The Steroids

Abstract

This chapter gives a general idea of the steroid hormones found in nonmammalian vertebrates both in their similarity and their differences from mammals. An evolutionary pattern may be discerned. Section 1.1 deals with the structure of the steroids and the rules for naming different hormones to enter the steroid ``language.'' Since endocrine studies rely on information obtained from both in vitro incubations and measurement of steroid levels in plasma, the advantages and limitations of these methods are given. Discussion of steroids is often divided into major classes, such as androgens, estrogens, corticosteroids, but here it is more helpful to discuss separately the steroids secreted by the individual endocrine glands. It must be emphasized, though, that it is no longer possible, especially in nonmammalian vertebrates, to link particular steroids to specific glands or to attach a particular biological activity in the sense, say, of male/female sex hormones. Even among the mammals there are major interspecific differences in the nature of steroids secreted by the endocrine glands. It is therefore to be expected that in the nonmammalian vertebrates, which include all species from the lampreys and hagfishes to the birds, that the variety will be enormous. In many classes of vertebrates only one or two species have been examined, and any conclusions deduced from these may well not be typical of the class as a whole.