Intercellular cytoplasmic transport during oogenesis of a moth midge, Tinearia alternata (Diptera: Psychodidae)

M. Mazurkiewicz & J. Kubrakiewicz

Zoological Inst., Univ. of Wroclaw, 50-335 Wroclaw, Sienkiewicza 21, Poland

Ovary of a psychodid, Tinearia alternata consists of several dozen (50-70), developmentally synchronized, polytrophic ovarioles, each containing one functional egg chamber. The egg chamber is composed of a cluster of 16 germ cells (cystocytes) interconnected by cytoplasmic bridges and invested by the layer of somatic follicular cells. In the pupal ovary the cystocytes forming particular egg chambers are of more or less equal size but they show clearly noticeable morphological differentiation. The most basally located cystocyte within the cluster becomes the oocyte and initiates vitellogenesis while all the remaining cells differentiate into nurse cells. The nucleus of each nurse cell contains one centrally located, prominent nucleolus and more peripherally disposed polytene chromosomes. Fine granular nuage material accumulates in close vicinity of the nucleus while other regions of cytoplasm are filled with densly packed ribosomes. Primary function of the nurse cells is to supply the growing oocyte with macromolecules (mainly RNPs) and organelles which are transported to the oocyte through intercellular bridges. Polarized nurse cell-oocyte transport proceeds in two distinct phases. Slower phase during early stages of oogenesis is followed by a massive and rapid flow of the nurse cell cytoplasmic content into the oocyte during the advanced stages of vitellogenesis In the ovaries of mature females the egg chambers grow considerably. Vitellogenic oocytes comprise an increasingly greater proportion of the egg chamber volume. Initially this growth is mainly due to yolk accumulation but later it depends also on the transport of nurse cell cytoplasm. During the final stages of oogenesis the nurse cells are almost devoid of cytoplasm and degenerate. Like in most other dipterans the contraction of nurse cells in Tinearia is mediated by actin cytoskeleton. Contrary to the situation found in the ovaries of brachyceran flies the nurse cells of Tinearia seem to lack extensive cytoskeletal networks. They contain only two populations of filamentous actin: subcortical actin and microfilaments associated with intercellular bridges. Thus the way the nurse cell nuclei are anchored in place during the rapid movement of cytoplasm remains unclear.

Index terms: nurse cells, oocyte, flow of cytoplasm, Nematocera

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