Ca++-induced structural changes in photoreceptor microvilli of Diptera

When the compound eyes of the fly Lucilia are fixed for electron microscopy with glutaraldehyde in common buffer solutions, artefactual whorls are liable to be formed from the photoreceptor microvilli. The whorls result from two factors: (i) a prolonged time interval prior to osmication, such as the "overnight" primary fixation or wash at 4 degrees C commonly used in studies of compound eyes; (ii) as little as 1-2 mM Ca++ in the primary fixative and wash solutions. Osmication after short (1h) glutaraldehyde fixation at 4 degrees C, or omission of Ca++ and addition of 2 mM EGTA, prevent whorl-formation. In the tipulid fly Ptilogyna, similar artefacts are produced, but are confined to the distal zone of the microvilli that sheds during turnover.