Tissues

Tissues. Cells that are similar functionally, differentiated in the same direction, and connected with each other (and with intermediate substance, if present) in a specific way, form systems that have received the name T. in histology. When making up T., cells are not merely components, but while remaining to some extent independent centers of metabolism, growth, secretion, differentiation, etc., at the same time they are interconnected and subject to the general laws of the given T. and organ; among other things, this dependence of them on general tissue laws is proved by the growth of pieces of T. in in vitro cultures: tissue cells in culture retain typical features of their differentiation and certain connections; in case of presence in the culture of elements of two T., e.g. epithelium and connective T., elements of both are arranged in such a way as to resemble the corresponding relations in the organism. At the earliest stages of embryo development, blastomeres obtained from the cleavage of the egg are so similar that morphological differences between them cannot be established; however, in some cases, e.g. in the development of determined eggs in some animals, one can ascertain that even before the first division of the fertilized egg, certain areas of the egg cell develop into certain tissues and organs, i.e. despite the absence of visible differences, there is already a certain difference in the prospective significance of certain areas of the protoplasm of the fertilized egg. In further development, this difference is manifested in the difference of morphological differentiation. Young T. possess great energy of growth, but later, with deepening of differentiation, the energy of growth of the entire T. and the ability to reproduce of its individual cellular elements decreases. Such a fall in the growth curve in various T. is observed to a very different degree: in cells of nervous T., with the completion of differentiation, cells completely lose the ability to reproduce, epithelial and connective T. in this respect show only a small fall in the curve. In animal histology, the generally accepted classification of T. establishes four main groups, sharply differing from each other both in morphological features and in origin from a certain rudiment, as well as functionally: THESE ARE 1) epithelial T. (covering and derived from them glandular), 2) supporting T. (connective T., cartilage and bone), 3) muscular T. and 4) nervous T.-Epithelial tissue is characterized by the absence of organized intercellular substance; its cells are joined by a small amount of homogeneous cementing substance or are connected with each other by intercellular bridges; it is derived from all three embryonic leaflets and according to its origin can be divided into groups having certain morphological and functional peculiarities: group of epithelium derived from ectoderm, epithelia derived from entoderm, epithelia of mesodermal origin; as has been clarified in recent years, a group of epithelium forming secretory elements of urinary organs can also be distinguished. The so-called germinal epithelium does not belong to any of these groups, since its elements detach from the common cellular mass of the embryo and, as part of the sexual pathway, go to the sexual organs outside the connection with the epithelial T. of the somatic part of the embryo. The group of supporting T. (connective T. in the broad sense) in contrast to epithelium contains much intercellular substance in the form of bundles of cementing fibers and elastic networks, so that in mass cells recede to the second place; it originates from the middle embryonic leaflet, in particular from mesenchyme. As the first stage of differentiation of mesenchyme, embryonic or mucous connective T. should be noted. It is characterized by the predominance of cellular elements and a relatively small amount of fibrous formations laid in a semi-liquid unformed ground substance; its cells are predominantly fibroblasts, formers of cementing and elastic fibers, gradually forming an ever larger mass of ground substance; semi-liquid ground substance in more or less noticeable amount remains only in fibrous connective T., especially in loose; parallel with the development of fibers, the relative amount of cells decreases, fibroblasts are replaced by fibrocytes not showing formative activity, and a picture of developed fibrous connective T. with its usual cellular forms is obtained.-In the group of supporting T. in the developed organism there are fibrous connective T., reticular or adenoid, cartilaginous and bony.-Muscular T. (see Muscles) is divided into smooth, striated of skeleton and cardiac muscles. Muscular T. is exclusively cellular: in smooth muscular T. cells are of small length, uninucleate, spindle-shaped, connected with each other by connective tissue formations into strands and plates arranged differently in different organs; skeletal striated muscles consist of relatively huge multinucleate cells, connected by fibrous connective T. into bundles, by their mutual arrangement giving this or that appearance to various muscles of the skeleton; T. of cardiac muscle does not consist of isolated uninucleate cells, as was thought earlier, but represents a syncytium in which contractile fibers, without interruption, pass from one "cell" to another.-Nervous T.-^:see Nerve cells, Neuroglia. Some authors also include blood among T., which is derived from the middle embryonic leaflet. However, there are no sufficient grounds for this, since morphological elements of blood, suspended in the liquid part-plasma, are in certain and constant relations to each other; in case of recognition of blood as T., it would be necessary to recognize lymph as such, which however no one does. In plant histology there is no generally accepted classification of T. and even the 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 T.: 1) embryonic T.--a) primary (meristem) and b) secondary (cambium)-and 2) differentiated T. (primary, if they come from meristem, and secondary, if they come from cambium). The latter are divided into: a) covering T.-cuticle, stomata, hairs, cork, lenticels, b) mechanical-sclerenchyma, collenchyma, stone cells etc., c) conducting T.-vessels, tracheids, sieve tubes etc., d) storage-parenchyma, prosenchyma, milk vessels etc. V. Fomin.

Tissues. Cells that are similar functionally, differentiated in the same direction, and connected with each other (and with intermediate substance, if present) in a specific way, form systems that have received the name T. in histology. When making up T., cells are not merely components, but while remaining to some extent independent centers of metabolism, growth, secretion, differentiation, etc., at the same time they are interconnected and subject to the general laws of the given T. and organ; among other things, this dependence of them on general tissue laws is proved by the growth of pieces of T. in in vitro cultures: tissue cells in culture retain typical features of their differentiation and certain connections; in case of presence in the culture of elements of two T., e.g. epithelium and connective T., elements of both are arranged in such a way as to resemble the corresponding relations in the organism. At the earliest stages of embryo development, blastomeres obtained from the cleavage of the egg are so similar that morphological differences between them cannot be established; however, in some cases, e.g. in the development of determined eggs in some animals, one can ascertain that even before the first division of the fertilized egg, certain areas of the egg cell develop into certain tissues and organs, i.e. despite the absence of visible differences, there is already a certain difference in the prospective significance of certain areas of the protoplasm of the fertilized egg. In further development, this difference is manifested in the difference of morphological differentiation. Young T. possess great energy of growth, but later, with deepening of differentiation, the energy of growth of the entire T. and the ability to reproduce of its individual cellular elements decreases. Such a fall in the growth curve in various T. is observed to a very different degree: in cells of nervous T., with the completion of differentiation, cells completely lose the ability to reproduce, epithelial and connective T. in this respect show only a small fall in the curve. In animal histology, the generally accepted classification of T. establishes four main groups, sharply differing from each other both in morphological features and in origin from a certain rudiment, as well as functionally: THESE ARE 1) epithelial T. (covering and derived from them glandular), 2) supporting T. (connective T., cartilage and bone), 3) muscular T. and 4) nervous T.-Epithelial tissue is characterized by the absence of organized intercellular substance; its cells are joined by a small amount of homogeneous cementing substance or are connected with each other by intercellular bridges; it is derived from all three embryonic leaflets and according to its origin can be divided into groups having certain morphological and functional peculiarities: group of epithelium derived from ectoderm, epithelia derived from entoderm, epithelia of mesodermal origin; as has been clarified in recent years, a group of epithelium forming secretory elements of urinary organs can also be distinguished. The so-called germinal epithelium does not belong to any of these groups, since its elements detach from the common cellular mass of the embryo and, as part of the sexual pathway, go to the sexual organs outside the connection with the epithelial T. of the somatic part of the embryo. The group of supporting T. (connective T. in the broad sense) in contrast to epithelium contains much intercellular substance in the form of bundles of cementing fibers and elastic networks, so that in mass cells recede to the second place; it originates from the middle embryonic leaflet, in particular from mesenchyme. As the first stage of differentiation of mesenchyme, embryonic or mucous connective T. should be noted. It is characterized by the predominance of cellular elements and a relatively small amount of fibrous formations laid in a semi-liquid unformed ground substance; its cells are predominantly fibroblasts, formers of cementing and elastic fibers, gradually forming an ever larger mass of ground substance; semi-liquid ground substance in more or less noticeable amount remains only in fibrous connective T., especially in loose; parallel with the development of fibers, the relative amount of cells decreases, fibroblasts are replaced by fibrocytes not showing formative activity, and a picture of developed fibrous connective T. with its usual cellular forms is obtained.-In the group of supporting T. in the developed organism there are fibrous connective T., reticular or adenoid, cartilaginous and bony.-Muscular T. (see Muscles) is divided into smooth, striated of skeleton and cardiac muscles. Muscular T. is exclusively cellular: in smooth muscular T. cells are of small length, uninucleate, spindle-shaped, connected with each other by connective tissue formations into strands and plates arranged differently in different organs; skeletal striated muscles consist of relatively huge multinucleate cells, connected by fibrous connective T. into bundles, by their mutual arrangement giving this or that appearance to various muscles of the skeleton; T. of cardiac muscle does not consist of isolated uninucleate cells, as was thought earlier, but represents a syncytium in which contractile fibers, without interruption, pass from one "cell" to another.-Nervous T.-^:see Nerve cells, Neuroglia. Some authors also include blood among T., which is derived from the middle embryonic leaflet. However, there are no sufficient grounds for this, since morphological elements of blood, suspended in the liquid part-plasma, are in certain and constant relations to each other; in case of recognition of blood as T., it would be necessary to recognize lymph as such, which however no one does. In plant histology there is no generally accepted classification of T. and even the 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 T.: 1) embryonic T.--a) primary (meristem) and b) secondary (cambium)-and 2) differentiated T. (primary, if they come from meristem, and secondary, if they come from cambium). The latter are divided into: a) covering T.-cuticle, stomata, hairs, cork, lenticels, b) mechanical-sclerenchyma, collenchyma, stone cells etc., c) conducting T.-vessels, tracheids, sieve tubes etc., d) storage-parenchyma, prosenchyma, milk vessels etc. V. Fomin.