Recent advances in understanding mechanisms of insect cuticle differentiation.

Insects possess a cuticle that covers all tissues exposed to the outside world including the body, the fore- and hindgut and the luminal side of the tracheae. The cuticle is a multifunctional device that protects its carriers against dehydration, arms them against predators, constitutes a physical barrier to prevent pathogen entry and serves as an exoskeleton allowing locomotion. Depending the developmental stage and the body part, the composition and function of the cuticle changes. The body cuticle of larvae of holometabolous insects for example is soft while their cuticular head skeletons used to chew food is hard. In spite of these differences, the basic architecture of the insect cuticle is evolutionarily well conserved between developmental stages and between species. The insect larval cuticle is formed at the apical site of a monolayer of polarised epithelial cells that differentiate concomitantly during embryogenesis. The stratified structure of the cuticle results from the concerted unfolding of basic cellular functions including timed transcription, biosynthetic enzymatic cascades, secretion and membrane trafficking as well as elaborate extracellular self-organization of the components. The aim of this review is to summarize recent advances in understanding these processes.