HISTOCHEMISTRY OF PLACENTA 



943 



lated to fibrinoid elsewhere in the body. 

 Altshuler and Angevine use the term "fibri- 

 noid" differently from Grosser and other 

 investigators. The staining seen by them 

 in Nitabuch's layer is probably referable to 

 metaehromasia of the ground substance of 

 the decidua and is most likely physiologic 

 rather than pathologic in nature, inasmuch 

 as similar metachromatic ground substance 

 occurs in the endometrium during the nor- 

 mal menstrual cycle (Bensley, 1934; Wis- 

 locki and Dempsey, 1948). 



Despite the fact that fibrin, placental 

 fibrinoid, and collagen seem to be identical 

 in reference to staining with acid and basic 

 dyes and isoelectric points (Singer and Wis- 

 locki, 1948; Sokoloff, Mund and Kantor, 

 1951) both fibrin and collagen are distin- 

 guishable in a number of respects from 

 placental fibrinoid. Moreover, although fi- 

 brinoid of pathologic lesions may be meta- 

 chromatic, the ground substance of the 

 peripheral trophoblast is not and the only 

 placental substance comparable to patho- 

 logic fibrinoid is possibly the metachromatic 

 ground substance of the stroma of degener- 

 ating chorionic villi. 



X. Placental Permeability with 



Respect to Morphologic 



Types 



The evidence that the placental barrier 

 in individual species becomes more permea- 

 ble to some substances as pregnancy pro- 

 gresses seems to be adequately established. 

 Utilizing rabbits, it has been shown that 

 permeability to antibodies (Rodolfo, 1934), 

 phenolsulfonphthalein (Lell, Liber and Sny- 

 der, 1932), neoarsphenamine (Snyder, 

 1943), and radioactive sodium (Flexner and 

 Pohl, 1941a) increases during the course of 

 gestation. In the rat, a similar increase in 

 permeability to insulin (Corey, 1932) and 

 radioactive sodium (Flexner and Pohl, 

 1941b) has been demonstrated, and the 

 same is true for sodium in the guinea pig, 

 sow, goat, and cat (Flexner and Pohl, 

 1941a-d; Pohl and Flexner, 1941). A pro- 

 gressive increase in rate of transfer of 

 heavy water has also been observed in the 

 guinea pig and man (Gellhorn and Flexner, 

 1942; Hellman, Flexner, Wilde, Vosburgh 

 and Proctor, 1948). Flexner and his associ- 



ates related their results to the progressive 

 thinning of the chorio-allantoic placenta of 

 the individual species studied with respect 

 to the reduction of the number and width of 

 the layers in the course of gestation. This 

 is a natural conclusion in view of the mor- 

 phologic observations reported in preceding 

 passages which show that in individual 

 species there is a diminution in both width 

 and number of layers of the chorio-allantoic 

 barrier in the course of gestation. For the 

 human, this correlation is well illustrated by 

 a series of drawings presented by Flexner, 

 Cowie, Hellman, Wilde and Vosburgh 

 (1948). 



Little comparative information exists 

 with respect to the relative permeability of 

 different types of placentas. What data are 

 available concern mainly the over-all ex- 

 change through the fetal membranes and 

 give few, excepting inferential, clues to the 

 exact regions and cytologic means of trans- 

 fer. Moreover, most of the substances fol- 

 lowed have been readily diffusible ones and 

 proteins of various kinds which afford no 

 histochemical means for their detection. 

 The manifold combinations of placental 

 structures in various animals and their 

 cytologic complexities have already been 

 outlined to some degree. However, the ma- 

 jority of investigators, speculating on the 

 comparative aspects of physiologic exchange 

 across the placental barrier, have generally 

 ignored all placental structures, excepting 

 the chorio-allantois. The popularity of 

 Grosser's morphologic scheme of the pro- 

 gressive differentiation of the chorio-allan- 

 toic placenta, to the almost complete ex- 

 clusion of the consideration of all other 

 routes of exchange, has been due doubtlessly 

 to its relative simplicity and its adaptability 

 to a concept, widely held until recently, 

 that all placental transmission can be ex- 

 plained on the basis of diffusion and filtra- 

 tion. Moreover, the fact that little is known 

 of the physiologic activity of the placental 

 structures other than the chorio-allantois 

 has also contributed to their neglect. 



It has been generally held that various 

 proteins are readily transmitted by the 

 hemochorial placentas of man and rodents, 

 whereas their passage is slow or entirely 

 prevented in epitheliochorial and syndes- 



