69transdifferentiation of supporting cells, and the molecular mechanisms underlying
them may also be investigated within this in vitro framework.
In the emerging era of precision medicine, organoids have already demonstrated
an important role: one study demonstrated variable responses to drug administration
in intestinal organoids derived from patients with cystic fibrosis. These findings
have strong implications for prediction of specific therapeutic effects in individual
patients with cystic fibrosis (Dekkers et al. 2013 ). Similar investigations aimed at
tailoring interventions for inner ear disease would prove highly useful in investigat-
ing treatments for deafness or vestibular dysfunction.
One major limitation present within the current inner ear organoid protocol is the
absence of cochlear cell types in the derived tissues. Further refinements to this
protocol may expand the diversity of inner ear cell types within the organoid and
more closely mimic the in vivo composition of the inner ear. This deficiency may be
rectified through use of human ESCs; successful adaptation of a mouse ESC-derived
protocol for optic cup formation into one using human ESCs resulted in human reti-
nal organoids that are much larger in size and contain a greater number of layers as
well as more diverse photoreceptors when compared to mouse organoids (Nakano
et al. 2012 ). It will be interesting to assess if significant differences in overall mor-
phology or types of derived cells are also observed between mouse and human inner
ear organoids. Additionally, to accurately quantify the effects of experiments, a
more robust endpoint analysis of organoids must be achieved. This may be accom-
plished by incorporating fluorescence-activated cell sorting or whole-mount immu-
nolabeling techniques into organoid analysis.
Acknowledgments The authors would like to thank Atsushi Shimomura for the schematic draw-
ing and Rachel DeJonge and Andrew Mikosz for some of the image data. This work was supported
by a National Institutes of Health grant R01 DC013294 (to E.H.), an Indiana Clinical and
Translational Research Institute predoctoral fellowship (to A.N.E.), and a Centralized
Otolaryngology Research Effort (CORE) grant (to R.F.N.).
References
Ahrens K, Schlosser G (2005) Tissues and signals involved in the induction of placodal Six1
expression in Xenopus laevis. Dev Biol 288(1):40–59
Bailey AP, Streit A (2005) Sensory organs: making and breaking the pre-placodal region. In:
Current topics in developmental biology, vol 72. Academic, San Diego/London, pp 167–204
Baker C, Bronner-Fraser M (2001) Vertebrate cranial placodes I. Embryonic induction. Dev Biol
232:1–61
Bhat N, Kwon H, Riley B (2013) A gene network that coordinates preplacodal competence and
neural crest specification in zebrafish. Dev Biol 373(1):107–117
Bouchard M, Andersson E, Novitch B, Muhr J (2004) Tissue-specific expression of cre recombinase
from the Pax8 locus. Genesis 38(3):105–109
Breneman K, Brownell W, Rabbitt R (2009) Hair cell bundles: flexoelectric motors of the inner ear.
PLoS One 4(4):e5201
4 Inner Ear Organoids: Recapitulating Inner Ear Development in 3D Culture