I 620 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



placental blood flow and therefore the internal en- 

 vironment of the fetus, consequent upon the retraction 

 of the cut uterine muscle around the uterine vessels, 

 still occurs. In the sheep, on which many of the 

 studies have been made, the uterine muscle is not so 

 reactive to mechanical stimuli as the uterus of the 

 rabbit, the guinea pig, or the monkey and man. 

 Spasmolytics have not been used to prevent this re- 

 sponse of the uterine muscle. The local application 

 of procaine or papaverine is used to prevent spasm of 

 the umbilical vessels during cannulation or sampling 

 of blood, but these maneuvers are best carried out on 

 the abdominal trunks of the vessels, which are less 

 contractile, or on the placental tributaries, in order 

 not to interfere with the main blood flow. 



Barcroft's (25) theme that it is dangerous to argue 

 from species to species about the relative stage of 

 physiological development in utero and at birth is 

 most applicable to a consideration of the cardiovascu- 

 lar system. It will become apparent that, in the spe- 

 cies which have been most extensively studied, func- 

 tional development does not depend upon gestational 

 age but corresponds most nearly to the requirements 

 of the newly born when it is, however, never so ad- 

 vanced as in the adult. 



THE FETAL PLACENTA 



Implantation 



What forces compel the fertilized ovum to satisfy 

 its high nutritive requirements in the superficial 

 layers of the endometrium? Or does the endometrial 

 epithelium have the power to attract inert particles 

 the size of the 7-day human blastocyst (106)? What 

 are the mutual relationships between the trophoblast 



and the endometrium once contact between the two 

 has been established? Both the anatomical and ex- 

 perimental aspects of implantation are beautifully 

 described, for many species, by Hamilton et al. 

 (102) and by Boyd & Hamilton (45). Under normal 

 conditions the blastocyst in utero implants at a defi- 

 nite size, at a prescribed time, and in special sites 

 (43); the presence of progesterone is essential hut the 

 mechanism of its action is unknown (148). Fawcett 

 et al. observe that, "the individual potentialities of the 

 ovum and uterine mucosa should not be thought of as 

 mutually exclusive but mutually supporting and 

 neither is "chiefly" responsible for implantation" 

 (94). These potentialities may, however, be observed 

 quite independently : the mouse ovum, once it has 

 reached a certain size, is capable of implanting ran- 

 domly in extrauterine sites such as the anterior cham- 

 ber of the eye and the abdominal cavity, regardless 

 of the sex of the host. The trophoblast causes extra- 

 vasation of blood in these sites before cellular inva- 

 sion has taken place and the substance responsible 

 must be actively penetrating, for secondary implan- 

 tation sites start to proliferate in the macaque before 

 any erosion of the uterine surfaces (191) and, in the 

 human, congestion also appears on the opposite side 

 of the uterus to the site of implantation (105). The 

 active substance may be a product of metabolism 

 of the dividing blastocyst, even carbon dioxide itself; 

 or it may be chorionic gonadotropin, known to 

 appear first at the time of implantation. Evidence 

 for a penetrating action of chorionic gonadotrophin 

 is suggested by perfusion experiments on full-time 

 human placentas; citrate metabolism is enhanced by 

 estradiol only when chorionic gonadotrophin is also 

 added to the perfusing fluid (180). The initial re- 

 sponses of the endometrium may also be observed in 



Fetal Capillary 



I v 1 



Epithelio- Maternal Syndesmo- Maternal Endothelio- Haemo- 



chorial Tissues chorial Capillary chorial chorial 



fig. I. Histological types of placenta arranged to emphasize the progressive breaking down of the 

 barrier between the maternal and fetal circulations. [Redrawn by Amoroso (8).] 



Maternal Haemoendo- 

 Blood thelial 



