HISTOLOGY

Histology is a branch of science that deals with microscopic study of cells structure, composition and tissues of higher organisms like plants and animals. The thin sheets or films of cells or tissues are viewed either under light microscope or electron microscope. Structure of cells and tissues can be characterized with the help of histological stains. This technology plays a  vital role in the field of medicine and biology.

Types of animal histological tissues

Major classifications of histological animal tissues are of four types.

• Nerve tissues
• Muscle tissues
• Connective tissues like blood cells
• Epithelial tissues

Preparation of histological stains

There are various ways of preparation of thin sheets of cells and tissues as mentioned below.

• Fixing
• Processing
• Embedding
• Sectioning
• Staining
• Cryosection

Fixing

Thin smears can be fixed with the help of chemical agents or by freezing. A chemical agent like formaldehyde helps tissues from further degradation. It is used as a fixative for light microscope observation. Fixatives maintain structure of cell membranes and cellular organelles. Glutaraldehyde is used as a fixative for electron microscopic observation. To rapidly fix the biological materials, frozen section is prepared with the help of cryostat.

Processing

This technique assists in removal of water from tissues and is replaced with medium. Size of thin films shall be 5 µm and 80-100 nm for light and electron microscope respectively. Water from tissues is removed by placing samples in alcoholic medium. Then, clearing agents like xylene is used to remove alcohol. Infiltration agents like paraffin wax, in case of light microscope, or epoxy resins/ acrylic resins, in case of electron microscope, are used to replace xylene.

Embedding

After processing tissues by dehydrating, use of clearing agents and infiltrating agents, they are embedded. Embedding agents like agar, wax or gelatin help in hardening of blocks. Thin sections are now ready for sectioning.

Sectioning

Two types of sectioning are, vertical and horizontal sectioning. Vertical sectioning of tissues is quite usual. Horizontal sectioning of tissues is used in evaluation.

Staining

Staining agents like hematoxylin and eosin are employed in light microscope. Basic dye hematoxylin, blue in colour, stains nucleus of the cell. Acidic dye eosin, pink in color, stains cytoplasm.

Cryosection

Tissues are frozen with the help of cryostat. Embedding agents like plastic or eosins are used in case of electron microscope.  Thin sections of tissues are cut in to blocks with the help of glass knives or diamond. They are stained using electron dense heavy metal staining agents like lead and uranium so that tissues can be easily seen through electron microscope.