The relationship between germline cells and apical complex in the testes of the milkweed bug throughout postembryonic development

Abstract

Summary The apical complex of Oncopeltus fasciatus is already fully developed at hatching of the first instar larva (L1). It consists of a vesicular structure containing 12 cells, which show an epithelial organization around a small lumen. These cells can be easily distinguished by their electron-dense cytoplasm and a nucleus which encloses a large condensed nucleolus; they do not divide further during postembryonic development. Their subcellular structure remains virtually unchanged from L1 until death of old age. Apical complex cells have a remarkably thin layer of cytoplasm around the nucleus, with many free ribosomes. Their most notable features are vesicular sER, which is continuous with the nuclear envelope, and the Golgi complex. The Golgi complex always faces the surface of the apical complex, where germline cells are apposed; however, no secretory processes involving the Golgi complex could be detected. In the newly hatched larva, anlagen of the testis follicles each contains only about four germline cells arranged around the apical complex, and surrounded by several (four to eight) follicle border cells. Even at this stage, the germline cells exhibit an unusual asymmetric shape. They each consist of a globular perikaryon, which at one side extends projections towards the apical complex where they terminate. During L1 and L2, germline cells divide symmetrically. At the beginning of L3, they start to generate spermatogonia, and spermatocyst development commences. Follicle border cells send extensions towards the spermatogonia and enwrap them, and thus become spermatocyst envelope cells. Usually, one cyst is surrounded by one envelope cell which becomes polyploid as the cyst enlarges. The relationship of apical complex cells and the terminals of germline cell projections have been analyzed in greater detail. The projection terminals often show bouton-like swellings at the surface of apical cells. The intercellular cleft is exceptionally narrow at these sites, but specialized cell-cell junctions are not discernible. Projection terminals contain many mitochondria, and autophagic vacuoles—defined by structure and enzymatic activity—are often present. Terminals are apparently constantly involuted and formed anew. The presence of sites of specialised contact between apical cells and germline cells prompts the conclusion that signals are exchanged at these sites which have an impact on the germline cells. Possibly they control the stem cell character of germline cells and the spermatogonial pathway.