Production Of Hair Cells Research Articles

Regenerated hair cells can originate from supporting cell progeny: evidence from phototoxicity and laser ablation experiments in the lateral line system

The mechanisms that lead to the production of sensory hair cells during regeneration have been investigated by using 2 different procedures to ablate preexisting hair cells in individual neuromast sensory epithelia of the lateral line in the tails of salamanders, then monitoring the responses of surviving cells. In one series of experiments, fluorescent excitation was used to cause the phototoxic death of hair cells that selectively take up the pyridinium dye DASPEI. In the other experiments, the ultraviolet output of a pulsed neodymium-YAG laser was focused to a microbeam through a quartz objective lens in epi-illumination mode and used to selectively kill individual unlabeled hair cells while the cells were simultaneously imaged by transmitted light DIC microscopy. Through observation of the treated neuromasts in vivo, these experiments demonstrated that mature sensory epithelia that have been completely depleted of hair cells can still generate new hair cells. Preexisting hair cells are not necessary for regeneration. Immediately after the ablations the only resident cells in the sensory epithelia were supporting cells. These cells were observed to divide at rates that were increased over control values, and eventually those cell divisions gave rise to progeny that differentiated as hair cells, replacing those that had been killed. Macrophages were active in these epithelia, and their phagocytic activity had a significant influence on the standing population of cells. The first new hair cells appeared 3-5 d after the treatments, and additional hair cells usually appeared every 1-2 d for at least 2 weeks. We conclude that the fate of the progeny produced by supporting cell divisions is plastic to a degree, in that these progeny can differentiate either as supporting cells or as hair cells in epithelia where hair cells are missing or depleted.

Development of location-specific hair cell stereocilia in denervated embryonic ears.

The developmental mechanisms that allow physiological coding of acoustic pitch have remained unexplained. Cochlear hair cells that have different structures respond to different sound frequencies and synapse with neurons that project to different locations in the brain. How do these hair cells develop appropriate structures, and how are the connections between specific hair cells and the neurons that code for their pitch sensitivities matched? We have investigated one aspect of this by denervating embryonic chicken ears, before the time of hair cell production, and then transplanting them to the aneural chorioallantoic membrane of host embryos where they have continued to develop. We report that vestibular and auditory hair cell phenotypes differentiate appropriately and that correct gradients of hair cell structural phenotypes, as expressed in stereocilia bundles, develop in the cochleae of these denervated ears. Therefore, the normal development of gradients in hair cell stereocilia properties must be controlled by location-specific cues originating in the ear itself. Neuronally directed modification of target cell phenotypes is not required for the quite specific phenotype development represented by the stereocilia bundles of individual hair cells and the connectional matching in the numerous distinct peripheral information lines of the auditory system.

The avian inner ear. Continuous production of hair cells in vestibular sensory organs, but not in the auditory papilla.

The avian inner ear. Continuous production of hair cells in vestibular sensory organs, but not in the auditory papilla.

Auditory neurons expand their terminal arbors throughout life and orient toward the site of postembryonic hair cell production in the macula neglecta in elasmobranchs.

The population of sensory hair cells in the macula neglecta auditory epithelium in skates increases from 500 to more than 3,000 postembryonically, but during the same time period the number of neurons innervating the epithelium changes by a much smaller amount, if at all. Morphometric analyses of the peripheral terminal arbors of these neurons demonstrate that the arbors expand in area through intussusceptive growth, so that each neuron contacts more hair cells as the epithelium grows by appositional addition of new hair cells at its outer edge. The synaptic contacts that these neurons make with hair cells may not be permanent. Many of the neurons that innervate the growing macula appear to shift their terminal arbors, breaking synaptic contacts with older hair cells in the center of the sensory epithelium as they branch to form new contacts with younger hair cells that are located in the periphery of the epithelium. Over 80% of the terminal branches of these auditory neurons are directed toward the outer edge of the macula, the site where new hair cells are produced. This suggests that the growth cones of these continually growing neurons are guided to newly produced hair cells by an active attraction mechanism.

Perpetual production of hair cells and maturational changes in hair cell ultrastructure accompany postembryonic growth in an amphibian ear.

Sensory hair cells are produced in the ears of birds and mammals only during early development, so that a programmed termination of hair cell proliferation leaves adult birds and mammals susceptible to irreversible deafness and balance disorders. This study reports that this is not an inherent feature of hair cells and is not shared through all the vertebrate classes. In toads (Bufo marinus) hair cells accumulate throughout life, increasing in the sacculus from approximately 400 cells at metamorphosis to more than 1600 in adulthood. In both embryonic and postembryonic ears new hair cells have been identified by scanning electron microscopy and through uptake of radioactively labeled thymidine. In the otic vesicle of postneurulation embryos there is a single primordial sensory epithelium that contains approximately 100 hair cells. Scanning electron microscopy has demonstrated that these newly formed hair cells all have stereocilia bundles that are shorter than 1.5 micron, whereas the majority of hair cells in postembryonic ears have stereocilia bundles that are at least 3 micron long. In the postembryonic sacculus the proliferation of hair cells never appears to cease, since newly produced hair cells identified by their short stereocilia have been found in a distinct peripheral growth zone at the edge of the sensory epithelium even in specimens from the oldest 1% of natural populations. This peripheral growth zone is also the site of the most frequent labeling of newly synthesized DNA in hair cells. It appears that the postembryonic enlargement of the toad sacculus occurs primarily through appositional addition of new hair cells at its edge, with few hair cells added within the existing structure of the epithelium.

Postembryonic growth of the macula neglecta auditory detector in the ray, Raja clavata: Continual increases in hair cell number, neural convergence, and physiological sensitivity

Quantitative scanning electron microscopy in an age series demonstrated that the macula neglecta auditory epithelium of the ray, Raja clavata, produces and accumulates sensory cells perpetually at 1-3 cells/day, so that the total increases from approximately 500 cells at birth to 6,000 at 7 years of age. The shape of the macula also changes with growth, and changes in the marginal zones of small and intermediate size hair cells are consistent with this differential growth and their proposed role as hair cell production sites. The neurons contacting the epithelium do not increase in number as animals age; instead they hypertrophy, increasing axon diameter and terminal field size. A hypothetical double-gradient interaction between the growing nerves and new hair cells is proposed to explain the development of synaptic connections and the continual production of individually oriented, functional hair cells. Electrophysiological recordings from the neurons demonstrated best sensitivities between 40 Hz and 200 Hz, directional receptive fields, and little or no effect of changes in the ear's position relative to gravity. The convergence ratio from sensory cells to neurons increases because of their unequal patterns of growth, and physiological sensitivity improves 500-fold and more as these animals age. These results contrast with current information on mammalian ears, where it appears that sensory cells are not produced at any time after birth.