Biology Reference
In-Depth Information
FIGURE 12.5
Orthogonal, reciprocal gradients of Ephrin-A
e
EphA and Ephrin-B
e
EphB proteins in the retina
(in each case, several proteins from the families are involved). The cartoon face shows the orientation of retinas,
which all represent the right eye; they have been 'flattened' for ease of drawing. The triangles represent changing
concentrations and are not meant to imply that the expression pattern is actually triangular (it extends across the
retina perpendicular to the gradient vector).
The centripetal chronotopic
)
relationship is not the only large-scale pattern in the retina;
there are also orthogonal gradients of Ephs and Ephrins (see Chapter 11). Ephrin-B proteins
and their receptors follow dorso-ventral gradients, Ephrin-B proteins being highest dorsally
and lowest ventrally while their EphB receptors are lowest dorsally and highest ventrally
(
Figure 12.5
). Similarly, Ephrin-A proteins are highest nasally and lowest temporally,
and their EphA receptors lowest nasally and highest temporally (
Figure 12.5
).
48
e
51
This effec-
tively sets up a coordinate system independent (at the time it is read) from the chronotopic
one. In some ways, the use of four additional gradients to create a system for specifying pos-
itional information seems surprising, since it would be possible to convert the radial coord-
inate chronotopic gradient into a complete position-specifying system by adding just one
extra component, a circumferential one, to make a polar coordinate system like the global
system of latitude and longitude as seen from above the North Pole. There is, however,
a problem with polar coordinate systems: they require a sharp discontinuity in an otherwise
steady circumferential gradient, like the 23h59min
0h0min transition on a clock face, and it
may have just been biologically easier for embryos to avoid this issue by using gradients
across opposite diameters; besides, evolution is not bound by any selection pressure for
mathematical elegance.
/
)
Chronotopic
ΒΌ
linking time and place.
