STRUCTURE OF THE SKIN

serves as a barrier to the environment, and some glands (sebaceous) may have weak anti-infective properties.
acts as a channel for communication to the outside world.
protects us from water loss, friction wounds, and impact wounds.
uses specialized pigment cells to protect us from ultraviolet rays of the sun.
produces vitamin D in the epidermal layer, when it is exposed to the sun's rays.
helps regulate body temperature through sweat glands.
helps regulate metabolism.
has esthetic and beauty qualities.

The skin consists of three functional layers:

Epidermis
Dermis or corium
Subcutis (hypodermis)

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 environmental influences such as fungi, bacteria and (UV) radiation.

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.

Skin Cell Types

Keratinocytes
The most abundant cell type of the epidermis is the keratinocyte. These cells produce keratin proteins that provide some of the rigidity of the outer layers of the skin. Keratinocytes also form the bulk of the material in hair follicles. Dandruff and hair are dead keratinocytes.

Fibroblasts
The dermis is produced largely by fibroblasts, which during embryonic development are part of the mesenchyme. The fibroblasts produce the collagens and elastins that make skin very durable, from within.

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 melanin is produced and distributed properly in the skin, dividing cells are protected from mutations that might otherwise be caused by harmful UV light.

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 development these cells migrate from the neural crest into the skin.

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 numerous in the palms and soles (feet). These cells are probably sensory mechanical receptors that respond to stimulus, such as pressure or touch.

Schematic Drawing of Human Skin

Drawing (transverse section) of human skin illustrates the epidermis, basement membrane, dermis, capillaries and major cellular components.

A: Epidermis

B: Dermis

C: Cornified layer of keratinocytes (stratum corneum)

D: Suprabasal keratinocytes

E: Basal layer of keratinocytes (stratum basale)

F: Basement membrane

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 melanocytes (approximately 1/30) called the stratum basale. This basal layer of keratinocytes is also called the stratum germinativum, because it is where new keratinocytes are generated by cell proliferation. Three types of keratinocytes in the stratum basale have been defined by kinetic analysis: stem cells, transient-amplifying cells and committed cells. Stem cells, which represent ~ 10% of the basal cell population, generate daughter cells from mitosis that are either stem cells themselves or transient-amplifying cells. Transient-amplifying cells, which represent ~40% of the basal cell population, replicate with much higher frequency than stem cells, but are capable of only a few population doublings. Transient-amplifying cells produce daughter cells that are committed to terminally differentiate. These committed cells detach from the basement membrane, differentiate, and ultimately cease to proliferate as they migrate toward the skin surface, where they are sloughed off as dead, cornified cells called squames.

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 time compared to transient-amplifying cells. Therefore, stem cells have been described as "label-retaining" cells. Because stem cells are undifferentiated, biochemical markers of stem cells are difficult to identify. However, keratin 19 expression has been suggested as a marker of keratinocyte stem cells, based on localization of keratin 19 expression to 3H-thymidine label-retaining cells. Keratinocyte stem cells may also express higher amounts of the a2 and a3 integrins, because an approximate 1.5-fold increase in the expression of these integrins has been observed in keratin 19-expressing cells relative to other epidermal basal cells. The retention and expansion of keratinocyte stem cells in culture is thought to be essential for using keratinocytes in ex vivo gene therapy.

The Epidermis

As the outermost skin layer, the epidermis forms the actual protective covering against environmental influences. Its thickness averages 0.1 mm. On the face it is only 0.02 mm, while on the soles of the feet between 1 and 5 mm.

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 nourishment, they flatten and shrink. They lose their nuclei, move out of the basal layer to the horny layer (the dead epidermis), and turn into a lifeless protein called keratin. After serving a brief protective function, the keratinocytes are imperceptibly sloughed off. This process of a living cell's evolution, called keratinization, takes about 4 weeks.

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 desmosomes. Keratinocytes function as a barrier, keeping harmful substances out and preventing water and other essential substances from escaping the body. The other 10 percent of epidermal cells are melanocytes, which manufacture and distribute melanin, the protein that adds pigment to skin and protects the body from ultraviolet rays. Skin color is determined by the amount of protein produced by these cells, not by the number of melanocytes, which is fairly constant in all races.

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 cosmetic in humans. The skin, however, is uniquely human, since it can betray emotion by blushing (embarrassment), turning red (anger), blanching (fear), sweating (tension), and forming goosebumps (terror).

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)

Schematic diagram of the epidermis: the basal cells change, through differentiation, into flat horny skin cells that are without nuclei.

1 Horny layer

2 Clear layer

3 Granular layer

4 Prickle-cell layer

5 Basal layer

6 Basal membrane

Basal layer (stratum basale)
The stratum basale (basal = basis, ground/lat.) is the lowest layer of the epidermis. The basal cells lie directly on the basal membrane that forms a definite border between the dermis and epidermis. The basal cells acting as mother-cells, by cell division, provide for the continuous regeneration of the skin. The daughter-cells are slowly driven, by the active cell division, into the outer lying layers where they undergo various development stages. In the basal layer are also found the melanocytes, which are the pigment producing cells.

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.

Prickle-cell layer (stratum spinosum)
The stratum spinosum (spino = thorn, prickle/lat.), the prickle-cell layer, is above the basal layer. In it are, visible for the first time, the keratinosomes, membrane-bounded vacuoles (Odland bodies). They contain the precursors of the epidermal lipids in the form of disk-like (lamellar) lipid bilayer membranes.

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. )

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.

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.

Granular layer (stratum granulosum)
Above the prickle-cell layer is the stratum granulosum (granula = grain/Lat.), where the cornification (keratinization) of the keratinocytes begins. It gets its name from its appearance, which is due to the presence of what are known as keratohyaline granules, a mixture of several smaller protein units. (Note: Besides keratohyaline, which is a precursor of keratin, the granules contain filaggrins - the intercellular cement of the skin structure.)

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.

Clear layer (stratum lucidium)
The stratum lucidium is also called the clear layer as it is highly refractive. The cells have been extremely flattened and are closely packed. The cell boundaries are no longer recognizable.

Also, the translucent or transitional layer, this is a translucent, thin layer of cells. This layer is sometimes visible in thick skin; however, nuclei and other organelles are not visible. The cytoplasm (the amorphous area between the nucleus and the outer membrane of the cell) is mostly made of keratin filaments.

Horny layer (stratum corneum)
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 permeability barrier, which is the skin's true barrier against exogeneous factors.

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.

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.

Scanning electron microscope image of scaling horny skin cells.

Differentiation and skin regeneration
Through differentiation, the living, cylindrical basal cells lose their nuclei and become flattened cornified cells, changing their shape and composition in the process. The cells pass through the barrier zone, the border zone between the living epidermal layers and the horny layer, where the epidermal lipids are released.

Did you know that 90% of household dust is dead skin cells? Keratinocytes contain structural protein (keratin) and become progressively flattened as they advance upward from the basal layer to the corneal layer. The epidermis renews itself every 28 days through continual reproduction, differentiation / cornification and desquamation (mechanical sloughing-off of the uppermost horny cell layer).

The epidermis is a stratified squamous epithelial tissue. This means that it has several layers of epithelial cells and that its outermost layer is made up of squamous (flat) epithelial cells.

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 time. It occurs primarily between midnight and 0400 hours.

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 become squamous in form, they also become hardened, or cornified, through the development of a special type of protein. As they approach the surface, they die. Thus, the outermost layers of the epidermis are dead, horny scales.

Keratinocytes

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).

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).

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. ).

The major proteins formed within keratinocytes are keratins (Table). Keratins are intermediate filament proteins that form the cytoskeleton of keratinocytes.Keratins are alpha-helical molecules and belong to 2 families: Type I (acidic keratins) and Type II (basic keratins). During keratin assembly, an acidic and basic keratin pair to form obligate heteropolymers which are then assembled into filaments. During epithelial differentiation the expression of keratins changes.

TABLE 1. Keratin Location

Type I (acidic)	Type II (basic)	Location
K10	K1	suprabasal epidermal keratinocytes
K9	K1	palmoplantar suprabasal keratinocytes
K10	K2e	granular layer of the epidermis
K12	K3	cornea
K13	K4	nonkeratinizing stratified squamous epithelia
K14	K5	basal layer keratinocytes
K15	K5	basal layer of non-keratinizing epithelia
K16	K6a	outer root sheath (hair), hyperproliferative keratinocytes, oral epithelium
K17	K6b	nail bed, myoepithelium, inflammatory conditions
	K7	various partners in transformed cells
K18	K8	simple epithelia
K19		bulge cells (hair follicle), simple epithelia
K21		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

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

Answers: D, C, D

Keratin

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°, 150A°, and 250A°, respectively. In addition, other intracytoplasmic filaments were noted, and since the diameter of these latter structures was found to be between 70 and 100A°, 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.

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.

Keratinocyte Maturation

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:

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.

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.

Melanocyte Function

http://sprojects.mmi.mcgill.ca/dermatology/physiology.htm#keratinocyte

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.

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.

The Horny Layer (Stratum Corneum)

1 Horny layer cells (corneocytes)
2 Epidermal lipids

The outermost layer of the epidermis - the horny layer - consists of a dense segment (pars compacta) with about 15 to 20 cell layers. The horny skin cells (corneocytes) are connected by a small number of desmosomes - protein-rich appendages of the cell membrane ("adhesive plates").

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 cement (brick and mortar model).

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 formed in the Golgi apparatus of the keratinocytes. Stored in these membrane-coated vacuoles - the Odland bodies - are the precursors of the skin-specific lipid barrier in the form of lamellar bilayer-lipid membranes. Through exocytosis, the contents of the Odland bodies are released into the extracellular space. Only then are the epidermal lipids formed: as a horny cell cement these bilayer-lipid membranes lend the horny layer stability.

At the same time these intercellular lipid membranes are the decisive permeability barrier of the horny layer: Regulation of the water and fluid content is its most important function, as elasticity and firmness of the horny layer depend on moisture content.

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 formed from high-molecular weight alcohols, primarily sphingosine, and various fatty acids such as linoleic acid.

Schematic diagram of the synthesis of epidermal lipids

1 Odland bodies
2 Stratum granulosum cells
3 Exocytosis
4 Bilayer lipid membrane
5 Stratum corneum cells

The permeability barrier
The epidermal lipids comprise 10 to 30 percent of the total volume of the horny layer (stratum corneum). That means they make up a 100 to 200 times more of the total volume of intercellular substance than that of other tissues. Accordingly, the horny layer makes an effective permeability barrier which fulfils two important functions: (Note: The acid pH of the skin plays an essential role in the creation of the permeability barrier.)

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 means of evaporation. The less intact the horny skin layer is, the higher the loss rate.)

If horny skin layers are removed and with them the epidermal lipids, the skin becomes more permeable to water (TEWL) and other substances, including toxins and allergens.

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 becomes increasingly fragile. The individual cells split apart from each other (pars disjunctiva), loosen and are sloughed off as scales. This unperceived, continuous process is called desquamation. An adult human sheds approximately 10 grams of skin scales a day.

The Dermis (Corium)

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).

The stratum papillare creates a well-defined, wave-shaped border to the epidermis.

1 Stratum papillare
2 Basal membrane
3 Basal cells
4 Epidermis

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 nourishment of the deepest layer of the epidermis - the basal cells - by way of the blood vessels running through the papillae.

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 meshwork contains a sort of "filling" made of long chains of sugar molecules (polysaccharides; poly = many, sacchar = sugar/Gk.). These are known as glycosaminoglycans (also mucopolysaccharides). With the help of fibronectins a type of "glue", they bind to the proteinous connective tissue matrix to form proteoglycans, which can bind water molecules. This gel-like mass functions like a sponge. Under pressure it can expel the bound water and in a reverse process take it up again. This process is the probable route of nourishment for the dermis. Hyaluronic acid (hyalo = glass/Gk.) belongs to the group of glycosaminoglycans and so contributes to the water-binding whole. Glycosaminoglycans are subject to a continuous waxing and waning. In contrast, the collagen fibres are only renewed when necessary, such as when injury is sustained.

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 times past. In the ear it forms a portion of what we see as earwax. It is also present under the arms, around the nipples and navel, and in the anal-genital area.

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 replacement of lost water can cause heat stroke. Eccrine glands sweat in response to physical activity and hot environments, but emotional stress and eating spicy foods can also cause perspiring.

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 mentioned previously.

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.

Epidermal Appendages

The epidermal appendages include the nails, hair and glands (glandulae cutis). They arise from invaginations of the epidermis into the dermis.

Schematic diagram of the follicle. The hair follicle and sebaceous gland form a structural and functional unit.

1 Hair shaft
2 Sebaceous gland
3 Bulbus
4 Hair papilla
5 Sweat gland

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 become a bulb (bulbus), which together with the underlying dermal hair papillae are responsible for the nourishment, development and growth of hair. A dermal sheath of connective tissue surrounds the whole hair root and together these form a hair follicle. The sebaceous glands open into the infundibular part of the hair follicles.

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 means they deliver their secretion directly to a surface such as the skin. In the case of the sebaceous glands this occurs with complete desintegration of lipid-rich cells. They are continually replaced through division of the basal cells (holocrine glands). In the case of apocrine glands, like the mammary glands or the sweat glands of the axilla (underarm), only the outer parts of the cell body are lost with the secretion. (Note: The secretion of the sebaceous and sweat glands contain important substances that help form the hydrolipid film.) The cells of the eccrine glands like the small sweat glands of the skin, show no loss of cytoplasm after the secretion process.

Together with the sweat glands, the sebaceous glands deliver vital substances, that - along with the epidermal lipids - form the hydrolipid film.

The Subcutis (Hypodermis)

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.

Nourishment, insulation and padding
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 acylglycerins, are the most plentiful and simplest fatty acid-containing lipids. They are esters of the triol alcohol, glycerine with three saturated and/or unsaturated fatty acids. Fats make up the main component of the fat depots.)

Fat distribution in men and women
The fat content of the subcutis is not the same in all body regions. Also men and women differ in the distribution of subcutaneous fat. An example is cellulite - it is characterized by a special arrangement of the subcutaneous fat tissue septa and predisposes to fat deposition on the hips, thighs and buttocks - which occurs mostly in women. Men on the other hand tend to store fat on the torso.

The fat content of the subcutis is not the same in all body regions. Also men and women show differing distributions of subcutaneous fatty tissues.

1 Adipocyte

SUMMARY

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 come the granular, clear and horny layers (stratum granulosum, lucidium and corneum) in that order. The horny layer consists of 15 - 20 cell layers that, together with the epidermal lipids, form the permeability barrier. This performs two important functions:

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 merges into the subcutis. The main constituents of the dermis are the proteinous connective tissue fibres which are connected to the glycosaminoglycans or mucopolysaccharides.

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 same time, the subcutis provides insulation and shock absorption.

References:

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 Kosmetik - Eine chemische Wechselwirkung. S. Hirzel Verlag Stuttgart *

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): Pflegekosmetik - Ein Leitfaden. Gustav Fischer Verlag Stuttgart, Govi- Verlag Frankfurt *

Schreiner, V., Maerker, U., Hoppe,U. (1995): Dependence of barrier repair in human skin on intra- and extracellular pH (poster). Gordon Conference, New York City.

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

Sandia National Laboratories

http://www.sandia.gov/media/NewsRel/NR2001/conjug.htm

color change

COLOR CHANGE — Thunderbird demonstrates how self-assembling nanostructures change color under certain stresses.

Conjugated polymers easily implanted in rigid structure

Intelligent nanostructures report on environment;
“nanoskin” may aid in inhabiting Mars

ALBUQUERQUE, N.M. — Intelligent nanostructures that report on their environment by changing color from blue to fluorescent red under mechanical, chemical, or thermal stress have been created by researchers at Sandia National Laboratories and the University of New Mexico.

Most immediately, the self-assembling structures — as durable as seashells — may lower costs by reducing the need for expensive manufactured devices like stress detectors, chemical analyzers, and thermometers. “The material can distinguish between different solvents,” says Sandia senior scientist and UNM professor Jeff Brinker. “There’s a high correlation of color with the polarity of the solvent.”

The material also can report changes in mechanical stress and temperature. When the environmental disturbance is removed, the structures change back to their original color in some cases, making them potentially reusable.

“The material is of interest to NASA — one of the sponsors of our research — 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 membrane with low permeability that could sense mechanical damage from hazards such as meteorites or sandstorms.”

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 method, which involves a technique that links monomers into polymers in an orderly fashion within a nanostructure, is published this week in the April 19 Nature.

Another possible use for the orderly arrangements is to form nanoscopic “wires” of organic polymers.

In seconds, robust housing for conjugated polymers
Underlying the immediate application described above, the Sandia/UNM method is a generic, efficient solution to a problem that has puzzled modern materials science: how to efficiently distribute conjugated polymers — inexpensive carbon-based polymers that due to special bonding patterns carry electrical current and produce changes in a material’s optical properties — within a hard, protective structure.

Conjugated polymers are prominent enough scientifically that the Nobel Prize was awarded this year to Alan J. Heeger (Univ. of Calif. at Santa Barbara), Alan G. MacDiarmid (Univ. of Pa.), and Hideki Shirakawa (Univ. of Tsukuba, Japan) for initially developing the field. In 1977, they oxidized polyacetylene (a solid polymer prepared from the flammable gas acetylene) with iodine to yield a material many times more electrically conductive than the untreated, semiconducting polyacetylene.

But a still-open question is how best to fashion a structure for these potentially useful but fragile extended molecules.

“Traditionally, bulk conjugated polymers are like a huge bowl of entangled spaghetti,” says Brinker. “Our method organizes this jumble by forcing them to adopt a particular conformation; that is, we organize them into nanostructures. We can force them into conformations, and so define where the polymer is and isn’t. Then we can control how interactions between polymer units will affect a material’s electrical and optical properties.”

A robust architecture that’s optically transparent and prevents oxidative degradation of the polymer can be patterned on surfaces and substrates.

“This is a simple means of forcing organization that should help us integrate conjugated polymers into devices,” says Brinker.

Conjugated polymers ‘while u wait’
It takes only seconds for the Sandia/UNM method to evenly pre-distribute monomers — simpler precursors of polymers — within a silica matrix through self-assembly. Exposure to UV light polymerizes the monomers into conjugated polymers housed in nanoscopic channels that penetrate the matrix of the material.

The result is a nanocomposite that is mechanically robust, optically transparent, and produces telltale changes of color under changing environmental conditions.

Technical discussion
Sandia researchers Alan Burns and Darryl Sasaki had characterized the responsiveness of two-dimensional films of these polymers to local stresses and temperature changes. However, their work, published last year in the American Chemical Society journal Langmuir, showed the organic materials to be “soft” and lacking the robustness required in harsh environments.

While this problem could be solved by incorporating the polymers in “hard” silica scaffolds, previous research groups at other institutions had found that implanting conjugated polymers into pre-existing silica structures to be a time-consuming, inaccurate, and relatively expensive process.

A significant step was performed by Sasaki, who was able to synthesize the precursor monomer of the polymer so that it became both substance and scaffold. His synthesized two-sided (detergent) molecule served as both the structure-directing agent for self-assembly and as the monomer of the conjugated polymer polydiacetylene. The method to self-assemble the detergent molecule with silica to form a nanocomposite was discovered by UNM postdoc Yi Yang and former Sandia postdoc and UNM graduate student Yunfeng Lu.

The self-assembly method is based on the scientifically well-known tendency of two-sided detergent molecules, composed of hydrophilic (water-loving) and hydrophobic (water-hating) portions, to spontaneously form spherical molecular assemblies and periodic three-dimensional nanostructures in solutions of water.

In the Sandia/UNM process, evaporation, exposure to ultraviolet light, and a low-temperature heat treatment polymerizes the organic surfactant monomers and the surrounding silica nanostructure. This process was used originally by Brinker and colleagues to fabricate a structure that mimicked the layered hard-soft construction of seashells.

In the current work, it is the polymers themselves — already evenly distributed through self-assembly — that are of interest. By compartmentalizing them within the aligned periodic pores of a silica nanostructure, it should be possible to align them and to control charge and energy transfer between them while providing mechanical and chemical stability.

Sandia’s Laboratory-Directed Research and Development program, the Department of Energy’s Office of Science, NASA and the Air Force Office of Scientific Research co-funded this work.

Thin films, nanoscopic spheres, intelligent ink, light-alterable pore sizes
The achievement is the group’s latest in making use of self-assembling two-sided molecules. The earliest, simplest version of the method was first reported in Naturein September 1997. In that paper the group described how detergent molecules, alcohol, silica, and water could be used to self-assemble a thin film with precisely defined pores for membranes, sensors, and low-k dielectrics.

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 medicine, intelligent ink that assembles during ink-jet printing, and self-assembled nanostructures with pore sizes alterable by light to a fineness of 0.2 angstroms.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy and environmental technologies, and economic competitiveness.