Abstract
The initiation of evagination of thoracic imaginal disks in Drosophila during metamorphosis has long been considered as being controlled by external factors such as an increase of the hemolymph pressure, a stimulus originating in the mesenchymal cells, or a contact between disk and larval hypoderm. In order to investigate the mechanism of evagination, isolated wing and leg disks from 3rd instar larvae have been cultivated in vitro in the absence of cerebral complex, ring gland and hypoderm. A synthetic medium M has been devised, that will allow isolated disks to evaginate and differentiate in culture. Results show that, in vitro, ecdysone is, among all the factors studied, the only one which is necessary for the initiation of evagination. In the absence of this hormone, disks are unable to evaginate. The osmolarity of the medium leading to complete evagination is lower than the osmolarity of hemolymph of 3rd instar larvae. Osmolarities lower or higher than optimal lead to partial evagination, in the presence of ecdysone. Disk fragments cultivated in medium M with ecdysone evaginate autonomously, which shows that evagination is an active process related to an intrinsic property of the cells. During the 3rd stage, disks progressively acquire the ability to evaginate under the influence of ecdysone. This capacity is lost within 48 hours, if disks are deprived of ecdysone. Internal structural integrity of the disk must be maintained for the preservation of the ability to evaginate. Mechanical damage will suppress this ability. This fact is discussed in view of Hadorn and coworkers' results in the field of disk transplantation. The results obtained from my cultivation experiments are compared to the data we have on the in situ development of imaginal disks in larva and pupa, as well as to the results of others.
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