Epithelium

Chapter 26

Epithelium

Epithelia are essentially surface tissues of ectodermal origin. The epidermis covering the surface of the skin on the outside of the body is a typical epithelium. However, epithelia cover the surfaces of other tissues and are not necessarily confined to the outside of the animal; oral epithelium and the epithelial lining of the intestine are examples of epithelia that line the wall of the alimentary canal and are kept moist by the secretion of mucous glands embedded in them. The functions of an epithelium are those that are clearly related to a superficial tissue, namely protection, absorption and secretion. The relative degree of development of these three functions varies considerably in different epithelia so that it is possible to classify these tissues according to which function predominates.

Protective epithelia

The best known protective epithelia are the epidermis of the skin together with related structures such as nails, hair and feathers, as well as analogous tissues, including some of specifically dental interest, such as the oral and gingival epithelium. Protective epithelia are often keratinized though not necessarily so. Thus, while the masticatory gingival epithelium is covered by a layer of keratin, the neighbouring crevicular gingival epithelium and the alveolar mucosal epithelium are not.

Epithelia usually form a covering to connective tissue from which they are separated by a basement membrane (Figure 26.1). Since epithelium has no blood supply of its own, it derives its nutrients from, and disposes of its waste products into, the neighbouring connective tissue which contains capillaries. There is considerable interaction between epithelial and connective tissue cells which influence each other’s division and differentiation and this interaction presumably takes place across the basement membrane. This structure stains strongly with the periodic acid–Schiff reagent which indicates the presence of glycoprotein which contains unsubstituted sugar residues. The basement membrane is shown by the electron microscope to consist of two distinct parts, a deeper layer consisting of reticulin and a more superficial part, which can in turn be divided into the lamina densa and the lamina lucida. Histologically, reticulin appears as a fine branching network which stains black when treated with solutions containing silver salts, probably as a result of the reducing behaviour of sugar residues in glycoprotein.

Chemical characterization of reticulin is difficult because it usually occurs rather sparsely and mixed with a variety of other tissue components. However, purified reticulin from the cortex of human kidneys has been shown to consist of a combination of collagen, carbohydrate (probably glycoprotein) and possibly lipid. It is thus a complex made up of high molecular weight substances. The lamina densa may influence the transfer of molecules between connective tissue and epithelium and hence would be expected to exert control over the epithelium. It has been suggested that both connective tissue and epithelium collaborate in building up the basement membrane and that the epithelial cells behave atypically by secreting a proline-rich protein into its outer layers.

The layer of epithelial cells in contact with the basement membrane is known as the stratum basale or germitivum. These basal cells undergo division to replace those that are continually shed from the outer surface of the epithelium (Figure 26.1). Differentiation also begins in the basal layer, most probably as a separate process. As cells differentiate they move away from the basement membrane and become flattened in a direction at right angles to their movement. The rate of cell division in the basal layer balances the rate at which cells are shed from the surface, and it has been suggested that this is controlled by a feedback mechanism involving a water-soluble heat-labile substance or chalone (page 366) produced in the outer layers which suppresses mitosis in the stratum basale.

As the cells are pushed towards the surface by cell multiplication in the basal layer, changes occur in their morphology. In both keratin-forming and non-keratinizing epithelia a prickle cell layer is first formed known as the stratum spinosum. In this layer the cells of the non-keratinizing crevicular gingival epithelium are slightly smaller than those in the same layer of the keratinizing masticatory gingival epithelium. The polygonal cells in this non-keratinizing epithelium then undergo a rapid further transition to a final layer of flattened cells parallel with the tooth surface. The attachment to the tooth enamel is by means of a glycoprotein ‘glue’ rather than by any visible structural element. Keratinization, on the other hand, involves a further series of changes affecting both cell morphology and the physical and chemical properties of the predominant proteins.

Keratinization

In epithelia which are about to keratinize, the prickle cell layer merges into the flatter stratum granulosum in which keratohyalin granules appear in the cytoplasm. The final stage of differentiation results in the formation of the stratum corneum, which is the keratin layer itself. It contains fully cornified, shrunken, flattened and featureless cells and is sharply demarcated from the underlying granular layer.

The systematic formation of keratin as an end product of many protective epithelia suggests that it is a highly organized process but, although the general nature of the changes that take place is known, the details and, in particular, the mechanism by which such changes occur are not understood. The primary function of keratin is to protect the surface with a layer that is physically strong, coherent and relatively impervious to both microorganisms and substances in solution. The chief difference between keratins and the underlying connective tissue proteins is that the structural elements of keratins are intracellular, whereas those of connective tissues are extracellular.

The keratin molecule in its native or α-keratin form is essentially long, thin and highly orientated and, as revealed by X-ray diffraction studies, contains long stretches of α-helix (page 57

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Dec 10, 2015 | Posted by in General Dentistry | Comments Off on Epithelium
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