Abstract: The present invention provides biological samples of interest that have been iteratively expanded in a method referred to herein as iterative direct expansion microscopy (id-ExM). In the id-ExM method, biological samples of interest are permeated with a swellable material that results in the sample becoming embedded in the swellable material, and then the sample can be expanded isotropically in three dimensions. The process of iteratively expanding the samples can be applied to expand samples one or more additional times such that, for example, a 5-fold expanded sample can be expanded again to achieve high expansion factors, for example, 20× to 100× or more linear expansion.