Fine structure ofDrosophila wing imaginal discs during early stages of metamorphosis.

Fine structural changes are described which occur in wing imaginal discs taken fromDrosophila prepupae at 120-128 hours of age. The first signs of cuticle formation appear at 124 hours in the tips of microvilli which extend into the peripodial cavity of the disc. A continuous epicuticular layer is evident at 126 hours, and the beginning of laminated endocuticle at 128 hours. Electron-lucid spaces appear in the apical regions of the cells at 122 and 124 hours. These are often clearly intercellular and contain membranous material, but sometime appear to be intracellular, or vacuoles. Their frequency decreases sharply after 122 hours, and begins to rise again at 126 hours. Junctional complexes between cells are present in all ages studied, consisting predominantly of long zonulae adhaerentes in the 120 h disc, of various attachment specializations, including septate desmosomes in the 128 h disc. Inclusions interpreted as lipid are found randomly scattered in the cytoplasm at 120 hours. They change in number, size, arrangement and location in later stages.Microtubules are widely distributed in discs of all ages reported; their function is thought to be cytostructural. A method of precise staging of prepupae based on ultrastructural criteria is proposed.