Development of the cochlea

Abstract

The cochlea, a coiled structure located in the ventral region of the inner ear, acts as the primary structure for the perception of sound. Along the length of the cochlear spiral is the organ of Corti, a highly derived and rigorously patterned sensory epithelium that acts to convert auditory stimuli into neural impulses. The development of the organ of Corti requires a series of inductive events that specify unique cellular characteristics and axial identities along its three major axes. Here, we review recent studies of the cellular and molecular processes regulating several aspects of cochlear development, such as axial patterning, cochlear outgrowth and cellular differentiation. We highlight how the precise coordination of multiple signaling pathways is required for the successful formation of a complete organ of Corti.

Keywords: Atoh1; Deafness; Hearing; Notch; Organ of Corti; Radial intercalation.

Fig. 1.

The inner ear and cochlea. The bony and membranous labyrinth of the inner ear includes both dorsal/vestibular structures, related to perception of balance and motion, and the ventral cochlear duct, which transduces sound. The membranous labyrinth (gray) is composed of epithelial cells and is surrounded by the bony labyrinth (beige), which is derived via the condensation of periotic mesenchyme. The cochlear duct includes three canals (or ‘scala’): the scala vestibuli, the scala media and the scala tympani. The scala media, part of the membranous labyrinth, is comprised of three walls: Reissner's membrane (RM), the stria vascularis and spiral ligament (SV), and the cochlear floor, which contains the sensory organ of Corti (OC) flanked by two regions of non-sensory cells, the inner sulcus (IS) and outer sulcus (OS). The organ of Corti contains two types of hair cells (inner hair cells and outer hair cells) and several different types of unique supporting cell types, including inner phalangeal cells adjacent to the inner hair cells, pillar cells separating the inner and outer hair cells, Deiters’ cells interdigitated among outer hair cells, and Hensen's cells lateral to the organ of Corti. Hair cells and supporting cells are arranged in a precise cellular mosaic. Hair cells have characteristic stereocilia bundles on their lumenal surface, and the stereocilia of the outer hair cells are in contact with the tectorial membrane (TM) within the scala media. Bipolar neurons of the spiral ganglion (SG) synapse with hair cells and project centrally to the cochlear nucleus.

Fig. 2.

Cochlear extension and patterning of the duct along the medial-to-lateral axis. (A) Diagram illustrating extension of the cochlear duct (CD) from the ventral region of the otocyst beginning at E11 through E14. A cross section through the cochlear duct at ∼E14 (shown on right) highlights the mature structures that arise from each region, with the transient embryonic structures indicated parenthetically. (B) Examples of gene/protein expression patterns in the cochlear duct during normal development. Overlapping expression is indicated by hatched colors. SG, though still present, is not shown in panels C-G. (C) Bmp4 expressed in the future OS potentially creates a morphogen gradient along the lateral-to-medial axis that regulates cell fates. Reduction of Bmp signaling (i.e. in the case of Alk3/6 conditional deletion) leads to a lateral shift of medial regions and the absence of lateral cell fate markers. (D) Loss of Fgf10 signaling in Fgf10^Δ2/Δ2 cochleae leads to the absence of RM and changes in the shape of the cochlear duct. KO is still present, as indicated by continued expression of Fgf10. (E) Otx2, expressed in the developing RM, is necessary for its formation. Absence of Otx2 results in expanded expression of KO and PS genes (Fgf10, Sox2) and formation of an ectopic PS domain, indicated by Lfng expression (star). (F) Lmo4, normally expressed in two regions just medial and lateral to the PS domain at E14, inhibits sensory cell fates. Loss of Lmo4 leads to formation of an ectopic mirror-image PS domain in the lateral cochlear duct (star). (G) In Esrp1 mutants, splicing defects in Fgfr2 result in disruption to FGF signaling, causing expansion of the RM domain at the expense of the SV. IS, inner sulcus; KO, Kölliker's organ; LER, lesser epithelial ridge; OC, organ of Corti; OS, outer sulcus; PS, prosensory domain; RM, Reissner's membrane; SG, spiral ganglion; SV, stria vascularis.

Fig. 3.

Convergent extension and radial intercalation regulate cochlear extension. At E14, post-mitotic cells within the prosensory domain, which form a pseudostratified epithelium, extend towards the apex of the duct as the cochlea grows. Extension is driven by both convergence of cells located at the medial and lateral edges of the prosensory domain towards the middle, and by radial intercalation-thinning of the epithelium as pseudostratified cells intercalate and shorten along the lumenal-to-basal axis. At E16, radial intercalation continues to generate movement of the sensory population towards the apex, but the width of the sensory domain along the medial-lateral axis expands as cell size increases. Between E16 and P0, cell growth, especially of hair cells, also contributes to ongoing extension. DC, Deiters’ cell; HC, Hensen's cell; IHC, inner hair cell; IPhC, inner phalangeal cell; L PrC, lateral prosensory cell; M PrC, medial prosensory cell; NS, nonsensory cells; OHC, outer hair cell; PC, pillar cell.

Fig. 4.

Regulation of cellular differentiation along the cochlear basal-to-apical (tonotopic) axis. Patterning of the long, tonotopic, axis of the spiral cochlea occurs via several events, which occur in gradients along its basal-to-apical axis. The upper panel shows the spiraled cochlea, highlighting its association with the spiral ganglion. The middle panel illustrates the duct ‘unrolled’, with the narrow base representing the high frequency-responding end and the wider apex representing the low frequency-responding end. The lower panel depicts some of the events that occur during various stages of cochlear development. Cochlear prosensory cells exit the cell cycle (in response to CDKN1B expression) in a gradient that initiates in the apex and extends towards the base of the cochlear duct between E12 and E14. In contrast, cellular differentiation (marked by Atoh1 expression) begins near the base at E13 and extends to the apex by E16. As a result, cells in the apex of the cochlea are maintained in a post-mitotic but uncommitted state for approximately 4 days. Timing of the onset of differentiation is mediated, at least in part, through a base-to-apex gradient of downregulation of Lin28b/Hmga2. The gradient of differentiation, and potentially downregulation of Lin28b/Hmga2, is regulated through a temporal base-to-apex decrease in Shh signaling from adjacent spiral ganglion neurons (SGNs).

Fig. 5.

The Notch and Fgf signaling pathways regulate cell fates in the organ of Corti. Developing hair cells (HCs) express the Notch ligands delta-like 1 (Dll1) and jagged 2 (Jag2), which bind to Notch1 (N1) in adjacent supporting cells (SCs), leading to the generation of Notch1 intracellular domains (N1-ICD) that translocate to the nucleus. N1-ICDs then induce the expression of Hes1/5, which act to inhibit Atoh1 activity and expression, diverting cells from an HC fate. Inner hair cells (IHCs) express Fgf8, whereas adjacent prosensory cells (PrCs) express Fgfr3 and the Fgf antagonist Spry2. Diffusion is thought to create a gradient of Fgf8 that extends away from the IHC. Exposure to a high concentration of Fgf8 is sufficient to activate Fgfr3 to levels that overcome the antagonistic effects of Spry2 and induce a pillar cell (PC) fate. In contrast, in SCs located more distant to the Fgf8 source, such as Deiters’ cells (DCs), activation of Fgfr3 is not sufficient to overcome the effects of Spry2, and cells are not induced to develop as PCs. Developing OHCs downregulate Fgfr3 and, as a result, are not affected by the gradient of Fgf8. An additional effect of Fgfr3 activation is to upregulate expression of the Notch target Hey2, which inhibits Atoh1, preventing the formation of ectopic HCs in the PC region.

Fig. 6.

Polarization of stereociliary bundles. At ∼E15, the lumenal surfaces of immature hair cells have a centrally located true cilium surrounded by many short microvilli. aPKC is localized throughout the lumenal surface of the cell, whereas Par3 is restricted to the lateral side. The core PCP molecules Dvl2 and Fz3/6 are asymmetrically localized to the medial (Fz3/6) and lateral (Dvl2) sides of the cell. Daple is localized to the lateral half of each cell. At E17, the developing kinocilium begins to migrate towards the lateral edge of the cell. Par6b and Cdc42 are co-localized with aPKC, whereas Gpsm2, Gα_i3, Insc and Dlg3 become localized to the lateral edge at the lumenal surface, leading to exclusion of aPKC/Par6b/Cdc42 from the lateral region. By P0, expression of Gpsm2/Gα_i3/Insc/Dlg3 has expanded concomitant with the formation of the fonticulus (the microvilli-free region between the kinocilium and the lateral edge of each HC), lateral to the developing stereociliary bundle. An accumulation of Gpsm2 and Gα_i3 also appears at the tips of the centrally located tallest stereociliary bundles. Finally, at P7, Gpsm2/Gα_i3 is localized to the tips of all stereocilia in the tallest row. A corresponding decrease in GPSM2/Gα_i3 is observed in the fonticular region.