MORPHOLOGICAL ASPECTS OF AGEING IN THE 



PLACENTA 



George B. Wislocki 



Department of Anatomy, Harvard Medical School, Boston, Massachusetts 



By modifying a previous definition devised by Flynn for 

 vertebrate placentas, Mossman (1937) proposed that "the 

 normal mammalian placenta is an apposition or fusion of the 

 fetal membranes with the uterine mucosa for physiological 

 exchange". Thus, the mammalian placenta is not a unified 

 organ composed of homologous units, as the liver and kidney, 

 but consists of several membranous structures which enter 

 variously into relationships with one another in different 

 groups of mammals. In any given animal, these membranous 

 tissues collectively form the placenta, although some differen- 

 tiate and function early in gestation, while others develop and 

 assume placental functions later in pregnancy. Hence, the 

 study of the ageing of the placenta is a matter of investigating 

 not a single structural unit, but a variety of organs which 

 may function either successively or concurrently. Thus, 

 while some of the placental structures of a given animal are 

 degenerating, others are growing and undergoing differentia- 

 tion. One need only mention the successive or concurrent 

 combination in mammals of a bilaminar yolk-sac placenta, a 

 choriovitelline placenta, an inverted yolk-sac placenta (com- 

 plete or incomplete), a chorionic placenta and a chorioallantoic 

 placenta. The combinations and sequences of these structures 

 in various placental types are catalogued and described in 

 masterly papers by Mossman (1937) and Amoroso (1952). 



Many of these placental structures are provisional or 

 transient and serve only temporarily to support the growing 

 conceptus. Thus, the bilaminar omphalopleure and chorio- 

 vitelline membranes may be eventually completely resorbed 



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