HISTOCHEMISTRY OF PLACENTA 



899 



basement membrane iii)on which the tropho- 

 bhist rests becomes increasingly thick and 

 continuous; it contains colhigenous fibers 

 which are strongly argyrojihilic and a 

 ground substance which stains intensely 

 with PAS reagents (r/. Figs. 15.29 and 

 15.50). 



As the human choi-ionic villi age, the cy- 

 toplasmic basophilia of the syncytium, re- 

 vealed by staining with methylene blue, 

 undergoes a steady decline, whereas the 

 affinity for acid dyes, as shown l)y the curve 

 of staining with orange G, increases sharply 

 up to the middle of pregnancy and then 

 continues at a constant level until full term 

 (Singer and Wislocki, 1948). Thus, as ribo- 

 nucleoprotein undergoes a gradual decline, 

 the basic protein component rises. 



As ribonucleoprotein diminishes, alkaline 

 phosphatase, which is minimal in amount in 

 the first 2 months of gestation, increases 

 tremendously, becoming maximal in the last 

 trimester (c/. Figs. 15.4, 15.41, and 15.42). 

 This reaction is confined mainly to the 

 syncytium, especially to its outer border 

 where it is extremely intense. It is apparent 

 that the amounts of ribonucleoi)rotein and 

 alkaline phosphatase in the syncytium arc 

 inversely related, the former being seem- 

 ingly peculiar to the period of rapid growth, 

 wdiereas the latter is associated with the 

 phases of maturity and aging of the pla- 

 centa. Acid phosphatase is also i)resent in 

 the cytoplasm of the syncytium, although it 

 does not increase to the degree to which 

 alkaline phosphatase does (c/. Figs. 15.39 

 and 15.40). 



Glycogen is not detectable in the syncy- 

 tium at full term, but a saliva-resistant PAS 

 reaction is evident in the cytoplasm of its 

 outer border (Fig. 15.50) (Wislocki, 1950). 

 This marginal staining occurs in minute, 

 stubble-like microvilli which have replaced 

 the luxuriant brush border of the first half 

 of pregnancy. This change in surface area is 

 clearly evident in both light and electron 

 microscopes. 



With age the stroma of the villi has be- 

 come more fibrous and less cellular, the fi- 

 bci's being markedly argyrophilic (Wislocki 

 and Bennett, 1943, Fig. 20). Associated with 

 the fibers, there is a ground substance which 

 is strongly periodic acid-Schiff positive and 



is contlensed to form a conspicuous basement 

 membrane upon which the syncytium rests, 

 besides forming sheaths which enclose the 

 sinusoidal fetal capillaries (Fig. 15.50). 



The lipid droplets present in the syncy- 

 tium in the first trimester (Fig. 15.13) be- 

 come reduced in size and relative number 

 (Wislocki and Bennett, 1943, Figs. 9, 10, 11, 

 12, and 21). Nevertheless, numerous, mi- 

 nute, sudanojihilic droplets are still demon- 

 strable at full term (Wislocki and Bennett, 

 1943, Figs. 14 and 21), and these also give a 

 positive Ashbel-Seligman reaction for car- 

 bonyl groups (Fig. 15.19), (Wislocki, 1952). 

 Besides the lipid droplets which react with 

 the Ashbel-Seligman reagents, the entire 

 syncytium exhibits a diffuse reaction (Fig. 

 15.19). In contrast to the above findings, 

 Ashbel and Hertig (1952) did not observe a 

 carbonyl reaction in 9 placentas examined at 

 term, although Ashbel and Seligman (1949) 

 reported a sparse reaction in 1 case at term. 



The syncytium at term exhibits a large 

 number of mitochondria (Wislocki and Ben- 

 nett, 1943, Fig. 19) and also gives a posi- 

 tive Bakei-'s acid hematein reaction for 

 phospholii)ids (Fig. 15.24). 



The consjMcuous sinusoidal capillaries of 

 the chorionic villi in the last trimester be- 

 come closely pi'csscd against the syncytium 

 in many i)laces. The latter becomes 

 stretched over the capillaries to form a thin 

 membrane. These thinned out meml)ranous 

 areas (Figs. 15.14, 15.24, and 15.50) (Wis- 

 locki and Bennett, 1943, Plates 5 and 6) 

 have been termed ''epithelial plates" and 

 have been equated with Bowman's cap- 

 sule of renal glomeruli (Bremer, 1916). It 

 has been suj^posed that through them at 

 this period the most active transfer of sub- 

 stances occurs. The thinnest areas of the 

 hemochoi'ial jilacental barrier, separating 

 the maternal and fetal blood streams, are 

 composed of the following layers: (1) a thin 

 lamina of syncytium which rests on a thick 

 basement membrane, (2) a connective tissue 

 space which contains ground substance, and 

 collagenous fibers, and (3) the wall of the 

 sinusoidal capillary composed of another 

 basement membi-ane which is lined inter- 

 nally by endothelium (Wislocki and Demp- 

 sey, 1955). The cytochemical and structural 

 comjilexity of the syncytium makes it more 



