Cells may be cultured as monolayers or, if a system simulating cell-cell interactions of the tissue of origin is required cells may be cultured in suspension with mild agitation or using rolling cell culture. Under these circumstances, cells form clusters, spheroids or reaggregates that may continue to grow proliferate over a period of several days or weeks, making them amenable to studies of long-term effects. For example, brain reaggregates pre- pared in this way have been maintained for up to sev eral weeks and used in studies of pesticide toxicity(Sales et al., 2000). However, many primary cultures tend to be very short lived. However, primary cell cultures do not necessarily exhibit identical cell cell interaction patterns