'59 2 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



emphasis has been laid upon the demands which 

 these several arrangements impose on the circulation. 

 Further, just as there are differences in gross mor- 

 phology, so there is still further sanation in the micro- 

 circulatory organization of the various types of 

 placentas. The generally recognized distinctions in 

 this regard are those of Grosser (88) who classifies 

 placentas on the basis of the number and type of tissue 

 layers interposed between the maternal and fetal 

 bloods. The beauty of this system is that tilings fall 

 neatly into place. For example, the epitheliochorial 

 type of placenta suggests that in general there are 

 six layers of tissue (maternal endothelium, connective 

 tissue, and epithelium apposed to layers of fetal 

 epithelium, connective tissue, and endothelium). The 

 simplest hemochorial placenta has only fetal epithe- 

 lium, connective tissue, and endothelium between 

 the two bloods. Intermediate between these two ex- 

 tremes are the syndesmochorial (lacking maternal 

 epithelium) and the endothelial chorial (lacking the 

 maternal connective tissue and epithelium). 



To complicate the apparent simplicity of this 

 system, there is no phylogenetic relationship between 

 the several types of placentas. Some rodents and 

 primates have hemochorial, and ungulates have 

 epitheliochorial and syndesmochorial types of pla- 

 centas. Carnivores do not fit into an)' one group. 

 More disturbing to the simple view of Grosser (88) 

 that a neat classification of placental types exists 

 which is based on their microcirculatory relationships 

 is the fact that careful re-evaluations by placentolo- 

 gists in recent years suggest that structural changes 

 take place throughout the life span of the placenta; 

 the morphology is not uniformly constant with regard 



to the tissues mentioned above [Amoroso (4), Huggett 

 & Hammond (116)]. Amoroso has enumerated a 

 wide range of types and patterns of changes of vascular 

 arrangements, not only in commonly used laboratorv 

 animals but in unusual ones as well. It is found that 

 capillaries "migrate" into intraepithelial locations. 

 One does not find any critical limitation of effective- 

 ness in the transfer of metabolites across the various 

 "types" of placentas in terms of growth and survival 

 of fetuses. 



TYPES OF PLACENTAS 



Just as there are distinguishing microscopic mor- 

 phological and physiological differences in the pla- 

 centas of the various species, there are wide differences 

 in the forms of placentas [Mossman (158)]. Apparently 

 the simplest form is the discoidal placenta, represent- 

 ing a single structure. This is seen in such widely 

 different species as hamsters, mice, rats, rabbits, 

 guinea pigs, and humans, to name a few. The opposite 

 extreme is seen in the placental structures of sheep, 

 goats, cows, mares, and many other species, all un- 

 gulates. In these, each developing embryo has a 

 complement of twenty or more discrete discoidal 

 placental parts, called cotyledons, each supplied by a 

 branch from one or two umbilical arteries and drained 

 by a vein that ultimately enters one of two umbilical 

 veins. These cotyledons have vascular connections 

 with others and lie scattered throughout the entire 

 interior surface of the uterus. 



Other types of placentas exist. In rhesus monkeys 

 there are two discoidal placentas, one lying on the 



fig. 7. Sodium transfer during preg- 

 nancy across placentas of four different 

 types. [From Flexner & Gellhorn (8i).j 



MOM No TRANSFERRED/GM PLACENTA/HR 

 AT MIODLE OF 9TH TENTH OF GESTATION 



SOW 



GOAT 



FETAL SIDE 



MATERNAL SIDE 



GUINEA RABBIT RAT MAN 



PIG 



FETAL SIDE 



<s> 





MATERNAL SIDE 



EPITHELIO- 

 CHORIAL 



SYNDESMO 

 CHORIAL 



ENDOTHELIO- 

 CHORI AL 



HEMO- 

 CHOR IAL 



