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histology(hĭstŏl`əjē), study of the groups of specialized cells called tissues that are found in most multicellular plants and animals. Histologists study the organization of tissues at all levels, from the whole organ down to the molecular components of cells. Animal tissues are classified as epithelium, with closely spaced cells and very little intercellular space; connective tissue, with large amounts of intercellular material; muscle, specialized for contraction; and nerve, specialized for conduction of electrical impulses. Blood is also sometimes considered a separate tissue type. These types are combined in different ways in the organism to form characteristic organs. Plants are composed of relatively undifferentiated tissue known as meristematic tissue; storage tissue, or parenchyma; vascular tissue; photosynthetic tissue, or chlorenchyma; and support tissue, or sclerenchyma and collenchyma. A variety of techniques are used for histological studies, including tissue culture, use of various fixatives and stains, the use of a microtome for preparing thin sections, light microscopy, electron microscopy, and X-ray diffraction. The field is divided into developmental histology, the study of tissue formation and specialization in growing embryos; histophysiology, the study of relations between morphological changes and physiological activity; and histochemistry, the study of the chemical composition of tissue structures. Genetic histological methodology utilizes in-situ hybridization of DNA probes that enable analysis of specific genetic sequences and polymerase chain reactions are used to identify single DNA molecules. Immunocyochemistry produces labeled antibodies that attach to specific parts of specified molecules, often used to quantify the available amount of substances (e.g., enzymes and receptor proteins). Histological investigation includes study of tissue death and regeneration and the reaction of tissue to injury or invading organisms. Because normal tissue has a characteristic appearance, histologic examination is often utilized to identify diseased tissue.
The study of the structure and chemical composition of tissues of animals and plants as related to their function. The primary aim is to understand how tissues are organized at all structural levels, including the molecular and macromolecular, the entire cell and intercellular substances, and the tissues and organs.
The four tissues of the animal body include cells and intercellular substances. They are (1) epithelium, in which the cells are generally closely applied to each other and separated by very little intercellular substance; (2) connective tissue, in which the cells are usually separated by greater amounts of intercellular substance, which may indeed form the great bulk of the tissue; (3) muscular tissue, whose cells are primarily concerned with contractility; and (4) nervous tissue, whose components are concerned primarily with rapid conduction of impulses. See Connective tissue, Epithelium, Muscular system, Nervous system (vertebrate)
The major fields of histological studies are morphological descriptions; developmental studies; histo- and cytophysiology; histo- and cytochemistry; and (5) fine (or submicroscopic) structure.
the study of tissues in multicellular animals and man. (The study of plant tissues belongs to the field of plant anatomy.)
The term “histology” was introduced by the German scientist C. Mayer in 1819. The tasks of histology are to obtain a clearer understanding of the evolution of tissues and their development in the body (histogenesis), the structure and function of specialized cells and interstitial environments, the interactions of cells within a tissue and between cells of different tissues, the regeneration of tissue structures, and the regulatory mechanisms that ensure the integrity and coordinated activity of tissues. The principal subjects of histological study are the complexes of cells in their interaction with each other and with their interstitial media. Present-day histology devotes a great deal of attention to study of the specific characteristics of the cells of various tissues; in this area histology, both in methods of investigation and in technique, has much in common with cytology, the science of the general properties of cells. Histology is commonly divided into general histology, which investigates the basic principles of the development, structure, and function of tissues, and specific histology, which investigates the properties of the tissue complexes that constitute the organs of multicellular animals. Specialized areas of general and specific histology include histochemistry, which undertakes the study of the chemistry of tissues, and histophysiology, the study of their mechanisms and activity.
Historical outline. The establishment of histology as an independent branch of science in the 1820’s is connected with the development of microscopy, although it had been noted long before that animal organs consist of components that differ in color and density. Aristotle, in the fourth century B.C.., distinguished “homogeneous parts” in the composition of an organ by these criteria. His classification of these homogeneous parts was repeated for centuries in the works of scientists of antiquity and the Middle Ages until the Renaissance. Mention of homogeneous parts is found in the works of the Roman physician and naturalist C. Galen (second century A.D.), the Middle Asian scientist Avicenna (tenth century), and the Italian physician and anatomist G. Fallopius (16th century). Invention of the microscope in the 17th century did not immediately affect the level of knowledge about the fine structure of organs. The first microscopists (R. Hooke and N. Grew in England, M. Malpighi in Italy, and A. Leeuwenhoek in Holland) saw some large cells, capillaries, and nerves, but observations of these were neither systematic nor linked to the other anatomical data of the time. Even by the beginning of the 19th century the notion of tissues was based, as in Aristotle’s time, on their appraisal with the naked eye. The “macroscopic” (premicroscopic) period in the development of histology was brought to a close with the fundamental work of the French anatomist and physiologist M. Bichat, General Anatomy in Its Application to Physiology and Medicine (1802). To designate parts of organs, Bichat used the term “tissue,” proposed earlier by N. Grew in his work The Anatomy of Plants (1672). In delimiting tissues, Bichat not only described the components of a section of the organ, but he attempted also to discover their properties—their relation to various reagents, heating, and other effects. Bichat distinguished 21 tissues. The classification he proposed was not complete, but it played a progressive role in establishing histology and made possible, along with the data accumulated from microscopic investigations, the formulation as early as the first quarter of the 19th century of the tasks of histology as an independent science. The work of the German scientist C. Mayer, On Histology and a New Subdivision of Human Tissues, appeared in 1819 and strengthened the concept of “tissue.” In this work, and especially in the monograph by the German scientist C. Heisinger, The System of Histology (1822), tasks of histology were formulated that were distinct from those of anatomy.
Intensive development of histology began in the 1830’s. In these and in the following years, the microscope was substantially improved. Techniques for the preparation of tissues for microscopy also developed. The cell theory, which was conclusively substantiated by the German biologist T. Schwann in 1839, became the methodological basis of histology. In the first half of the 19th century a great amount of data was obtained on the microscopic structure of organs and tissues by the Czech scientist J. Purkinje, the German scientists J. Müller, F. G. J. Henle, T. Schwann, and R. Remak, and the Russian scientists N. M. Iakubovich and N. F. Ovsiannikov. Generalization of the extensive literature and their own research permitted the German histologists F. von Leydig (1853) and R. A. von Kolliker (1855) to compile a rational classification of tissues, which has been preserved in its general features to this day. In Leydig’s and Kolliker’s system four groups of tissues were isolated, not only according to structure but also according to their functional significance in the body: epithelial, connective, muscle, and nerve. Subsequent extension of the morphophysiological classification of Leydig and Kolliker (chiefly in studying the development of tissues) laid the groundwork for modern histology.
In the second half of the 19th and the beginning of the 20th century essential data were obtained on epithelial tissues (by Kolliker, French scientists E. Laguesse and L. Ranvier, and the Russian scientist, S. G. Chasovnikov), on tissues of the internal environment (by the German scientist V. Ebner von Rofenstein and the Russian scientists E. Metchnikoff, F. Goier, V. Danchakova, and in particular A. A. Maksimov, who created and substantiated in detail the original theory of the histogenetic unity of the tissues of the internal environment, which subsequently—particularly in the 1950’s and 1960’s—received numerous experimental confirmations), on muscle tissues (by the German histologist M. Heidenhain, the Russian biologist A. I. Babukhin, and L. Ranvier), and on nerve tissue (by the Italian histologist C. Golgi, the Russians M. D. Lavdovskii, V. la. Rubashkin, and A. S. Dogel’, and the Spaniard S. Ramon y Cajal). Major discoveries in the field of general cytology also belong to this period, including a description of the indirect division of nucleus and cell, or mitosis (by the Russian scientists A. Schneider and I. D. Chistiakov and the Germans W. Flemming and E. Strasburger), and the discovery and study of cytoplasmic organoids, such as the mitochondria and the Golgi complex (by the German scientists R. Altmann and K. Benda and the Italian C. Golgi). The discovery by E. Metchnikoff of the cellular nature of the inflammatory process drew cytology and histology closer to the problems of pathology. This was fostered in large measure by the works of the German scientist R. Virchow. Histology also progressively became more closely tied to physiology; this may be traced in the works of the French scientists A. Prenant and A. Policard, the Germans O. Hertwig and M. Heidenhain, and the Russian scientist I. F. Ognev. Of great significance for the development of histology and cytology was O. Hertwig’s Cells and Tissues (1893-98), in which numerous microscopic investigations were summarized and it was concluded that the in-depth study of cells is the path to the solution of many biological problems, including those of tissue interrelationships.
In Russia, histology developed in the universities of St. Petersburg (N. M. Iakubovich, M. D. Lavdovskii, and A. S. Dogel’), Moscow (A. I. Babukhin, I. F. Ognev, and V. P. Karpov), Kazan (N. F. Ovsiannikov, K. A. Arnshtein, and A. N. Mislavskii), and Kiev (M. I. Peremezhko). After the October Revolution the study of histology began to develop in medical institutes (where the schools of A. A. Zavarzin, N. G. Khlopin, B. I. Lavrent’ev, and M. A. Baron were formed) as well as in university departments. Histological research is also conducted in institutes and laboratories of the Academy of Sciences of the USSR and the Academy of Medical Sciences of the USSR. Soviet histologists have made a great contribution to the knowledge of the properties of tissues; they have also revealed a number of important patterns in the histogenesis and peculiarities of function of tissue structures. Substantial improvements have been made in histochemical methods of research, by means of which data have been gathered on the development, function, and pathology of tissues.
Matter, problems, and methods of histology. The historical development (phylogeny) of multicellular animals led to the differentiation and specialization of cells and to the isolation of cell systems and complexes that perform particular functions. Phylogenetically formed systems of cells, united by a common structure, function, and origin, are considered to be tissues. According to these characteristics, the following tissues are distinguished: epithelia, which form the outer or inner coverings of the body, as well as of the various glands that perform protective, digestive, and endocrine functions; tissues of the internal environment (connective tissue, blood), which play a fundamental part in the internal nutritional system and perform supportive functions; muscle tissue, which performs contractive functions; and nerve tissue, which effects the fundamental regulation of life processes of all body systems. The various tissues coexist and interact closely in any organ of a multicellular animal.
In modern histology, especially in histophysiology, experimental approaches to the study of tissue properties are widely used. Often employed among these approaches are the reproduction in experimental animals of the processes of regeneration and inflammation; transplant of organs and parts of organs; experimental denervation of tissues; and stimulation and inhibition of tissue activity by influence on the nervous and endocrine systems or on certain syntheses, transport of substances, or tissue energetics. Tissue and organ cultures are also used for solving a number of problems in histology.
Cytological technique is widely used in studying tissues. Electron microscopy makes it possible to study the submicroscopic structure of tissue cells, their morphological contacts with each other and with intercellular tissue components. The task of histochemistry is the elucidation of the specific features of metabolism in various tissues. The advantage of this method over biochemical analysis lies in the possibility of precise localization of tissue processes. One of the histological methods, known as autoradiography, permits investigation of the kinetics of cell populations, histogenesis, and the metabolic activity of tissues. Cytogenetic analysis (for example, the use of chromosome markers) is used in experiments in tissue transplantation.
An important problem in general histology is the clarification of the potential for development inherent in each type of differentiated cell and of the mechanisms that regulate maintenance of constancy and changes of differentiation. In every tissue one may distinguish a few stable types of cell differentiation, such as fibroblasts (which form the principal substance of connective tissue) and erythroid cells (which form and carry respiratory pigments). Each type of differentiation is achieved in the course of the multistage process of tissue development, or histogenesis. Only a small part of the possibilities provided by the genetic program of the organism is realized in the cells that perform specialized functions. The remaining, unrealized portion of the genetic information in the differentiated cells is preserved, but in an inactive, or repressed, state. Under certain external influences on the cell, derepression may occur and the character of the differentiation of the cells may be altered. Such changes occur constantly in many tissues, in particular when there is normal maturation of the component cells and when the variability of the cells does not exceed the limits typical to each tissue. Under pathological conditions, however, more significant changes begin to occur in the differentiation of tissue cells. These changes are known as metaplasia.
General histology studies histogenesis during formation of tissues in embryonic development, during the natural renewal of tissues in adult animals, and during regeneration after injuries that cause extensive destruction of cells. Associated with this is the problem of determining which cells participate in renewal of tissues and the factors that regulate the direction and tempo of the renewal process. Cell populations of some tissues (for example, of nerve tissue in adult animals) basically do not renew themselves. Nerve cells usually are long-lived, but some of them die with age nonetheless as a result of stresses, diseases, and so forth. However, in the majority of tissues (epithelia and tissues of the internal environment), some of the cells retain their ability to divide. In such tissues the processes of cell replacement occur constantly. Under the normal conditions of renewal of cell composition, the destruction of some cells is compensated by the reproduction of others. This process is conditioned by a number of regulatory mechanisms, which act both within the tissue and in the organism as a whole.
Prolonged maintenance of equilibrium in those tissues whose cells are short-lived (several days or weeks) is ensured by special, so-called stem cells, which are capable of repeated division. Stem cells divide and maintain their own line in the body practically throughout the entire course of the life of the organism; they are the cells that initiate the development of various specialized cells in a given tissue. Elucidation of the factors that regulate the reproduction and differentiation of stem cells and of the mechanisms that determine the path of their development is an important problem of general histology.
Another essential task of histology is understanding of the mechanisms of tissue interactions and determination of the nature of intratissual and intertissual regulation. The properties of cells and the coordinated activity of cell complexes that form tissue are to a significant degree determined by external effects from surrounding cells and from nervous and humoral influences.
An important problem of histology is the elucidation of the paths of the historical development of tissues. Evolutionary histology provides valuable material for the analysis of histogenesis and the mechanisms of tissue differentiation. In the field of general evolutionary histology, the most important conclusions have been drawn by A. A. Zavarzin, on the basis of a comparative study of normal histogenesis and inflammatory reactions in various protostomes and deuterostomes (the theory of the parallelism of tissue evolution and the uniform development of homologous tissues in animals of phylogenetically distant groups) and by N. G. Khlopin, on the basis of the behavior of tissues in culture (the theory of divergent evolution of tissues—the gradually increasing complexity and specialization of tissues that originate from the same embryonic rudiments).
The problems indicated above are directly connected with the behavior of cells and tissues under pathological conditions, such as inflammation, metabolic disturbances, tumor growth, regeneration after injury, and premature aging. Tissue incompatibility in organ transplants is determined by characteristic reactions of the host-organism’s cells to the transplanted tissue. Hence, the problems of general histology have medical as well as biological significance.
General histology thus provides basic information about individual tissues and about the principles of their interconnections. These data are supplemented by study of the development, structure, and activity of tissues in the various organs of a multicellular organism, which constitutes the subject of specific histology (the study of the tissue architecture of an organ, the interaction within it of various tissues, intratissual and intertissual regulation, the histological equivalents of various functional states of the organ, and the development and regeneration of its tissual components). The aim of specific histology is knowledge of the histological and cellular structure of the organ, its histochemical and histophysiological characteristics, and, with the aggregate of this knowledge, determination of the mechanisms of the organ’s activity.
Along with the individuality of structure of various organs, certain general principles are also being revealed, especially in higher animals, about their tissue organization. One may isolate the principle of microanatomical polymerism in a number of internal organs (their construction from repeated cell complexes of various tissues). Each complex performs all the main functions of an organ, acting as its structural-functional unit. The structural-functional unit of the small intestine is the villus; of the liver, the lobulus; of the kidney, the nephron; of the lung, the alveolus; of the pancreas and salivary glands, the acinus; and of the thyroid, the folliculus.
Internal anatomicophysiological polymerism of organs is the result of an evolutionarily determined increase in the reliability of their structure and activity. The multiplicity of structural-functional units (from hundreds to millions) serves as the basis for the formation of optimal work regimes for the organ—its rhythmic activity and the alternation of the active and resting phases in its various parts. Despite the relative lack of reliability of each separate component (the cell and the structural-functional units), the organ as a whole is rather reliable in the performance of functions important to the whole body and in the maintenance of the dynamic equilibrium of its own components, which are interconnected by a common blood supply system and innervation.
The principle of microanatomical polymerism is as a rule characteristic of the complex organs of the digestive, excretory, respiratory, and in part the endocrine systems in higher animals. Body coverings (and their simple derivatives) and the circulatory and nervous systems are constructed differently. The biological function of coverings presupposes continuity of structure; similarly, elements of the circulatory and nervous systems run through the entire body, ensuring its general nutrition and basic regulation of activity and introducing necessary components to the various histological structures.
The tasks of specific histology are (1) determination of the schema of blood supply and the innervation structure of the organ in connection with its histological topography and the characteristics of its specialized cells; (2) elucidation of the nature and significance of the internal polymerism of organs, the intertissual and intercellular interactions in the system of the structural-functional unit, and the regulatory mechanisms of the coordinated work of these units; (3) study of the histological and cytological mechanisms of the restorative processes that occur in organs when they have been injured (reparative regeneration) or upon age-related changes in their structure and activity (physiological regeneration); (4) elucidation of the histological and cytological foundations of the secretory processes, especially of the interaction of terminal secretory sections and ducts and of the mechanisms of formation and regulation of the rhythmic operation of glandular elements; and (5) investigation of the structure and nutrition of pathologically altered organs and of the histological bases for the development of pathological processes (for example, myocardial infarction or malignant tumors). In order to solve the above problems (and their number may be considerably expanded) it is important both to make a comparative study of analogous and homologous organs for the purpose of knowing their historical development and also to study organogenesis in individual development.
A basic tendency in modern histology is the transition from descriptive research to the experimental. Knowledge of the mechanisms of development, activity, and pathology of body tissues poses the main problem. In conformity with this is the trend in many histological works toward greater knowledge of the submicroscopic structures of tissue and specialized cells, as well as of the qualitative and quantitative characteristics of their metabolism in various functional states (usually set up in the experiment). Also typical in these experiments is the simulation of tissual and organic processes, including both development and operating activity (for example, in tissue and organ cultures or upon transplantation). The goal of these works is synthesis of information from different levels of investigation (cell, tissue, tissue complexes, organ) in relation to the properties of the total organism.
The results of histological research are being discussed at the meetings of scientific and medical societies (all-Union and republic) of anatomists, histologists, and embryologists, and at cytological, histochemical, and other conferences. The principal histological journal in the USSR is the Arkhiv anatomii, gistologii, i embriologii, published since 1916. Leading foreign journals include the Journal of Anatomy (London, since 1866); Acta Anatomica (Basel-New York, since 1945); Experimental Cell Research (New York, since 1950); and the Journal of Cell Biology (New York-Baltimore, since 1963; from 1955-1962, the Journal of Biophysical and Biochemical Cytology).
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V. IA. BRODSKII and A. IA. FRIDENSHTEIN