Some highly enriched synthetic media permit the culture of as few as 25-50 cells/ml, while some media supplemented with casamino acids and coconut milk support cell division at 1-2 cells/ml. It is postulated that some biochemicals essential for cell division leach into the culture medium when they are subcultured; this produces the lag phase.

Cells begin to divide only when an equilibrium is established for these metabolites between the medium and the cells; this happens much later at lower cell densities and below a critical cell density it may not be achieved. For this reason conditioned or specially enriched media are required for culture of cells at low densities.