FOETAL-MATERNAL RELATIONSHIPS: ENDOCRINOLOGY

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Although the growth and development of the fetus is hormonally modulated, there is little evidence to suggest that maternal hormones play a major role. Thus a relatively impermeable placenta preserves an autonomous status for the endocrine regulation of fetal growth. However indirect effects such as the influence of maternal diabetes upon fetal pancreatic function and of maternal thyrotoxicosis should not be overlooked.On the other hand, the intervention of chorionic hormones into the physiology of the mother is well established, varied, and in some species such as the human and rat, highly complex. It would seem that in gaining the advantages of a longer gestation mammals have faced greater evolutionary pressures to develop mechanisms to safeguard the endometrial-chorial relationship. Although in some species this has been accomplished by mechanisms which simply extend the life of the corpus luteum, in others, chorionic hormones themselves not only ensure the continuation of the progestational state, but actually marshall maternal intermediary metabolism for the benefit of the fetus.Luteotrophic responsibility resides in chorionic gonadotrophins such as hCG and PMSG. It may also be a property of placental lactogen, as for example in the rat where it replaces maternal pituitary hormones on about day 12, and possibly in the human female.More striking roles for placental lactogens are (1) in the alteration of maternal metabolism to fetal advantage and (2) in the stimulation of lobulo-alveolar growth in the mammary gland. In one species at least the subsequent lactational performance is dependent upon the intensity of this chorionic stimulus.Control of chorionic polypeptide hormone secretion is poorly understood. There is little doubt that it is influenced by fetal genotype as in the control of PMSG output in the mare, bPL in the cow and possibly hCG in man.The steroidogenic capability of the conceptus has permitted the fetus to participate in the control of birth. The influence is most complete in sheep where adrenal glucocorticoid induction of placental enzymes permits oestrogen synthesis to increase markedly at the expense of that of progesterone. In rats, where there is evidence of strong maternal influence on the onset of labour, the key event, namely the reduction of ovarian progesterone secretion, may again be determined by the conceptus. In man a fall in progesterone is not obligatory for labour but fetal adrenal participation in chorionic oestrogen biosynthesis may be important. The significance of oestrogen is not clear but may reside in actions on the myometrium and on the cervix, both of which may involve the participation of relaxin.