CHANGES IN THE MICROVILLUS CYTOSKELETON DURING RHABDOM FORMATION IN THE RETINA OF THE CRAYFISH PROCAMBARUS-CLARKII

Changes in the microvillus cytoskeleton during the formation of the light-receptive rhabdom in the crayfish retina were examined at four structurally distinct stages. The cytoskeleton of microvilli in early rhabdoms is composed of a regularly packed bundle of 12-25 actin filaments. The polarity of S1 decorated filaments indicates that the plus end of the actin filaments is located at the microvillus tip. The hexagonal packing of filaments within the bundle, their spacing, and the presence of cross-striations along the bundle in longitudinal sections indicate the filaments are held together by cross-linking proteins. Electron microscopic observations and data from three-dimensional reconstructions of individual microvilli indicate that the filaments arise from a concentration of dense material at the tip of the microvillus and extend into the cytoplasm as a rootlet. Over the four developmental stages examined there is an increase in the number of microvilli forming the rhabdomeres and a 50% decrease in the mean cross-sectional area of individual microvilli. During this same period the number of actin filaments forming the microvillus cytoskeleton also decreases. Following this decline, microvilli of late stage rhabdoms, which are structurally similar to adults, contain only two to four filaments. These changes are discussed in relation to the three phases of growth described for stereocilia and brush border microvilli.