I feel pretty confident that your issue is the cryostat temperature, not your freezing technique. Tissue + OCT in mold over isopentane and liquid nitrogen is standard. We use powdered dry ice which isn’t standard and I did have issues with the grey matter shattering like that when I first started sectioning/histology on hSpinal cord… and it was completely resolved by optimizing the cryostat/sectioning conditions. For me, the chuck around 14 degrees, and chamber around -18 to -20 usually works great for me. I usually swap out the blade for a warmer one when I see the grey matter shattering like that, and you can even put your thumb against the sample to warm it slightly then try sectioning again. Faster speed works best - I think its just the abrupt difference in tissue density between white and grey matter.