Histology Lab Outline
Introduction
Histology
Histology is the study of the cellular organization of body tissues and organs. The term is derived from the Greek “histos” meaning web or tissue, and refers to the “science of tissues”. The compound light microscope is the tool used most widely for clinical applications of histology. However, the advent of the electron microscope greatly extended the detail at which subcellular structure can be studied. Thus, histology now embraces the study of the structures of both tissues and cells, and the relationship between these structures and physiological function.
The structure of cells and tissues can be distinguished at two levels. The fine structure is that which can be distinguished at the level of compound light microscopy (a magnification of 1000x or less). Electron microscopes are generally employed to study ultrastructure, the detailed structure of the cell cytoplasm, organelles and membranes that is not discernable with a compound light microscope.
Many techniques have been developed which are designed to preserve the structural integrity of a specimen so that it can be viewed microscopically. The process through which cell structure is preserved is called fixation. Since cells rapidly deteriorate after a tissue has been removed from the body, achieving adequate fixation is often the most difficult task confronting a histologist. “Artifacts” are changes to the original structure of cells and tissues that arise from tissue deterioration and from the fixation process itself. Thus, a skilled histologist employs techniques that minimize the formation of artifacts in different types of tissues, and has the ability to distinguish artifacts from normal cell structures.
Cell structure is most commonly studied in slices of the tissue, called sections, which are thin enough to allow transmission of light or an electron beam. There are many methods of sectioning tissues, and sometimes particular tissues require special techniques. The method most widely employed is called the paraffin method. Although this technique is not universally applicable, e.g., it does not work well with hard tissues such as woody parts of plants or bones from animals, it does present many advantages over alternative methods. The necessary reagents are inexpensive, readily available, and much less toxic to humans than those used in most other techniques.
How do we see specific parts of a organism, organ or tissue?
Sectioning a tissue into a thin slice (e.g.. 5 – 10 µm) is required in order to examine the tissue using transmitted light microscopy because the light needs to pass through the section in order to visualize the different structures. However, it is important to understand that the orientation of the tissue while sectioning will determine the parts of the cells/structure/tissues that can be visualized on the microscope slide. This is largely related to the anatomical planes and the symmetry (or asymmetry) found in living orga