Fig. 6. Proposed polar cell lineage. Given that polar cells and stalk cells have been shown to have common precursors, we propose the existence of at least 3 stalk and polar cell precursors (white circles, first row) for each polar cell pair and neighboring stalk cells. The stalk cells considered are not necessarily those immediately adjacent to the polar cell pair since intercalation events disperse the cells giving rise to an interfollicular stalk. Such precursors would first generate groups of pre-polar (pink circles) and pre-stalk (blue circles) cells. Divergence between pre-stalk and pre-polar cells could occur either in the course of stalk cell/polar cell precursor divisions (1, arrow), or after completion of divisions (2, arrow). A subsequent step involving apotosis-mediated selection of the final polar cell pair (red) would then take place among groups of up to 6 pre-polar cells (indicated by Xs in boxed area). Although we do not have any evidence yet for the existence of a selection process among pre-stalk cells, such a mechanism could also be part of the stalk cell maturation program (X ? in boxed area).

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