Stratified squamous epithelial cells

The epithelium (the biological and medical collective term for covering and glandular tissues) is composed of layers of cells that line the outside and inside surfaces of organs. The outermost layer of the skin is composed of stratified squamous epithelial cells, while other epithelial cells line the insides of the respiratory, gastrointestinal, reproductive and urinary tracts, and also comprise the exocrine and endocrine glands. The functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection and selective permeability. The endotheUum-the irmer lining of blood and lymph vessels-is a specialized form of epithelium. 

The skin is composed of two distinct but intimately associated layers of tissues, namely the epidermis and dermis (Fig. 1). The epidermis is further divided into the viable epidermis (a layer of stratified squamous epithelial cells in various stages of differentiation) and the stratum comeum (a layer of dead cells that form the outer layer of skin). In contrast to most tissues, the epidermis is in a state of flux. The cells at the surface of the skin are replaced continuously by cells migrating up from the lower levels of the skin in a process known by several names, including keratinization, comification, or differentiation. 

Stratified squamous epithelial tissue is a frequently found epithelium in the human body (nasopharyngeal and oral cavities, esophagus, urethra, vagina, cervix, and others). It is a multilayered structure consisting of a layer of actively dividing basal cells anchored to the basement membrane, beneath which one finds connective tissue (stroma). The daughter cells created by the division of the basal cells form the parabasal layer and mature and migrate to the surface layer. 

A single layer of epithelial cells is termed simple epithelium, whereas those composed of more than one layer are termed stratified epithelia. Stratified epithelia are found in areas which have to withstand large amounts of wear and tear, for example the inside of the mouth, orthe skin. Epithelial cells may be, for example, squamous (flattened), columnar (tall), cuboidal (intermediate between squamous and columnar) and may contain surface specializations, such as cilia in the nasal epithelium and keratin in the skin. 

In common with all epithelial interfaces (see Section 1.3.2), the epithelium of the oral cavity is supported by a basement membrane, which separates the epithelium from the underlying connective tissue layer (the lamina propria) (Figure 7.1). Oral epithelium is broadly similar to stratified squamous epithelia found elsewhere in the body, for example the skin (see Section 8.2.1), in that cells are produced by mitosis in the basal layer of the epithelium and these proliferating cells push existing cells towards the surface. The phases of this dynamic process are represented in four morphological layers ... 

The epithelium is stratified and composed of five to seven layers of interconnected squamous cells of various types, sizes, and shapes The deepest layer of epithelial cells. 

The human nasal cavity, bearing a total surface area of about ISOcm and a total volume of about 15mL, is divided by a midline septum into two non-connected parts. As a cross-sectional view is schematically shown in Fig. lA, the nasal cavity consists of several major differentiated regions. " The nasal vestibule is situated just inside of the nostrils, with an area of about 0.6 cm. The epithelial cells in this region are stratified, squamous, and keratinized. The atrium located at the back of the vestibule is the narrowest region, and has stratified squamous cells anteriorly and pseudostrati-fied cells with microvilli posteriorly. The olfactory... 

The ultrastructures of the taste buds of the above mentioned species are now compared to each other. With the exception of the catfishes (where the taste buds are located also in the body s outer skin, especially on the barbels) taste buds occur only in the lips and oropharyngobranchial cavity. In each case, taste buds consist of an elongated, pear-shaped sensory epithelium that is composed of different electron-lucent and electon-dense cells and that rests in a vertical position in the stratified squamous epithelium. At their apical poles these cells bear microvilli, the so-called receptor villi, which form together the receptor area of the taste buds. The border with the adjacent and non-specialized epithelial cells is formed by marginal cells. Besides this relatively big uniformity of the taste buds sensory epithelium, the taste buds of different fish vary in their innervation, in the number, morphology and position of their basal cells, and their position within the epidermis and on the top of a dermal papilla. The results of this comparison are listed in Table 1 and schematically de-... 

While epithelia are basically simple arrangements made up of single (in simple epithelia) or several (in stratified epithelia) layers of characteristically shaped, e.g. cuboidal, columnar or squamous (i.e. flattened) cells, they are capable of modification to serve a variety of functions in different organs. Thus the epithelial cells in the small intestine have numerous finger-like microvilli projecting from their surfaces. These microvilli greatly increase the surface area and are correlated with the absorptive function of the tissue. 

From a cellular viewpoint, the respiratory tract is extraordirrarily complex, consisting of more than 40 different individual cell types. The airways of the nasopharyngeal and tracheobronchial compartments are lined by epithelial cells. The most anterior portion is lined by stratified squamous epithelium that in a short distance transitions to respiratory epithelium, with numerous ciliated cells and occasional goblet cells that serve as a source of mucus. 

Squamous cell carcinomas of different sites of origin are generally characterized by a predominance of stratified-epithelial/keratinocyte-type keratins but may coexpress certain simple-epithelial keratins (for details, see Moll 1998). Most of these... 

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