Tissues

TISSUES. Cells that are functionally similar, differentiated in the same direction, and connected to each other (and to the interstitial substance, if present) in a specific way, form systems that have received the name of tissues in histology. While being part of a tissue, cells are not merely components, but, while remaining to a certain extent independent centers of metabolism, growth, secretion, differentiation, etc., they are at the same time mutually connected to each other and are subject to the general laws of the given tissue and organ; incidentally, this dependence of theirs on general tissue laws is proven by the growth of pieces of tissue in in vitro cultures: tissue cells in culture also retain the typical features of their differentiation and specific connections; in the case of the presence of elements of two tissues in a culture, e.g., epithelium and connective tissue, the elements of both are arranged in such a way that they resemble the corresponding relationships in the organism. In the earliest stages of embryo development, the blastomeres obtained from the cleavage of the egg are so identical that it is impossible to establish morphological differences between them; however, in some cases, e.g., during the development of determinate eggs in some animals, one can be convinced that even before the first division of the fertilized egg, specific tissues and organs develop from specific areas of the egg cell, i.e., despite the absence of visible differences, there is already a specific difference in the prospective significance of specific areas of the cellular protoplasm of the fertilized egg. In further development, this difference also manifests itself in the difference in morphological differentiation. Young tissues possess great growth energy, but subsequently, in parallel with the deepening of differentiation, the growth energy of both the entire tissue and the ability of its individual cellular elements to reproduce decreases. Such a drop in the growth curve in various tissues is observed to a very different degree: in the cells of nervous tissue, with the completion of differentiation, the cells completely lose the ability to reproduce; epithelial and connective tissues show only a small drop in the curve in this respect. In animal histology, the generally accepted classification of tissues establishes four main groups, sharply differing from each other both in morphological features and in origin from a specific primordium, as well as functionally: 1) epithelial tissues (covering and the glandular ones derived from them), 2) supporting tissues (connective tissue, cartilage, and bone), 3) muscular tissues, and 4) nervous tissue. Epithelial tissue is characterized by the absence of formed interstitial substance; its cells are cemented by a small amount of homogeneous cementing substance or connected to each other by intercellular bridges; it is a derivative of all three germ layers and, according to its origin, can be divided into groups having specific morphological and functional features: a group of epithelium originating from the ectoderm, epithelia originating from the endoderm, and epithelia of mesodermal origin; as has been clarified recently, it is possible to distinguish another group of epithelium forming the secretory elements of the urinary organs. The so-called germinal epithelium does not belong to any of these groups, because its elements detach from the general cellular mass of the embryo and, as part of the germ line, enter the reproductive organs without connection to the epithelial tissue of the somatic part of the embryo.

The group of supporting tissue (connective tissue in the broad sense), in contrast to epithelium, contains much intercellular substance in the form of bundles of collagenous fibers and elastic networks, so that in terms of mass, the cells recede into the background; it originates from the middle germ layer, in particular from the mesenchyme. As the first stage of differentiation of the mesenchyme, one must note the embryonic or mucous connective tissue. It is characterized by a predominance of cellular elements and a relatively small amount of fibrous formations embedded in a semi-liquid, unformed ground substance; its cells are predominantly fibroblasts, the formers of collagenous and elastic fibers, gradually forming an ever-increasing mass of ground substance; the semi-liquid ground substance remains in a more or less noticeable amount only in fibrous connective tissue, especially in loose connective tissue; in parallel with the development of fibers, the relative number of cells falls, fibroblasts are replaced by fibrocytes that do not exhibit formative activity, and a picture of developed fibrous connective tissue with its usual cellular forms is obtained. The group of supporting tissue in the developed organism includes fibrous connective tissue, reticular or adenoid, cartilage, and bone. Muscular tissue (see Muscles) is subdivided into smooth, striated skeletal, and cardiac muscles. Muscular tissue is exclusively cellular: in smooth muscular tissue, the cells are of small length, mononuclear, spindle-shaped, connected to each other by connective tissue formations into strands and layers, which are arranged differently in different organs; skeletal striated muscles consist of relatively huge multinucleated cells, connected by fibrous connective tissue into bundles, which, by their mutual arrangement, give one or another appearance to the various muscles of the skeleton; cardiac muscle tissue does not consist of isolated mononuclear cells, as was thought earlier, but represents a syncytium in which contractile fibrils, without interruption, pass from one "cell" to another. Nervous tissue (see Nerve cells, Neuroglia). Some authors also include blood, which is a derivative of the middle germ layer, among the tissues. However, there are no sufficient grounds for this, because the morphological elements of blood, suspended in the liquid part—plasma—are not in specific and constant relationships to each other; in the case of recognizing blood as a tissue, it would be necessary to recognize lymph as such as well, which, however, no one does. In plant histology, there is no generally accepted classification of tissues, and even the very principle of classification is not established. There are three main points of view—morphological, physiological, and genetic. The most modern and widespread can be considered the following division of tissues: 1) embryonic tissues—a) primary (meristem) and b) secondary (cambium)—and 2) differentiated tissues (primary, if they originate from meristem, and secondary, if they originate from cambium). The latter are divided into: a) covering tissues—epidermis, stomata, hairs, cork, lenticels; b) mechanical—sclerenchyma, collenchyma, stone cells, etc.; c) conducting tissues—vessels, tracheids, sieve tubes, etc.; and d) storage—parenchyma, prosenchyma, laticifers, etc. V. Fomin.