A new look at the nature of insect juvenile hormone with particular reference to studies carried out in the Czech Republic

This article is a comprehensive summary of the 50-year history of physiological investigations in the Czech Republic into the mode of action of the corpus allatum hormone (CAH) in insects, which is commonly known as the juvenile hormone (JH). During this period 4000 synthetic JH-mimetic bioanalogues were tested. The sesquiterpenoid epoxy-homofarnesoate (JH-I), which is generally thought to be the true JH of insects, is an excretory product of the male colleterial gland, not an insect hormone. There are two principal hormones produced by the insect neuroendocrine system: activation hormone (AH) produced by neurosecretory cells in the brain and JH secreted by the corpora allata. The prothoracic glands are a subordinated target of JH, not PTTH; they are not involved in the regulation of moulting in insects. The development of larval, pupal and adult structures depends primarily on inherited instructions encoded within the genome, not on high, medium or low concentrations of JH. At the level of epidermal cells, the responses to JH are always all-or-none with intermediate forms mosaic mixtures of cells of previous and future developmental stages. There are two alternative theorical explanations of the action of insect hormones. The Gilbert-Riddiford theory proposes that insect development is stimulated by a moulting hormone (ecdysone) released from the PG in response to the prothoracicotropic hormone (PTTH) from the brain. Formation of larval, pupal and adult epidermal strucures depends, respectively, on high, medium or low concentrations of JH. The alternative hormonal theory of Novak-Slama argues that PG is not involved in the regulation of insect moults. High, medium or low concentrations of JH have nothing in common with the selective formation of larval, pupal or adult structural characters. The immature stages of certain insect groups undergo an autonomic (hormone independent), genetically programmed morphogenesis, which can be extended over several moulting cycles. Comparative endocrinological studies revealed several common evolutionary links between the neuroendocrine systems of insects and humans. The neurosecretory cells (NSC) in the insect brain and the NSC in the human hypothalamus are homologous as are the neurohaemal organs (CC) in insects and the neurohypophysis in humans, and the glandular CA in insects is anatomically and physiologically homologous with the human adenohypophysis. The centrally produced hormones (AH and CAH) alter the transcriptional potential the cellular genome. AH switches on genes at the most suitable moments for initiating inherited developmental programmes; JH determines in which direction the development should proceed, i.e. larval somatic growth or morphogenesis.