In vitro Time-lapse Live-Cell Imaging to Explore Cell Migration toward the Organ of Corti

Existing in vitro method to investigate migration of MSCs include Transwell or wound healing protocols. The method described here can be used to track migration of MSCs to explant in real time. In terms of culture of explant, the advantage of this method compared to others is that it achieves a reduction in time and cost, as well as a simplification of experimental steps.

This technique can be used to observe stem cell homing to target in real time leading to development of models with optimized homing efficiency. Removal of tectorial membrane is difficult for unexperienced researchers. Depending on the development stage, understanding the right time to remove the tectorial membrane is important for the success of this protocol.

Demonstrating the procedure will be Dr.Jeong-Eun Park and Dr.Su Hoon Lee, researcher professors from my lab. Begin by sterilizing the laminar flow tissue culture hood by turning on the ultraviolet light for approximately 30 minutes. Spray all surfaces with 70%ethanol prior to use and allow them to dry.

Place dissection instruments in 70%ethanol for 10 minutes, then dry them before using. After decapitating a three to four-day-old mouse, soak the ahead in 70%ethanol and place it under a serial microscope in the laminar flow hood. Quickly soak the tissue in tissue dissection solution to remove the ethanol.

Cut the center line of the skull with the surgical blade, then expose the skull by pulling down the skin anteriorly and cutting the external auditory canal of the ear. Cut from the anterior to the posterior part of the cranium across the eye line, then open the skull and remove the forebrain, cerebellum, and brainstem with blunt forceps. Using microforceps, separate the cochlea from the temporal bone and transfer it to a Petri dish containing tissue dissection solution.

Carefully dissect all of the cochlear otic capsule leaving only the internal cochlear soft tissue. Hold the modiolus of the cochlea with forceps and the cochlear duct with another pair of forceps, then slowly separate the two tissues. Remove the stria vascularis and tectorial membrane by gently peeling them away.

Place a sterilized plastic coverslip in new tissue dissection solution, then place the organ of Corti on the coverslip making sure that the basilar membrane faces downward. Immobilize the tissue by pressing the Reissner's membrane and the remaining modiolus tissue onto the coverslip with forceps. Transfer the coverslip with the embedded tissue to the center of a 35 millimeter confocal dish.

Place the glass cloning cylinder on the dish with the cochlear explant positioned in the center of the dish and add 100 microliters of explant culture medium into the cylinder. Place 5, 000 bone marrow derived GFP-tagged mesenchymal stem cells or MSCs in two milliliters of culture medium outside the glass cylinder and put the dish in the incubator. After culturing the cells as described in the text manuscript, aspirate all medium inside and outside the cylinder and remove the glass cylinder from the confocal dish.

Add two milliliters of fresh culture medium to the dish. Place it in a humidified incubator until ready for analysis. Use a confocal microscopy system with a stage top incubator.

Turn on the confocal microscope, fluorescent light and computer. Set the conditions of the stage top incubator to 37 degrees Celsius and 5%carbon dioxide atmosphere. Place the sample dish on the dish fixing vessel, cover it with the dish fixing lid and close the chamber with the top heater lid.

Adjust the zoom and focus to localize the organ of Corti and MSCs in the field of view. Open the image processing software. Under the locate option, select a 20X plan-apochromat objective or numerical aperture 0.8 and a 0.5X crop area.

Under acquisition, click on smart setup and select EGFP. Open the channel tab under acquisition and set the laser power to 0.2%the pinhole to 44 micrometers, the master gain to 750 volts, and the digital gain to 1.0. Click on ESID under imaging setup and set the ESID gain to four and the digital gain to 7.5.

Click on tiles and stake to produce 210 tiles. Open focus strategy and select the focus mode. Under time series, set the duration to 24 hours and the interval to 10 minutes.

Under acquisition, set the frame size to 512 by 512 pixels, scan speed to eight, the direction to bidirectional, the averaging to 4X, and the bits per pixel to 16. Click on start experiment to begin the experiment. The two-dimensional path of mesenchymal stem cells or MSCs was tracked in a wound healing method by using a glass cylinder as a barrier between the MSCs and the organ of Corti.

When the organ of Corti was cultured, fibroblasts grew very quickly outward from the outer hair cells in the outermost layer. Most GFP-labeled MSCs were pushed out by fast-growing fibroblasts. Nonetheless, some penetrated the layer of fibroblasts and successfully migrated to the organ of Corti.

The results from 72 hours of incubation showed that MSCs migrated to the organ of Corti and were mainly localized in the modiolus and the area with the cochlear nerve fibers separated from the modiolus. The morphology of the MSCs was altered to a radial shape similar to that of nerve fibers. The cells were transformed into a linear shape along the limbus line, rather than the hair cell area.

The MSCs were able to migrate and grow in the nerve fiber collection area by penetrating the various cell layers and physical barriers. When damage to hair cells was induced by a 16-hour treatment with kanamycin and cells were cultured for a further 72 hours after the removal of kanamycin, MSCs were localized not only in the modiolus, but also in the outer hair cells. Immobilizing the tissue is an important step in this protocol was the organ of Corti is attached.

It does not fall off easily. This is a trivial, but helpful tip for successful explant culture. This method may be adapted to study the efficacy of exosomes secreted from MSCs.