HISTOCHEMISTRY OF PLACENTA 



909 



series both morphologically and function- 

 ally. 



In some groups of eutherian mammals 

 (carnivores, some insectivores ) a further 

 provision for placental transfer exists in 

 the paraplacental and central hematomas. 

 These structures, illustrated by the para- 

 placental ''green" and "brown" borders in 

 dog and cat, consist of extensive extravasa- 

 tions of maternal blood between the chorion 

 and endometrium. The extravasated blood 

 is absorbed by the chorionic epithelium, 

 with the result that the iron of the j^hago- 

 cytized red blood cells becomes available to 

 the fetus. These paraplacental structures, 

 which are epitheliochorial according to 

 Grosser's classification but which occur 

 mainly in association with endotheliochorial 

 placentas, represent another important but 

 poorly studied route of nutritive exchange 

 between mother and fetus which has not 

 been adequately evaluated with reference to 

 Grosser's doctrine. 



Hemotrophe is the name given to the 

 nutritive materials absorbed by the pla- 

 centa or fetal membranes directly from the 

 circulating maternal blood stream. Histo- 

 trophe, on the contrary, refers to secretions 

 and degradation products of the endome- 

 trium, as well as extravasated maternal 

 blood, which undergo absorption. According 

 to Grosser (1927), there is a correlation be- 

 tween the kind of nutriment supplied to the 

 fetus and the degree of association between 

 the maternal and fetal blood streams. Thus, 

 in epitheliochorial placentas, histotrophe in 

 the form of secretions and transudations is 

 stated to be the almost exclusive form of 

 nourishment. However, the higher the or- 

 ganization of the placenta is, the more im- 

 portant hemotrophic nourishment is said to 

 become, so that it is maximal in species 

 with hemochorial placentas. In man, and 

 possibly the hedgehog, transmission becomes 

 exclusively hemotrophic, so that according 

 to Grosser, the human placenta in this re- 

 spect represents a developmental end-stage. 

 Amoroso (1952) concluded similarly that 

 "in the hemochorial and hemo-endothelial 

 placentas of man and higher rodents, his- 

 totrophic nutrition is insignificant after the 

 early stages of development and nourish- 

 ment of the foetus becomes possible, largely 



by direct absorption from the maternal 

 blood." In regard to man, this conclusion 

 seems entirely warranted, but in respect to 

 rodents it should be borne in mind that they 

 possess, in addition to a hemochorial pla- 

 centa, a well developed yolk sac placenta 

 which engages actively throughout gestation 

 in the absorption of transudate and secre- 

 tion, representing histotrophe derived from 

 the endometrium. This illustrates again the 

 paradox that, in the presence of what is 

 regarded as the most highly developed and 

 "efficient" type of placenta, namely, the 

 hemochorial one of the higher rodents, a 

 complex yolk sac placenta is also present 

 which certainly functions principally by the 

 absorption of histotrophe, a process which 

 is generally regarded as being most primi- 

 tive. 



B. ULTRASTRUCTURE OF THE CHORIO-ALLANTOIC 

 PLACENTA 



A knowledge of the ultrastructure of the 

 placental barriers of mammals is necessary 

 to form hypotheses for the mechanism of 

 transport across these membranes. Although 

 our information on ultrastructure is frag- 

 mentary now, many significant observations 

 have already been presented. 



The ])lacenta of the pig is classified as 

 epitheliochorial since it consists of a simple 

 apposition of the chorion to the endometrial 

 epithelium without erosion. Observations 

 have been made on the ultrastructure of the 

 definitive pig placenta in which the cor- 

 rugated surfaces of the chorion and uterine 

 mucous membrane interdigitate closely 

 (Dempsey, Wislocki and Amoroso, 1955}. 

 Chorionic ridges fit into matching uterine 

 depressions (Figs. 15.23, 15.28, 15.64). These 

 macroscopic interdigitations are further ex- 

 tended by the "submicroscopic" interdigita- 

 tion of the free surfaces of the chorionic and 

 uterine epithelia. In the regions of the cho- 

 rionic ridges, the chorionic surface is thrown 

 into numerous microvilli which align with 

 the uterine microvilli to form simple in- 

 terdigitations. The bovine fetal-maternal 

 junction is similarly constructed (Bjorkman 

 and Bloom, 1957). In the chorionic fossae, 

 the fetal-maternal junction is similarly con- 

 structed; however, in addition, there are 

 deep, thread-like invaginations of the fetal 



