Structure of the germinal vesicle during oogenesis in leechGlossiphonia heteroclita (Annelida, Hirudinea, Rhynchobdellida)

Abstract

Oogenesis in the glossiphoniid leech Glossiphonia heteroclita (Hirudinea, Rhynchobdellida) is nutrimental, i.e., the growing oocyte is supported by specialized germline cells, the nurse cells. The main function of the nurse cells is to provide oocytes with cell organelles and RNAs (mainly rRNA). However, in studied leech species, irrespective of the nutrimental mode of oogenesis, the germinal vesicle (GV = oocyte nucleus) seems to be very active in rRNA production. As shown in the present study, during early previtellogenesis in the GV the meiotic chromosomes and prominent primary nucleoli occur. In late previtellogenesis the chromosomes condense and occupy a limited space of nucleoplasm in close vicinity to primary nucleolus, forming a karyosome. At the onset of vitellogenesis several prominent extrachromosomal DNA bodies appear in close association with the karyosome. At the same time, the primary nucleolus is no longer visible in the GV. As vitellogenesis proceeds the extrachromosomal DNA bodies undergo fragmentation and numerous spherical, RNA- and AgNOR-positive inclusions occur in the nucleoplasm. They are regarded as multiple nucleoli. Finally, in late oogenesis numerous accessory nuclei are formed in close proximity to the nuclear envelope. They usually contain one dense body, morphologically similar to multiple nucleoli. The amplification of rDNA genes, the occurrence of extrachromosomal DNA bodies, as well as the presence of multiple nucleoli and accessory nuclei are described for the first time in the phylum Annelida.

Formation of germ-line cysts with a central cytoplasmic core is accompanied by specific orientation of mitotic spindles and partitioning of existing intercellular bridges

“…They can be spherical and so resemble male cysts (as in P. geometra; Fig. 1c) or their shape might be irregular with a voluminous and branched cytophore (as in Glossiphonia heteroclita and other glossiphoniid leeches, see Fernández et al 1992;Świątek 2005a, 2005b. In D. veneta, female germ cells cluster around a poorly developed cytophore that resembles a thin cytoplasmic strand (Siekierska 2003).…”

“…The clusters are enveloped by an extended somatic follicular epithelium only (Siekierska 2003). In leeches, female germ-cell clusters with their follicular envelope are enclosed within coelomic cavities of the ovisacs (Fernández et al 1992;Świątek 2005a, 2005b, 2008. During oogenesis, the oocytes significantly grow and protrude from the clusters into the coelomic cavity.…”

“…In all clitellate species, each cystocyte in the cluster is connected to a central cytophore via its own intercellular bridge (Fernández et al 1992;Jamieson 2006;Krüger et al 2008;Siekierska 2003;Świątek 2005a, 2005b, 2008Fig. 1a-d).…”

Accessory nuclei in insect oogenesis: in search of the function of enigmatic organelles

“…Polyxenus lagurus Kubrakiewicz, 1991 Annelida/ Hirudinea Rhynchobdellida Glossiphonia heteroclita Swiatek, 2005 TABLE 1 standing AN function(s), and they are discussed at greater length in the next section.…”

Similarities between oocytes and macrophages. An overview

“…In leeches, nurse cells, which are also called trophocytes, provide cytoplasmic organelles to the oocytes by way of transport through the oolemma, and some of them are even sacrificed for this purpose [13,14].…”