936 



SPERM, OVA, AND PREGNANCY 



must be accepted with considerable reserva- 

 tion. 



From the foregoing summary of the evi- 

 dence regarding the localization of chorionic 

 gonadotrophic hormone, it seems probable 

 that in the human the trophoblast is the site 

 of its formation. However, it is not clearly 

 established whether it is localized in the 

 Langhans cells, syncytium, peripheral cy- 

 totrophoblast, or in several of these ele- 

 ments. Evidence favors the peripheral tro- 

 phoblasts. The Langhans cells are a less 

 likely site because they are chromophobic 

 and decline perceptibly in size and number 

 by the beginning of the last trimester, 

 whereas chorionic gonadotrophin and the 

 peripheral trophoblasts persist until full 

 term. Involvement of the syncytium seems 

 unlikely, because it is the probable site of 

 formation of the placental steroid hormones, 

 and because it gradually loses its cytoplas- 

 mic basophilia, whereas the production of 

 chorionic gonadrotrophin continues until 

 full term. 



In attempting to evaluate the possible 

 nature of granules, vacuoles, lipid droplets, 

 mitochrondria, and other organelles in the 

 syncytium, the entire role of the placental 

 barrier must be kept in mind. The syncytial 

 trophoblast is chiefly involved in the ab- 

 sorption and transfer of metabolites from 

 the maternal to the fetal blood stream, be- 

 sides serving as a means of excreting certain 

 waste products. Many of the organelles 

 which its cytoplasm contains are related in 

 some manner to these functions, although it 

 is not possible at present to assign specific 

 roles to many of them. Wislocki and Streeter 

 (1938) and' Wislocki and Bennett (1943) 

 suggested that a considerable number of the 

 vacuoles seen in the syncytium, especially 

 early in gestation, might be related to ab- 

 sorption. They based their opinion on the 

 probability that much of the syncytial cyto- 

 plasm is in a state of motion and flux, with 

 the likelihood that maternal plasma is ab- 

 sorbed by a process of pinocytosis in the 

 manner visualized by Lewis (1931) in cells 

 growing in tissue culture. Recent observa- 

 tions of the human placenta with the elec- 

 tron microscope (Boyd and Hughes, 1954; 

 Wislocki and Dempsey, 1955a) bear out 

 this interpretation. On the other hand, waste 



products of fetal metabolism, such as cre- 

 atine, creatinine, and urea, are so readily 

 diffusible that their excretion would in all 

 probability not be associated with the for- 

 mation of granules which liquefied and 

 formed vacuoles. There is considerable 

 justification for associating the formation of 

 placental steroid hormones with the sudan- 

 ophilic, birefringent, lipid droplets present 

 in the syncytium, but there is no evidence 

 that steroid hormones are liberated from 

 cells, in the adrenal glands or elsewhere, in 

 a visible sequence of liquefying granules and 

 discharging vacuoles. In regard to placental 

 gonadotrophin, the slight evidence which 

 can be assembled regarding its localization 

 tends to place it in the peripheral tropho- 

 blasts rather than in the Langhans cells and 

 syncytium. On the other hand, early in 

 gestation during implantation and subse- 

 quent invasion of the uterine wall by the 

 trophoblast, cytolytic substances and en- 

 zymes are quite possibly released by the 

 trophoblast and these probably account for 

 some of the granules and vacuoles seen in 

 parts of the trophoblast at that period (Wis- 

 locki and Bennett, 1943). 



Considering all of the evidence, chorionic 

 gonadotrophic hormone of the human is 

 most likely produced by the peripheral 

 trophoblasts. This opinion is based on the 

 observed results of culturing trophoblast, 

 as well as on several histochemical similari- 

 ties of the basophils of the pituitary gland 

 with the peripheral trophoblasts (cytoplas- 

 mic basophilia, PAS reaction). Of impor- 

 tance for this concept is the observation 

 that, whereas the majority of the Langhans 

 cells diminish in size and number by the 6th 

 month of gestation, many of the peripheral 

 trophoblasts remain viable and functional 

 until full term (Wislocki, 1951), thus coin- 

 ciding with the continued production of 

 chorionic gonadotrophin. According to these 

 observations, the Langhans cells, clothing 

 the secondary villi and located in the proxi- 

 mal ends of the trophoblastic cell columns, 

 represent a germinal bed composed of chro- 

 mophobic trophoblasts, from which the line- 

 ages of the syncytium and the chromophilic 

 peripheral trophoblasts are separately de- 

 rived. A further parallelism with the pitui- 

 tary is apparent here, in that in both organs 



