Discussion 115 



deal with the endometrium first — we have been investigating the histo- 

 logical changes that are occurring in its blood vessels during pregnancy: 

 we find that there are very extensive changes occurring in the so-called 

 spiral arteries and particularly in the endothelium of those arteries. 

 In the early stages, the endothelium, of course, is quite thin but as 

 gestation proceeds it becomes thickened in many places and indeed 

 many-layered. Why this occurs is not very easily answered but it is a 

 very striking feature indeed. 



The changes in the placenta are also striking. The villi in the early 

 stages of gestation are oedematous but as pregnancy advances the 

 mesenchyme of these villi becomes fibrotic. I regard this as being in the 

 nature of an ageing phenomenon as we progress from the early stages of 

 the placenta to the later stages. The question of the cause of these 

 quasi pathological changes in the placenta are very difficult to deter- 

 mine. We have not found the fibrinous deposits on the villi that have 

 been described by the pathologists. 



We also found that syncytium invades the mouths of endometrial 

 vessels, both veins and arteries, that are opening into the intervillous 

 spaces, or what we would rather regard as labyrinthine spaces than 

 purely intervillous spaces. 



Amoroso: In your allusion to the endometrium, are you making a 

 distinction between the maternal part of the placenta and other parts 

 of the endometrium ? 



Hamilton: I am. 



Harrison: One of the points I would like to raise is the question of 

 increase in surface area of the placenta as it ages or as it grows. Prof. 

 Hamilton and I have been working on a series of Fallow Deer which 

 have been obtained throughout pregnancy and we have specimens 

 covering extensively the period of gestation in this animal. We find 

 that the number of villi, if you count them, increases; not only that 

 but their branches also increase very remarkably in number about the 

 end of the first quarter of pregnancy ; and later there is a steady increase 

 in the number of tertiary branches. If you remember, the villi in this 

 animal are long and straight and bifurcate into equally straight branches. 

 I am convinced that some of the so-called changes which have been 

 called increases in efficiency of the placenta are, in fact, reflections of 

 this increase in surface area, which at least I know occurs with one 

 animal, and I would like to ask Prof. Wislocki if he has any observations 

 to make on that in other mammals. 



Wislocki: That question was extensively discussed at the First Con- 

 ference on Gestation of the Josiah Macy Jr. Foundation, 1955. It was 

 stressed that unit weights do not form a valid basis for comparison 

 of the relative rates of placental exchange in different mammals. The 

 respective surface areas through which metabolic exchange occurs, 

 involving either the effective surface of the placental membranes 

 (trophoblast, vitelline membrane) or the surface area of the placental 

 capillaries, would seem to be the most important factors. 



Dawes: In the whole animal perhaps what matters is the total amount 

 of exchange. If you define "efficiency" in terms of surface area, you 



