Three-dimensional reconstruction of root cells and interdental cells in the rat inner ear by serial section scanning electron microscopy

Store-Operated Ca2+ Channels Contribute to the Generation of Ca2+ Waves in Interdental Cells in the Cochleae

Cochlear calcium (Ca2+) waves are vital regulators of the cochlear development and establishment of hearing function. Inner supporting cells are believed to be the main region generating Ca2+ waves that work as internal stimuli to coordinate the development of hair cells and the mapping of neurons in the cochlea. However, Ca2+ waves in interdental cells (IDCs) that connect to inner supporting cells and spiral ganglion neurons are rarely observed and poorly understood. Herein, we reported the mechanism of IDC Ca2+ wave formation and propagation by developing a single-cell Ca2+ excitation technology, which can easily be accomplished using a two-photon microscope for simultaneous microscopy and femtosecond laser Ca2+ excitation in any target individual cell in fresh cochlear tissues. We demonstrated that the store-operated Ca2+ channels in IDCs are responsible for Ca2+ wave formation in these cells. The specific architecture of the IDCs determines the propagation of Ca2+ waves. Our results provide the mechanism of Ca2+ formation in IDCs and a controllable, precise, and noninvasive technology to excite local Ca2+ waves in the cochlea, with good potential for research on cochlear Ca2+ and hearing functions.

Three-dimensional visualization of osteoclasts in embryonic mouse mandibles using SEM array tomography

OBJECTIVE

Three-dimensional (3-D) images of osteoclasts in vivo have been elusive, due to their large size and intricate morphology. The present study was designed to reconstruct the 3-D morphology of whole osteoclasts in developing mouse mandibles using scanning electron microscopy (SEM) array tomography.

METHODS

Mandibles of 16 days post coitum mouse embryos were fixed and embedded in epoxy resin after decalcification. Epoxy blocks were trimmed, and serial sections of 1 μm in thickness were cut with an ultramicrotome and mounted on glass microscope slides. Consecutive images of every fourth or fifth serial section were obtained by SEM after electron staining and platinum coating. Three dimensional reconstruction of osteoclasts was performed using these consecutive images.

RESULTS

Multinucleated osteoclasts were observed to cluster around developing bone in the embryonic mouse mandible. The outlines of osteoclasts and their sealing zones were identified in the serial sections. The reconstructed 3-D image revealed whole osteoclast morphology with the sealing zone. Osteoclasts were adherent to bone with the anchoring structure between the osteoclast and the bone.

CONCLUSIONS

SEM array tomography with our modification revealed 3-D imagery of a whole osteoclast and its sealing zone in vivo for the first time. This methodology could provide useful information on in vivo structures and dynamics of large cells, such as osteoclasts.

Lack of collagen α6(IV) chain in mice does not cause severe-to-profound hearing loss or cochlear malformation, a distinct phenotype from nonsyndromic hearing loss with COL4A6 missense mutation

Congenital hearing loss affects 1 in every 1000 births, with genetic mutations contributing to more than 50% of all cases. X-linked nonsyndromic hereditary hearing loss is associated with six loci (DFNX1-6) and five genes. Recently, the missense mutation (c.1771G>A, p.Gly591Ser) in COL4A6, encoding the basement membrane (BM) collagen α6(IV) chain, was shown to be associated with X-linked congenital nonsyndromic hearing loss with cochlear malformation. However, the mechanism by which the COL4A6 mutation impacts hereditary hearing loss has not yet been elucidated. Herein, we investigated Col4a6 knockout (KO) effects on hearing function and cochlear formation in mice. Immunohistochemistry showed that the collagen α6(IV) chain was distributed throughout the mouse cochlea within subepithelial BMs underlying the interdental cells, inner sulcus cells, basilar membrane, outer sulcus cells, root cells, Reissner's membrane, and perivascular BMs in the spiral limbus, spiral ligament, and stria vascularis. However, the click-evoked auditory brainstem response analysis did not show significant changes in the hearing threshold of Col4a6 KO mice compared with wild-type (WT) mice with the same genetic background. In addition, the cochlear structures of Col4a6 KO mice did not exhibit morphological alterations, according to the results of high-resolution micro-computed tomography and histology. Hence, loss of Col4a6 gene expression in mice showed normal click ABR thresholds and normal cochlear formation, which differs from humans with the COL4A6 missense mutation c.1771G>A, p.Gly591Ser. Therefore, the deleterious effects in the auditory system caused by the missense mutation in COL4A6 are likely due to the dominant-negative effects of the α6(IV) chain and/or α5α6α5(IV) heterotrimer with an aberrant structure that would not occur in cases with loss of gene expression.

Petrosal morphology and cochlear function in Mesozoic stem therians

Here we describe the bony anatomy of the inner ear and surrounding structures seen in three plesiomorphic crown mammalian petrosal specimens. Our study sample includes the triconodont Priacodon fruitaensis from the Upper Jurassic of North America, and two isolated stem therian petrosal specimens colloquially known as the Höövör petrosals, recovered from Aptian-Albian sediments in Mongolia. The second Höövör petrosal is here described at length for the first time. All three of these petrosals and a comparative sample of extant mammalian taxa have been imaged using micro-CT, allowing for detailed anatomical descriptions of the osteological correlates of functionally significant neurovascular features, especially along the abneural wall of the cochlear canal. The high resolution imaging provided here clarifies several hypotheses regarding the mosaic evolution of features of the cochlear endocast in early mammals. In particular, these images demonstrate that the membranous cochlear duct adhered to the bony cochlear canal abneurally to a secondary bony lamina before the appearance of an opposing primary bony lamina or tractus foraminosus. Additionally, while corroborating the general trend of reduction of venous sinuses and plexuses within the pars cochlearis seen in crownward mammaliaforms generally, the Höövör petrosals show the localized enlargement of a portion of the intrapetrosal venous plexus. This new vascular feature is here interpreted as the bony accommodation for the vein of cochlear aqueduct, a structure that is solely, or predominantly, responsible for the venous drainage of the cochlear apparatus in extant therians. Given that our fossil stem therian inner ear specimens appear to have very limited high-frequency capabilities, the development of these modern vascular features of the cochlear endocast suggest that neither the initiation or enlargement of the stria vascularis (a unique mammalian organ) was originally associated with the capacity for high-frequency hearing or precise sound-source localization.

Age-dependent alterations of Kir4.1 expression in neural crest-derived cells of the mouse and human cochlea

Age-related hearing loss (or presbyacusis) is a progressive pathophysiological process. This study addressed the hypothesis that degeneration/dysfunction of multiple nonsensory cell types contributes to presbyacusis by evaluating tissues obtained from young and aged CBA/CaJ mouse ears and human temporal bones. Ultrastructural examination and transcriptomic analysis of mouse cochleas revealed age-dependent pathophysiological alterations in 3 types of neural crest-derived cells, namely intermediate cells in the stria vascularis, outer sulcus cells in the cochlear lateral wall, and satellite cells in the spiral ganglion. A significant decline in immunoreactivity for Kir4.1, an inwardly rectifying potassium channel, was seen in strial intermediate cells and outer sulcus cells in the ears of older mice. Age-dependent alterations in Kir4.1 immunostaining also were observed in satellite cells ensheathing spiral ganglion neurons. Expression alterations of Kir4.1 were observed in these same cell populations in the aged human cochlea. These results suggest that degeneration/dysfunction of neural crest-derived cells maybe an important contributing factor to both metabolic and neural forms of presbyacusis.

Role and mechanisms of a three-dimensional bioprinted microtissue model in promoting proliferation and invasion of growth-hormone-secreting pituitary adenoma cells

Growth-hormone-secreting pituitary adenoma (GHSPA) is a benign tumour with a high incidence and large economic burden, which greatly affects quality of life. The aetiological factors are yet to be clarified for GHSPA. Conventional two-dimensional (2D) monolayer culture of tumour cells cannot ideally reflect the growth status of tumours in the physiological environment, and insufficiencies of in vitro models have severely restricted the progress of cancer research. Three-dimensional (3D) bioprinting technology is being increasingly used in various fields of biology and medicine, which allows recapitulation of the in vivo growth environment of tumour cells. In this study, a GHSPA microtissue model was established using 3D bioprinting. Tumour cells in the 3D environment exhibited more active cell cycle progression, secretion, proliferation, invasion, and tumourigenesis compared with those in the 2D environment. Furthermore, the molecular mechanisms of the 3D-printed microtissue model were explored. We demonstrated that the 3D-printed microtissue provides an excellent in vitro model at the tissue level for oncological research and may facilitate in-depth studies on the aetiology, treatment, drug resistance, and long-term prognosis of GHSPA .

The Stress Response in the Non-sensory Cells of the Cochlea Under Pathological Conditions-Possible Role in Mediating Noise Vulnerability

Various stressors, such as loud sounds and the effects of aging, impair the function and viability of the cochlear sensory cells, the hair cells. Stressors trigger pathophysiological changes in the cochlear non-sensory cells as well. We have here studied the stress response mounted in the lateral wall of the cochlea during acute noise stress and during age-related chronic stress. We have used the activation of JNK/c-Jun, ERK, and NF-κB pathways as a readout of the stress response, and the expression of the FoxO3 transcription factor as a possible additional player in cellular stress. In the aging cochlea, NF-κB transcriptional activity was strongly induced in the stria vascularis of the lateral wall. This induction was linked with the atrophy of the stria vascularis, suggesting a role for NF-κB signaling in mediating age-related strial degeneration. Acutely following noise exposure, the JNK/c-Jun, ERK, and NF-κB pathways were activated in the spiral ligament of the lateral wall of CBA/Ca mice. This activation was concomitant with the morphological transformation of macrophages, suggesting that the upregulation of stress signaling leads to macrophage activation. In contrast, C57BL/6J mice lacked these responses. Only the combination of noise exposure and a systemic stressor, lipopolysaccharide, exceeded the threshold for the activation of stress signaling in the lateral wall of C57BL/6J mice. In addition, we found that, at the young adult age, outer hair cells of CBA/Ca mice are much more vulnerable to loud sounds compared to these cells of C57BL/6J mice. These results suggest that the differential stress response in the lateral wall of the two mouse strains underlies, in part, the differential noise vulnerability of their outer hair cells. Together, we propose that the molecular stress response in the lateral wall modulates the outcome of the stressed cochlea.

Methods for array tomography with correlative light and electron microscopy

The three-dimensional ultra-structure is the comprehensive structure that cannot be observed from a two-dimensional electron micrograph. Array tomography is one method for three-dimensional electron microscopy. In this method, to obtain consecutive cross sections of tissue, connected consecutive sections of a resin block are mounted on a flat substrate, and these are observed with scanning electron microscopy. Although array tomography requires some bothersome manual procedures to prepare specimens, a recent study has introduced some techniques to ease specimen preparation. In addition, array tomography has some advantages compared with other three-dimensional electron microscopy techniques. For example, sections on the substrate are stored semi-eternally, so they can be observed at different magnifications. Furthermore, various staining methods, including post-embedding immunocytochemistry, can be adopted. In the present review, the preparation of specimens for array tomography, including ribbon collection and the staining method, and the adaptability for correlative light and electron microscopy are discussed.