Biology Reference
In-Depth Information
FIGURE 16.8
The cooperative sharing of a sense of direction among epithelial cells. The North end of cell A
forces the adjacent end of cell B to take on a South character, by activating Strabismus and Prickle and thus sup-
pressing Dishevelled and preventing location of Frizzed in that part of the membrane. The opposite end of the cell is
free to take on North character, with active Dishevelled and Frizzled, and the extracellular parts of these North
complexes force the adjacent part of cell C to be South, and so on. The diagram depicts the proteins currently
thought to be most important in maintaining and propagating planar polarity (the 'core' proteins as they are often
called): there may turn out to be more.
To be active in mediating suppression of 'North' complexes, Prickled has to be activated,
possibly via Strabismus. This activation is driven by signals from the extracellular domains of
the North-type complexes on an adjacent cell. In a row of cells A, B, C
, the North end of cell
Awill therefore activate Prickled behind the adjacent membrane of its immediate neighbour
B, thus excluding 'North'-like activity from that end of cell B and defining it as South
(
Figure 16.8
). The opposite side of cell B is free to take on 'North' character, and this will affect
the next cell along, C, and so on, leading to a chain of cells all lined up like little magnets, the
North end of one meeting and stabilizing the South end of the next.
The importance of cell-cell cooperation in propagating polarity information when setting
up aligned chains of plane-polarized cells has been illustrated by mosaic epithelia that
include clonal islands of cells mutant in one of the critical proteins in a sea of wild-type cells.
When mutants lack Frizzled they cannot broadcast to a neighbouring cell that any part of the
mutant periphery has North character. Wild-type neighbours therefore orientate their own
Northern ends towards the mutant patch. In contrast, mutant cells lacking Strabismus do
not activate Prickled so cannot prevent all of their surface having, and broadcasting, a North
character.
15
They therefore force neighbouring wild-type cells to orientate their South ends
towards the mutants.
)
.
)
This experiment, which used 'hair'-bearing cells of the insect ectoderm to indicate polarity, can be simu-
lated using a tray of small magnetic compasses, of the kind used in school physics teaching. These will
naturally align if close together. If one is replaced by the end of a long bar magnet held vertically, the
surrounding compasses will realign in much the same way as cells realigned in the cell mosaic experiment.
