Mounting media and slide coatings

Introduction

In order to provide the maximum degree of transparency to stained tissue sections, the refractive index of the mounting medium must approximate to that of dried protein, i.e. between 1.53 and 1.54. This is especially important for photographing slides. To visualize detail in unstained tissues, it may be desirable to employ a medium with lower or higher refractive index. The refractive index of a mounting medium may change on drying due to evaporation of solvents. Air bubbles should not be permitted to remain under coverslips since these tend to expand.

Mountants

Most pathology laboratories currently utilize commercial mounting media, usually non-aqueous. From the standpoint of safety and costs, this may be a laboratory’s best approach to mounting sections.

Potassium acetate may be added to mountants, almost to the point of saturation, in order to reduce the ‘bleeding’ of cationic dyes, at the same time giving a pH of approximately 7.0.

Aqueous mounting media are required for some special stains, e.g. Oil Red O where using solvents would dissolve the fats being demonstrated. In direct immmunofluorescent stained slides the aqueous mountant should be fluorescence free. Few aqueous mountants have a refractive index higher than 1.5, most being in the range 1.4 to 1.45. Higher refractives are usually achieved by the use of high concentrations of sugars.

For other specialized mountants, see the appendices of the 6th edition of this text.