Spindles from the gonads of five insect species were examined after conventional preparation for electron microscopy. The aim of the study was to determine (1) the range of variation of the spindle membranes between mitotic and meiotic cells and (2) the correlation of possible differences with the microtubule content of the spindles. The study involved four moth species, Ephestia kuehniella, Phragmatobia fuliginosa, Orgyia thyellina, Orgyia antiqua, and one fly, Megaselia scalaris. Somatic and gonial mitoses in all species examined showed a sparse spindle membrane inventory. In contrast, spermatocytes consistently had a multi- layered spindle envelope. In spermatocytes of all Lepidoptera species examined, but not in those of M. scalaris, diverse forms of intraspindle membranes existed in addition to the spindle envelope. Microtubule counts in serially cross-sectioned spindles of E. kuehniella revealed an about 6-fold increase in the mass of polymerized tubulin during the transition from spermatogonia to primary spermatocytes. The increase was 3.3-fold in O. thyellina and less than 3-fold in M. scalaris. The density of intraspindle membranes in E. kuehniella was higher than in O. thyelhna by factors of 1.8 to 3.0. The correlation between the amount of spindle membranes and the microtubule content of the spindle indicates a functional relationship. Spindle membranes are believed to influence microtubule stability via the regulation of the Ca2+ concentration within the spindle area. The high microtubule mass in spindles from Lepidoptera spermatocytes may result from the membrane-dependent lowering of the Ca2+ level within the spindles. Finally, an unconventional idea on the role of intraspindle membranes is offered. This concept is not intended to challenge the function of spindle-associated membranes as Ca2+-sequestrating compartments. Intraspindle membranes are considered as stuffing material in sheathed spindles. Membranous compartments reduce the free volume within the spindle. Thereby, monomeric tubulin is concentrated and the formation of microtubules is favoured.

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