The allatotropic activity of the larval brain of Galleria mellonella cultured in vitro

The brains of last instar larvae of Galleria mellonella can be successfully cultured in vitro. The high allatotropic activity of 0–8 hr brains, as measured by their ability to elicit supernumerary larval molts upon implantation into 0–8 hr last instar larval hosts, remains constant through 4 days in vitro. The allatotropic activity of 120 hr brains increases during in vitro culture. The number of supernumerary larvae resulting from the implantations of 120 hr brains rises from 20 to 30% of the hosts implanted with fresh brains, to 90% of the hosts supplied with brains maintained in vitro for 24 hr prior to implantation. There is a progressive loss of paraldehyde-fuchsin-stainable neurosecretory material over 72 hr of in vitro culture from the three groups of neurosecretory cells visible in whole mounts of the brains. Nevertheless, the ultrastructural picture of the medial neurosecretory cells, which are the probable source of the allatotropic factor, indicates synthetic activity after 72 hr of culture. In 0–8 hr last instar brains which are cultured for 48 hr as complexes with attached corpora cardiaca and corpora allata, stainable neurosecretory material accumulates in all three groups of cerebral neurosecretorycells. The brains from the cultured complexes have low allatotropic activity. It is concluded on the basis of the bioassay and ultrastructural observations that under our conditions of in vitro culture, the neurosecretory cells of the last instar larval brain not only survive but also maintain synthetic activity through at least 72 hr in vitro. The relationship between stainable neurosecretory material and neurosecretory activity in this system is discussed, as are possible feedback interactions involving the corpora cardiaca-corpora allata and the brain.