Again, Cruz brought similar data. This time Rubel told him to change his counting criteria. Even then, the regenerated cells were still there. “Well, maybe I better look in the microscope,” Rubel said. Cruz was right, of course. But no one understood the mechanism. “What’s going on here?” they asked themselves.
Around the same time, Doug Cotanche, then a post doc at the University of Pennsylvania, saw the same results in chicks after damage due to intense noise exposure. Rubel and Cotanche published separate papers in different scientific journals, but continued communicating and soon got together to publish dual papers in the prestigious journal Science, showing, as Rubel said, that these were indeed brand-new hair cells “due to new cell division and the creation of new cells in the inner ear.” This was a stunning scientific development. “And wow, we had a new field.”
The next step was to figure out how chickens did it. Studying the cochlea of chicks and other birds, Rubel and others found eventually—over eighteen years!—that bird hair cells do indeed regenerate. Over the same period they discovered many other important molecular and functional details related to this remarkable ability. He showed some slides. The first slide showed the condition of the hair cells shortly after the animals were exposed to noise: “It looks just terrible. All these hair cells are blebbing out and being discarded.” Five days later, however, they could see “baby cells” budding, some of them with the distinctive hair, or microvillus, on top. Then, after a few days, a high-power scanning electron microscope showed that all the hair cells were back. Not perfect, a few small abnormalities, but perfectly functional.
Interestingly, Rubel went on, they found that this happened no matter the age of the bird. Brenda Ryals, a former student of Rubel’s, had a colony of senile quail that, they found, “regenerate cells just as well as a baby chicken does,” Rubel said. New cells are created not only in the cochlea but also in the vestibular epithelium, important for balance. And, perhaps most important, the new cells are appropriately connected to the brain. “The new cells restore near-normal hearing and perfectly normal vestibular reflexes. They restore perception and production of complex vocalizations. Birds lose their song when they lose their hearing, but they gain their song back when they restore their hearing.”
In 2001 Rubel joined forces with David Raible, also at UW, who was using zebrafish, a popular aquarium species, to study development of the nervous system. Eleven years later the two labs are still collaborating on understanding how to prevent and cure hearing loss, and on hair cell regeneration.
Zebrafish proved to be an even better animal model for studying some aspects of hearing-loss prevention and regeneration than birds. In addition to hair cells in the inner ear, aquatic vertebrates like fish have hair cells on the outside of the body, in something called the lateral line. The lateral line is used for detecting change in water currents and its cells are physiologically very similar to human inner ear cells. At electron microscope level, intracellular structure is similar. It turns out that fish and reptiles, like birds, regenerate hair cells, as do frogs and other animals. “So why can’t we?” Rubel asked.
The Rubel/Raible team subjected the zebrafish larvae to ototoxic screening, again using aminoglycoside antibiotics. They tested drugs and druglike compounds to find ones that inhibit hair cell death in the fish. This work may lead to the development of protective cocktails to preserve hair cells before exposure to antibiotics or ototoxic chemotherapy drugs. They may also be given to humans after ototoxic assaults, which include noise exposure.