role regulat ion steroidogenic enzyme enzyme level steroidogenic cell normal physiological fluctuation steroidogenlc enzyme adrenal cortex cell-specific expression final part hormonal signal parallel change steroid oxidoreductase major type basal expression steroidogenlc enzyme varies cell specific expression gene expression major physiological variation selective activation trophic hormonal stimulation steroid output cytochrome p450 steroidogenic capacity uring tissue cell differentiation steroid hormone biosynthesis biosynthesis ofspecific steroid steroidogenic p450s specific steroid quantitative capacity relative role
The pattern of steroid synthesis in the placenta is complicated by the fact that the placenta is an ‘incomplete endocrine gland’: it lacks the key enzymes required to synthesise the full range of steroid hormones. Most notably, the placenta does not express the CYP17A enzyme necessary to convert progestogens to androgens. As a consequence, for the placenta to synthesise estrogens, it has to work in concert with the fetal adrenal gland and maternal liver, with steroids passing back and forth between the placenta, fetal and maternal circulations. Importantly, 16 α -hydroxyandrostenedione secreted from the fetal zone of the fetal adrenal cortex can, in the placenta, be aromatised to estriol, a unique steroid that is synthesised only by the placenta and that can be used as a surrogate marker to assess the functional status of the fetal adrenal gland.