Early Steps towards Hearing: Placodes and Sensory Development

Abstract

Sensorineural hearing loss is the most prevalent sensory deficit in humans. Most cases of hearing loss are due to the degeneration of key structures of the sensory pathway in the cochlea, such as the sensory hair cells, the primary auditory neurons, and their synaptic connection to the hair cells. Different cell-based strategies to replace damaged inner ear neurosensory tissue aiming at the restoration of regeneration or functional recovery are currently the subject of intensive research. Most of these cell-based treatment approaches require experimental in vitro models that rely on a fine understanding of the earliest morphogenetic steps that underlie the in vivo development of the inner ear since its initial induction from a common otic-epibranchial territory. This knowledge will be applied to various proposed experimental cell replacement strategies to either address the feasibility or identify novel therapeutic options for sensorineural hearing loss. In this review, we describe how ear and epibranchial placode development can be recapitulated by focusing on the cellular transformations that occur as the inner ear is converted from a thickening of the surface ectoderm next to the hindbrain known as the otic placode to an otocyst embedded in the head mesenchyme. Finally, we will highlight otic and epibranchial placode development and morphogenetic events towards progenitors of the inner ear and their neurosensory cell derivatives.

Keywords: epibranchial; gene regulatory network; hair cell; inner ear; sensory neuron; sensory placode; taste buds.

Figure 1

Comparison of the critical steps identified in distinct steps of neural induction of pre-placode. Initially, Nodal, Activin, BMP and Wnt are expressed throughout the ectoderm. Gmnn, Zic and Foxd4 become expressed, upstream from Irx, and upregulate Sox expression. An early expression of Shh drives Gli expression ventral and is counteracted by the roof plate/choroid plexus. Lmx1a/b drives the dorsal expression of Wnts and BMPs, including Gdf7. Additional downstream genes of Fgf3/10 and Foxi3 are needed for the earliest pre-placode definition. Downstream and largely independent include genes such as Eya1 and Pax2/8, among others. Modified after [37,38,44,46,47,48,49].

Figure 4

Hair cells and taste buds are independently derived. Progenitors of hair cells depend on Eya1, Sox2 and Atoh1 that are needed for their development. Downstream are Pou4f3 and Gfi1 that are needed to maintain hair cells. Tbx1,2,3 interacts with Neurog1, and Foxg1 regulates the number and distribution of cochlear to make it shorter and has increased the number of hair cells. A loss of all cochlear hair cells is revealed in Pax2, Gata3 and Lmx1a/b mice. Two types of vestibular hair cells exist that have a mixed distribution of type I and type II HCs. In contrast in the cochlea, we have a single row of IHCs and three rows of OHCs. Tbx2, Srrm3/4 and Fgf’s are needed for differentiation and viability of IHC. Insm1, Ikzf2 and Fgf20 are needed to differentiate OHCs or requires for forming three rows of OHCs. A very different sequence of genes is needed in taste buds. Starting is the upregulation Krt8/14 that is prior to Shh. Downstream is Sox2 that is required for taste bud differentiation. Overlapping are other genes some of which seem to differentiate into distinct taste sensory input. Compile with permission from Refs. [4,8,16,52,84,110,140,141,142,143,144,145].

Figure 5

Progenitors of mammals are dependent by initial Eya1, Sox2 and Neurog1 that are needed for several genes to differentiate neurons of either the otocyst or the epibranchial placodes. In the ear, downstream are a large set of genes, among them are Neurod1, Isl1 and Pou4f1. A split is dependent on Tlx3 that forms VGNs, whereas SGNs are not dependent on Tlx3. After that, the upregulation of Sall3 is needed to make two different kinds of VGNs but has a third population that is unclear as to its origin. SGNs split into four populations: Ia, Ib, Ic and II. A unique set of genes and their expression needs to be verified to develop each type of SGN, notably Pax2, Gata3 and Lmx1a/b. For the epibranchial neurons, unique common genes are deviate in the epibranchial neurons by switching the Neurog1 into Neurog2 and have early expressions of Phox2b, Isl1, Neurod1 and Foxg1. Image is compiled with permission from Refs. [4,17,108,116,142,180,186].

Figure 2

Otic placode (OP) and epibranchial placode (EP) induction. Fgf3 and Wnt1/3a/8a from the hindbrain downstream of Lmx1a/b as well as Fgf8 and Fgf10 from the surrounding somite’s combined with Foxi3 are believed to be sufficient to induce the otic and epibranchial placode. The subsequent mechanism is not fully elucidated but may involve Jagged1 and Notch1 inhibition of Foxi2. Upregulation of Dlx5/6, Eya1, Foxg1, Gata3, Gbx2, Hes2, Hmx2/3, Lmx1a/b, Pax2/8, Six1, Sox9, Spry1 and others marks the otic and epibranchial placode and is essential for later otic vesicle specification and morphogenesis. Foxi2 delineates the ectoderm. Adapted with permission from Refs. [48,49,72,79].

Figure 3

Generating the otic vesicle and the epibranchial placode. (A) The otic vesicle is depicted in a mouse using 3D reconstruction of the ear. An early formation segregates from the endolymphatic duct (white). Separate color is defined as the canal cristae (blue) next to the utricle (white) that interconnects the saccule (lilac). Note the progression of the length of the cochlear duct can about E16.5 (yellow). (B) The neuronal differentiation of epibranchial placodes starts with Eya/Six next to the otic vesicle (OV) followed by Pax2 and Sox2, which initiate transformation of the geniculate (G), petrosal (P) and nodose (N). As neuroblasts migrate from the ectoderm to deeper locations, additional factors are expressed in sequence, Neurog2 (in mice, Neurog1 in chicken) followed by Neurod1, Isl1, Foxg1, Pou4f1 and Phox2b. Abbreviations: OP, olfactory placode; OV, otic vesicle, T, trigeminal neurons; VA, vestibular and auditory neurons. Reprinted with permission from Refs [17,105,128].