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Steroids are widely dispersed throughout the biosphere where they fulfil a plethora of roles as bioregulators in both plants and animals. Indeed the simplest organism yet discovered to be utilizing them is a water mold which deploys a pheromone, oogoniol, which is a steroid. Within the animal kingdom, true hormonal use of the oestrogens appears to be restricted to the phyla Arthropoda and Chordata whereas their elaboration appears to have originated as early as ≈400 million years ago with the phyla Mollusca and Echinodermata which developed the cytochrome P450arom necessary to convert androstenedione to oestrone and testosterone to oestradiol. Until relatively recently it was assumed that, in the human, the endocrine oestrogens were largely if not exclusively involved in reproductive physiology. They were known to be responsible, with other agents, for the complex signalling and signal transduction which prepares the female organism for the key events of ovulation, blastocyst implantation, maternal adaptation to pregnancy and lactation—all of which, being central to the survival of the species, are highly conserved. However, the last half century has seen a major expansion in our understanding of the range of oestrogenic activity in physiology. The physico-chemical characteristics of the oestrogens and their receptors provide a signal and signal transduction system of high utility which has found a role in many systems unrelated to reproduction. |
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