The Differentiation of Hair Cells during Development

Abstract

A series of experiments were devised to investigate the sequence of events that led to the differentiation of hair cells and their synapses. In particular, we inquired whether the presence of nerve fibers in the developing sense organ is necessary for induction of differentiation. Lateral line organs in the translucent tail of salamander tadpoles (Ambystoma mexicanum) were chosen for this study because here cellular structures can be observed in considerable detail in vivo with interference contrast optics. Furthermore, during degeneration of a new tail after amputation, new neuromasts develop and can be taken for electron microscopy at desired stages of development. At the first identified stage, nerve terminals are already present above the basement membrane. A cupula, afferent synapses, kinocilia, and stereocilia then appear. In another set of experiments, the tail was amputated and at the same time a segment of the lateral line nerve which innervates these organs was surgically removed behind the front limb. The nerve was prevented from invading the growing tail by repeated sectioning. New organs were now looked for and were also found. The ultrastructure of such organs is described and discussed in relation to the development of the inner ear.