Structural polarity and dynamics of male germline stem cells in the milkweed bug (Oncopeltus fasciatus)

Abstract

The male germline stem cells (GSCs) of the milkweed bug present an extraordinary structural polarity that is, to our knowledge, unequalled by any other type of stem cells. They consist of a perikaryon and numerous projections arising from the cell pole directed toward the apical cells, the proposed niche of the GSCs. The projections can traverse a considerable distance until their terminals touch the apical cells. From hatching until death, the GSC projections undergo conspicuous changes, the sequence of which has been deduced from observations of all developmental stages. Projection formation starts from lobular cell protrusions showing trabecular ingrowths of the cell membrane. Finger-like projections result from a process of growth and "carving out". The newly formed projections contain mostly only free ribosomes other than a few mitochondria. A stereotyped degradation process commences in the projection terminals: profiles of circular, often concentric, cisternae of rough endoplasmic reticulum appear and turn into myelin bodies, whereas mitochondria become more numerous. The cytoplasm vesiculates, lysosomal bodies appear, and mitochondria become swollen. At the same time, the projection terminals are segregated by transverse ingrowths of the cell membrane. Finally, autophagic vacuoles and myelin bodies fill the segregated terminals, which then rupture. Simultaneously, new projections seem to sprout from the perikaryon of the GSCs. These dynamics, which are not synchronized among the GSCs, indicate that a novel type of signal exchange and transduction between the stem cells and their niche is involved in the regulation of asymmetric versus symmetric division of GSCs.

Evidence against a germ plasm in the milkweed bugOncopeltus fasciatus, a hemimetabolous insect

“…This sex-specific function may relate to a putative role in stem cell function. As in other hemimetabolous insects (Büning, 1994), in Oncopeltus germ line stem cells are likely active in the apex of the testes (Schmidt and Dorn, 2004) but are not thought to be present in adult ovaries. One caveat to this hypothesis is that vasa transcript was not detected by in situ hybridisation in the primary spermatogonia (Fig.…”

Cystic spermatogenesis in three species of theprolixuscomplex (Hemiptera: Triatominae)

“…This behaviour is quite well known in insects of the order Hemiptera, as a result of cytogenetic studies. The cystic cells, or polyploid nucleus, make up part of the testicular follicle wall and provide nutritional support for the germ cells during spermatogenesis (Schmidt & Dorn 2004). This nucleus passes through polyploidization events, which support the high metabolic activity observed in some species of triatomines such as T. lenti (Alevi et al 2013c).…”

MT-Nanotubes: Lifelines for Stem Cells

“…The common feature of these situations is that cellular projections are used to extend the range of signaling, unlike that described by Inaba et al, wherein MTnanotubes serve to restrict BMP activation only at the niche-stem cell interface. It is notable that other insect species display extensions from GSCs toward neighboring niche cells, although it is not clear whether they are analogous to MT-nanotubes (Schmidt and Dorn, 2004). Nevertheless, GSC projections may represent a common feature of niche-stem cell communication in insects and other organisms.…”