THE FETAL AND NEONATAL CIRCULATION 



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the absence of a fertilized ovum. In the rat, but not 

 in the guinea pig, the endometrium is able to implant 

 inert objects, such as glass beads the size of the blasto- 

 cyst normally implanted (36). Electrical and me- 

 chanical stimulation of the pregnant rat uterus can 

 produce the formation of a maternal placenta, iden- 

 tical in structure with the decidua of pregnancy; 

 these deciduomata may bleed into the uterine cavity 

 and the early normal extravasation of blood in the 

 endometrium is therefore probably not dependent 

 upon the fetal trophoblast (125, 170). 



The implanted blastocyst probably receives its 

 nourishment for a short time from the glycogen, 

 lipid, and other materials stored in the stromal 

 cells of the uterine mucosa which have become en- 

 larged by the decidual response; in some ruminants 

 the mucosa secretes uterine milk for this purpose. The 

 formation of the true placenta, containing the fetal 

 and maternal circulations is due to a balance in activ- 

 ity of the fetal trophoblast and maternal decidua; any 

 disturbance in this balance may result in the rejection 

 of the blastocyst or nonspecific, even malignant, 

 growths. Progesterone and estrogen are required for 

 placental and fetal development, the ovary and 

 placenta itself contributing to the production of these 

 hormones to varying extents in the different species 

 (10): an adequate placenta may develop in the per- 

 itoneal cavity and viable infants be delivered at lap- 

 arotomy (81); it has been shown in the mouse and the 

 cat that the fetus is not necessary for the development 

 of the placenta (11, 141). 



Placentation and Placental Function 



Amoroso (8) describes fully the structure of the 

 tissue which separates the fetal from the maternal 

 blood streams in mammalian placentas, following the 

 classification of Grosser (100) modified by Mossman 

 (138). Figure 1 demonstrates how this scheme is 

 based on the number of tissue lavers between the two 



fig. 2.A: section through a villus from a human placenta 

 of 9 weeks gestation. (X 7,040.) The surface projections and a 

 bulbous promontory are illustrated. In the apical part of the 

 syncytium some large vacuoles, filled with granular material, 

 are seen. These are interpreted as absorption vacuoles formed 

 by pinocytosis. They are different from the smaller vesicles with 

 homogenous centers which are thought to be ergastoplasmic. 

 B: section through a thin portion of a human villus from a 

 placenta delivered at term. (X 10,110.) There are well-de- 

 veloped microvilli on the surface of the syncytium. Beneath the 

 syncytium is a zone of lighter cytoplasm. Such a zone has been 

 shown to be continuous with residual Langhans cells. The base- 

 ment membranes are present, separated by a space in which 

 collagenous fibrils can be seen. Beneath the second of these is 

 the endothelium of a fetal capillary. [From Amoroso (9).] 



