Apoptosis in cavitation of middle ear space

The adhesion molecule cadherin 11 is essential for acquisition of normal hearing ability through middle ear development in the mouse

Cadherin 11 (Cdh11), a member of the cadherin adhesion molecule family, is expressed in various regions of the brain as well as the head and ear. To gain further insights into the roles of Cdh11 in the development of the ear, we performed behavioral tests using Cdh11 knockout (KO) mice. KO mice showed reduced acoustic startle responses and increased thresholds for auditory brainstem responses, indicating moderate hearing loss. The auditory bulla volume and ratio of air-filled to non-air-filled space in the middle ear cavity were reduced in KO mice, potentially causing conductive hearing loss. Furthermore, residual mesenchymal and inflammatory cells were observed in the middle ear cavity of KO mice. Cdh11 was expressed in developing mesenchymal cells just before the start of cavitation, indicating that Cdh11 may be directly involved in middle ear cavitation. Since the auditory bulla is derived from the neural crest, the regulation of neural crest-derived cells by Cdh11 may be responsible for structural development. This mutant mouse may be a promising animal model for elucidating the causes of conductive hearing loss and otitis media.

The development of the mammalian outer and middle ear

The mammalian ear is a complex structure divided into three main parts: the outer; middle; and inner ear. These parts are formed from all three germ layers and neural crest cells, which have to integrate successfully in order to form a fully functioning organ of hearing. Any defect in development of the outer and middle ear leads to conductive hearing loss, while defects in the inner ear can lead to sensorineural hearing loss. This review focuses on the development of the parts of the ear involved with sound transduction into the inner ear, and the parts largely ignored in the world of hearing research: the outer and middle ear. The published data on the embryonic origin, signalling, genetic control, development and timing of the mammalian middle and outer ear are reviewed here along with new data showing the Eustachian tube cartilage is of dual embryonic origin. The embryonic origin of some of these structures has only recently been uncovered (Science, 339, 2013, 1453; Development, 140, 2013, 4386), while the molecular mechanisms controlling the growth, structure and integration of many outer and middle ear components are hardly known. The genetic analysis of outer and middle ear development is rather limited, with a small number of genes often affecting either more than one part of the ear or having only very small effects on development. This review therefore highlights the necessity for further research into the development of outer and middle ear structures, which will be important for the understanding and treatment of conductive hearing loss.

Eya4-deficient mice are a model for heritable otitis media

Otitis media is an extremely common pediatric inflammation of the middle ear that often causes pain and diminishes hearing. Vulnerability to otitis media is due to eustachian tube dysfunction as well as other poorly understood factors, including genetic susceptibility. As EYA4 mutations cause sensorineural hearing loss in humans, we produced and characterized Eya4-deficient (Eya4(-/-)) mice, which had severe hearing deficits. In addition, all Eya4(-/-) mice developed otitis media with effusion. Anatomic studies revealed abnormal middle ear cavity and eustachian tube dysmorphology; thus, Eya4 regulation is critical for the development and function of these structures. We suggest that some human otitis media susceptibility reflects underlying genetic predisposition in genes like EYA4 that regulate middle ear and eustachian tube anatomy.

Apoptosis and regression of embryonic mesenchyme in the development of the middle ear spaces

Apoptosis was studied using temporal bones from three fetuses representing different times of gestation and from three neonates. Paraffin-embedded sections 20-microm thick were studied using the terminal deoxynucleotide transferase-mediated dUTP nick-end labeling method based on 3'-end-labeling of fragmented DNA. Phenotyping of the immune cells was performed using regular monoclonal antibodies. In the bone marrow the granulocyte series dominated and the number of cells in the macrophage series was noticeably fewer, with apoptotic cells occurring in both. In the embryonic mesenchyme, solitary apoptotic cells occurred in all locations in both the fetuses and neonates. Apoptosis is a basic factor in the regression of embryonal mesenchyme, but may not be preprogrammed. Basic scientific data obtained from modified tissue cultures show that mechanical forces cause cells to switch between different genetic programs. It is suggested that the act of swallowing causes periodic changes in the amniotic fluid pressure and provides the necessary force for regression of the mesenchyme by apoptosis.

Apoptosis in the developing rat cochlea and its related structures

Mammalian development involves proliferation and programmed cell death (apoptosis). This study was undertaken to analyse the spatial and temporal organisation of apoptosis in developing rat cochlear and associated tissues using in situ terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labelling of DNA fragments (TUNEL), and light and electron microscopy. Embryonic (E12-E19 days) and postnatal rats (P0-P21 days) were studied. Fixed tissues were stained for apoptosis using TUNEL technique and the cytomorphology of apoptosis was confirmed by light and electron microscopy. Apoptotic cells were detected predominantly during the embryonic and early postnatal development of the cochlea. Apoptosis occurred in embryonic precursors of the cochlear duct epithelium, mainly in the region of its outgrowth between E12 and E16. In the periotic mesenchyme, apoptosis occurred in areas committed to develop into the middle ear cavity (peaking at E16) and perilymphatic compartments (peaking around E18-E19). Apoptosis in the VIIIth nerve (statoacoustic) ganglion was detected throughout the embryonic and early postnatal periods, peaking at E18-E19, around the time when the cochlear neural connections are being established. At later postnatal days, apoptosis was seen only occasionally in cochlear tissues, predominantly in tissues lining the middle ear cavity and sporadically in cells of the otic capsule. Therefore, apoptosis appears to occur in areas of remodeling, in areas of cavitation and in areas of differentiation. These findings provide a template for studying the molecular mechanisms involved in the development of the rat inner ear.