The epidermis is formed from many layers of closely packed cells, the most superficial of which are flattened and filled with keratins; it is therefore a stratified squam-ous epithelium. It adheres to the dermis partly by the interlocking of its downward projections (epidermalridges or pegs) with upward projections of the dermis(dermal papillae) (Fig. 2.1).
The epidermis contains no blood vessels. It varies in thickness from less than 0.1 mm on the eyelids to nearly 1 mm on the palms and soles. As dead surface squames are shed (accounting for some of the dust in our houses), the thickness is kept constant by cells dividing in the deepest (basal or germinative) layer. A generated cell moves, or is pushed by underlying mitotic activity, to the surface, passing through the prickle and granular cell layers before dying in the horny layer. Thejourney from the basal layer to the surface (epidermal turnover or transit time) takes about 60 days. During this time the appearance of the cell changes. A vertical section through the epidermis summarizes the life history of a single epidermal cell (Fig. 2.2).
The basal layer, the deepest layer, rests on a base-ment membrane, which attaches it to the dermis. It is a single layer of columnar cells, whose basal surfaces sprout many fine processes and hemidesmosomes, anchoring them to the lamina densa of the basement membrane.
In normal skin some 30% of basal cells are prepar-ing for division (growth fraction). Following mitosis, a cell enters the G1 phase, synthesizes RNA and protein, and grows in size (Fig. 2.3). Later, when the cell is triggered to divide, DNA is synthesized (S phase) and chromosomal DNA is replicated. A short postsynthetic (G2) phase of further growth occurs before mitosis (M). DNA synthesis continues through the S and G2 phases, but not during mitosis. The G1 phase is then repeated, and one of the daughter cells moves into the supra-basal layer. It then differentiates (Fig. 2.2), having lost the capacity to divide, and synthesizes keratins. Some basal cells remain inactive in a so-called G0 phase but may re-enter the cycle and resume proliferation.
The cell cycle time in normal human skin is controversial; estimates of 50–200 h reflect differing views on the duration of the G1 phase. Stem cells reside amongst these basal cells and amongst the cells of the external root sheath of the hair follicle at the level of attachment of the arrector pili muscle but cannot be identified by histology. These cells divide infrequently, but can generate new proliferative cells in the epidermis and hair follicle in response to damage.
The spinous or prickle cell layer (Fig. 2.4) is composed of keratinocytes. These differentiating cells, which synthesize keratins, are larger than basal cells. Keratinocytes are firmly attached to each other by small interlocking cytoplasmic processes, by abundant desmosomes and by an intercellular cement of glycoproteins and lipo-proteins. Under the light microscope, the desmosomes look like ‘prickles’. They are specialized attachment plaques that have been characterized biochemically. They contain desmoplakins, desmogleins and desmocollins. Autoantibodies to these proteins are found in pemphigus, when they are responsible for the detachment of keratinocytes from one another and so for intraepidermal blister formation. Cytoplasmic continuity between keratinocytes occurs at gap junctions, specialized areas on opposing cell walls. Tonofilaments are small fibres running from the cytoplasm to the desmosomes.
They are more numer-ous in cells of the spinous layer than of the basal layer, and are packed into bundles called tonofibrils. Many lamellar granules (otherwise known as membrane-coating granules, Odland bodies or keratinosomes), derived from the Golgi apparatus, appear in the super-ficial keratinocytes of this layer. They contain poly-saccharides, hydrolytic enzymes and, more importantly, stacks of lipid lamellae composed of phospholipids, cholesterol and glucosylceramides. Their contents are discharged into the intercellular space of the granular cell layer to become precursors of the lipids in the intercellular space of the horny layer (see Barrierfunction below).
Cellular differentiation continues in the granular layer, which normally consists of two or three layers of cells that are flatter than those in the spinous layer, and have more tonofibrils. As the name of the layer implies, these cells contain large irregular basophilic granules of keratohyalin, which merge with tonofibrils. These keratohyalin granules contain proteins, includ-ing involucrin, loricrin and profilaggrin, which is cleaved into filaggrin by specific phosphatases as the granular cells move into the horny layer.
As keratinocytes migrate out through the outer-most layers, their keratohyalin granules break up and their contents are dispersed throughout the cytoplasm, leading to keratinization and the formation of a thick and tough peripheral protein coating called the hornyenvelope. Its structural proteins include loricrin andinvolucrin, the latter binding to ceramides in the sur-rounding intercellular space under the influence of transglutaminase. Filaggrin, involucrin and loricrin can all be detected histochemically and are useful as markers of epidermal differentiation.
The horny layer (stratum corneum) is made of piled-up layers of flattened dead cells (corneocytes)a the bricksastuck together by lipidsathe mortarain the intercellular space. The corneocyte cytoplasm is packed with keratin filaments, embedded in a matrix and enclosed by an envelope derived from the keratohyalin granules. This envelope, along with the aggregated keratins that it encloses, gives the corneocyte its tough-ness, allowing the skin to withstand all sorts of chem-ical and mechanical insults. Horny cells normally have no nuclei or intracytoplasmic organelles, these having been destroyed by hydrolytic and degrading enzymes found in lamellar granules and the lysosomes of granular cells.
All cells have an internal skeleton made up of microfila-ments (7 nm diameter; actin), microtubules (20–35 nm diameter; tubulin) and intermediate filaments (10 nm diameter). Keratins (from the Greek keras meaning ‘horn’) are the main intermediate filaments in epithe-lial cells and are comparable to vimentin in mesenchy-mal cells, neurofilaments in neurones and desmin in muscle cells. Keratins are not just a biochemical curiosity, as mutations in their genes cause a number of skin diseases including simple epidermolysis bul-losa and bullous ichthyosiform erythroderma.
The keratins are a family of more than 30 proteins, each produced by different genes. These separate into two gene families: one responsible for basic and the other for acidic keratins. The keratin polypeptide has a central helical portion with a non-helical N-terminal head and C-terminal tail. Individual keratins exist in pairs so that their double filament always consists of one acidic and one basic keratin polypeptide. The inter-twining of adjacent filaments forms larger fibrils.
Different keratins are found at different levels of the epidermis depending on the stage of differenti-ation and disease; normal basal cells make keratins 5 and 14, but terminally differentiated suprabasal cells make keratins 1 and 10 (Fig. 2.2). Keratins 6 and 16 become prominent in hyperproliferative states such as psoriasis.
During differentiation, the keratin fibrils in the cells of the horny layer align and aggregate, under the influence of filaggrin. Cysetine, found in keratins of the horny layer, allows cross-linking of fibrils to give the epidermis strength to withstand injury.
Firm cohesion in the spinous layer is ensured by ‘stick and grip’ mechanisms. A glycoprotein intercellular sub-stance acts as a cement, sticking the cells together, and the intertwining of the small cytoplasmic processes of the prickle cells, together with their desmosomal attachments, accounts for the grip. The cytoskeleton of tonofibrils also maintains the cell shape rigidly.
The typical ‘basket weave’ appearance of the horny layer in routine histological sections is artefactual and deceptive. In fact, cells deep in the horny layer stick tightly together and only those at the surface flake off; this is in part caused by the activity of cholesterol sulphatase. This enzyme is deficient in X-linked recessive ichthyosis, in which poor shedding leads to the piling up of corneocytes in the horny layer. Desquamation is normally responsible for the removal of harmful exogenous substances from the skin surface. The cells lost are replaced by newly formed corneocytes; regeneration and turnover of the horny layer is therefore continuous.
The horny layer prevents the loss of interstitial fluid from within, and acts as a barrier to the penetration of potentially harmful substances from outside. Solvent extraction of the epidermis leads to an increased per-meability to water, and it has been known for years that essential fatty acid deficiency causes poor cutan-eous barrier function. These facts implicate ceramides, cholesterol, free fatty acids (from lamellar granules;), and smaller quantities of other lipids, in cutaneous barrier formation. Barrier function is also impaired when the horny layer is removed experiment-ally, by successive strippings with adhesive tape, or clinically, by injury or skin disease. It is also decreased by excessive hydration or dehydration of the horny layer and by detergents.
The rate of penetration of a substance through the epidermis is directly proportional to its concentration difference across the barrier layer, and indirectly pro-portional to the thickness of the horny layer. A rise in skin temperature aids penetration. A normal horny layer is slightly permeable to water, but relatively impermeable to ions such as sodium and potassium. Some other substances (e.g. glucose and urea) also penetrate poorly, whereas some aliphatic alcohols pass through easily. The penetration of a solute dis-solved in an organic liquid depends mainly on the qualities of the solvent.
Both the thickness of the normal epidermis, and the number of cells in it, remain constant, as cell loss at the surface is balanced by cell production in the basal layer. Locally produced polypeptides (cytokines), growth factors and hormones stimulate or inhibit epidermal proliferation, interacting in complex ways to ensure homeostasis. Cytokines and growth factors (Table 2.2) are produced by keratinocytes, Langerhans cells, fibroblasts and lymphocytes within the skin. After these bind to high affinity cell surface receptors, DNA synthesis is controlled by signal transduction, involving protein kinase C or inositol phosphate. Catecholamines, which do not penetrate the surface of cells, influence cell division via the adenosine 3′, 5′-cyclic monophosphate (cAMP) second messenger system. Steroid hormones bind to receptor proteins within the cytoplasm, and then pass to the nucleus where they influence transcription.
The steroid 7-dehydrocholesterol, found in ker-atinocytes, is converted by sunlight to cholecalciferol. The vitamin becomes active after 25-hydroxylation in the kidney. Lack of sun and kidney disease can both cause vitamin D deficiency and rickets.
Keratinocytes make up about 85% of cells in the epidermis, but three other types of cell are also found there: melanocytes, Langerhans cells and Merkel cells. (Fig. 2.5).Melanocytes are the only cells that can synthesize melanin. They migrate from the neural crest into the basal layer of the ectoderm where, in human embryos, they are seen as early as the eighth week of gestation. They are also found in hair bulbs, the retina and pia arachnoid. Each dendritic melanocyte associates with a number of keratinocytes, forming an ‘epidermal melanin unit’ (Fig. 2.5). The dendritic processes of melanocytes wind between the epidermal cells and end as discs in contact with them. Their cytoplasm contains discrete organelles, the melanosomes, containing vary-ing amounts of the pigment melanin (Fig. 2.6).
The Langerhans cell is a dendritic cell (Figs 2.5 and 2.7) like the melanocyte. It also lacks desmosomes and tonofibrils, but has a lobulated nucleus.
The specific granules within the cell look like a tennis racket when seen in two dimensions in an electron micrograph (Fig. 2.8), or like a sycamore seed when reconstructed in three dimensions. They are plate-like, with a rounded bleb protruding from the surface.
Langerhans cells come from a mobile pool of pre-cursors originating in the bone marrow. There are approximately 800 Langerhans cells per mm2 in human skin and their dendritic processes fan out to form a striking network seen best in epidermal sheets (Fig. 2.7). Langerhans cells are alone among epidermal cells in possessing surface receptors for C3b and the Fc por-tions of IgG and IgE, and in bearing major histocompat-ibility complex (MHC) Class II antigens (HLA-DR, -DP and -DQ). They are best thought of as highly spe-cLangerhans cells have a key role in many immune reactions. They take up exogenous antigen, process it and present it to T lymphocytes either in the skin or in the local lymph nodes. They probably play a part in immunosurveillance for viral and tumour antigens. In this way, ultraviolet radiation can induce skin tumours both by causing mutations in the epidermal cells, and by decreasing the number of epidermal Langerhans cells, so that cells bearing altered antigens are not recognized or destroyed by the immune system. Topical or systemic glucocorticoids also reduce the density of epidermal Langerhans cells. The Langerhans cell is the principal cell in skin allo-grafts to which the T lymphocytes of the host react during rejection; allograft survival can be prolonged by depleting Langerhans cells.
Merkel cells are found in normal epidermis (Fig. 2.5) and act as transducers for fine touch. They are non-dendritic cells, lying in or near the basal layer, and are of the same size as keratinocytes. They are con-centrated in localized thickenings of the epidermis near hair follicles (hair discs), and contain membrane-bound spherical granules, 80–100 nm in diameter, which have a core of varying density, separated from the membrane by a clear halo. Sparse desmosomes connect these cells to neighbouring keratinocytes. Fine unmyelinated nerve endings are often associated with Merkel cells, which express immunoreactivity for various neuropeptides .
The skin appendages are derived from epithelial germs during embryogenesis and, except for the nails, lie in the dermis. They include hair, nails and sweat and sebaceous glands.