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.
