Ultrastructure and function of nurse cells in phthirapterans. Possible function of ramified nurse cell nuclei in the cytoplasm transfer

The structure of nurse cells as well as the distribution of cytoskeletal el ements (actin filaments, microtubules) in three representatives of phthirap terans: the pig louse, Haematopinus sids (Anoplura) and bird lice, Eomenaca nthus stramineus, Columbicola columbae (Mallophaga) were investigated. All three species have polytrophic-meroistic ovaries which means that each oocy te remains connected with a group of nurse cells via specialized cytoplasmi c canals-intercellular bridges (ring canals). Throughout vitellogenesis, va rious macromolecules as well as organelles (mitochondria, endoplasmic retic ulum vesicles, ribosomes) are transferred from the nurse cells to the oocyt e. During this flow, the nurse cell nuclei do not enter the oocyte and are retained in the cell centers. In holometabolous insects (e.g. Drosophila, h ymenopterans), the central position of nurse cell nuclei is maintained by c ytoskeletal elements (actin filaments or microtubules). In the investigated species, the nurse cells are equipped with large, highly extended (irregul arly lobed) nuclei. The inner nuclear membrane is lined with a relatively t hick layer of nuclear lamina. Ultrastructural analysis and staining with rh odamine-labeled phalloidin revealed that the nurse cell cytoskeleton is poo rly developed and represented only by: (1) single microtubules in the perin uclear cytoplasm, and (2) the F-actin layer in the cortical cytoplasm. In t he light of this, we postulate that in phthirapterans the position of nurse cell nuclei during the cytoplasm transfer is maintained not by the cytoske letal elements, but by a largely extended shape of the nuclei (i.e. their e longated extensions). (C) 2001 Published by Elsevier Science Ltd.