Figure 2.

Basic overview of Drosophila spermatogenesis. (A) Testes were dissected from male flies and visualised by phase contrast microscopy. The overview of spermatogenesis in Drosophila is remarkably similar to that in mammals [29]. The process of spermatogenesis begins at the apical tip of the testes with a small number of self-renewing stem cells (germ line) that continually divide to maintain the stem cell number. These cells go through 4 synchronous mitotic amplifications with incomplete cytokinesis which result in a cyst of 16 interconnected spermatogonia. These spermatogonia (or primary spermatocytes) go through cellular growth, differentiation and is particularly characterised by high levels of gene expression. All 16 spermatocytes exit the cell growth program and undergo meiosis I and II to form a cyst of 64 inter-connected primary spermatids which have a single phase-light nucleus and a single phase-dark mitochondrial derivative (Nebenkern). In cross-section the Nebenkern has concentric rings of mitochondrial membranes that resemble an onion and give this stage its name ('onion stage'). These inter-connected haploid spermatids undergo cellular remodelling, elongating as the sperm axonemes are formed inside to form long spermatids that stretch almost the entire length of the testis. During this elongation step the spermatid flagella extend down the lumen towards the apical tip and the nuclei move down towards the basal end. The final step in spermatogenesis is a highly complex process of membrane remodelling called individualisation. In this process an actin cone assembles around the spermatid nuclei and moves synchronously from the heads to the tips of the tail, enclosing cytosol and vesicles in a cystic bulge. It is within this cystic bulge that membrane remodelling occurs to enclose each sperm axoneme to yield 64 individual sperm that are then transferred to the seminal vesicle. (B) Distinct stages in spermatogenesis can be easily visualised by Hoechst DNA staining. Bar, 20 μm.