Fig. 2. Chitin is absent in the mmy mutant cuticle. (A-C) Comparison of wild-type and mmy mutant larval (end of stage 17) epidermis by transmission electron microscopy (TEM). The wild-type epidermis (A) consists of an epithelial monolayer of flattened cells (inset) beneath a stratified cuticle. The cuticle has three distinct layers: the outermost envelope (env), the middle protein-rich epicuticle (epi) and the underlying chitin-rich lamellar procuticle (pro) attached to the apical plasma membrane (pm). The epidermis of mmy^IK63 mutant larvae (B) is an epithelium with cuboidal cells (inset) covered by a disorganised cuticle. Arrows in the insets of A and B indicate the lateral membrane that is undulated in the wild-type but not in the mmy^IK63 mutant epidermal cell. The mmy^IK63 procuticle varies in thickness and is devoid of lamellar chitin microfibrils, the overlying epicuticle is irregular, and the outermost envelope is reduced to three, instead of five, alternating electron-dense sheets. The envelope and the epidermis of larvae homozygous for the weaker mmy^IL07 allele (C) appear to be normal, the epicuticle is, however, discontinuous and the procuticle harbours electron dense inclusions, probably orphan proteins. (D-F) Detection of chitin in wild-type, mmy^IK63 and mmy^IL07 mutant cuticles with gold labelled WGA. Chitin is detected in the wild-type and mmy^IL07 mutant procuticle (D,F; black dots), but is absent in the mmy^IK63 cuticle (E). Scale bars: 0.5 µm (bar in A applies to A-C; bar in D applies to D-F); 1 µm for insets.