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 fi