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It is widely believed that during the relatively short duration of a normal pregnancy the placenta progressively ages and is, at term, on the verge of a decline into morphological and physiological senescence.1-3 This belief is based on the apparent convergence of clinical, structural, and functional data, all of which have been taken, rather uncritically, as supporting this concept of the placenta as an aging organ with, all too often, no distinction being made between time related changes and true aging changes. I will review some of these concepts and consider whether the placenta truly undergoes an aging process. For the purposes of this review an aging change is considered to be one which is intrinsic, detrimental, and progressive and which results in an irreversible loss of functional capacity, an impaired ability to maintain homeostasis, and decreased ability to repair damage.
The placenta is unusual in so far as its basic histological structure undergoes a considerable change throughout its lifespan. For some time it has been customary to describe the appearances of the placental villi in terms of their changing appearance as pregnancy progresses, comparing, for instance, typical first trimester villi with those in third trimester placentas. It has often been implied that this changing appearance is an aging process, but it is now recognised that this temporal variability in villous appearances reflects the continual development and branching of the villous tree (fig 1) In recent years the relation between the growth of the villous tree and the villous histological appearances has been formally codified5-8with identification of five types of villi (fig 2).
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