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.

It has been proposed that supporting cells may be the progenitors of regenerated hair cells that contribute to recovery of hearing in birds, but regeneration is difficult to visualize in the ear, because it occurs deep in the skull. Hair cells and supporting cells that are comparable to those in the ear are present in lateral line neuromasts, and in axolotl salamanders these cells are accessible to microscopic observation in vivo. After amputation of a segment of the tail that contains neuromasts, cells from the posteriormost neuromast on the tail stump divide rapidly and form a migratory regenerative placode. The cells of the regenerative placode represent a lineage that eventually produces both hair cells and supporting cells in replacement neuromasts. We sought to identify the progenitors of the regenerative placode by using differential interference contrast microscopy combined with time-lapse video recording in living axolotl salamanders. In response to amputation, the mantle-type supporting cells at the posteroventral edge of the neuromast that is nearest to the wound increased their frequency of cell division, and gave rise to the first cells of the placode. The increase in mitotic activity of mantle-type supporting cells was accompanied by an unexplained decrease in the frequency of divisions in the same neuromast's population of internal supporting cells. The time-lapse records suggested that the changes in the mitotic activity of supporting cells might have been linked to the presence of phagocytic leukocytes in the vicinity of the neuromast that was nearest to the wound. Leukocytes were evenly distributed around control neuromasts, but during regeneration leukocyte activity increased significantly in the vicinity of the posterior half of the posteriormost neuromast. The redistribution of leukocytes occurred early in the regenerative response, but a causal role for the leukocytes has not been conclusively established. It is possible that the leukocytes could contribute to the formation of the regenerative placode at that location by breaking down the glycocalyx that ensheaths the outermost cells of the neuromast, or through the secretion of mitogenic growth factors.

The conversion efficiency for planar Al0:7GaAs/ GaAs heterostructure barrier varactor triplers is shown to be reduced from a theoretical efficiency of 10% to 3% due to selfheating. The reduction is in accordance with measurements on planar Al0:7GaAs/GaAs heterostructure barrier varactor (HBV) triplers to 261 GHz at room temperature and with low temperature tripler measurements to 255 GHz. The delivered maximum output power at 261 GHz is 2.0 mW. Future HBV designs should carefully consider and reduce the device thermal resistance and parasitic series resistance. Optimization of the RF circuit for a 10-m diameter device yielded a delivered output power of 3.6 mW (2.5% conversion efficiency) at 234 GHz.

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