Histochemical and biochemical investigations concerning the function of larval oenocytes of Tenebrio molitor L. (Coleoptera, Insecta)

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Abstract
Larval oenocytes of Tenebrio molitor were investigated histochemically. In contrast to the lipid droplets of the fat body, they did not stain with Sudan black. A positive reaction for lipoproteins appeared only after destructive oxidation with sodium hypochlorite. These lipoproteins are the remnants of degenerated membranes, as revealed by ultrastructural analysis. Polyphenols could be identified in the exocuticle of exuvia, and in the newly formed procuticle. Endocuticle, epidermis and oenocytes showed no staining reaction. In oenocytes a great amount of lipase is also present which could be detected with several Tweens as substrates. The significance of these lipases remains unclear, since only few glycerides are synthesized in the cells, as shown below. They may play a role in the extended membrane turnover observed in this cell type. In vitro incubation of oenocytes of the larval generation demonstrated that 14C-labeled acetate was only incorporated into the paraffin fraction. A negligible amount of the label was found in glycerides; wax esters were free of label. Larval epidermis is also capable of paraffin formation, but only to a small degree. Oenocytes of the imaginal generation located between the sternal epidermis cells of pupae and adults do not synthesize paraffins, but other more polar compounds not yet identified. Labeled waxes in cuticular lipids were detected only when 14C-acetate was injected into whole larvae, and the lipids extracted some hours later. Autoradiographs demonstrated that 14C-acetate was intensively incorporated into larval oenocytes, the rate varying in different cells. Incorporation into the epicuticle, probably into the wax layer, was clearly shown. Cuticulin and dense layer do not show an intensive label. The lamellated cuticle also seems to be impregnated with acetate derivatives.