ABSTRACT
E yesta lk H orm on e D irectly In h ib its V ite llo g e n in S y n th e sis
Much research on endocrine systems has been conducted on the inhibi tion of reproductive maturation by eyestalk hormone (s) since the pio neering work in the 1940s. Eyestalk ablation stimulates ovarian matura tion in penaeid shrimp. This treatment reduces the production of a vitellogenesis-inhibiting hormone (VIH), also known as the gonad-inhib iting hormone, and thus permits maturation of the ovaries in female penaeid shrimp. Many workers showed that reproductive maturation in penaeid shrimp is regulated by a VIH from the X-organ-sinus gland complex in the eyestalks. In this complex, the acidophilic sinus gland is connected with axons from the neurosecretory cell of the X-organ, which is located in the medulla terminalis of penaeid shrimp and the other crustaceans. In general, the sinus gland is easily recognizable on the neurilemma, surrounding the optic ganglia. However, in the penaeid shrimp, kuruma prawn (P. japonicus), and white prawn (P. vannamei), which are members of the most primitive family in the Natantia, the sinus gland is present between the medulla externa and medulla interna (Fig. 5). Unilateral eyestalk ablation stimulates Vg synthesis and its secre tion into the blood in previllogenic immature P. japonicus. Eyestalk abla tion also induces a rapid increase in yolk protein synthesis in P. vannamei. These findings suggest that VIH, secreted by the X-organ-sinus gland complex, inhibits Vg synthesis and secretion into the blood in female penaeid shrimp. Also in the kuruma prawn, P. japonicus, Vg synthesis can be induced in isolated previtellogenic ovarian pieces incubated in vitro (Yano and Hoshino, unpublished data). This indicates that Vg syn thesis in previtellogenic ovary can be initiated when the ovary is not affected directly by VIH from eyestalks. Therefore, these results suggest that Vg synthesis in the ovary may be inhibited directly by vitellogenesisinhibiting hormone (VIH) secreted by the X-organ-sinus gland complex of eyestalks (Fig. 7). Unilateral eyestalk ablation is not effective in in creasing serum Vg in vitellogenic female kuruma prawn. This suggests that the VIH level decreases quickly immediately before the initiation of vitellogenesis and stays at a low level until after vitellogenesis is com pleted. Therefore, eyestalk ablation may no longer be effective in regulat ing the production of VIH after vitellogenesis has been initiated. Bomirski el al. (1981), Quackenbush and Hermkind (1983), Chamiaux-Cotton (1985),
and Meusy et al. (1987) partially purified VIH and characterized the bioactive factor as a peptide of three different molecular sizes: 2000, 5000, and 7000 Da. Rotllant et al. (1993) demonstrated by immunocytochemistry that VIH neuropeptides are located in the perikarya of neuroendocrine cells belong ing to the X-organ of the medulla terminalis, in their tract joining the sinus gland, and in the neurohemal organ itself in Homarus gammarus larvae.
