. Wepf, I. Moll & W.W. Franke. 2004.
Sealing the live part of the skin: the integrated mesh-
work of desmosomes, tight junctions and curvilin-
ear ridge structures in the cells of the uppermost
granular layer of the human epidermis. Eur. J. Cell
Biol. 83: 655–665.
8. Hadj-Rabia, S., L. Baala, P. Vabres, et al. 2004.
Claudin-1 gene mutations in neonatal sclerosing
cholangitis associated with ichthyosis: a tight junc-
tion disease. Gastroenterology 127: 1386–1390.
9. Nelson, W.J. 2003. Adaptation of core mechanisms
to generate cell polarity. Nature 422: 766–774.
10. Perez-Moreno, M. & E. Fuchs. 2006. Catenins: keep-
ing cells from getting their signals crossed. Dev. Cell
11: 601–612.
11. Tinkle, C.L., T. Lechler, A.H. Pasolli & E. Fuchs.
2004. Conditional targeting of E-cadherin in skin:
insights into hyperproliferative and degenerative re-
sponses. Proc. Natl. Acad. Sci. USA 101: 552–527.
12. Young, P., O. Boussadia, H. Halfter, et al. 2003. E-
cadherin controls adherens junctions in the epidermis
and the renewal of hair follicles. Embo J. 22: 5723–
5733.
13. Tunggal, J.A., I. Helfrich, A. Schmitz, et al. 2005.
E-cadherin is essential for in vivo epidermal barrier
function by regulating tight junctions. Embo J. 24:
1146–1156.
14. Evangelista, F., A.D. Dasher, A.L. Diaz, et al. 2008.
E-cadherin is an additional immunological target for
pemphigus autoantibodies. J. Invest. Dermatol. 128:
1710–1718.
15. Sprecher, E., R. Bergman, G. Richard, et al.
2001. Hypotrichosis with juvenile macular dystro-
phy is caused by a mutation in CDH3, encoding
P-cadherin. Nat. Genet. 29: 134–136.
16. Kjaer, K.W., L. Hansen, C.G. Schwabe, et al. 2005.
Distinct CDH3 mutations cause ectodermal dys-
plasia, ectrodactyly, macular dystrophy (EEM syn-
drome). J. Med. Genet 42: 292–298.
17. Radice, G.L., M.C. Ferreira-Cornwell, D.S. Robin-
son, et al. 1997. Precocious mammary gland develop-
ment in P-cadherin-deficient mice. J. Cell Biol. 139:
1025–1032.
18. Perez-Moreno, M., A.M. Davis, E. Wong, et al. 2006.
p120-catenin mediates inflammatory responses in the
skin. Cell 124: 631–644.
19. Vasioukhin, V., C. Bauer, L. Degenstein, et al. 2001.
Hyperproliferation and defects in epithelial polarity
upon conditional ablation of alpha-catenin in skin.
Cell 104: 605–617.
20. Tinkle, C.L., A.H. Pasolli, N. Stokes & E. Fuchs.
2008. New insights into cadherin function in epider-
mal sheet formation and maintenance of tissue in-
tegrity. Proc. Natl. Acad. Sci. USA 105: 15405–15410.
21. Fromm, M., D.J. Schulzke & U. Hegel. 1985. Ep-
ithelial and subepithelial contributions to transmural
electrical resistance of intact rat jejunum, in vitro.
Pflugers Arch. 405: 400–402.
22. Macara, I.G. 2004. Parsing the polarity code. Nat.
Rev. Mol. Cell Biol. 5: 220–231.
23. Mertens, A.E., P.T. Rygiel, C. Olivo, et al. 2005. The
Rac activator Tiam1 controls tight junction biogene-
sis in keratinocytes through binding to and activation
of the Par polarity complex. J. Cell Biol. 170: 1029–
1037.
24. Helfrich, I., A. Schmitz, P. Zigrino, et al. 2007. Role
of aPKC isoforms and their binding partners Par3
and Par6 in epidermal barrier formation. J. Invest.
Dermatol. 127: 782–791.
25. Turksen, K. & C.T. Troy. 2002. Permeability bar-
rier dysfunction in transgenic mice overexpressing
claudin 6. Development 129: 1775–1784.
26. Leyvraz, C., P.R. Charles, I. Rubera, et al. 2005. The
epidermal barrier function is dependent on the serine
protease CAP1/Prss8. J. Cell Biol. 170: 487–496.
27. Gareus, R., M. Huth, B. Breiden, et al. 2007. Normal
epidermal differentiation but impaired skin-barrier
formation upon keratinocyte-restricted IKK1 abla-
tion. Nat. Cell Biol. 9: 461–469.
28. Gumbiner, B., B. Stevenson & A. Grimaldi. 1988.
The role of the cell adhesion molecule uvomorulin
in the formation and maintenance of the epithe-
lial junctional complex. J. Cell Biol. 107: 1575–
1587.
29. Suzuki, A., T. Yamanaka, T. Hirose, et al. 2001. Atyp-
ical protein kinase C is involved in the evolutionarily
conserved par protein complex and plays a critical
role in establishing epithelia-specific junctional struc-
tures. J. Cell Biol. 152: 1183–1196.
30. Seifert, K., H. Ibrahim, T. Sodtmeister, et al. in press.
An adhesion independent, aPKC dependent function
for cadherins in morphogenetic movements. J. Cell.
Sci.
31. Gumbiner, B.M. 2005. Regulation of cadherin-
mediated adhesion in morphogenesis. Nat. Rev. Mol.
Cell Biol. 6: 622–634.