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Fig. 7. Model of restriction of size and patterning of animal plate.
(A) The egg has a neuralizing potential in the animal half. The size
and patterning of the neural tissue depends on the ß-catenin-mediated
specification of endomesoderm. (B) During cleavage, specification of
endomesoderm in vegetal blastomeres also specifies ectoderm as ciliary band,
which restricts neurogenic ectoderm to a small animal plate. (C) As the
ectodermal domain is specified as ciliary band, oral and aboral ectoderm,
neuron formation is patterned in the animal plate. Nodal acts as a paracrine
repressor of serotonergic neuron formation where oral ectoderm is adjacent to
the animal plate. Serotonergic neurons form where the animal plate is adjacent
to ciliary band and aboral ectoderm. Red, animal plate; blue, endomesoderm;
purple, ciliary band; yellow, oral ectoderm; light green, aboral ectoderm;
green cells, serotonergic neurons.
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