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 |
|
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 fi