Understanding the whole human brain at a cellular level, particularly its connectivity, has been a long-standing question in neuroscience. Classical approaches such as cryoprotection and frozen section cutting have proven invaluable in anatomically studying the brain at a cellular level, offering clear visualization of its diverse cells and structures. Diverse high-throughput, integrated digital histology and computational pipelines have been developed to study small brains such as mice, rats and primates such as marmosets. As the size of the brain tissue increases from the mouse (1 cm^3) to the human brain (1500 cm^3), freezing the tissue uniformly for further processing poses unique challenges, mainly non-uniform freezing, tissue cracking, formation of freezing artifacts such as ice crystal formation and misalignment during the freezing process.

This talk focuses on the freezing dynamics, exploring how techniques successful in smaller brains can be adapted and optimized for larger brains. Experiments were conducted to characterize the freezing process, specifically determining the appropriate cryoprotection medium, the duration of cryoprotection, and the freezing source. The outcome is an effective and robust freezing technique utilizing an appropriate choice of cryoprotection and leveraging engineering tools such as brain master patterns, custom-designed molds, and a continuous temperature monitoring system. Notably, brain tissues cryoprotected with graded sucrose solutions, allowed to sink completely, and frozen at a controlled rate of 3-4°C/min, resulted in high-quality histology with minimal artifacts.