546 



SPERM, OVA, AND PREGNANCY 



six square yards (Dodcls, 1924; Rech, 

 1924) up to twice that figure (Christoffer- 

 sen, 1934) . At present, there would seem to 

 be no clear demonstration of a correlation 

 between Grosser's four morphologic types 

 and the relative rates at which placental 

 exchange takes place. 



It is, of course, quite probable that read- 

 ily diffusible substances, such as oxygen, 

 water, and salts, are exchanged through the 

 thinnest parts of the placental membranes 

 which are usually confined to the chorio- 

 allantoic placenta. However, it is probably 

 not true, as previously generally believed, 

 that almost all substances capable of trans- 

 mission, including proteins, traverse the 

 thinnest regions. It would seem likely that 

 all of the substances requiring regulation, 

 including most carbohydrates, lipids, and 

 proteins, are exchanged through thicker and 

 more specialized regions. 



Cunningham (1920, 1922) was one of the 

 first to discern, from experiments on the 

 differential permeability of the placental 

 barriers of cats and rabbits to potassium 

 ferrocyanide and iron ammonium citrate, 

 that substances which traverse the placenta 

 are divisible into three categories. (1) Those 

 which are diffusible and which meet with no 

 mechanism in the placenta capable of acting 

 on them. These pass by diffusion from 

 mother to fetus, or in the reverse direction, 

 without any mediation on the part of the pla- 

 centa. (2) Those which meet with a definite 

 preformed, regulatory mechanism. These in- 

 clude most of the substances which are de- 

 signed for the fetal metabolism, including 

 iron compounds. (3) Finally, those to 

 which the maternal or fetal surfaces of the 

 placental barrier are impermeable. These 

 include most formed substances, such as 

 cells and particulate matter. 



From all of these considerations it would 

 seem most likely that readily diffusible 

 substances, such as water, oxygen, and 

 some salts, are exchanged through the thin- 

 ner parts of chorio-allantoic placentas, 

 whereas more complex substances are 

 transferred mainly through thicker regions, 

 including the paraplacental borders and 

 yolk sac placentas of animals which pos- 

 sess them. If it is true that readily diffusible 

 substances traverse principally the thinner 



portions of the placental barrier, then with 

 respect to them, the sequence of the several 

 placental types defined by Grosser would 

 continue to be significant. Nevertheless, it 

 is apparent, as Huggett and Hammond 

 (1952) and others have recently empha- 

 sized, that the exchange of each substance 

 will have to be individually investigated 

 with reference to its mode of transfer and 

 the factors affecting it, before a clear pic- 

 ture of placental physiology can be drawn. 



In view of the results of Brambell and 

 his associates which indicate that anti- 

 bodies are transmitted through the yolk 

 sac placenta of rodents rather than through 

 the thin placental membrane of the allan- 

 toic placenta, one wonders what prevents 

 antibodies and some other proteins from 

 traversing the syndesmochorial and epithe- 

 liochorial placentas of ungulates. Is it so 

 much that the placental barrier in ungu- 

 lates is too thick to permit their passage, or 

 is it perhaps mainly that the barrier lacks 

 the particular provisions which in the ro- 

 dent's yolk sac facilitate their passage? 



The human placenta is interesting in that 

 it i)rovides only one general avenue for the 

 transfer of substances from mother to fetus. 

 The chorionic villi transmit both readily 

 diffusible substances and those requiring 

 chemical mediation of various kinds for 

 their transfer. This raises the question as 

 to whether, here, all substances follow the 

 same morphologic route through the pla- 

 cental membrane. This cannot be answered, 

 except to suggest that possibly they do not, 

 because of slight histologic differences be- 

 tween the chorionic villi in various seg- 

 ments of the villous tree and of possible 

 differences in the relationships of the seg- 

 ments to the maternal circulation. Simi- 

 larly, the arrangement of the fetal blood 

 vessels and the mode of circulation of the 

 blood within them might also result in differ- 

 ences in permeability and functional activity 

 in different regions of the villous trees. These 

 are questions which should be investigated 

 further. It seems reasonable to anticipate 

 that future biochemical investigations will 

 define transport mechanisms in the placenta, 

 as they have in the kidney tubules and small 

 intestine. 



Another jioint of interest concerning the 



