In this laboratory exercise, the students learn about oogenesis in two every different systems: the mammal and the fruit fly, Drosophila virilis.  Microscope slides of human and cat ovaries will be used to illustrate the basic sequence of human oogenesis and the histology of the cells involved in that process. The fruit fly, however, offers us the chance to learn about an unusual and illustrative molecular event that occurs during insect oogenesis.  Specifically, the 15 nurse cells attached to each fruit fly oocyte possess polytene chromosomes, chromosomes that each consist of 512 copies (2n=1024).  The function of these unusual chromosomes is to facilitate the massive accumulation of mRNA that is needed for later embryo development.  Unfortunately we can’t easily isolate and examine nurse cells and their polytene chromosomes, but we can look at these chromosomes in the larval salivary gland.  Thus, this is the only time during the semester that the students look at something other than gametes or developing embryos!

The students start by isolating salivary glands from individual third instar Drosophila virilis larvae; this requires some patience, as the students have to learn to pull off the head of each larva without destorying or losing the salivary glands.  When they do this correctly, the salivary glands trail behind the head (see image below).  The glands are then placed on a microscope slide, fixed in acetic acid for 30 seconds, and stained in aceto-orcein stain for up to 5 minutes.  A coverslip is placed on top of the glands and firm pressure is applied to the coverslip.  This pressure forces the salivary gland cells to burst open and the chromosomes to spread out.