ONTOGENY OF F-ACTIN IN HAIR-CELLS

This report describes the ontogenesis of cochlear stereocilia using sc anning electron microscopy for analysis of cilia appearance, and fluor escence microscopy of phalloidin, a label for F-actin, to determine th e maturation of the cilia framework. Surface and frozen-sectioned prep arations of the otic capsule were obtained from several stages of rat pup development beginning at the 16th gestational day and at various s tages until adulthood. In the earliest stage investigated, strong fluo rescence labeling was visible on the apical part of Kolliker's organ, revealing a reticular outline of cell junctions. Hair cells started to differentiate at the 18th day of gestation from cells within the prim ordial receptor area. Phalloidin labeling revealed a sequential appear ance of F-actin as the hair cells differentiated from the cells within the Kolliker's organ. The differentiation of receptor cells occurred first with the appearance of a junctional complex between the hair cel l and the surrounding cells. Then a cuticular plate appeared followed by the progressive emergence of stereocilia. The F-actin labeling also revealed a progressive differentiation of receptor cells from the coc hlear base to its apex. There was also an inner to outer hair cell dev elopmental gradient of label. Inner hair cells developed stereocilia b efore outer hair cells. The third row of outer hair cells was the last to acquire stereocilia. The adult pattern of stereocilia was reached around the 6th postnatal day. We conclude that the appearance of actin filaments in developing receptor cells and the emergence of stereocil ia can be regarded as markers for correlating function and other struc tural differentiation.