According to Wikipedia – Histology is a branch of biology that studies the structure, vital activity, and development of tissues of living organisms. This is usually done by dissecting the tissue into thin layers and using a microtome. Unlike anatomy, histology studies the structure of the body at the tissue level.
Histology is the science of the structure, development, and life of the tissues of animal organisms. Tissues are part of organs and are composed of cells and non-cellular structures. Therefore, histology includes cytology (the study of the cell), general histology (the study of tissues), and private histology (the study of the microscopic structure of organs).
Histology (Greek histo – tissue + logos – research) is the science that studies the tissues of the body and how these tissues are located, forming organs. There are four main tissues known: epithelial tissue, connective tissue, muscle tissue, and nerve tissue. Histology as the science of the structure, development and vital activity of human and animal tissues, acquaintance with the historical stages of development. General characteristics of the methods of disclosing the mechanisms of molecular genetic regulation of cell differentiation.
The study of the basics of histology is an important link in the knowledge of the structure of the human body, since tissues are one of the levels of organization of living matter, the basis for the formation of organs. The history of the development of histology at the end of the 19th century. in Russia was closely associated with the formation of university education.
The aim of the work is to define histology as a science.
The stated goal defines the research objectives:
- To study the objects and methods of research of histology;
- Outline the historical stages in the development of histology.
From this definition, it follows that the main subject of the study of histology is tissue. There are 5 main tissues in the human and animal body:
In Histology, each of which has its own characteristics.
Tissues which is a system of cells and non-cellular structures that have united and specialized in the process of phylogenesis and ontogenesis to perform the most important functions in the body. Thus, the basis for the development and structure of tissues are cells and their derivatives – non-cellular structures.
Histology, as an academic discipline, includes the following sections:
- General histology; (studies the structure and function of tissues);
- Private histology (studies the structure and function of tissues).
The subject of general histology (the doctrine of tissues itself) is both general patterns and distinctive features of the structure of specific tissues, the subject of private histology is the regularities of vital activity and interaction of tissues in specific organs.
The actual tasks of histology are:
- Development of a general theory of histology, reflecting the evolutionary dynamics of tissues and patterns of embryonic and postnatal histogenesis;
- Study of histogenesis as a complex of processes of proliferation, differentiation, determination, integration, adaptive variability, programmed cell death, coordinated in time and space;
- Elucidation of the mechanisms of homeostasis and tissue regulation (nervous, endocrine, immune), as well as age-related dynamics of tissues;
- Study of the patterns of reactivity and adaptive variability of cells and tissues under the influence of unfavorable environmental factors and in extreme conditions of functioning and development, as well as during transplantation;
- Development of the problem of tissue regeneration after damaging influences and methods of tissue replacement therapy;
- Disclosure of the mechanisms of molecular-genetic regulation of cell differentiation, inheritance of a genetic defect in the development of human systems, development of methods of gene therapy and transplantation of embryonic stem cells;
- Elucidation of the processes of human embryonic development, critical periods of development, reproduction and the causes of infertility.
But the main task of histology, like other biological sciences, is to identify the essence of life, the structural organization of vital processes for a purposeful impact on them, which is very important for medical practice. Proceeding from the main task, histology studies the patterns of formation, the mechanisms of regulation of the processes of tissue morphogenesis and the role of the nervous, endocrine and immune systems in these processes. The most important tasks solved by histology are the study of cellular and tissue compatibility during blood transfusion, tissue and organ transplantation. Histology is closely related to other biomedical sciences – biology, anatomy, physiology, biochemistry, pathological anatomy, and clinical disciplines. In addition, modern histology makes heavy use of the achievements of physics, chemistry, mathematics, cybernetics, which determines its close relationship with these sciences.
Objects of research of histology
Research objects are divided into:
- Living (cells in a drop of blood, cells in culture and others);
- Dead or fixed, which can be taken both from a living organism (biopsy) and from corpses.
To study living micro-objects, methods of implantation of transparent chambers with the studied cells into the body of an animal, transplantation of cells into the fluid of the anterior chamber of the eye and observation of their vital activity through the transparent cornea of the eye are used. The most common methods of in vivo study of structures are cell and tissue cultures – suspension (suspension in a liquid medium) and monolayer (formation of a continuous layer on glass). For long-term maintenance of cells in culture, it is required to create an optimal temperature corresponding to body temperature and a special nutrient medium (blood plasma, embryonic extract, growth stimulants) to maintain the main vital signs: growth, reproduction, movement, differentiation.
To study dead, or fixed, cells and tissues, they must, as a rule, undergo special processing in order to obtain a histological specimen for examination in a light or electron microscope.
A histological specimen can be in the form:
- A thin stained section of an organ or tissue;
- Smear on glass (for example, blood smear, bone marrow);
- An imprint on glass from a fracture of an organ (for example, the mucous membrane of the oral cavity, vagina, etc.);
- A thin film preparation (for example, peritoneum, pleura, meninges).
Preparation of histological preparations
A histological specimen of any form must meet the following requirements:
- Preserve the vital state of structures;
- Be thin and transparent enough to study it under a microscope in transmitted light;
- Be contrasting, that is, the studied structures should be clearly defined under the microscope;
- Preparations for light microscopy should be stored for a long time and used for re-examination.
These requirements are achieved when preparing the drug.
The following stages of the preparation of a histological specimen are distinguished.
Taking material (a piece of tissue or organ) for preparing the drug. The following points are taken into account:
- The sampling of material should be carried out as early as possible after the death or slaughter of the animal, and, if possible, from a living object (biopsy), so that the structures of the cell, tissue or organ are better preserved;
- The collection of pieces should be done with a sharp instrument so as not to injure the tissue;
- The thickness of the piece should not exceed 5 mm, so that the fixing solution could penetrate into the thickness of the piece;
- The piece must be marked (the name of the organ, the number of the animal or the surname of the person, the date of collection, and so on).
Fixation of the material is necessary to stop metabolic processes and preserve structures from decay. Fixation is achieved most often by immersing a piece in fixing liquids, which can be simple alcohols and formalin and complex Carnois solution, Zinker’s fixative and others. The fixator causes protein denaturation and thereby suspends metabolic processes and preserves the structures in their vital state. Fixation can also be achieved by freezing (cooling in a stream of CO 2 , liquid nitrogen, and others). The duration of fixation is selected empirically for each tissue or organ.
Pouring the pieces into sealing media (paraffin, celloidin, resins) or freezing for the subsequent production of thin sections.
Preparation of slices on special devices (microtome or ultramicrotome) using special knives. Sections for light microscopy are glued onto glass slides, and for electron microscopy they are mounted on special meshes.
Coloring of sections or their contrasting (for electron microscopy). Before coloring the sections, the sealing medium is removed (dewaxing). The color achieves the contrast of the studied structures. Dyes are classified as basic, acidic and neutral. The most widely used are basic dyes (usually hematoxylin) and acidic dyes (eosin). Complex dyes are often used.
Clarification of sections (in xylene, toluene), encapsulation in resins (balsam, polystyrene), covering with a cover glass. After these successively performed procedures, the preparation can be examined under a light microscope.
For the purposes of electron microscopy, there are some peculiarities in the preparation stages, but the general principles are the same. The main difference is that a histological specimen for light microscopy can be stored for a long time and used repeatedly. Electron microscopy sections are used once. In this case, the objects of interest of the preparation are first photographed, and the study of the structures is carried out already on the electron diffraction patterns.
From tissues of a liquid consistency (blood, bone marrow and others), preparations are made in the form of a smear on a glass slide, which are also fixed, stained, and then studied.
From fragile parenchymal organs (liver, kidney and others), preparations are made in the form of an organ imprint: after a break or rupture of an organ, a glass slide is applied to the site of the break of the organ, on which some free cells are glued. Then the drug is fixed, stained and studied.
Finally, from some organs (mesentery, pia mater) or from loose fibrous connective tissue, film preparations are made by stretching or crushing between two glasses, also with subsequent fixation, staining and pouring into resins.
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The main method for studying biological objects used in histology is microscopy, that is, the study of histological preparations under a microscope. Microscopy can be an independent study method, but recently it is usually combined with other methods (histochemistry, historadiography, and others). It should be remembered that different designs of microscopes are used for microscopy, allowing to study different parameters of the objects under study. There are the following types of microscopy:
- Light microscopy (resolution 0.2 microns) the most common type of microscopy;
- Ultraviolet microscopy (resolution 0.1 microns);
- Luminescence (fluorescence) microscopy for the determination of chemicals in the structures under consideration;
- Phase-contrast microscopy for studying structures in unstained histological preparations;
- Polarizing microscopy to study mainly fibrous structures;
- Microscopy in a dark field for the study of living objects;
- Microscopy in incident light for studying thick objects;
- Electron microscopy (resolution up to 0.1-0.7 nm), two of its types are transmission (transmission) electron microscopy and scanning or scanning microscopy provides a display of the surface of ultrastructures.
Histochemical and cytochemical methods make it possible to determine the composition of chemical substances and even their amount in the studied structures. The method is based on carrying out chemical reactions with the used reagent and chemicals in the substrate, with the formation of a reaction product (contrast or fluorescent), which is then determined by light or luminescence microscopy.
The method of histo-auto-radiography makes it possible to reveal the composition of chemical substances in the structures and the intensity of exchange by the inclusion of radioactive isotopes in the studied structures. The method is used most often in animal experiments.
The method of differential centrifugation allows you to study individual organelles or even fragments isolated from the cell. For this, a piece of the investigated organ is rubbed, poured with saline, and then dispersed in a centrifuge at different speeds (from 2 to 150 thousand) and the fractions of interest are obtained, which are then studied by various methods.
The interferometry method allows you to determine the dry mass of substances in living or fixed objects.
Immunomorphological methods allow, using previously conducted immune reactions, based on the antigen-antibody interaction, to determine subpopulations of lymphocytes, to determine the degree of foreignness of cells, to carry out histological typing of tissues and organs (to determine histocompatibility) for organ transplantation.
Cell culture method (in vitro, in vivo) – growing cells in a test tube or in special capsules in the body and then studying living cells under a microscope.
Units of measurement used in histology
To measure structures in light microscopy, micrometers are mainly used: 1 μm is 0.001 mm; electron microscopy uses nanometers: 1 nm is 0.001 microns.
Historical stages in the development of science
In the history of the development of histology, three periods are conventionally distinguished:
- The pre-microscopic period (from the 4th century BC to 1665) is associated with the names of Aristotle, Galen, Avicenna, Vesalius, Fallopia and is characterized by attempts to isolate heterogeneous tissues in the body of animals and humans (hard, soft, liquid, etc. ) and the use of methods of anatomical preparation;
- Microscopic period (from 1665 to 1950). The beginning of the period is associated with the name of the English physicist Robert Hooke, who, firstly, improved the microscope (it is believed that the first microscopes were invented at the very beginning of the 17th century), and secondly, he used it for the systematic study of various, including biological objects and published the results of these observations in 1665 in the book “Micrography”, thirdly, he first introduced the term “cell” (“cellulia”). In the future, there was a continuous improvement of microscopes and their increasingly widespread use for studying biological tissues and organs. Particular attention was paid to the study of the structure of the cell. Jan Purkinje described the presence of “protoplasm” (cytoplasm) and nucleus in animal cells, and somewhat later R. Brown confirmed the presence of a nucleus in most animal cells. Botanist M. Schleiden became interested in the origin of cells by cytogenesis. The results of these studies allowed T. Schwan, on the basis of their reports, to formulate the cellular theory (1838-1839) in the form of three postulates:
- all plant and animal organisms are composed of cells;
- all cells develop according to the general principle from the cytoblastoma;
- each cell has an independent vital activity, and the vital activity of the organism is the sum of the activity of cells.
However, soon R. Virkhov (1858) clarified that the development of cells is carried out by division of the original cell (any cell from a cell). The provisions of the cell theory developed by T. Schwan are still relevant, although they are formulated in a different way.
Modern provisions of cell theory:
- A cell is the smallest living unit;
- Cells of animal organisms are similar in structure;
- Multiplication of cells occurs by dividing the original cell;
- Multicellular organisms are complex ensembles of cells and their derivatives, united into systems of tissues and organs, interconnected by cellular, humoral and nervous forms of regulation.
Further improvement of microscopes, especially the creation of achromatic objectives, made it possible to identify smaller structures in cells:
- Hertwig cell center, 1875;
- mesh apparatus or lamellar Golgi complex, 1898;
- Bend mitochondria, 1898
- The modern stage of development of histology begins in 1950 with the beginning of the use of an electron microscope to study biological objects, although the electron microscope was invented earlier (E. Ruska, M. Knol, 1931). However, the modern stage of development of histology is characterized by the introduction of not only an electron microscope, but also other methods:
- Cyto- and histochemistry;
- Other modern methods listed above.
In this case, a complex of various techniques is usually used, which makes it possible to compile not only a qualitative idea of the structures under study, but also to obtain accurate quantitative characteristics. At present, various morphometric techniques are especially widely used, including automated systems for processing the received information using computers.
In Histology, The main objects are living or dead tissue. Research methods include a microscope, histochemical and cytochemical methods, histoautoradiography, differential centrifugation, interferometry, immunomorphological methods and cell culture.
There are three stages in the history of the development of histology: pre-microscopic, microscopic and modern.