THE MOLTING GLAND OF CRUSTACEANS - LOCALIZATION, ACTIVITY, AND ENDOCRINE CONTROL (A REVIEW)

Y-organs are paired cephalic endocrine organs of higher Crustacea (Malacostraca). In lower groups (e.g., Entomostraca), they are absent. They were demonstrated as molting glands by Echalier in 1954. They originate from epidermis, and they may either remain attached to epidermis (e.g., in crayfishes) or become fully independent organs (e.g., in crabs); this is taken as an example of the formation of an endocrine gland from part of the target tissue of neurohormones (Buckmann, 1984). Their anatomical features show very large variations among species; these are extensively discussed here. Their ultrastructural characteristics were recently the subject of two excellent reviews (Birkenbeil, 1990; Spaziani, 1990) and are not fully described here. Y-organs secrete three different ecdysteroids, identified as ecdysone (E), 25-deoxyecdysone (25dE), and 3-dehydroecdysone (3DE). Usually these organs produce either E + 25dE or E + 3DE. The significance of these variations is unclear at the moment. These ecdysteroids are derived from dietary cholesterol, and our knowledge of the biosynthetic pathway is far from complete. Apart from the first step (conversion of cholesterol to 7-dehydrocholesterol) and the three last steps (hydroxylations at positions 25, 22, and 2), the reactions are still unknown, as is also the case in insects. The regulation of Y-organ activity is rather unusual, as it seems to be mainly exerted by an inhibitory neuropeptide secreted from the X-organ-sinus gland complex, the so-called molt-inhibiting hormone or MIH. MIH, recently isolated and sequenced in two species, is a member of a new 7-9 kDa peptide family that appears unique to crustaceans. The inhibitory mechanism of action of MIH on Y-organs has been extensively investigated, but it still remains controversial concerning the transduction mechanism involved, and the regulated steps are unknown. More recently, it has been shown that xanthurenic acid (XA), a derivative of tryptophan isolated from eyestalks, displays also an inhibitory effect on ecdysteroid biosynthesis that would be caused by a direct inhibition of cytochrome P-450 monooxygenase(s). The elucidation of the regulatory mechanisms of Y-organ activity by MIH (and other factors) should contribute to a better understanding of fundamental aspects of the regulation of steroidogenesis in general.