The saccus vasculosus of the neotropical cichlid fish Cichlasoma dimerus: characterization, developmental studies and its response to photoperiod

Abstract

The saccus vasculosus is an organ present in gnathostome fishes, located ventral to the hypothalamus and posterior to thepituitary gland, whose structure is highly variable among species. In some fishes, this organ is well-developed; however, itsphysiological function is still under debate. Recently, it has been proposed that this organ is a seasonal regulator of reproduction.In the present work, we examined the histology, ultrastructure, and development of the saccus vasculosus in Cichlasomadimerus. In addition, immunohistochemical studies of proteins related to reproductive function were performed. Finally,the potential response of this organ to different photoperiods was explored. C. dimerus presented a well-developed saccusvasculosus consisting of a highly folded epithelium, composed of coronet and supporting cells, closely associated with bloodvessels, and a highly branched lumen connected to the third ventricle. Coronet cells showed all the major characteristicsdescribed in other fish species. In addition, some of the vesicles of the globules were positive for thyrotropin beta subunit,while luteinizing hormone beta subunit immunostaining was observed at the edge of the apical processes of some coronetcells. Furthermore, neuropeptide Y and gonadotropin inhibitory hormone innervation in the saccus vasculosus of C. dimeruswere shown. Finally, animals exposed to the long photoperiod showed lower levels of thyrotropin beta and common alphasubunits expression in the saccus compared to those of animals exposed to short photoperiod. All these results support thehypothesis that the saccus vasculosus is involved in the regulation of reproductive function in fish.