Recherches sur les ecdysteroïdes présents dans les cocons de la sangsueHirudo medicinalisau cours de l'embryogenèse / Investigations on the ecdysteroids in the cocoons of the leechHirudo medicinalisduring embryogenesis

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Abstract
In earlier studies, we demonstrated that leeches contain ecdysone and 20-hy- droxyecdysone. The titre of these molecules was found to fluctuate during the moult/intermoult cycle which is suggestive of a role of ecdysteroids in the control of cuticulogenesis in Hirudinea similar to that observed in Arthropods. We have now extended our investigations to embryonic development in the leech Hirudo medicinalis. During this period of development, which lasts some 30 days, 5–25 embryos grow inside a large cocoon at the expense of a proteolipidic gel (‘albumen’) synthesized by clitellian glandular cells of the parent leech. We have found that the albumen already contains ecdysteroids before the onset of embryogenesis (‘parental ecdysteroids’). At this time, and during the early stages of embryogenesis, several as yet unidentified low polar ecdysteroids predominate in the albumen; the titre of these molecules shows a dramatic decrease in the albumen at mid-embryogenesis which is concomitant with a marked rise in the concentration of free ecdysone and 20-hydroxyecdysone. In the embryos, this stage coincides with a remarkable transformation of the first structures (‘the cryptolarval metamorphosis’). At all stages investigated the albumen contains significant amounts of Helix hydrolysable conjugates. At least in early stages, the hydrolysis of these conjugates yields free ecdysone and a low polarity ecdysteroid. We suggest that in Hirudo the ecdysteroids synthesized before egg-laying by the adult leech play a role in the control of embryonic development and possibly also in the as yet not understood cycle(s) of embryonic cuticulogenesis.