cell fates within these patches may have involved cell interactions
mediated by the Notch system.
It is still not clear whether the ancestral sensory neuron evolved
into the specialized hair cell, or whether the hair cell arose de novo
(as did the ciliary photoreceptor cells in the vertebrate retina,
whose ganglion cells seem to be homologous to the rhabdomeric
photoreceptors of invertebrates, Arendt et al., 2002). One possibil-
ity is that the ancestral sensory neuron duplicated, with one copy
becoming specialized in the sensory function (the hair cell), and the
other retaining the signal transmitting function (the neuron). In-
deed, the hair cell retains many properties of a true neuron,
including its presynaptic specialization. Not only could this separa-
tion of functions have facilitated cell specialization, but it would also
open the possibility of coupling a single neuron to a larger number
of sensory cells, thereby increasing the sensitivity of the system.
Interestingly the same situation occurs in the visual system, where
single ganglion cells are also coupled to a large number of ciliated
photoreceptors.