Midgut epithelium in molting silkworm: A fine balance among cell growth, differentiation, and survival

• The morphology and functional activity of the silkworm midgut during larva–larva molting was studied.
• ISCs mainly proliferate during the first half of the molt period to widen the midgut epithelium.
• The structure of the midgut epithelium is maintained and its functional activity is partially retained throughout the molt.
• Midgut cells do not need autophagy to cope with nutrient deprivation.
• The mobilization of long-term energy storage nutrients provides a trophic support to the midgut epithelium during molt.

The midgut of insects has attracted great attention as a system for studying intestinal stem cells (ISCs) as well as cell death-related processes, such as apoptosis and autophagy. Among insects, Lepidoptera represent a good model to analyze these cells and processes. In particular, larva–larva molting is an interesting developmental phase since the larva must deal with nutrient starvation and its organs are subjected to rearrangements due to proliferation and differentiation events. Several studies have analyzed ISCs in vitro and characterized key factors involved in their division and differentiation during molt. However, in vivo studies performed during larva–larva transition on these cells, and on the whole midgut epithelium, are fragmentary.In the present study, we analyzed the larval midgut epithelium of the silkworm, Bombyx mori, during larva–larva molting, focusing our attention on ISCs. Moreover, we investigated the metabolic changes that occur in the epithelium and evaluated the intervention of autophagy.Our data on ISCs proliferation and differentiation, autophagy activation, and metabolic and functional activities of the midgut cells shed light on the complexity of this organ during the molting phase.

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