The application of incident light polarization microscopy for the visualization of vertebrate sensory hair cells in vivo.

We have examined the feasibility of incident light microscopy for visualizing the poorly reflecting sensory epithelium in the macula of the fish lateral line in vivo. This investigation demonstrates that the apical cell surface and the hair bundles of the sensory hair cells in the epithelium can be visualized with incident light polarization microscopy (ILPM). Under the most favourable conditions of illumination, individual sensory hairs with a diameter of about 0.3 micron can be distinguished. Also, in the superficial layers of the macula up to a depth of about 200 microns branching nerve fibres and blood capillaries can be readily discerned. From measurements of the polarized light flux as a function of focal depth it followed that the image of the sensory epithelium is formed by the light reflected from the underlying nerves. Thus although the preparation is illuminated with incident light, the image of the object is actually formed under transmitted light conditions. We demonstrate further that the present microscopical technique can be successfully applied to several other poorly reflecting biological objects, which are unsuited for transmitted light microscopy because of their thickness.