Epidermal Cells

Abstract

Everything that we recognize easily about insects depends upon their integument. Their segmentally repeating patterns of sense organs, muscle insertions, cuticle colours and sculpturing, and their cyclical growth with ecdyses, all result from the activities of the monolayer of cells that covers them. Regional patterns differentiate in the plane of the surface, which varies in the kind of cuticle secreted and in the nature of cell specializations as muscle insertions, sense organs and secretory cells. This is made possible by the signals specifying position in the two dimensions of the epidermal monolayer and the local reading and response to those signals. There is also differentiation in the thickness of the cuticle representing change in epidermal activity with time during the intermoult/moult sequence. In sequence or concurrently, an epidermal cell may be growing, dividing, changing a developmental commitment to be manifest later, secreting cuticle as a nutritional reserve, changing its position and the body form, or moulting and ecdysing. The interacting hormonal cues for these temporal changes have allowed the evolution of a sequential polymorphism of great complexity. This concept of the insect integument as a transcript of spatial (positional information) and temporal signals (hormones) is an elaboration of Wigglesworth's idea of local and general factors operating in the epidermis (Wigglesworth 1940). The operation of local and general factors is easily illustrated in lepidopteran epidermal development. At particular times in the last larval stadium, epidermal cells vary in the size of their nuclei indicating their ploidy (Locke 1984). Local factors determine which cells replicate their DNA and where they divide. However, the nucleoli in these nuclei dance to a different tune. They follow a temporal pattern of enlargement for RNA synthesis that is uniform through the epithelium whatever the ploidy of the cells at that moment (Locke and Huie 1980). The stimulus for RNA synthesis and activity in relation to the intermoult/moult sequence is a general one, presumably hormonal, and is superimposed over the local stimuli controlling cell number. There is also an interaction between local and general factors. The environment allowing mitosis has itself a hormonal component.