Nurse cell-oocyte interaction: a new F-actin mesh associated with the microtubule-rich core of an insect ovariole.

Fluorescent staining of Rhodnius prolixus ovarioles with rhodaminylphalloidin revealed an elaborate interconnecting meshwork of F-actin encasing the microtubule rich core. The thick actin struts have branches extending partially into the nurse cell lobes extending from the syncytical core. Basally the network ends at the nurse cell-pre-follicular interface and does not extend into the trophic cords. Light microscopy, TEM and SEM show the dense fibrous struts have numerous branch points creating a porous cylindrical collar at the core periphery. The filaments are organized in a dense felt-like anisotropic meshwork rather in parallel arrays. This morphology coupled with the stability of the entire structure in extraction procedures suggests the actin filaments are crosslinked, presumably by associated proteins. Detergent extraction experiments indicate isolation of the mesh is possible. Comparison of Nomarski DIC images with polarizing microscopy images of the same preparations show the F-actin struts are not birefringent while the microtubules of the core are highly birefringent and resistant to nocodazole. Indirect immunocytochemical staining of sections for actin confirmed the F-actin distribution visualized with phalloidin and contrasted markedly with staining pattern for tubulin. The discovery of this prominent actin meshwork must now be incorporated into models attempting to explain the mechanisms underlying polarized nurse cell-oocyte transport.