THE PROTHORACIC GLAND REVISITED

Ecdysone and ecdysteroids have always been defined as insect hormones produced by the prothoracic glands to stimulate molts. I have recently reread some old papers about the prothoracic glands and found that th e widely used brain-prothoracic gland theory on the hormonal control o f insect metamorphosis might be misleading Removal of prothoracic glan ds has no effect on molt cycles. The quantitative responses induced by ecdysteroids are profoundly different from the qualitative, all-or-no ne responses which are induced by the centrally produced metamorphosis hormones. Ecdysteroids are produced in a number of peripheral target tissue and organs themselves, and this fact contradicts the exact defi nition of the term hormone. The true ecdysteroid status can be defined as peripheral feedback tissue factors, which synchronize tissue growt h with some important developmental events like ecdysis or oviposition . It also has been concluded that the endogenous peak; of ecdysteroids , which always occur in a nonfeeding period with minimal metabolic int ensity, have their origin in polar metabolites of sterol that is retri eved from the old disintegrating tissues; prothoracic gland contributi on is very small. The partly hydrophilic, polyhydroxylated sterol (ecd ysteroid) can be reutilized as an essential sterol for growth of the n ewly developing adult tissues and organs. The transport and reutilizat ion hypothesis of polyhydroxylated polyhydroxylated sterol is consiste nt with a widespread distribution of ecdysteroids, not only in insects but in most if not in all living organisms-bacteria Fungi, ferns, hig her plants, invertebrates, vertebrates, and also in the human body. In sects and other arthropods evolved special feedback mechanisms, in whi ch ecdysteroid acquired a role of essential peripheral growth factors coordinating proliferation among various tissue and organs.