Organ Regeneration Based on Developmental Biology

59

The differentiation of embryonic stem cells (ESCs) into inner ear organoids
comprising sensory epithelia, including hair cells, has emerged through translation
of normal developmental cues by way of in vitro application of proteins and small
molecules at precise time points in a three-dimensional (3D) cell culture system
(Koehler et al. 2013 ; Koehler and Hashino 2014 ).

4.2 Inner Ear Development

The anatomic arrangement of inner ear components reflects the immense intricacy
in the embryologic transition from ectoderm to the fully developed and functional
inner ear. The relevant, sequential stages of inner ear development may be generally
ordered as definitive ectoderm, nonneural ectoderm (NNE), preplacodal region/
ectoderm (PPR), otic epibranchial placode domain (OEPD), and otic placode
(Fig. 4.2). Further development of the otic placode gives rise to hair cells with struc-
tural and functional properties of native mechanosensitive hair cells, sensory neu-
rons, and supporting cells of the inner ear.
During the process of gastrulation, in which the blastula is rearranged into three
germ layers, the definitive ectoderm is divided into neural and nonneural compo-
nents (Litsiou et al. 2005 ; Saint-Jeannet and Moody 2014 ). The neuroectoderm is
destined to form the central nervous system and neural crest derivatives, whereas
the nonneural ectoderm gives rise to the inner ear, other placodal derivatives, and
the epidermis. The lineage choice between neuroectoderm and nonneural ectoderm
is mediated by bone morphogenetic protein (BMP) signaling—in which high levels
of BMP in the lateral ectoderm induce NNE formation (Kwon et al. 2010 ; Leung
et al. 2013 ). BMP-mediated induction of NNE may be facilitated by transcription
factors from the Dlx and GATA families (Pieper et al. 2012 ). Analysis of zebrafish

ESC

Mesoderm Cranial

Placodes

Cranial

Placodes

Ectoderm NNE PPR

Mesoderm NENNEE EpidermisEpidermisEpidermis PlacodesCranial

Otic

Placode

TGFβi BMP BMPi/FGF Wnt/FGF

Fig. 4.2 Temporal sequence of embryological steps and pertinent signals involved in the induction
of the inner ear within described 3D stem cell differentiation protocol. ESC embryonic stem cell,
NNE nonneural ectoderm, NE neural ectoderm, PPR preplacodal region, TGFβi inhibition of
TGFβ signaling, BMPi inhibition of BMP signaling

4 Inner Ear Organoids: Recapitulating Inner Ear Development in 3D Culture