Morphological Aspects of Ageing in the Placenta 109 



placenta (Padykula, 1955). Similarly, it has been suggested 

 (Wislocki, Dempsey and Fawcett, 1948) that the intense 

 cytoplasmic basophilia of the early human placenta, which is 

 attributable to ribonucleoprotein, represents a provision for 

 the synthesis of proteins necessary for foetal growth until 

 such time as this function is taken over by the foetal liver. 



A progressive increase in permeability of the placental 

 barrier has been reported by various investigators. In rabbits, 

 placental permeability to antibodies (Rudolfo, 1934), phenol- 

 sulphonphthalein (Lell, Liber and Snyder, 1932), neoars- 

 phenamine (Snyder, 1943) and radioactive sodium (Flexner 

 and Pohl, 1941a) increases during the course of gestation. 

 Similarly, in the rat there is a gradual increase in permeability 

 to insulin (Corey, 1932) and radioactive sodium (Flexner and 

 Pohl, 19416). Moreover, Flexner and his associates have 

 observed an increase in the rate of transfer of heavy water 

 and sodium across the placental barrier in other mammals 

 (sow, goat, cat, man). These increases in the rate of placental 

 exchange have been ascribed to a progressive reduction in the 

 number of layers of the placenta accompanied by a diminu- 

 tion in width of the barrier, a combination of changes which 

 has generally been regarded as favouring a progressive in- 

 crease in placental "efficiency". 



Aside from these gradual morphological and physiological 

 changes, the question arises as to whether, beginning some 

 time late in gestation, the definitive placental structures 

 undergo regressive, terminal changes which have an unfavour- 

 able effect on placental exchange, limit the length of gestation, 

 or influence the time of parturition. Such changes, if present, 

 would comply with Lansing's (1952) definition of ageing as a 

 process of unfavourable progressive change which becomes 

 apparent after maturity, is inversely related to growth 

 and involves a decrease in efficiency of the mechanism for 

 reconstruction. 



There are relatively few substantial morphological grounds 

 upon which to evaluate to what degree the definitive placental 

 structures undergo unfavourable terminal changes which 



