Careful histological observation of the development of the
anlage of the inner ear in chicken embryos led us
to question the traditional view of otic placode (OP) formation.
First, morphological studies in the cephalic
region carried out on stages preceding the appearance of the
placodal epithelium revealed that the medial
placodal cells are continuous temporally and spatially with cells
belonging to the neural fold (NF). Second,
both the formation of the basal lamina between the dorsal region
of the neural tube (NT) and ectoderm and
the pattern of formation of the neural crest present distinctive
characteristics between otic levels and regions
located anteriorly and posteriorly. Third, numerical comparisons
of parameters for the NT and the OP
between different levels of the rhombencephalon allowed us to
assign a differential behaviour in the growth
pattern of the otic region. These results indicated that the
medial part of the OP is not derived from already
independent ectoderm that increases in thickness under the
influence of the NT (as previously accepted) but
that it develops directly from the NFs. Although we do not
exclude other possibilities, we propose that at
least a proportion of the OP cells originate directly from
cells committed to be neural crest. After this
incorporation, basal laminal formation would delimit the NT
from the OP without transition of the otic
cells to ectoderm. This hypothesis would imply that part of the
otic cells originate directly from
neuroepithelial cells having a neuroectodermal (rather than the
previously established ectodermal) origin.