To begin, aspirate the media from the wells where the hydrogel-encapsulated cells are cultured in the 24-well plate. Rinse the hydrogels by pipetting 500 microliters of PBS directly onto each well containing the hydrogels, and then gently aspirate the PBS. Using a 1, 000 microliter pipette, add 500 microliters of fixative solution per well to fix the spheroids in the 24-well plate.
Allow the fixative to soak the gels for 30 minutes at room temperature, before pipetting out the fixative solution and discarding it in a designated waste container. Next, rinse the hydrogels with PBS three times as demonstrated previously. If not used immediately, store the hydrogels in 500 microliters of PBS per well at four degrees Celsius for up to one week.
To stain in the hydrogel-encapsulated cells, first, prepare appropriately diluted primary antibodies for nestin and SOX2. After aspirating the PBS from the wells, add 50 microliters of the diluted antibodies to each well. Incubate the cells with the antibodies for 24 hours to stain them completely.
Then using a 1, 000 microliter pipette, remove the staining solution and discard the waste appropriately. After rinsing the hydrogels three times, store them in PBS at four degrees Celsius for up to two weeks prior to imaging, or image immediately. To clear the stained spheroid for improving imaging transparency, first aspirate the PBS from each well.
Then sequentially treat the spheroids with 500 microliters of 20%40%and 80%formamide per well for 90 minutes each. Finally incubate the hydrogels in 100%formamide for 24 hours prior to imaging. The spheroids were imaged at varying z-stack depths, allowing for the cell viability assessment at each location within the z-stack.
A maximum projection utilizing the spheroid stack represented the highest point of light intensity within each location, simplified into one image. Representative images of stained spheroids revealed that the self-renewal transcription factor, SOX2, was co-localized with DAPI in the nucleus. On the contrary, stem cell marker nestin was present throughout the cells.
There was no difference between the free floating spheroid without gel and the encapsulated spheroids, possibly due to the inertness of the PEG gel used. The representative images of optical sections at about 90 microns into cleared and uncleared spheroids revealed that when clearing was performed, the spheroid core could be imaged about 30 microns deeper compared to an uncleared spheroid.
