Structure of
Skin
(1) Function of Skin
(2) Skin Functional Layers
(3) Skin Cell Types
(4) Schematic Drawing of Human Skin
(1) Keratinocytes
(2) Keratin
3.
The Horny Layer (Stratum
Corneum)
4.
Dermis
6. The
Subcutis (Hypodermis)
7. Summary
8. Reference
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Function of Skin
The skin performs a complex role in human physiology:
The
skin consists of three functional layers:
Epidermis
Dermis or corium
Subcutis (hypodermis)
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1 Epidermis 2 Dermis 3 Subcutis 4 Hair follicle 5 Sebaceous gland 6 Sweat gland |
In
these layers are found the epidermal appendages: nails, hair and glands. (Note: Sebaceous
and sweat glands belong to the exocrine glands. Sebaceous glands are nearly
always connected to hair follicles. Sweat glands deliver their secretions
directly to the skin surface.) The skin performs various functions such
as temperature regulation and insulation, energy storage, sensory perception
and protection from environ
The
skin is composed of several layers. The lowest layer is called the
dermis. This layer is composed of connective
tissue, blood vessels, nerve
endings, hair follicles,
and sweat and oil glands.
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Skin Cell Types
Keratinocytes
The most abundant
cell type of the epidermis is the keratinocyte. These cells produce keratin
proteins that provide so
Fibroblasts
The dermis is
produced largely by fibroblasts, which
during embryonic develop
Melanocytes
Melanocytes are cells
in low abundance in the epidermis that produce the pigment melanin. The
pigment made in melanocytes is transferred to the cells of the hair or
epidermis. The melanin granules are injected into (or ingested by) the
keratinocyte cells. There, the melanin granules accumulate around the nucleus of each keratinocyte.
Melanin
absorbs harmful ultraviolet (UV) light before the UV radiation can reach the nucleus.
Melanin protects the DNA
in the nucleus from UV radiation damage. When
Differences
in skin color are due mostly to differences in the types and amount of pigment in our
keratinocytes. Skin darkening (tanning) from sun exposure is caused by
the movement of existing melanin into keratinocytes, and by increased
production of melanin by the melanocyte.
During
embryonic develop
Langerhans cells
These are star-shaped
resident immune cells, macrophages. A macrophage is a cell that protects
your body from injury or illness. Macrophages break up or destroy
(phagocytise) the invading organisms. These macrophages process the
invading organisms and present antigens to the T-lymphocytes. The T-lymphocytes
are immune-system cells which ultimately identify a substance as foreign or
dangerous to the body.
Merkel's Cells
Only a few of these
cells are present in skin; they are more nu
Schematic Drawing of Human Skin
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Drawing (transverse section) of human skin illustrates the epidermis,
base A: Epidermis B: Dermis C: Cornified layer of keratinocytes (stratum
corneum) D: Suprabasal keratinocytes E: Basal layer of keratinocytes (stratum basale) F: Base G: Collagen fibers in dermis H: Capillary (enclosed by a single microvascular
endothelial cell) I: Melanocyte J: Dermal Fibroblast |
The great majority of cells in the epidermis are keratinocytes, which
are arranged in stratified layers. At the dermal-epidermal junction is a single
layer of keratinocytes with a small number of interspersed
Keratinocyte stem cells (like stem cells from other
tissues) are relatively undifferentiated, both biochemically and
histologically. Although keratinocyte stem cells have a high capacity for cell
division, they divide with much lower frequency than transient-amplifying
cells. Thus, when labeled with 3H-thymidine, stem cells retain nuclear label
for long periods of ti
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Though
paper thin, the epidermis is composed of many layers of cells. In the basal
layer (the living epidermis), new cells are constantly being reproduced,
pushing older cells to the surface. As skin cells move farther away from their
source of nourish
The
epidermis consists of up to 90 percent keratinocytes, the actual epidermal
cells or dead skin cells, that are held together by what are called desmoso
Hair
and nails are specialized keratin structures and are considered part of the
epidermis. While animals use fur and claws for protection and defense, these
corresponding structures are largely cos
On
the skin surface are the sweat gland pores (100-200/cm2) and the
openings of the sebaceous glands (50-100/cm2). Their secretions
ensure skin moisture and oiliness, and thus maintain the hydrolipid film. The
epidermis itself has no blood vessels, so the nutrients are supplied through
the fine blood vessels in the dermal papillae.
The
epidermis is differentiated into five layers:
Horny layer (stratum corneum)
Clear layer (stratum lucidum)
Granular layer (stratum
granulosum)
Prickle-cell layer (stratum
spinosum)
Basal layer (stratum basale)
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Schematic diagram of the epidermis: the basal cells
change, through differentiation, into flat horny skin cells that are without
nuclei. 2 Clear layer 3 Granular layer 4 Prickle-cell layer 5 Basal layer 6 Basal |
The stratum basale (basal = basis, ground/lat.) is the lowest layer of the
epidermis. The basal cells lie directly on the basal
The
basal cell layer is comprised mostly of keratinocytes which are either dividing
or non-dividing. The cells contain keratin tonofibrils and are
secured by hemidesmosomes to the basement membrane.
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The stratum spinosum
(spino = thorn, prickle/lat.), the prickle-cell layer, is above the basal
layer. In it are, visible for the first ti
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Section through the spinous layer or stratum
spinosum. Individual epithelial cells (EC) are attached to one another by
numerous intercellular junctions. These junctions can best be seen as
spiny projections bridging the intercellular spaces (ICS) if you click on the
figure to the left to see the picture at higher magnification. The dense array
of intercellular junctions prevents the intercellular spaces from becoming
markedly dilated. Inflammation may damage the intercellular junctions,
causing them to rupture and allowing the intercellular spaces to
enlarge. Inflammatory cells will often occupy these enlarged
intercellular spaces. (The following five TEM images were copied from http://www.temple.edu/dentistry/perio/periohistology/gu0203m.htm.
) |
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Transmission electronmicrograph of an intercellular junction between adjacent cells in the spinous layer of the oral epithelium. The junction is mediated primarily by desmosomes. The desmosomes (D) are arranged in a sawtooth pattern. Bundles of tonofilaments or tonofibrils (TF) extend from the desmosomes into the adjacent cytoplasm. Tonofibrils serve as a cytoskeleton that help to dissipate mechanical stresses placed on the desmosomes and adjacent cell membranes. The bar in the upper part of the figure measures 0.1 micrometer. |
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The epithelial cells in the superficial portion of the stratum spinosum become flattened. Tonofibrils take up an increasing volume of the cytoplasmic contents. Relatively sparse, round cytoplasmic granules appear in the stratum granulosum (SG). The stratum corneum (SC) is characterized by an increased packing of the tonofibrils within a markedly flattened cell. Nuclei and most organelles disappear and the staining characteristics of the cells in this layer are markedly altered. |
Above the prickle-cell layer is the stratum granulosum (granula = grain/Lat.),
where the cornification (keratinization) of the keratinocytes begins. It gets
its na
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Transmission electronmicrograph of stratum
granulosum. Note the electron-dense
keratohyalin granules (KHG) within the cytoplasm of the flattened cells.
The degree of flattening of the cells can be estimated by the
proximity of adjacent intercellular junctions (ICJ). The increased
density of tonofibrils within the cytoplasm can be observed when the image on
the right is magnified by clicking on it. |
The stratum lucidium is also called the clear layer as it is highly refractive.
The cells have been extre
Also, the
translucent or transitional layer, this is a translucent, thin
layer of cells. This layer is so
The stratum corneum (cornea = horny skin/Lat.) is the uppermost layer of the
epidermis. Between the cornified cells (corneocytes) lie the epidermal lipids.
The horny layer - especially the bottom third - forms the per
This layer is made of flattened epithelial cells in multiple layers.
These layers are called keratinized layers because of the build-up of the
protein keratin in those cells. Keratin is a strong protein that is
specific to the skin, hair and nails. This layer of skin is, for
the most part, dead--it is composed of cells that are almost pure protein.
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The stratum
corneum consists of tightly packed cornified cells. Intercellular
junctions (ICJ) between the flattened cells are still distinguishable.
The cells contain densely packed tonofilaments.
No nuclei or cytoplasmic organelles are detectable. This form of
keratinization is referred to as orthokeratinization,
i.e. complete keratinization of the epithelial cells. Orthokeratinized
epithelia provide the best protection against mechanical injury. The
most superficial cells peel off or desquamate into the oral cavity, taking
with them any bacteria that may have colonized the epithelial surface.
This constant renewal of the epithelial surface is an important defense
mechanism against bacterial infection. |
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Scanning electron microscope image
of scaling horny skin cells. |
Through differentiation, the living,
Did you know that 90% of household dust is
dead skin cells? Keratinocytes contain structural protein (keratin) and beco
The
epidermis is a stratified squamous epithelial tissue. This
Mitotic Activity: The layer adjacent to the dermis is known
as the basal layer. The basal layer is made up of columnar epithelial cells.
Since all of the mitotic (cell-multiplying) activity of the epidermis occurs in
the basal layer, the basal layer is often called the germinative layer. This
mitotic activity involves about 4 percent of the cells in the basal layer at
any given ti
Migration
of Cells to the Surface:
Over a period of weeks,
new cells gradually migrate from the basal layer to the surface. During this
migration to the surface, the cells change in shape from the original columnar
to cuboidal and then finally to squamous. As the cells beco
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http://www.aad.org/education/keratinocytes.htm
Keratinocytes
are stratified, squamous, epithelial cells which comprise skin and mucosa,
including oral, esophageal, corneal, conjunctival, and genital epithelia.
Keratinocytes provide a barrier between the host and the environment. They
prevent the entry of toxic substances from the environment and the loss of
important constituents from the host. Keratinocytes differentiate as they
progress from the basal layer to the skin surface. The normal turnover time for
keratinocytes is around 30 days but epidermal turnover may be accelerated in
some skin diseases such as psoriasis.
Keratinocyte
stem cells reside in the basal layer. These cells have a low rate of mitosis
and give rise to a population of transient amplifying cells. (Figure 1) Transient amplifying cells go through a
limited number of divisions, differentiate, and move up in the epidermis. The
cells above the basal layer are known as the spinous layer. Under routine
microscopy small bridges, resembling spines, can be seen between the
keratinocytes which represent intercellular adhesion complexes known as
desmosomes. As the cells further differentiate, they synthesize keratohyaline
granules, a prominent feature of cells in the granular layer. Proteins
synthesized in the granular layer are important in the final stages of
epidermal differentiation and include profilagrin, loricrin, involucrin, and
cornifin. These molecules are important in the formation of the stratum
corneum, the outer most layer of the epidermis (Figure
2).
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Figure 1 A diagram of the cell
cycle. The cycling component consists of cells in the G1 phase,
the most variable part of the cycle. Cells then move into the S phase during
which the DNA content of the cell is doubled. Subsequently, cells enter the
second gap phase (G2), which leads to mitosis and the production
of two daughter cells. The daughter cells may proceed through another replicative
cycle, enter the differentiation pathway or, according to some investigators,
enter a resting phase (G0). |
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Figure 2. Schematic
representation of the heterogeneity in basal keratinocytes. The nonserrated
(NS) cells at the tips of the deep rete ridges are believed to be the slowly
cycling stem cells. These give rise to suprabasally located transient
amplifying cells (TA) cells, which actively incorporate [3H]
thymidine. The TA cells give rise to the more superficial nonlabelled post
mitotic (PM) cells. The serrated (S) cells located in the more shallow rete
ridges are believed to play a role in anchoring of epidermis to dermis.
B=basal; S=spinous; G=granular; SC=statum corneum. From Lavker and Sun
(reference: Fifth edition. Freedberg IM, Eisen AZ, Wolff K, Goldsmith LA,
Katz SI and Fitzpatrick TB (eds), New York: McGraw-Hill, 1999, pp. 133-143.
). |
TABLE 1. Keratin Location
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Type I (acidic) |
Type II (basic) |
Location |
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K10 |
K1 |
suprabasal epidermal keratinocytes |
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K9 |
K1 |
palmoplantar suprabasal keratinocytes |
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K10 |
K2e |
granular layer of the epidermis |
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K12 |
K3 |
cornea |
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K13 |
K4 |
nonkeratinizing stratified squamous
epithelia |
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K14 |
K5 |
basal layer keratinocytes |
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K15 |
K5 |
basal layer of non-keratinizing epithelia |
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K16 |
K6a |
outer root sheath (hair),
hyperproliferative keratinocytes, oral epithelium |
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K17 |
K6b |
nail bed, myoepithelium, inflammatory
conditions |
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K7 |
various partners in transformed cells |
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K18 |
K8 |
simple epithelia |
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K19 |
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bulge cells (hair follicle), simple
epithelia |
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K21 |
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intestinal epithelium |
Basal cells express keratins 5 and 14. As keratinocytes leave the basal layer, they become larger and synthesize keratins 1 & 10 (Figure 3). Different keratins are associated with hair and nail formation. In hyperprolific epidermis, such as psoriasis and atopic dermatitis, keratin 6 & 16 predominate. A congenital blistering disease, epidermolysis bullosa simplex, is due to defects in keratins 5 & 14 resulting in blistering at the basal layer. Other keratin pairs are involved in a variety of diseases of epidermis, hair, and nails
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Figure 3. The epidermis and keratin
expression. On the left is a histologic cross-section of human skin and on
the right a cartoon representing the process of epidermal differentiation.
The four major steps in epidermal differentiation are 1) an innermost basal
layer of mitotically active cells; 2) three to six layers of spinous cells
that are still transcriptionally active but are no longer dividing; these
cells can devote most of their translational machinery to expressing
keratins; 3) one to three layers of granular cells that are transcriptionally
active and deposit a cornified envelope of crosslinked proteins beneath the
plasma membrane; and 4) 5-20 layers of stratum corneum, which consist of
metabolically inert, enucleated squames that are sloughed from the skin
surface. Basal epidermal cells express keratins 5 and 14. As basal cells
commit to terminally differentiate, they switch off the expression of K5 and
K14 and induce the expression of K1 and K10. As epidermal cells move up
through the spinous, they express K2e, which can pair with K10. Squames
sloughed from the skin surface are merely dead sacs, chock full of keratin
macrofibrils. |
Questions - Keratinocytes
1. Stem
cells in the epidermis are found in the:
A) stratum corneum
B) granular layer
C) spinous layer
D) basal layer
E) spinous and basallayers
2. The
keratin pair in the basal layer keratinocytes consists of keratins
A) 1 and 2
B) 1 and 10
C) 5 and 14
D) 8 and 18
E) 6 and 16
3. Which
of the following epidermal layers is transcriptionally inert?
A) basal layer
B) spinous layer
C) granular layer
D) stratum corneum
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Answers: D, C, D
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http://telemedicine.org/anatomy/anatomy.htm#keratin
Electron
microscopical examination of cells from all tissues reveals that they contain a
complex, heterogenous, intracytoplasmic system of filaments. The components of
this system include actin, myosin, and tubulin, whose diameters average
approximately 60A¢X, 150A¢X, and 250A¢X, respectively. In addition, other
intracytoplasmic filaments were noted, and since the diameter of these latter
structures was found to be between 70 and 100A¢X, they were called intermediate
filaments.
Intermediate
filaments form a major part of the cytoskeleton of most cells and fulfill a
variety of roles related to cell shape, spatial organization, and perhaps
informational transfer. The nucleus contains structures related to these
intermediate filaments and many intracellular components including
polyribosomes, mitochondria, nucleic acids, enzymes, and cyclic nucleotides are
attached to the cytoskeleton.
Based
on their biochemical, biophysical, and antigenic properties, a number of
classes of intermediate filaments can be recognized in different cell types:
desmin (skeletin) in muscle cells, glial fibrillary acidic filaments in glial
cells, neurofilaments in neurons, vimentin in mesenchymal cells, and keratin in
epithelial cells. In cultured epidermal cells, keratins account for up to 30%
of the cellular protein, while in stratum corneum, keratin accounts for up to
85% of the cellular protein.
At
least 19 keratin proteins can be identified ranging in molecular weight from
approximately 40,000 to
68,000 micrograms. Moll and his coworkers published their human keratin
catalogue in 1982. According to this catalogue, there are two keratin subfamilies. The molecular weight of the
members of one (the basic subfamily) is
relatively larger than that of the members of the other (the acidic subfamily). Each of the keratins is the
product of a unique gene and, in essentially all situations, the keratins are
expressed as pairs containing one member of each subfamily. The two members of
each pair are in the same size rank order within their respective family, e.g.,
the largest acidic keratin is expressed with the largest basic.
The
type of keratin differs in different tissues, i.e, there are different types of
keratin for keratinized epidermis, hyperproliferative epidermis of palms and
soles, corneal epithelium, stratified epithelium of the esophagus and cervix,
and simple epithelium of the epidermal glands. As mentioned before, keratin is
the main structural protein of the epidermis.
The keratinocytes in the basal layer and prickle cell layer synthesize keratin filaments (tonofilaments) which aggregate into bundles (tonofibrils). Eventually, in the cells of the stratum corneum, these bundles of keratin filaments form a complex intracellular network embedded in an amorphous protein matrix. The matrix is derived from the keratohyaline granules of the granular layer. Epidermal keratinization results in the production of a barrier which is relatively impermeable to substances passing in or out of the body.
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Tonofilaments (blue arrowheads) are filamentous structures and are part of the
cytoskeleton of cells. These filaments are abudantly present in keratinocytes
and are found at desmosomal junctions. |
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http://sprojects.mmi.mcgill.ca/dermatology/physiology.htm#keratinocyte
The aging of basal cells into the corneocytes (dead cells) is
crucial. The stratum corneum is important in
preventing all manner of agents from entering the skin, including
micro-organisms, water and particulate matter. It's the epidermis that also prevents loss of vital body fluids.
The dividing basal cell on average replicates every 200 to 400 hours,
and the resulting cell takes 14 days to differentiate and 14 days to be shed.
Keratinocyte
maturation can be divided into five sequences:
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1)
In the basal
layer (stratum basale), undifferentiated cells and cells in
the layer immediately above divide continuously. Half of these cells progress
upwards and differentiate, while the other half remain behind to divide
again. 2)
In the prickle
cell layer (stratum spinosum), the shape of cells change from
columnar to polygonal. Differentiating keratinocytes synthesize keratins
which aggregate to form tonofilaments. Condensations of these tonofilaments
form desmosomes which connect keratinocytes. Desmosomes maintain a distance
of 20 nm between adjacent cells and distribute structural stresses throughout
the epidermis. 3)
In the basal
layer (stratum basale), undifferentiated cells and cells in
the layer immediately above divide continuously. Half of these cells progress
upwards and differentiate, while the other half remain behind to divide
again. |
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4)
In the prickle
cell layer (stratum spinosum), the shape of cells change from columnar
to polygonal. Differentiating keratinocytes synthesize keratins which aggregate
to form tonofilaments. Condensations of these tonofilaments form desmosomes
which connect keratinocytes. Desmosomes maintain a distance of 20 nm between
adjacent cells and distribute structural stresses throughout the epidermis.
5)
In the granular
layer (stratum granulosum), enzymes induce degradation of nuclei and
organelles. Keratohyalin granules mature the keratin and provide an amorphous
protein matrix for the tonofilaments. Membrane coating granules attach to the
cell membrane and release an impervious lipid containing cement which
contributes to cell adhesion and to the horny layer barrier.
6)
In the horny
layer (stratum corneum), the dead flattened corneocytes have
developed thickened cell envelopes enclosing a matrix of keratin tonofibrils.
The disulphide bonds of keratin provide the strength to the layer, but the
horny layer is also flexible and can absorb up to three times its weight in
water. However, if the layer dries out (below 10% water content), pliability
fails.
7)
Corneocytes are
shed from the skin surface.
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http://sprojects.mmi.mcgill.ca/dermatology/physiology.htm#keratinocyte
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Melanocytes are located in the basal layer. In this location, they produce the
pigment melanin in elongated, membrane-bound organelles known as melanosomes. Melanin is packaged into granules
which are moved down dendritic processes and transferred by phagocytosis to
adjacent keratinocytes. In the inner layers of the
epidermis, melanin granules form a protective cap over the outer
part of keratinocyte nuclei. In the stratum corneum,
melanin granules are uniformly distributed to form a UV-absorbing
blanket which reduces the amount of radiation penetrating the skin. |
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UV radiation - mainly the
wavelengths of 290 to 320
nm (UVB) - darkens the skin firstly by immediate photo-oxidation of
preformed melanin, and secondly over a period of days by stimulating
melanocytes to produce more melanin.
UV radiation also thickens the epidermis by inducing keratinocyte
proliferation.
Contrary to popular belief, variations in racial
pigmentation are not due to differences in melanocyte numbers, but to the number
and size of melanosomes produced.
Red-haired people have
the pigment phaeomelanin and their melanosomes are spherical,
rather than the more common eumelanin pigment and oblong melanosomes.
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The Horny Layer (Stratum Corneum)
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1 Horny layer cells (corneocytes) |
The outermost layer of the
epidermis - the horny layer - consists of a dense seg
The
brick and mortar modell
Between the cells lie the epidermal lipids, the horny skin cells are thought of
as bricks, and then lipids fill the spaces between the cells like mortar or ce
Formation
and function of the epidermal lipids
The lipid composition and moisture content of the epidermis change with
increasing differentiation of the skin cells. Lipids are for
At the sa
Composition
of the epidermal lipids
Ceramides form the largest fraction with 40 percent. Also found are free fatty
acids (25%) and cholesterol (25%) as well as cholesteryl sulphate. The
ceramides are primarily responsible for the barrier forming and
moisture-binding functions of the complex lipid mixture. Chemically, the
ceramides are a group of sphingolipids. These are compounds for
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Schematic diagram of the synthesis of epidermal lipids 1 Odland bodies |
The
per
The epidermal lipids comprise 10 to 30 percent of the total volu
It prevents invasion by certain
substances such as microorganisms, chemical substances and allergens.
It minimizes
transepidermal water loss (TEWL) and thus protects the body from
dehydration. (Note; Transepidermal water is the
water diffusing to the skin surface. There it is removed from the body by
If
horny skin layers are removed and with them the epidermal lipids, the skin beco
Natural moisturizing factors (NMF)
The ability of the skin to store water depends in
large part on the make-up of the barrier lipids in the horny layer. The protein
structure of the horny cells, including the presence of the amino acid
arginine, also influences the water-binding capacity of the skin. These
substances that occur physiologically in the body and that retain water in the
horny layer, are called natural moisturizing factors (NMF). The substances
originate from the cornification (differentiation) of the keratinocytes (e.g.
pyrrolidine carboxylic acid) and secretions from the sweat and sebaceous glands
(including urea, salts, organic acids).
Desquamation and skin renewal
Towards the surface, the horny layer of the skin beco
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The dermis (derm = skin/Gk also corium) forms a well-defined border with
the epidermis (scarf skin) and a more fluid border with the subcutis
(subcutaneous fatty tissue).
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The stratum papillare
creates a well-defined, wave-shaped border to the epidermis. 1 Stratum papillare |
The
dermis, or the "true skin," is composed of gel-like and elastic
materials, water, and, primarily, collagen. Embedded in this layer are systems
and structures common to other organs such as lymph channels, blood vessels,
nerve fibers, and muscle cells, but unique to the dermis are hair follicles,
sebaceous glands, and sweat glands.
Stratum papillare and stratum reticulare
The stratum reticulare (reticular = net-like/Lat.) makes up the lower part of
the dermis and shows a continuous transition to subcutis. The stratum papillare
(papillae = protuberance/Lat.) is the upper layer which is clearly demarcated
from the epidermis by an undulated border. The wave-like structure increases
the contact area with the epidermis, thus ensuring optimal nourish
The connective tissue of the dermis
The main constituent of the dermis is the proteinous connective tissue made up
of arc-shaped, elastic fibres and undulated, nearly inelastic collagen fibres.
These are responsible for the high elasticity and tensile strength of the
dermis.
Young collagen fibre - glycosaminoglycan
- can bind large amounts of water and so determine the high intrinsic tension
of young skin. As the skin ages, the interweaving of the collagen fibres
increases and the water-binding capacity diminishes. The skin tends to wrinkle.
(Note: Glycosaminoglycans (mucopolysaccharides)
bind with the proteinous connective tissue matrix to form proteoglycans. These
form a gel-like mass that can absorb and expel water like a sponge.)
Connective tissue, glycosaminoglycane and water-binding capacity
The space within the dermal
Other
constituents of the dermis are various types of cells such as fibroblasts, mast
cells and other tissue cells, as well as a multitude of blood and lymph
vessels, nerve endings, hot and cold receptors as well as tactile sensory
organs.
Like
the epidermis, the hair follicle manufactures a keratin structure, hair. These
follicles are found everywhere on the body except for the palms and soles,
though most of the hairs produced are fine, light hairs that, quite unlike the
hair of the scalp, are scarcely visible to the naked eye. The sebaceous glands
are attached to the hair follicles and through the follicles excrete an oily
substance called sebum, which both lubricates and protects the skin. On most of
the skin surface sebum appears constantly and imperceptibly, but in areas with
a higher concentration of sebaceous glands, such as the face and back, there
are wide variations in the amount of sebum produced.
There
are two distinctive sweat-producing glands, the apocrine and the eccrine. The
apocrine gland is best known for producing body odor but otherwise has no known
physiological function and is apparently a holdover from ti
The
eccrine glands are an advanced and extensive system of temperature control.
Several million of these glands are distributed over the entire body, with the
highest concentration in the palms, soles, forehead, and underarms.
Sweat,
a dilute salt solution, evaporates from the skin's surface to cool the body.
Excessive sweating without replace
The
dermis also regulates heat through a network of tiny blood vessels. In hot
weather these vessels dilate to give off heat, causing the skin to flush. In
cold weather, they constrict, conserving heat, causing pallor. The blood in
these vessels nourishes the skin and provides protection for the cellular and
fluid systems. Like the eccrine glands, blood vessels in the dermis are
responsive to emotional stress, causing the color changes
Nerve
endings in the dermis are the source of the body's sense of touch. They sense
heat, cold, and pressure, providing both pain and pleasure.
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The epidermal appendages include the
nails, hair and glands (glandulae cutis). They arise from invaginations of the
epidermis into the dermis.
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Schematic diagram of the follicle.
The hair follicle and sebaceous gland form a structural and functional unit. 1 Hair shaft |
Nails
The nails are horny plates firmly attached to the nail bed. They are about 0.5
mm thick and consist of the front free edge - the body of the nail - and the
nail matrix, which is embedded in the proximal nail fold.
Hair
The hair is divided into the protruding hair shaft and the hair root. The
latter thickens at the end to beco
Glandulae cutis
The glands of the skin (glandulae cutis) include the sweat, scent, sebaceous
and milk glands. The sebaceous glands are nearly always connected to hair
follicles which deliver the lipid-containing secretion to the surface through
their funnel shaped openings. The size of the sebaceous gland and therefore the
amount of sebum itself differ according to body
region. (Note: Except on the palms of the
hands and the soles of the feet, sebaceous glands are found everywhere on the
skin. On the face they are larger than those on the arms or legs.)
The glands found on the face, for example, are bigger than those found on the
arms or legs. An important influencing factor in sebaceous gland activity is
the androgens.
Sebaceous
and sweat glands are exocrine glands (exo = outer, external/Gk.), which
Together
with the sweat glands, the sebaceous glands deliver vital substances, that -
along with the epidermal lipids - form the hydrolipid film.
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The subcutis (sub = under; cutis = skin/Lat.) refers
to the fat tissue below the skin. It consists of spongy connective tissue
interspersed with energy-storing adipocytes (fat cells).
Fat cell clusters
Fat cells are grouped together in large cushion-like clusters held in place by
collagen fibres called connective tissue septa or sheaths.
Nourish
The subcutis is heavily interlaced with blood vessels, ensuring a quick
delivery of stored nutrients as needed. The functions carried out by the
subcutaneous fatty tissue, beside the storage of nutrients in the form of
liquid fats, include the insulation of the
body from cold and shock absorption. On the palms of the hand, the soles of the
feet and the buttocks, fat padding serves almost exclusively for shock
absorption. (Note: Fats, also triglycerides or a
Fat distribution in
The fat content of the subcutis is not the sa
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The fat content of the
subcutis is not the sa 1 Adipocyte |
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The skin consists of three functional layers:
Epidermis
Dermis
or corium
Subcutis
(hypodermis)
The
epidermis is divided into 5 layers. The basal layer (stratum basale) contains
the basal or mother cells that ensure continual regeneration of the skin
through cell division (proliferation). Above lie the cells of the prickle cell
layer (stratum spinosum). Next co
It hinders
the invasion of certain substances such as microorganisms, chemical irritants
and allergens.
It
minimizes transepidermal water loss (TEWL) and so is of great importance to the
body.
The
dermis is divided into two layers, the stratum papillare forming the
distinct undulated border with the epidermis and the stratum reticulare, which
continually
The
epidermal appendages include the nails, hair, and skin glands (glandulae
cutis). Especially the sweat and sebaceous glands play an important role in
formation of the hydrolipid film.
The
subcutis serves foremost as the energy reservoir of the skin: here
nutrients in the form of liquid fats are stored in the adipocytes. At the sa
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Main Source: http://www.eucerin.co.uk/skin/skincell_1.html
Others:
Fritsch,
P. (1990): Dermatologie. 3. Auflage. Springer-Verlag *
Heymann,
E. (1994): Haut, Haar und Kos
Mauro,
T. et al. (1995): Extracellular pH controls barrier repair. J Invest Dermatol
104 (4): 687-97 *
Meyer,
J., Grundmann, H., Knabenhans, S. (1990): Properties of acid phosphatase in
human stratum corneum. Dermatologica 180: 24-29 *
Osborne,
D. W., Friberg, S. E. (1987): Role of stratum corneum lipids as moisture
retaining agent. J Disp Sc a Tech 8 (2): 173-179 *
Raab,
W., Kindl, U. (1991): Pflegekos
Schreiner,
V., Maerker, U., Hoppe,U. (1995): Dependence of barrier repair in human skin on
intra- and extracellular pH (poster). Gordon Conference,
http://www.cascadebio.com/Epilife/Html/epilife%20pages/Background%20info.html
http://www.sweethaven.com/academic/lessons/physiol01/module0305.htm
http://www.aad.org/education/keratinocytes.htm
http://telemedicine.org/anatomy/anatomy.htm#keratin
http://sprojects.mmi.mcgill.ca/dermatology/physiology.htm#keratinocyte
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http://www.sandia.gov/media/NewsRel/NR2001/conjug.htm
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COLOR CHANGE ¡X
Thunderbird demonstrates how self-assembling nanostructures change color
under certain stresses. |
Conjugated
poly
Intelligent nanostructures report on
environ
¡§nanoskin¡¨ may aid in inhabiting Mars
Most
im
The
material also can report changes in
¡§The
material is of interest to NASA ¡X one of the sponsors of our research ¡X for a
thin film for an inflatable structure that would aid in the inhabitation of
Mars,¡¨ says Brinker. ¡§The structure¡¦s skin would require a very thin yet strong
The
color change of the coating would also be sensitive to the composition of
chemicals hitting the structure¡¦s ¡§skin,¡¨ or to dangerous increases in temperature.
The
elegantly simple
Another
possible use for the orderly arrange
In
seconds, robust housing for conjugated poly
Underlying the im
Conjugated
poly
But
a still-open question is how best to fashion a structure for these potentially
useful but fragile extended molecules.
¡§Traditionally,
bulk conjugated poly
A
robust architecture that¡¦s optically transparent and prevents oxidative
degradation of the poly
¡§This
is a simple
Conjugated
poly
It takes only seconds for the Sandia/UNM
The
result is a nanocomposite that is
Technical
discussion
Sandia researchers Alan Burns
and Darryl Sasaki had characterized the responsiveness of two-di
While
this problem could be solved by incorporating the poly
A
significant step was perfor
The
self-assembly
In the
Sandia/UNM process, evaporation, exposure to ultraviolet light, and a
low-temperature heat treat
In
the current work, it is the poly
Sandia¡¦s
Laboratory-Directed Research and Develop
Thin
films, nanoscopic spheres, intelligent ink, light-alterable pore sizes
The achieve
Since
then, this inexpensive process has been used in increasingly complex
procedures, all reported in Natureand in Science.The process has
produced a seashell-like layering at once very strong and nonbrittle,
nanoscopic spheres that can hold catalysts or
Sandia
is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Depart