944 



SPERM, OVA, AND PREGNANCY 



mochorial types of placentas (Needham, 

 1931, Table 227). To give a well known ex- 

 ample, immune bodies are not transmitted 

 through placentas of ungulates, whereas 

 in animals possessing hemochorial placen- 

 tas their transfer takes place readily (Kutt- 

 ner and Ratner, 1923; Ratner, Jackson 

 and Gruehl, 1927; Ratner, 1943). How- 

 ever, the hemochorial placenta of the rat 

 and rabbit is not involved in antibody 

 transfer; the inverted yolk sac placenta 

 handles this function exclusively in these 

 species. In the monkey which lacks a yolk 

 sac placenta, Bangham, Hobbs and Terry 

 (1958) obtained experimental evidence 

 that the hemochorial chorio-allantoic disc 

 handles antibody transfer. Thus, the routes 

 of antibody transfer are different in pri- 

 mates and rodents. 



Flexner and his associates have related 

 the variations in rates of transfer of sodium 

 per unit weight of placenta in pigs, goats, 

 cats, rodents, and man to the four morpho- 

 logic types of Grosser. On this basis they 

 found that approximately 320 times as 

 much sodium passes across a unit weight 

 of the rat's placenta per hour as across the 

 sow's placenta, and that between the sow 

 and the goat the difference is approximately 

 16 times, whereas between the goat and the 

 cat, both of which possess syndesmochorial 

 placentas (see Section IV C), the difference 

 is slight (Gellhorn, 1943; Flexner, Cowie, 

 Hellman, Wilde and Vosburgh, 1948j. The 

 inference is that the fewer the layers of tis- 

 sue intervening between the circulations the 

 greater is the rate of transfer per hour of a 

 readily diffusible substance across a unit 

 weight of placenta. However, these results 

 are based on transfer per unit weight instead 



of transfer per unit absorbing surface of the 

 placental barrier. Weight as a unit of meas- 

 urement would be more acceptable if the 

 placentas of different animals were essen- 

 tially alike in their internal structure, so 

 that unit weights contained approximately 

 similar amounts of transmitting surfaces. 

 Actually a gram of pig's placenta contains 

 a large amount of edematous chorionic 

 stroma and much endometrium, including 

 uterine glands, and hence cannot be truly 

 compared or equated with a gram of human 

 placenta which contains relatively closely 

 packed chorionic villi. As a consequence, 

 any representative part of pig's placenta 

 by weight contains little effective absorb- 

 ing surface and a large amount of extra- 

 neous tissue, whereas a similar amount by 

 weight of human placenta contains a rela- 

 tively large amount of effective absorbing 

 surface. Thus, comparisons of the relative 

 rates of transmission of substances across 

 various types of placental barriers based 

 upon units of weight are relatively unsat- 

 isfactory. 



Measurements based on the relative 

 areas of the effective transmitting surface 

 would be more significant. However, the 

 rates of transmission of substances ex- 

 changed b}'' diffusion would depend not 

 alone on the surface area of the placental 

 membrane but more exactly on the areas 

 of contact with the maternal and fetal 

 capillaries variously associated with the 

 membrane. To obtain accurate measure- 

 ments of these complex surfaces would be 

 well nigh impossible and no attempts to do 

 so have been made, with the exception of 

 the surface area of the human placenta, 

 for which crude measurements vary from 



Plate 15.XIX 



Fig. 15.81. Election micTograph of a \'isceral endodermal cell of the rat .yolk sac at 13 days. 

 The junction of two endodermal cells occurs at J. Only a portion of the cell at the right is 

 shown. Cholesterol rich lipid occurs in different forms both above and below the nucleus. A 

 portion of a large homogeneous droplet (D) is seen in close association with the basal surface 

 of the nucleus. Above the nucleus, lipid clusters {LC) of various sizes are conspicuous. The 

 indi\idual lipid units which comprise the cluster are irregular in shape and are bound together 

 by membranes. Long filamentous mitochondria (M) are oriented roughly parallel to the 

 long axis of this columnar cell. The perinuclear cytoplasm is rich in typical elements of the 

 endoplasmic reticulum, which are often longitudinally oriented. A vacuole (V) similar to 

 those seen in Figure 15.80 occurs near the free surface. The supeificial cytoplasm is composed 

 of pleomorphic, anastomosing microvilli. The plasma membrane of the microvilli {MV) is 

 continuous with that which lines the canaliculi (C). These canaliculi which lie beneath the 

 surface have dense walls, and they anastomose in a complicated fashion. The free surface 

 is further enlarged in Figure 15.82. X 12,800. 



