Confocal microscopy allows for optical sectioning of tissues, thus
obviating the need for physical sectioning and subsequent registration
to obtain a three-dimensional representation of tissue architecture.
However, practicalities such as tissue opacity, light penetration, and
detector sensitivity have usually limited the available depth of
imaging to 200 μm. With the emergence of newer, more powerful
systems, we attempted to push these limits to those dictated by the
working distance of the objective. We used whole-mount
immunohistochemical staining followed by clearing with benzyl
alcohol-benzyl benzoate (BABB) to visualize three-dimensional
myocardial architecture. Confocal imaging of entire chick embryonic
hearts up to a depth of 1.5 mm with voxel dimensions of 3 μm was
achieved with a 10× dry objective. For the purpose of screening
for congenital heart defects, we used endocardial painting with
fluorescently labeled poly-L-lysine and imaged BABB-cleared hearts with
a 5× objective up to a depth of 2 mm. Two-photon imaging of
whole-mount specimens stained with Hoechst nuclear dye produced clear
images all the way through stage 29 hearts without significant signal
attenuation. Thus, currently available systems allow confocal imaging
of fixed samples to previously unattainable depths, the current
limiting factors being objective working distance, antibody
penetration, specimen autofluorescence, and incomplete clearing.