Reduced Silver Impregnations Derived from the Holmes Technique

Abstract

Techniques for the reduced silver impregnation of vertebrate central nervous tissues have a long history, but until recently none of them could be made to give consistent results with arthropod materials, and the best results obtained were usually far inferior to those from vertebrate preparations. This chapter describes a number of modifications of Holmes’ method for vertebrate peripheral nerve (Holmes, 1947). The original method was first applied to honey-bee brains by Vowles (1955), but although the staining obtained was promising and allowed a certain amount of gross anatomy to be resolved, it still fell far short of the standard routinely required in vertebrate neuroanatomical work. Blest (1961) introduced modifications that employed alkylated pyridines as vehicles for the silver, and they were further modified by Strausfeld and Blest (1970 cf. Blest, 1976). Weiss (1972) held, with justification, that the published results of these techniques still failed to meet modern requirements, and he made a number of adjustments to the fixation, embedding, and impregnating routines that yielded, in his hands, far greater resolution and differentiation from background. Meanwhile, Rowell (1963) substituted a ‘‘physical” developer for the hydroquinone-sulfite developer used in the other variants.