The very rapid induction of filopodia in insect cells.

Many insect cells, including epidermis, fat body, ocnocytcs and pericardial cells, can very easily be induced to form long fine processes or filopodia. Filopodia contain microfilaments hut differ from epidermal feet in lacking microtubules and in having a much smaller and uniform diameter. Although they may be 10-30 mum long they are less than 0.1 mum wide. They often form straight connections like guy-ropes between their origins and their tips, and when freed from their surface attachments they may contract into helices, as though capable of generating tension. The basal lamina helps to keep the basal surfaces of epidermal cells together. In Rhodnius epidermis, filopodia form only seconds after its removal. They arise at the cell margins and extend to distant part of neighbouring cells where they adhere particularly at their tips. Such filopodia retract and disappear in 20-60 min with the reformation of the basal lamina as though they have functioned to pull neighbouring cells back together. In Calpodes epidermis, filopodia form from the lateral faces as well as the cell margins after trypsin digestion of desmosomes and hemidesmosomes. The observations suggest that filopodia are induced in response to cell separation and function to restore cell to cell continuity. Filopodia also form in the normal course of development where cells separate prior to their rearrangement to make new tissues as in epidermal and fat body metamorphosis. Filopodia are probably ubiquitous agents for the sensing and movement of cells relative to one another in tissue morphogenesis.