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Structure And Function

The epidermis is made up entirely of cells. The maincell of the epidermis is the keratinocyte which produceskeratin. In addition, melanocytes produce the black pigment of the skin, the melanin, while Langerhans cells areresponsible for phagocytosing the agents which invade theskin. The keratinocytes lie side by side in several layersthe number of such layers varying from region to region.The deeper-most layer of the epidermis is called basal celllayer or stratum germinativum and consists of a single layerof columnar cells. The cells in this layer keep dividing ata fixed rate and progressively move towards the surface.The layer superficial to the basal cell layer is called tprickle cell layer or the malpighian cell layer. This is madeup of several layers of cells which are polygonal in shapeand attached to each other as well as to the cells of thebasal cell layer by means of specialized cell-wall structurescalled desmosomes. In addition, these cells are coveredwith a proteinous substance called the intercellular cement.The cytoplasm of the basal cells as well as the prickle cellscontains very fine filamentous structures called tonofilaments which consist of long chains of aminoacids. Thesefilaments are attached to the desmosomes and form a crisscross pattern in the cytoplasm of the cells. Tonofilamentsdo not cross from one cell to the other but constitute a sortof skeleton for the cell. The cells in the most superficiallayer of the prickle cell layer develop small circular orgae called membrane coating granules or Odland bodieswhich progressively move towards and fuse with the cellmembrane and discharge their content in the intercellularspace. Superficial to the prickle cell layer lies the granularcell layer which consists of two layers of rhomboid shapedcells which are flatter than the cells of the prickle cell layer. These cells characteristically contain irregular shapedmasses of dark-staining material lying in between thetonofilaments. The tonofilaments progressively becomeparallel to each other, develop interfilamentous bonds andtogether with the cement substance constitute larger fibrilscalled tonofibrils.

The most superficial layer of the epidermis is calledthe corneal cell layer or the stratum corneum. It consistsof several layers of flat cells lying over each other. Thesecells have lost their nuclei and other cellular inclusions butcontain a protein called keratin which consists of bundlesof tonofibrils embedded in the interfibrillar cement substance. The deeper layers of stratum corneum are morecompact while the superficial layers are loose. The cellsin the superficial layer become fragmented and fall offfrom the surface: Thus there is a continuous outwardmovement of the epidermal cells, the new cells being generated in the basal cell layer and the old cells falling offrom the surface. This process is called epidermopoiesiswhich continues at a fixed rate. The time taken for anepidermal cell to move from the basal cell layer and falloff from the surface is approximately 28 days. This ratehowever gets modified if the skin gets injured and also insome disease processes. The entire process is under a localcontrol.

The continuous outward movement of the epidermalcells constitutes a barrier for the agents which tend topenetrate the skin from outside. The stratum corneum alsoconstitutes a mechanical barrier against the invadingorganisms unless it has been destroyed by injury or disease.Stratum corncum in addition is a highly selective barrierfor chemicals in as much as it does not allow even waterto penetrate the skin from outside while it can allow certainlarger molecules to penctrate the intact skin.

The dermis can be broadly divided into two layers, themore superficial papillary dermis, and the deeper layercalled reticular dermis. The major component of dermisis the collagen fibres which lie singly or in smaller bundlesin thc papillary dermis, while the lower part of the dermiscontains thick and larger bundles lying in a criss-crospattern. Another important component of the dermis is theelastic fibres which form a horizontal network at thejunction of the papillary dermis with the reticular dermis,from where smaller elastic fibres rise vertically towards theepidermis. The collagen and the elastic fibres are embedded in a ground substance made-up of water, clectrolytesand proteoglycans (mucopolysaccharides). The proteoglycans in dermis consist of hyaluronic acid, dermatansulphate (chondroitin sulphate B), chondroitin 6-sulphate(chondroitin sulphate C) and heparansulphate.
In addition,the dermis contains the cellular elements which are mainlyof three types:

  • 1] Fibroblasts which are spindle shaped cells with acentral oval nucleus. These cells are concemed with theformation of collagen and elastic fibres.
  • 2] Histiocytes which are amoeboid in shape and concerned with phagocytosis and immunologic functions.
  • 3] Mast cells which contain the mast cell granules and canrelease histamine and several other substances with Ipotent biologic functions. These cells have a role in ninflammatory, immunologic and repair mechanisms.

The dermis is also richly supplied with blood vessels which form one dense network at the level of the reticular ncrmis, and a second network at the junction of the fiapillary dermis with the reticular dermis. These two plnetworks are inter-connected by means of vertical bloodvessels. The papillary portion of the dermis is nourishedby the vertical vessels which arise from the superficialplexus and form a loop in each of the dermal papillae. Thevenous blood is returned by the corresponding veins whichrm venous plexuses around the arterial plexuses. Thee no blood vessels in the epidermis which is nourishedby the seepage of nutrients from the dermis through thedermo-epidermal junction.

The skin is also supplied with sensory nerves which are present in between the epidermal cells as free nerveendings and also as specialised nerve endings in the dermand around the cutaneous appendages. These nerveendings subserve the sensations of touch, pain, temperature, wetness and itch. The autonomic nerves innervate theblood vessels and the sweat glandsThe subcutaneous tissue consists mainly of fat cells andforms a padding between the dermis and the underlyingstructures. The fat cells are circular cells, which inhistopathological sections appear as closely packed emptysacs. The nucleusof the cell lies alongside the cell wall.

The interphase between the dermis and the epidermisthe dermo-epidermal junction is a specialised structure. The upper surface of the dermis has numcrousprojcctionscalled dermal papillae which project into the under surfaceof the epidermis. A vertical scction through the skintherefore shows the dermo-epidermal junction to be wavy. The corresponding epidermal projections into the dermisare called rete pegs or rete ridges. This arrangement helpsto strengthen the dermo-epidermal junction. This interphase is further strengthened by a basement membranezone which is constituted by the plasma membrane of theells of the basal cell laycr. This membrane has structureslike desmosomes which serve to attach the lower surfacesof the basal cells to the dermis. These structures are calledhemi-desmosomes because they do not have the secondcomponent from the adjoining cell. The basement membrane is a homogenous layer rich in collagen III. Thismembrane has an upper electron translucent zone calledlamina lucida and a lower electron dense layer called thelamina densa. The fourth component of the basementmembrane zone is the sub-lamina densafibrillar zonewhich contains the anchoring fibrils, microfibrils and singlecollagen fibres. These fibrils traverse through the basementmembrane zone and provide attachment between the elasticfibres in the dermis and the hemi-desmosomes on theplasma membrane on the lower surface of the basal cells.

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