The use of imidazole-osmium (C3H4N2/OsO4) to stain lipids in salivary gland histological sections of Rhipicephalus sanguineus sensu lato (Acari: Ixodidae) female ticks

Histological techniques used to stain lipids in invertebrate histology are sometimes inefficient to demonstrate the presence of these elements in cells, once these methods leave a residual background, making the separation of lipid from the remaining cytoplasmic content difficult. The use of imidazole-osmium is common in transmission electron microscopy (TEM), and it has provided very accurate label of the lipid. Aiming to optimize histochemical analyses, this study proposes a lipid staining protocol using the imidazole-osmium technique, modified to allow the observation of lipids in histological sections under light microscopy. In this experiment, two concentrations of imidazole-osmium (1% and 2%) were applied in salivary glands sections from Rhipicephalus sanguineus sensu lato (s.l.) ticks. The results showed that both concentrations allow accurate detection of the lipid content in cells, in the endomembranes in general and in the plasma membrane as well. In addition, different staining intensities were observed here, varying according to the different feeding periods (2, 4 and 6 days), once the salivary glands undergo alterations in the secretory cycle. The cells submitted to the concentration of 2% imidazole-osmium displayed a greater variation in the lipid staining intensity when compared with those exposed to 1%, indicating that this methodology under conventional light microscopy can be pointed as an efficient alternative for the analysis of invertebrate cells and tissues, providing clearer and more specific results in comparison with other techniques.