Midgut epithelium formation in Thermobia domestica (Packard) (Insecta, Zygentoma)

During the first instar larval stage more and more energids migrate to the yolk periphery and the cell membrane starts to form numerous foldings surrounding the groups of energids, which in turn lead to formation of isolated regenerative cell groups. Eventually the cell membrane invaginations reach the center of the yolk mass. Large cells of the primary epithelium, surrounding the newly formed midgut lumen are formed. The cells of the primary epithelium are filled with yolk and are equipped with microvilli pointing to the midgut lumen. As the yolk is being digested, the process of the primary epithelium cells degeneration begins. The cells are getting shorter and start to degenerate. The definitive midgut epithelium is formed from proliferating regenerative cells. It consists of regularly spaced regenerative cell groups as well as the epithelial cells. The ultrastructure of both these cell groups has been described.