Alginate encapsulation as a novel strategy for the cryopreservation of neurospheres

Primary cultures of brain cell neurospheres are valuable in vitro models for neurotoxicology and brain cell research. Such applications would greatly benefit from the development of efficient cryopreservation protocols that assure the availability of viable and genetically stable stocks of functional neurospheres. In this work we aimed at developing an integrated strategy allowing for long-term culture and cryopreservation of brain cell neurospheres with high viability and reduced recovery time postthawing. Microencapsulation in clinical-grade, ultrahigh viscous, highly purified alginate uniformly cross-linked with Ba2+ was evaluated as the main strategy to avoid the commonly observed loss of cell-cell and cell-matrix interactions with consequent aggregate's fragmentation and decrease in cell viability that occurs postthawing. Brain cells isolated from 16-day-old fetal rats were cultured in spinner vessels as neurospheres, encapsulated at the 5th day of culture, and cryopreserved at day 19. Culture characterization and assessment of postthawing recovery, concerning cell metabolism, aggregate's cell type composition, and neuron-astrocyte interactions were performed through analysis of membrane integrity, metabolic activity assays, and immunohistochemistry. Our results show that the encapsulation process does not affect cell viability's central metabolism; neither cell differentiation nor cell extensions into cell networks are usually observed between neurons and astrocytes within the neurosphere structure. Neurosphere encapsulation resulted in reduced recovery time postthawing and significantly less fragmentation. Further, the use of serum-free CryoStor (TM) solution provided further protection for both nonencapsulated and encapsulated aggregates compared with serum-supplemented culture medium as the cryopreservation medium.