Growth and differentiation of the larval midgut epithelium during molting in the moth, Manduca sexta

The number of epithelial cells comprising larval midgut of the tobacco hornworm moth, Manduca sexta increases 200-fold in development from the first to the fifth instar. We have examined larvae periodically before and during molting to follow epithelial cell proliferation and differentiation. The midgut epithelium in Manduca sexta consists predominantly of columnar and goblet cells. These are arranged in a characteristic pattern with each goblet cell surrounded by a single layer of 4-6 columnar cells (Hakim et al., (1988)). While undifferentiated basal stem cells are infrequently seen in intermolt larvae, just prior to the period when external signs of molting are visible, their number increases and mitotic figures become common. Proliferation continues for several hours and then these stem cells differentiate following a pattern similar to that seen during embryogenesis (Hakim et al., (1988)). Here, however, the newly differentiating cells become intercalated among the mature differentiated cells already present in the epithelium. Since the pattern of individual goblet cells surrounded by a reticulum of columnar cells is maintained after the addition of new cells, the midgut epithelium of molting larvae appears to be a useful model for studying pattern formation in development.