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Fig. 7. Model of progression of cell identity. Progression of cell
identities (red) proceeds independently of the Wg and Hh patterning signals.
Patterning signals (green) stabilize already specified cell identities and
prevent their switch towards different identities. Differentiation acts on the
cells according to their current cell identity (blue). The already
differentiated second row of groove cells switch from groove cells to cells
producing hairs, showing that differentiation does not block the other two
mechanisms. Interplay between cell identity progression and patterning signals
ensures that a cell identity is specified when the previous and adjacent one
is patterned. Newly specified cells take the patterned cells as a reference,
so the anterior and the posterior side of a group of cells is patterned at
different stages by distinct signals. The progression of cell identity is
therefore visible in the spatial sequence of cell types present in the
wild-type embryo. Wild-type Odd cells normally receive Hh early. This early
influence maintains the population that will later transition to
hair-producing cells, explaining why these cells are missing in Hh
mutants.
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