An aggregation of cells more or less similar morphologically and functionally. The animal body is composed of four primary tissues, namely, epithelium, connective tissue (including bone, cartilage, and blood), muscle, and nervous tissue. The process of differentiation and maturation of tissues is called histogenesis. See Histology
in biology, an aggregation of cells that are similar in origin, structure, and function. Tissues also include intercellular substances and structures that are products of cellular activity.
Animal tissues. Four types of animal tissues have been isolated, corresponding to the body’s principal somatic functions. Epithelial tissue comprises the skin and the membranes of the internal organs; its derivatives perform secretory functions and constitute, for example, the bulk of the liver and pancreas. Connective tissue, including that of the blood, performs nutritive and protective functions. Derivatives of connective tissue—cartilage and bone—comprise the supporting skeletal structure of vertebrates. Muscular tissue performs motor functions, moving the body and producing contractions of the organs. Nerve tissue regulates and coordinates the activity of all tissues, receives signals from the external environment, and determines the body’s respective reactions.
The development of each type of tissue is the result of a specific type of histogenesis occurring during the embryonal period. In many tissues, histogenesis continues in adult life, ensuring the regeneration and sometimes the growth of tissues. The functions specific to a given organ are usually performed by a single type of tissue, or by a number of specialized cells of that tissue. In all organs, different types of tissues interact to promote the nutritive and coordinative functions of the organ. The activity of tissue cells depends on their interrelationship within the tissue, as well as on outside hormonal and neural influences.
In lower organisms the tissues are not as distinctly differentiated as in higher ones. The evolution of organisms led to the specialization of cells and to the mutual interdependence of the cells’ functions and very existence within a system containing different types of tissues. However, by making a model of the cell environment, it is possible to keep cells alive outside the body and to develop tissues through tissue culture, which has become one of the principal methods of studying tissues. Histology is the study of animal tissues.
V. IA. BRODSKII
Plant tissues. The growth of a plant and the development of its internal structure depend on the activity of formative tissue, or meristem, whose derivatives undergo complex structural and functional differentiation when converting to elements of permanent tissues. The classification of permanent tissues is based on morphological, functional, and genetic features; an example is the distinction between parenchyma and prosenchyma. Permanent tissues belong to one of three systems: dermal, conductive, and fundamental (ground). In the ontogenesis of plants, these systems reflect the major stages in the internal differentiation of the plant during the process of evolution.
The most widely accepted physiological classification of plant tissues, that of G. Haberlandt, divides permanent tissues into a number of systems. These include the dermal system, composed of epidermis, cork, and cortex, and the mechanical system, composed of collenchyma (parenchyma cells with walls of irregular thickness) and sclerenchyma (lignified fibers and more or less iso-diametric sclereids). The absorptive system absorbs substances by means of rhizoids and root hairs made of epiblem. In Orchida-ceae, this function is performed by the multilayered epidermis (velamen) of the aerial roots. The assimilatory tissue system consists of parenchyma cells with many chloroplasts, and the conductive system consists of the xylem, which conducts water, and the phloem, which conducts the movement of organic matter. Further tissue systems are the storage system, consisting of parenchyma cells, the secretory system, which includes hydathodes, lati-cifers, and reservoirs for discharged substances, and the aeration system, consisting of intercellular spaces, stomata, and lenticels. All tissues except those of the dermal, conductive, and aeration systems are variants of fundamental (ground) tissue. Plant anatomy is the discipline that studies plant tissues.
L. I. LOTOVA