Researchers Create Inner Ear Structures From Stem Cells

Indiana University scientists have used mouse embryonic stem cells to make key structures of the inner ear. This accomplishment provides new insights into the sensory organ’s developmental process and sets the stage for laboratory models of disease, drug discovery and potential treatments for hearing loss, and balance disorders.

Eri Hashino, professor of otolaryngology at the University of Indiana School of Medicine, and his co-workers, were able to use a three-dimensional cell culture method that directed the stem cells to form inner-ear sensory epithelia that contained hair cells and supporting cells and neurons that detect sound, head movements and gravity.

In the past, other attempts to grow inner-ear hair cells in standard culture systems have not succeeded. Apparently the cues required to form inner-ear hair bundles, which are essential for detecting auditory or vestibular signals, are absent in cell-culture dishes.

Inner ear hair cells

To conquer this barrier, Hashino and his team changed their culture system. The suspended the cells as aggregates in a specialized culture medium and this mimicked conditions normally found in the body as the inner ear develops.

Another strategy that paid off was to precisely time the application of several small molecules that coaxed the stem cells to differentiate from one stage to the next into precursors for the inner ear.

a, Schematic of vestibular end organs and type I/II vestibular hair cells. vgn, vestibular ganglion neurons. b, c, Pax2 (b) and Calb2 (c) are expressed in all Myo7a+ stem-cell-derived hair cells on day 20. CyclinD1 (cD1) is expressed in supporting cells. d–g, The structural organization of vesicles with Calb2+ Myo7a+ hair cells mimics the E18 mouse saccule (sagittal view) in vivo. nse, nonsensory epithelium. h, Tuj1+ neurons extending processes to hair cells. i, The synaptic protein Snap25 is localized to the basal end of hair cells. j, The postsynaptic marker Syp colocalizes with Ctbp2 (arrowheads and inset). hcn, hair cell nucleus. k, Quantification of synapses on day 16, 20 and 24 hair cells (n > 100 cells, *P < 0.05, ***P < 0.001; mean ± s.d.). l, Overview of in vitro differentiation. Scale bars, 50 μm (d, f, h), 25 μm (b, c, e, g), 10 µm (i), 5 µm (j). Also, BMP = Bone morphogen protein, FGF = fibroblast growth factor, LGN = Small molecule that inhibits BMP signaling, Wnt = small secreted glycoprotein involved in cell signaling.

Even though the added growth factors made a big difference to the success of this experiment, it was the three-dimensional suspension culture system that provided many important mechanical cues. The tension caused by the pull of the cells on each other played a very important role in directing the differentiation of the cells to become inner-ear precursors.

Karl A Koehler, first author of this paper and a graduate student in the medical neuroscience program at IU School of Medicine said: “The three-dimensional culture allows the cells to self-organize into complex tissues using mechanical cues that are found during embryonic development.”

Hashino added that they were “surprised to see that once stem cells are placed in 3-D culture, these cells behave as if they knew not only how to self-organize into a pattern remarkably similar to the native inner ear.” Hashino continued: “Our initial goal was to make inner-ear precursors in culture, but when we did testing we found thousands of hair cells in a culture dish.”

Electrophysiological testing of these stem cell-derived hair cells showed that they were, in fact, functional, and were similar to those that sense gravity and motion. Moreover, neurons like those that normally link the inner-ear cells to the brain had also developed in their cell culture system, and were connected to the hair cells.

Hashino thinks that additional research is needed to determine how to derived inner-ear cells involved in auditory sensation might be made from stem cells, and how such techniques might be adapted to make human inner ear cells.