FACTORS ALTERING CONCENTRATION OF BLOOD LIPIDS 409 



free and esterified) can pass into the fetus. However, Popjak 329 recorded a 

 unilateral permeability of the rabbit placenta to cholesterol; according to 

 this concept, the placenta may take up large amounts of cholesterol from 

 the maternal circulation, but it does not pass it to the fetus. Popjak and 

 Beeckmans, 331 using phospholipids tagged with P 32 , proved that, although 

 the fetal placenta took up appreciable amounts of phospholipid from the 

 maternal circulation, it did not transmit the whole phospholipid molecule 

 either to the fetal liver or to the rest of the fetus. Moreover, these investi- 

 gators 332 later demonstrated that glycerophosphate, which is the first 

 decomposition product of lecithin in the tissues, does not pass unhydrolyzed 

 through the placenta. It is therefore believed that a unilateral permea- 

 bility for phospholipids exists in the placenta of rabbits similar to that for 

 cholesterol to some extent, but that the degree of transfer of these sub- 

 stances through the placenta is definitely limited. A comparison of the 

 composition of the lipids in maternal and in fetal blood at birth and during 

 the lactation period is found in the following section. 



i. The Effect of Lactation. At the start of the lactation cycle, the level 

 of lipids in the maternal blood is high, while that in the fetal blood has a 

 low value. Slemons and Stander 249 reported that, at the time of delivery, 

 the blood of the mother had total cholesterol values varying between 195 

 and 330 milligram per cent, as contrasted with figures of 120 and 230 milli- 

 gram per cent in the fetal blood. These figures correspond closely to 

 those reported by Rosenbloom, 250 which were 223 and 120 milligram per 

 cent, respectively. 



As lactation proceeds, there is a rapid change in the levels of blood lipid 

 in the mother and child. This results from the lowering of the high values 

 in the mother's blood, as well as from an augmentation in the low figures 

 of the infant's blood. The colostrum of women contains considerably 

 more cholesterol than does the milk secreted later. 333-335 



During lactation, the blood lipids remain elevated over much longer 

 periods in the cow than in the human subject. Maynard and associates 336 

 found that the total fatty acids, phospholipids, and cholesterol in cows 

 rose rapidly following parturition in an approximately parallel manner; 

 this was followed by a gradual drop to the prelactation figure as the dry 



331 G. Popjak and M. L. Beeckmans, Biochem. J., 45, x (1949). 



332 G. Popjak and M. L. Beeckmans, Biochem. J., 46, 99-103 (1950). 



333 F- W Fox and j A . Gardner, Biochem. J., 18, 127-135 (1924). 



334 L. Wacker and K. F. Beck, Berl. klin. Wochschr., 58, 452-457 (1921). 

 338 L. Wacker and K. F. Beck, Z. Kinderheilk., 27, 288-292 ( 1921). 



336 L. A. Maynard, E. S. Harrison, and C. M. McCay, J. Biol. Chem., 92, 263-272 

 (1931). 



