Clarifying Tissue Clearing

Abstract

Biological specimens are intrinsically three dimensional; however, because of the obscuring effects of light scatter, imaging deep into a tissue volume is problematic. Although efforts to eliminate the scatter by "clearing" the tissue have been ongoing for over a century, there have been a large number of recent innovations. This Review introduces the physical basis for light scatter in tissue, describes the mechanisms underlying various clearing techniques, and discusses several of the major advances in light microscopy for imaging cleared tissue.

Figure 1. Wave optic view of light passing through a homogenous scattering medium

(A) In a material with a uniform density of scattering molecules (shown here as stars) such as glass, water, or air, light is not scattered orthogonal to the direction of the incoming light wave (arrows). In this situation all the scattering molecules that are excited simultaneously (colored stars) are aligned orthogonal to the direction of the plane wave. (B) A wave view to explain what is happening in panel A. Each of these simultaneously excited molecules generate wave energy in all directions but by virtue of their different positions there is always a molecule that is exactly ½ a wavelength out of phase with every other molecule in the vertical direction causing destructive interference. (C) The scatters that are aligned in the direction of the incoming light wave (arrow) are activated sequentially (colored stars). Their excitations sum constructively in the forward direction causing no attenuation of the light moving through the medium. (D) A wave view to explain what is happening in panel C. Shown is the same light wave a four successive time points 1 fs apart. At the first time point (top) the wave imparts vibrational energy to a scattering molecule (red star). 1 fs later the molecule emits the absorbed energy (red wave, 1/2 phase delay from incoming light). At this time another molecule (blue star) is first excited by the incoming light wave. At 2 fs the molecule denoted by the blue star emits its scattered light (blue wave) that is in phase with the scattered light from the red wave and a third molecule (green star) is excited by the incoming light wave. At 3 fs the light from all three scatters are vibrating in phase (superimposition of the red, blue and green waves) and another molecule (yellow star) absorbs energy from the incoming light wave. In this way all forward moving scattered light remains in phase causing no attenuation in light energy in this direction.

Figure 2. Methodology of tissue clearing techniques

(A) Left, Solvent based clearing is a two-step process. First, the tissue is dehydrated and lipid is removed. Second, the tissue is moved to a high refractive index solvent where additional lipid solvation and clearing occurs. Right, Molecules commonly used for solvent based clearing along with the refractive indices (RI) of the pure chemical. (B) Left, For simple immersion, the tissue to be cleared is placed in an aqueous clearing solution for days to months. During this time the solution is exchanged repeatedly. Right, Molecules commonly used for simple immersion along with the refractive indices (RI) at the commonly used concentration. (C) Hyperhydration involves submerging the sample in an aqueous solution and allowing it to passively clear. During this clearing step, urea or formamide in the clearing solution can enter tightly folded regions of high refractive index proteins creating an osmotic gradient that pulls in water as well. This partially denatures the protein, hydrates it and decreases its overall refractive index. Some hyperhydration methods contain detergent which is used to disrupt membranes and remove lipid from the sample. (D) Left, Hydrogel embedding is most often performed on an entire animal by perfusing with a fixative, a temperature sensitive crosslinker, and the hydrogel monomer. Alternatively, these chemicals can be passively diffused into an isolated tissue sample. Once fixed, the tissue of interest is warmed to induce hydrogel crosslinking. The sample is then placed in a detergent solution to remove lipid material passively or via an electrophoretic charge. Finally the lipid-free sample is placed in a high-refractive index matching solution for clearing. Histodenz is one high refractive index molecule that can be a component of this clearing solution. Glycerol, TDE or Diatrizoic acid also can play this role.