RELATION OF EFFECTIVE THICKNESS AND REFRACTIVE INDEX TO PERMEABILITY OF TISSUE COMPONENTS IN FIXED SECTIONS.

Abstract

SYNOPSISAccurate and reproducible interference microscopy of unstained sections requires (a) the use of a shearing system, preferably with a half‐shade eye‐piece; (b) complete removal of embedding wax from the section; (c) complete replacement of one mounting medium by any successive medium; (d) accurate measurement of the refractive index (RI) of the mounting media at the temperature and wave‐length used for microscopy. The optical path through a specimen mounted in water is consistently lower than expected from measurements in non‐polar fluids—this is attributed to swelling of the tissues in water. Cellulose, glycogen and some mucins have a relatively low RI, and secretory granules and cytoplasmic chromidial substance a relatively high one. The highest RI observed was in calcified cartilage matrix. The “effective thickness” of tissue components, especially certain mucins, is often considerably less than the geometrical thickness of the section. This is attributed to the presence in the tissues of substantial spaces containing mounting medium.The relation of the RI and the effective thickness of tissue components in sections to their permeability to histological dyes is discussed. It is concluded that both the RI and the effective thickness may be of some value in predicting the ease with which dye particles of a given size can penetrate the tissue.