Subcortical microtubule network separates the periplasm from the endoplasm and is responsible for maintaining the position of accessory nuclei in hymenopteran oocytes

Oocytes of hymenopterans are equipped with peculiar organelles termed accessory nuclei. These organelles originate from the germinal vesicle (oocyte nucleus) and gather preferentially at the anterior pole. To gain insight into the mechanism of uneven (asymmetrical) distribution of accessory nuclei, the organization of the microtubule cytoskeleton in the oocytes of two hymenopterans Chrysis ignita and Cosmoconus meridionator has been studied. It is shown that during late previtellogenesis two networks of microtubules are present along the contact zone between the oocyte and enveloping follicular epithelium. The external one is associated with belt desmosomes connecting neighbouring follicular cells. The internal network is composed of randomly orientated microtubules and separates transparent, organelle-free periplasm from the endoplasm. All cellular organelles and the germinal vesicle are localized in the endoplasm. Accessory nuclei are accumulated in the anterior endoplasm; they always lie in direct contact with the subcortical network. Treatment with colchicine results in the disappearance of the periplasm as well as in the redistribution of cellular organelles including accessory nuclei. Presented findings suggest that subcortical microtubules play an important role in the positioning of accessory nuclei throughout the ooplasm.