Optical clearing methods: an overview of the techniques used for the imaging of 3D spheroids.

Spheroids have emerged as in vitro models that reproduce in a great extent the architectural microenvironment found in human tissues. However, the imaging of 3D cell cultures is highly challenging due to its high thickness, which results in a light scattering phenomenon that limits the light penetration. Therefore, several optical clearing methods, widely used in the imaging of animal tissues, have been recently explored to render spheroids with enhanced transparency. These methods are aimed to homogenize the microtissue refractive index (RI) and can be grouped into four different categories, namely i) simple immersion in an aqueous solution with high RI; ii) delipidation and dehydration followed by RI matching; iii) delipidation and hyperhydration followed by RI matching; and iv) hydrogel embedding followed by delipidation and RI matching. In this review, the main optical clearing methods, their mechanism of action, advantages and disadvantages are described. Furthermore, the practical examples of the optical clearing methods application for the imaging of 3D spheroids are highlighted. This article is protected by copyright. All rights reserved.