Human Histology

Histology and its Study:

Histology is the study of cell, tissues and organs as seen with a microscope.

Traditional Histological Methods:

The earliest histological observations were made on unfixed tissue (usually teased make a flat preparation). The first advance was the discovery of chemicals for fixation and staining of tissues. The next major development was invention of instruments (called microtome) for cutting thin sections of tissue. These sections could me mounted on glass slides and stained.

The process of function preserves a tissue by denaturing its proteins. It also makes the handling to tissue, and the preparation and staining of sections, more efficient. Numerous fixatives are knows, the most commonly used being formaldehyde (Formaldehyde is a gas, this gas dissolved in water is called formalin).

Before a tissue can be sectioned it has to be given a firm consistency, one way of doing this to freeze the tissue and cut sections while it is still frozen (such sections being called frozen sections).

Techniques for the production of frozen sections have undergone great refinement and at present they are prepared using a microtome enclosed in a refinement is called cryostat.

Preparation of frozen sections is the fastest method of examining a tissue. The technique allows the examination of pieces of tissue removed by a surgeon, while the patient is still on the operating table, making it possible for the surgeon to plan his operation his keeping in mind the nature of disease.

Apart from freezing a tissue, it can be made suitable for sectioning by the embedding it in a suitable medium, the most common being paraffin wax. Such paraffin sections can be thinner than frozen section and reveal more details of structure. However some materials (e.g., fat) or last during the process of embedding tissues in paraffin wax.

The commonest staining procedure used in histology is baemataxylin-eosin staining. In sections stained with most other components are seen in verging shades of pink. Numerous other staining methods are available for demonstrating specific tissue elements.

Electron microscopy:

In the last few decades many new discoveries in field of histology have become possible because of the development of the electron microscope (usually abbreviated to EM). This microscope uses an electric fields in place of lenses. With the EM magnifications in excess of 100,000 times can be achieved. The structure of a cell or tissue as seen with the EM is referred to a ultra structure.

For EM studies small pieces of tissues are fixed very rapidly after removal from the animal body. Special fixatives are required (the most common being glutaraldehyde). Very thin sections are required, and for this purpose thin tissues have to be embedded in media that are harder than wax. The microtome used for cuttings sections are such more sophisticated versions of traditional microtome’s and are called ultramicrotomes. Thin sections prepared in this way are also very useful in light microscopy. This reveal much more detail than can be seen in conventional paraffin sections.

Before sections are examined under an electron microscopy they are often treated with solution containing uranium or lead, to increased contrast of the image. Osmium tetraoxide acts both as fixative and staining agent and has been extensively used for preparing tissues for electron-microscopy.

In conventional EM studies (or transmission electron-microscopy) images are formed by electron passing through the section. Wide use is also made of scanning electron-microcopy in which the surface of tissue can be seen, and three dimensional images can also be obtained. Especially useful details of some tissues (e.g., membranes) can be obtained by freezing a tissue and then fracturing it to view the fractured surface.

Histochemistry:

In many cases the chemical nature of cellular and intra-cellular constituents can be determined by the use of staining techniques. Lipids and carbohydrates (glycogen) present in the cells are easily demonstrated. The presence of many enzymes can be determined by placing section in solutions containing the substrate of the enzyme, and by observing the product formed by action of enzyme on substrate. The product is sometimes visible, or can be made visible using appropriate staining agents.

For enzyme studies the use of frozen sections in essential. Good frozen sections can be obtained by using cryostats 9mentioned above).

Immunocytochemistry:

Specific molecules within cells can be identified by treating tissue sections with antibodies specific to the molecules. The technique enables chemical substances o be localized in cells with great precision. Such studies have greatly enhanced our knowledge of chemical transformations taking place within the cells.

Autoradiography:

Many molecules (e.g. aminoacids) injected into the animal become incorporated into the tissues of the animal. Sometimes it is possible to replace a normal aminoacid with a radioactive substitute. For example if a radioactive isotope of thymidine. The sites of pressence of the radioactive material can be determined by covering tissue sections with a photographic emulsion. Radiations emerging frm radioactive material act on the emulsion.

Units of measurement in Histology:

The study of histology frequently involves the measurement of microscope distances. The units used for this purpose are as follows.

1 micrometer or micron (um) = 1/1000 of a millimeter (mm).

1 nanometer (nm) = 1/1000 of a micrometer.

Cells, Tissue And Organs:

The human body, like that of most other animals and plants, is made up of units called cells. Cells can differ greatly in their structure.