Introduction

INTRODUCTION

  • Histopathology is the study of pathological changes that are occurring in the tissues at cellular level. Histopathological changes have been widely used as biomarkers in the evaluation of the health of fish infected with pathogens, parasites and exposed to contaminants. One of the great advantages of using histopathological biomarkers in environmental monitoring is that this category of biomarkers allows examining specific target organs, including gills, kidney and liver, that are responsible for vital functions, such as respiration, excretion and the accumulation and biotransformation of xenobiotics in the fish.
  • In order to observe the cellular changes, the tissues need to be sectioned at 4-7 mm thickness. The tissue sections are observed under the microscope following staining. The changes in the tissues observed are compared to that of the normal ones to find out the histopathological changes present in the tissue at cellular level.
  • Since the animal cells are very fragile and easily damaged, the histological technique involves preserving the structural integrity of the cells at the time of cell death and making it available for observation under microscopy. Generally, the principle followed includes fixation of the cellular structure by a suitable fixative (buffered formalin and Davison’s fixative  are normally used for fish and shrimp) and impregnation of wax into the tissues and cutting it using a microtome at desired thickness. The thin sections cut are then stained and observed under a microscope.
  • Since the tissues have considerable amount of water and as the water and wax are not miscible, 2 additional solvents are used following fixation to bridge this lacunae. The aqueous nature of the tissues is changed to wax impregnated nature by the steps of dehydration of the fixed tissue through a series of alcohol, which is miscible with water and second intermediate solvent xylene. Following dehydration, the tissues are cleared of alcohol by xylene (toluene or chloroform could also be used). The xylene, which is miscible with wax is then removed from the tissues and impregnated with wax by putting the tissue in molten wax. The wax impregnated tissues are made into blocks and cut by sectioning using microtome. The sections are later observed after staining conventionally by hematoxylin and eosin and mounting in a mountant.
Last modified: Saturday, 17 September 2011, 7:15 AM