Endocrine regulation of reproduction in elasmobranchs: Archetype for terrestrial vertebrates

Abstract

The reproductive biology of the chondrichthyan fishes is remarkably sophisticated. Using both oviparous and viviparous reproductive modes, the group in general has adapted the strategy of producing relatively few young at one time. The oviparous species foreshadow the situation common in oviparous reptiles and universal in birds; viviparous species range from internal incubators to species in which the complexity of placentation and egg yolk reduction approaches the eutherian condition. In certain viviparous elasmobranchs histiotrophic nutrition attains an importance not seen in any other vertebrate group. Associated with the elasmobranch priority on these female reproductive adaptations, a typically vertebrate type of reproductive endocrinology is seen for the first time. This laboratory has focused on ovarian endocrine function and the regulation of the reproductive tract. We report on the steroidogenic capacity of theca, granulosa, and luteal elements and their regulation and correlate this with the natural ovarian cycle and plasma hormone levels. In addition, we describe the biochemistry of the ovarian peptide hormone relaxin and some of its actions on the reproductive tract. As in terrestrial species, relaxin effect on the elasmobranch reproductive tract is steroid hormone-dependent. In general, the results support the concept that the basic endocrine regulatory elements for vertebrate reproductive processes were established in the earliest gnathostomes, the elasmobranchs. These species may therefore provide useful models from which to gain insights into the complexities of higher vertebrate systems that may be obscured by subsequent adaptation to a terrestrial environment and homeothermy.