The people behind the papers - James O'Sullivan and Zoë Mann.

To build a functional inner ear, hair cell morphology must be precisely controlled along the proximo-distal axis. A new paper in Development shows that differential mitochondrial dynamics in proximal versus distal cells impacts on the apical cell surface area - a key aspect of morphology. To find out more about this work, we spoke to first author James O'Sullivan and senior author Zoë Mann, both at King's College London, UK.

Emx2 lineage tracing reveals antecedent patterns of planar polarity in the mouse inner ear.

The planar polarized organization of hair cells in the vestibular maculae is unique because these sensory organs contain two groups of cells with oppositely oriented stereociliary bundles that meet at a line of polarity reversal (LPR). EMX2 is a transcription factor expressed by one hair cell group that reverses the orientation of their bundles, thereby forming the LPR. We generated Emx2-CreERt2 transgenic mice for genetic lineage tracing and demonstrate Emx2 expression before hair cell specification when the nascent utricle and saccule constitute a continuous prosensory domain. Precursors labeled by Emx2-CreERt2 at this stage give rise to hair cells located along one side of the LPR in the mature utricle or saccule, indicating that this boundary is first established in the prosensory domain. Consistent with this, Emx2-CreERt2 lineage tracing in Dreher mutants, where the utricle and saccule fail to segregate, labels a continuous field of cells along one side of a fused utriculo-saccular-cochlear organ. These observations reveal that LPR positioning is pre-determined in the developing prosensory domain, and that EMX2 expression defines lineages of hair cells with oppositely oriented stereociliary bundles.

In utero adeno-associated virus (AAV)-mediated gene delivery targeting sensory and supporting cells in the embryonic mouse inner ear.

In vivo gene delivery to tissues using adeno-associated vector (AAVs) has revolutionized the field of gene therapy. Yet, while sensorineural hearing loss is one of the most common sensory disorders worldwide, gene therapy applied to the human inner ear is still in its infancy. Recent advances in the development recombinant AAVs have significantly improved their cell tropism and transduction efficiency across diverse inner ear cell types to a level that renders this tool valuable for conditionally manipulating gene expression in the context of developmental biology studies of the mouse inner ear. Here, we describe a protocol for in utero micro-injection of AAVs into the embryonic inner ear, using the AAV-PHP.eB and AAV-DJ serotypes that respectively target the sensory hair cells and the supporting cells of the auditory sensory epithelium. We also aimed to standardize procedures for imaging acquisition and image analysis to foster research reproducibility and allow accurate comparisons between studies. We find that AAV-PHP.eB and AAV-DJ provide efficient and reliable tools for conditional gene expression targeting cochlear sensory and supporting cells in the mouse inner ear, from late embryonic stages on.

In vitro study of morphological changes of the cultured otocyst isolated from the chick embryo / Estudio in vitro de los cambios morfológicos del otocisto cultivado aislado del embrión de pollo

The aim of this study was to observe morphological changes of the cultured otocysts isolated from various stages of the chick embryo. Isolated otocysts were dissected from embryonic day, E2.5-4.5 of incubation (HH stage 16-26) according to stages of developing inner ear. Morphology of the chick otocyst exhibited an ovoid shape. The width and height of the otocyst were 0.2 mm and 0.3 mm, respectively. Elongation of a tube-like structure, the endolymphatic duct, was found at the dorsal aspect of the otocyst. The cultured otocyst is lined by the otic epithelium and surrounding periotic mesenchymal cells started to migrate outwards the lateral aspect of such epithelium. Notably, the acoustic-vestibular ganglion (AVG) was observed at the ventrolateral aspect of the otocyst. Appearance of AVG in vitro can be applied for studying chemical-induced ototoxicity and sensorineural hearing loss. It was concluded that the organ-cultured otocyst of the chick embryo could be used as a model to study sensory organ development of avian inner ear.

El objetivo de este estudio fue observar los cambios morfológicos de otocistos cultivados aislados en las diversas etapas del desarrollo del embrión de pollo. Otocistos aislados fueron obtenidos de embriones día, E2.5-4.5 de incubación (HH etapa 16-26) de acuerdo a las etapas de desarrollo del oído interno. El otocisto de pollo presentó una morfología ovoide. El ancho y la altura del otocisto fue de 0,2 mm y 0,3 mm, respectivamente. En la cara dorsal del otocisto se visualizó el alargamiento de una estructura similar a un tubo, el conducto endolinfático. El otocisto cultivado está revestido por epitelio ótico y células mesenquimatosas perióticas que comienzan a migrar hacia el exterior de la cara lateral en búsqueda del epitelio. En particular, el ganglio acústico-vestibular (GAV) fue observado en la parte ventrolateral del otocisto. La aparición de GAV in vitro puede ser aplicado para el estudio de la ototoxicidad inducida por productos químicos y la pérdida de audición neurosensorial. Se concluyó que el otocisto cultivado de embrión de pollo podría ser utilizado como un modelo para estudiar el desarrollo de órganos sensoriales del oído interno aviar.

Desarrollo del oído medio humano entre la 5a y la 17a semana de gestación / Human middle ear development between the 5th and the 17th week of pregnancy

Se realizó un estudio de la embriogénesis del oído medio humano entre la 5a y la 17a semana de gestación, utilizando 50 individuos (9 embriones y 41 fetos), los cuales fueron fijados, realizándose técnicas de tinción corrientes (hematoxilina-eosina), histoquímicas, tricrómicas e inmunohistoquímicas (anticuerpos monoclonales antilaminina, anticolágeno IV, antineurofilamentos). Seis fetos fueron procesados para microscopía electrónica de barrido, intentando obtener la mayor información posible del material humano. El análisis de los resultados reveló que el desarrollo del oído medio humano es mas tardío que el de los animales, distinguimos un crecimiento interesante de la tuba auditiva y como esta recubre a los huesecillos formando sus mesos, describimos la fijación de la platina del estribo a la pared medial de la caja timpánica en un período específico del desarrollo y la posterior formación, por muerte celular programada de la ventana oval y un nuevo pericondrio, hecho llamativo en la embriogénesis. En los embriones y fetos estudiados se describe la formación de nudos epidérmicos en la membrana timpánica (en su capa epitelial), dentro del desarrollo normal de esta estructura. Este hallazgo, no ha sido comunicado en la literatura. Con el conocimiento actual y el avance en la tecnología, se puede utilizar técnicas mas avanzadas de tinciones histológicas, inmunohistoquímicas y el uso de microscopio de barrido, para esclarecer las interrogantes del desarrollo del oído medio humano, agregando nueva información a lo tradicionalmente conocido