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. 2019 Mar 8;10:204. doi: 10.3389/fphys.2019.00204

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Copyright © 2019 Bonelli, Bruno, Caccia, Sgambetterra, Cappellozza, Jucker, Tettamanti and Casartelli.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schematic representation of H. illucens larval midgut in which the main morphofunctional features of each region of the organ are reported. The luminal pH of the three regions is very different, being acidic, strongly acidic, and alkaline in the anterior, middle, and posterior midgut, respectively. The AMG is characterized by columnar cells with secretory activity; in this region ingested polysaccharides begin to be degraded by soluble amylases and lipids are hydrolyzed by lipase. Copper cells, which are responsible for the highly acidic pH in the luminal content of the middle midgut, are localized in the first tract of the middle midgut (MMG1). The epithelium of the second part of the middle midgut (MMG2) is formed by large, flat cells; in this tract digestive processes do not occur, but the high activity of lysozyme and the strong luminal pH indicate an important role of this region in killing ingested pathogens. The posterior region of the midgut plays a fundamental role in protein digestion thanks to endo-and eso-peptidases, and it is responsible for further digestion of lipids. The first tract of this region (PMG1) is characterized by columnar cells with secretory activity, while the second one (PMG2) presents columnar cells with microvilli that are longer than in other regions, suggesting a main role of these cells in nutrient absorption.