Selective sensitivity to methylmercury of specific cell aggregates and validity of the "nurse" cell concept

Meristematic cells of carrot (Daucus carota--Ca68-10) were grown in a heterogeneous suspension culture in the absence of (2,4-dichlorophenoxy)acetic acid in 71-V medium in the dark. Freshly inoculated cell cultures were treated daily with methyl mercury for the first 4 days and harvested on Day 7. The cultures were separated (by selective filtration) into groups of aggregates varying in size from 44 to 900 microns. Methyl mercury severely decreased the cell mass of the dominant (300-microns) fraction. When separate aggregates were cultured (44 to 900 microns) and treated as before, the inhibition of the 300 microns fraction was more evident. Separately cultured cell groups of 200- or 300-microns size developed into larger heterogeneous populations and contained the dominant 300-microns size group which was the most sensitive to methyl mercury. Other cell groups (900-, 500-, and 44-microns) failed to produce a large heterogeneous cell population when cultured separately. Without the 300-microns-sized aggregate, neither the whole culture nor the separate aggregates culture (900, 500, or 44 microns) could survive, suggesting the "nurse" cell role of the 300-microns cell group.