Tyrosine storage vacuoles in insect fat body

Abstract

The watery vacuoles first described from larval insect fat body ( Chironomus, Voinov, 1927; Aedes, Wigglesworth, 1942; Rhodnius, Wigglesworth, 1967) have been studied in 4th and 5th stage Calpodes larvae. The vacuoles arise at the beginning (E+6–24 hr) of the 4th stadium from plasma membrane infolds that separate from the cell surface as provacuoles less than 1 μm in diameter. These provacuoles grow and fuse with one another through the intermolt until about half the volume of each fat body cell is occupied by a single, large vacuole. The vacuoles begin to disappear at molting. Their membrane is either incorporated into the plasma membrane by exocytosis or fragmented into vesicles that fuse to become lamellar bodies where the membranes are presumably digested. All the vacuoles have gone by a few hours after ecdysis. The tyrosine content of the fat body increases and decreases in proportion to the size of the vacuoles. As the vacuoles decrease at molting the titre of tyrosine in the hemolymph is transiently elevated at the time when there is most demand for phenolics for cuticle stabilization. Crystals having the form of tyrosine crystallize out from vacuoles separated from the fat body. In fat body extracts separated by thin layer chromatography, similar crystals occur only in the eluates from spots corresponding to tyrosine. The vacuoles are therefore presumed to be tyrosine stores used in cuticle stabilization at molting. They correspond to a type of aqueous storage compartment that is well known in plants but hitherto little recognized in animal cells.