Extending from the apical surface of each hair cell of the chick cochlea
are from 75 to 200 microvilli or stereocllia and one true cllium, the
kinocilium. The stereocllia are arranged in rows of progressively increasing
length (Fig. 1). Within each tapering sterocilium is a bundle of actin
filaments with over 900 filaments near the tip yet only approximately 25 at
the base where filaments are enmeshed in a dense material (Fig. 1); from
here some of the filaments enter the apical surface of the cell (cuticular
plate) as a rootlet. Examination of longitudinal sections of the stereocilia
(Fig. 2) show that the filaments are aligned parallel to each other and show
considerable order. Examination of an optical diffraction pattern of this
bundle (Fig. 4) reveal that the actin filaments are packed such that the
crossover points of adjacent actin filaments are inregister. A prominent
reflection at 125Å−1 demonstrates that the filaments
are cjossbridged by a macromolecular bridge situated at an average of
125Å−1 intervals (Fig. 4) in transverse sections
the filaments appear hexagonally packed although there are regions where the
filaments are less ordered (Fig. 3). In images processed in the computer to
remove, noise and enhance detail periodic nature of the bridge can be
clearly seen (see arrows Fig. 5). This image resembles that of an actin
paracrystal formed from sea urchin extract composed of bundles of actin
filaments crossbridged by a second protein. Thus the actin filaments in the
bird stereocilia by being cross-bridged and packed with a high degree of
order and produces a structure with considerable structural rigidity.
Embryos were studied at various stages in development in an attempt to
determine how the stereocilia form and how does the actin packing develops.
These stages will be discussed.
MITR DDE End Fitting Structural Rigidity Analysis
