Epithelium

Epithelium (from Greek epi- on and thele- nipple), a term introduced by Ruysch (1703) that originally designated the outer covering of the nipple. Subsequently, the term "Ep." came to denote very diverse histological structures consisting of cells, for the most part densely joined to one another in solid complexes, and having the character of layers that cover the external surface of the body or form the lining of tubular passages and cavities of the organism that have communication with the external environment or had such communication at certain stages of their development. In multicellular organisms, Ep. includes a number of tissues that make up and to a large extent determine the morphological and functional significance of many vital organs: the skin, digestive, respiratory, and excretory apparatuses, genital tracts, body cavity, and a number of endocrine organs. At the same time, epithelial tissues develop from certain embryonic rudiments—the ectoderm, rudiment of the anterior intestine, entoderm, and parts of the mesoderm that give rise to excretory organs and the wall of the body cavity with some derivatives. Epithelial tissues have primarily the functional significance of histological structures that, on one hand, ensure connection between the organism and the environment (absorption, secretion, excretion, transmission of irritations, etc.), and on the other hand, have covering, delimiting, or protective significance. Cellular complexes of epithelium may secondarily lose their connection with the external environment and with those covering layers from which they formed, become completely immersed in the internal environment of the organism, and enter into the composition of various endocrine organs. One of the characteristic features of epithelial layers is polarity, i.e., the morphological and functional inequality of their so-called basal surfaces, bordering the internal environment of the organism and resting on connective tissue, and free or apical surfaces; this polarity is manifested for the most part also in the internal structure of individual elements. In elements of epithelial complexes that have lost their delimiting significance and become immersed in the internal environment of the organism, polarity may be weakened or not expressed at all. For designating the form and mutual arrangement of epithelial cells, there exists a certain terminology. Epithelial cells may be flat; their height may be equal to their width (cubical, or more correctly, isoprismatic cells) or exceed the latter (cylindrical, or more correctly, high-prismatic cells). In addition, one can speak of conical, or pyramidal, spindle-shaped, polyhedral (many-sided) cells, etc. Cells usually have one nucleus of round, oval, or disk-shaped form; in their cytoplasm are located organelles (Golgi apparatus, chondriosomes, cell center). In epithelial layers, phenomena of wear, death, and desquamation of highly differentiated cells continuously occur, as well as compensatory reproduction of less differentiated (so-called cambial) cells. Among elements of epithelial complexes, cells may be encountered in greater or lesser quantity in which certain inclusions accumulate in their protoplasm, for example, secretory inclusions in glandular cells, pigment grains, etc. The mutual arrangement of cells in layers may be single-layered, single-rowed, multirowed, or multilayered. In so-called single-layered epithelia, which in turn may be flat, iso- or high-prismatic, all cells of the layer have a free surface and a surface resting on connective tissue; such epithelia, consisting of cells of approximately the same shape, are also called isomorphic. Multirowed and multilayered Ep., on the contrary, are anisomorphic, i.e., formed from elements of different shapes. Multirowed epithelia consist of long and short cells; the former with their ends enter into the composition of both the basal and free surfaces of the layer; in the latter, only one end (more often basal, more rarely apical) enters into the composition of the surface of the layer, while the other ends in the thickness of the latter. Multilayered epithelia consist of basal elements lying on the border with connective tissue and covering elements forming the free surface of the layer. Between the former and latter are located, forming a varying number of layers, intermediate cells not in contact with the surfaces of the layer. In multilayered and multirowed Ep., basal elements have the significance of cambial elements, while elements entering into the composition of the free surface, on the contrary, are usually the most differentiated and often even lose the ability to reproduce. Elements of multilayered epithelia are connected with one another by means of so-called intercellular protoplasmic bridges, fragments of which on artificially isolated cells have the appearance of short spine-like processes. Cells provided with bridges of the intermediate layers of multilayered Ep. received the name of spinous or prickly. When characterizing multirowed and multilayered Ep., usually only the form of cells forming their free surface is taken into account, for example, one says: multirowed prismatic, multilayered flat, multilayered prismatic Ep. Epithelial layers, the free surfaces of which are formed by flat cells of regular polygonal shape, are called pavement-like. A special variety of anisomorphic Ep. is represented by the so-called transitional Ep., consisting of two or more cellular layers, having among covering cells characteristic multinuclear elements and changing, for example in the urinary bladder, its thickness depending on the degree of stretching of the organ wall. In addition, the total thickness of the epithelial layer is sometimes designated by special terms (Merkel); very thin layers are called leptodermal, layers of medium thickness—metriodermal, thick—batidermal. Under the basal surface of epithelial elements, which is often provided with protoplasmic spines or teeth, one can notice more or less distinctly the so-called basal, or basement, membrane. It represents a network of thin fibers, apparently derivatives of connective tissue, although according to some authors the possibility of some participation of epithelial cells in its formation is not excluded. Free surfaces of epithelial layers or tubes as a whole or of individual cells entering into their composition may exhibit various differentiations: closing strips or plates, cilia, various types of cuticular and horny formations, as well as often less constant structures connected, for example, with the secretory activity of cells. Closing (also cementing) strips may be observed between the free ends of epithelial cells; they probably ensure a firm mutual connection of free cellular ends and actually close from the outside capillary intercellular spaces between the surfaces of epithelial elements facing each other. Cilia are hair-like protoplasmic processes, usually beginning from so-called basal bodies and in continuous vibratory movement, which causes either the flow of liquid on the free epithelial surface or the movement of an organism clothed with ciliated epithelium in a liquid medium. Cuticles represent products of transformation or secretion of the protoplasm of epithelial cells and may have very different structures, thicknesses, and functional significance. Horny formations (see Skin, Hair, Nail), characteristic only of multilayered Ep. of higher vertebrates, consist of dead epithelial cells, the protoplasm of which has undergone a specific horny degeneration, and have protective significance. The level of differentiation of the above-mentioned epithelial structures and the character of their constituent elements may change under experimental conditions; in this case, between some structures, related features may be discovered, between others—qualitative differences. Until recently, the concept of Ep. and the classification of epithelial tissues had a completely formal character and were based exclusively on the above-mentioned structural features. It is more correct, however, to speak of several types of tissues of "epithelial appearance," each of which is characterized by a combination of its biological peculiarities, i.e., has its phylogenetic and ontogenetic history of development, and is capable of certain morphological and functional transformations. 1. Epidermal tissues, differing in the greatest diversity of structures, are in different animals at very different levels of development and enter into the composition of the skin, anterior, and sometimes posterior sections of the digestive apparatus, respiratory apparatus, as well as in some cases excretory and genital apparatuses. Most primitive are probably the single-layered—ciliated epithelia of the skin of many invertebrates. A more differentiated character is possessed by single-layered prismatic Ep. of the skin, provided with a continuous protective cuticle (for example, many annelids). In the composition of such Ep., nerve cells are often encountered, the basal end of which continues into a proper nerve fiber. Such relationships have been preserved in vertebrates in the so-called olfactory Ep.

Cuticular epithelia reach their highest development in arthropods, forming a chitinous carapace-like covering. In vertebrates, primitive forms include the single-layered and multi-rowed ciliated prismatic epithelia of the respiratory passages, as well as, for example, the amphibian esophagus. This is followed by the non-keratinized multi-layered epithelia rich in glandular cells of the skin and the anterior part of the digestive tract of many lower vertebrates, and finally the multi-layered flat epithelium of the skin, oral cavity, and esophagus of higher vertebrates with more or less pronounced keratinization. Glands belonging to the epidermal type in vertebrates have a two-rowed epithelial lining. The evolution of the cutaneous epithelial cover is characterized by the acquisition of increasingly pronounced protective properties. In mammals, this group also includes the multi-layered flat epithelium of the vagina, the transitional epithelium of the urinary passages, and the two-rowed epithelium of the seminal ducts. 2. Digestive and absorptive enterodermal tissues, which make up the gastrointestinal section of the digestive apparatus, have entodermal origin and consist of a single-layered high prismatic or (in corresponding glands) also isoprismatic, and in places even flat epithelium. The lining of the digestive tube itself in vertebrates consists of absorptive cells, each provided with a separate cuticular border, and glandular elements that are partly protective (for example, mucous goblet cells) and partly of a digestive nature. In many invertebrates and for example in lampreys, ciliated cells are also included in the composition of enterodermal tissues. In the intestine, certain cambial areas of the epithelium can be noticed in depressions of various shapes. 3. Nephrodermal tissues are represented by the excretory epithelia of nephridia or kidneys (see). In mammals, they are possibly related to the single-layered, partly ciliated epithelia of the uterus and oviducts. 4. Coelodermal tissues unite in vertebrates genetically related mesothelium of serous membranes, follicular and Sertoli cells of the gonads, and cells of the interrenal organs. All are characterized by a more or less far advanced loss of demarcation properties and polarity. All the listed tissue types differ in a high degree of specificity, at least in highly organized animals, such as vertebrates, and are not capable of transforming into each other or into any other tissues. How to view such epithelial-like tissues as the ependyma, the pigment layer of the retina of the eye, and the covering of the vascular plexuses of the brain, which develop from the same rudiment as nerve elements and are related to neuroglia, is difficult to say for now. In any case, designating them with the term 'epithelium' says nothing about their histological nature. The so-called endothelium (see) of the vascular system has nothing in common, except for a very superficial external resemblance, with the tissues described above and, as a mesenchymal derivative, belongs to the same group of tissues of the internal environment as blood.

N. Khlopin.

Pathological changes in the epithelium are quite diverse and consist either in degenerative-destructive processes or in hyperplastic, neoplastic ones. The first category includes the so-called desquamative phenomena (see Desquamation) of mucous membranes, vacuolar degeneration, etc. (see Skin). Hyperplastic processes are observed during regeneration, especially in tumors (see Cancer), where along with the multiplication of epithelial cells, significant morphological deviations of epithelial cells from their usual structure are noted (see Anaplasia), up to their resemblance to mesenchymal cells. Some authors point to the possibility of the transformation of epithelium into mesenchymal cells, speaking of 'desmollasia,' 'mesenchymoplasia' of the epithelium, etc. (Kromayer, Koritsky).