Steroid transport through the surface of the prothoracic gland cells in Galleria mellonella L.

Steroid transport through the cell surface of the giant polyploid prothoracic gland cells of Galleria mellonella L. was studied by an ultracytochemical method. The alkaloid digitonin, known to form a complex with all sterols having a free-OH radical in position 3, proved to be suitable for studying the interiorisation of moulting hormone precursors and the release of synthesized hormones. The results suggest that cholesterol uptake in the last larval instar occurs by macropinocytosis during the feeding period, while the release of the steroids produced by the gland occurs by reverse micropinocytosis mostly on days 5-7 of the instar. The two processes are not simultaneous. The intracytoplasmic localisation of the reaction product confirms the steroidogenic role of the prothoracic gland.