ABSTRACT
Steroid hormones play a major role in the endocrine systems in all vertebrates, with actions that include regulation of mineral and water balance, modulation of immune response, and regulation of various facets of reproduct ion. Steroids are synthesized from the base molecule cholesterol via an enzymatically mediated cleavage pathway that involves sequential loss of carbon atoms, and the loss or addition of various active groups. There is considerable diversity of specific steroids produced by different vertebrate classes and taxa within classes, but some general considerat ions apply. T w e n t y - o n e - c a r b on (C2 1) steroids occur as cort icosteroids which are primarily concerned with metabol ic and osmoregulatory function and only have indirect action in reproduction, and progestational steroids which have a primary role in reproduction in all groups. Effects of progestins variously include regulation of oocyte maturation, ovulation, and maintenance of pregnancy or fertilized eggs
in internally fertilizing species. In aquatic vertebrates, progestins or their metabolites also appear to have widespread action as sexual pheromones, probably as a result of their capacity to unambiguously signal the occurrence of 'way points' in the reproductive process. steroids (collectively termed androgens) are commonly considered to be 'male' steroids but, in fact, regulate a suite of reproductive processes in both sexes with expression of 'male' effects essentially being dependent on the metabolic fate of the androgens produced. Effects of androgens include regulation of gonadal growth and development of secondary sexual characteristics in males, and central effects on behaviour, hypothalamic and pituitary function in both sexes. C 1 8 steroids (estrogens) can be synthesized in both sexes but occur at much higher levels in females where they regulate aspects of ovarian growth, development of female secondary sexual characteristics, and the hepatic synthesis of the eggyolk precursor vitellogenin (Vtg) and egg shell proteins (collectively termed zona pellucida proteins).
