884 



SPERM, OVA, AND PREGNANCY 



Between 1951 and 1955 George B. Wis- 

 locki worked enthusiastically on the original 

 manuscript of this review. He viewed it as 

 one of his most important contributions. A 

 long delay in its pul3lication, since his death 

 in October 1956, has necessitated consider- 

 able revision to permit current publication. 

 Helen A. Padykula, who worked in close 

 association with Dr. Wislocki, agreed to 

 make this revision. Although the original 

 manuscript has been altered in many ways, 

 a genuine attempt has been made to preserve 

 the dynamic approach to the study of pla- 

 centation which characterized Dr. Wislocki. 



I. Introduction 



Histochemistry has elucidated the com- 

 plicated architectural relationships of the 

 mammalian placenta to an important de- 

 gree. The localization of chemical constitu- 

 ents in tissue sections has provided a frame- 

 work of evidence and suggestions for the 

 function of various parts of the maternal- 

 fetal placental complex. Evidence for the 

 sites of hormonal synthesis is principally of 

 a histochemical nature. Furthermore, as has 

 been true in the study of many tissues and 

 organs, histochemical methods have also 

 permitted further definition and differentia- 

 tion of placental cell types. This is well il- 

 lustrated in the use of histochemical meth- 

 ods to study the cellular intermingling which 

 occurs at the maternal-fetal junction in the 

 human placenta (Wislocki, 1951). 



Descriptions of the ultrastructure of the 

 placenta appeared soon after the introduc- 

 tion of methods for preparing sections thin 

 enough to allow study with the electron 

 microscope (Boyd and Hughes, 1954; Wis- 

 locki and Dempsey, 1955a; Wislocki and 

 Dempsey, 195513; Dempsey, Wislocki and 

 Amoroso, 1955; Dempsey and Wislocki, 



1 956 ) . There was immediate interest in de- 

 fining the fine structure of membranes of 

 great physiologic exchange (Low, 1953, pul- 

 monary alveolar lining; Pease, 1955, ne- 

 phron; Yamada, 1955, glomerulus; Palay 

 and Karlin, 1959a, b, intestinal epithelium). 

 Some comparisons of ultrastructure are al- 

 ready possible between the placental mem- 

 branes and those mediating exchange in the 

 kidney, intestine, and lung. Observations on 

 ultrastructure and histochemistry are closely 

 related, and this correlation will be delin- 

 eated here for the placenta. 



In this review, emphasis will be placed 

 on the human and rodent placentas, al- 

 though there will also be a consideration of 

 the comparative histochemistry and histo- 

 physiology of the placental barrier. Amo- 

 roso's chapter in Marshall's Physiology of 

 Reproduction gives an excellent account of 

 comparative placentation, and thus no at- 

 tempt will be made in the j^resent review to 

 cover that subject, except for a brief intro- 

 ductory description of placental histology in 

 man and the rhesus monkey. Huggett and 

 Hammond ( 1952) , in a chapter in Marshall's 

 book, drew, to a slight degree, on cytologic 

 and histochemical data in discussing various 

 aspects of fetal nutrition, placental me- 

 tabolism, and the placental barrier. The en- 

 docrine functions of the placenta and the 

 transport activities of the placental barrier 

 will be considered only in so far as studies 

 of the morjihology have contributed to an 

 understanding of them. 



Placental morphology is relatively diffi- 

 cult to understand, because of the complex 

 structural relationships between the de- 

 veloping embryo and the uterus. These com- 

 prise a succession of stages of placental 

 development involving implantation or ni- 

 dation, followed by the gradual formation of 



Pl.\te 15.1 



Fig. 15.1. A normal human gestation sac containing an embryo approximately 1 month of 

 age. The chorion laeve has been dissected away to repeal the embryo, amnion, exocoelom, 

 yolk sac and chorionic membrane. Numerous delicate chorionic ^■illi extend outward from 

 the chorion, constituting the fetal placenta. X 3'/2. (Carnegie Institution of Wavshington.) 



Fig. 15.2. A section through a 16-day-old human ovum. Observe the embryonic shield (h) 

 with the amniotic cavity above it and the yolk sac below (i). The dark stained chorion (ch) 

 encloses the large exocoelomic cavity (;). Secondary villi (b) containing cores of mesoderm 

 are in process of development. The villi are separated from one another by the intervillous 

 space (/) which contains maternal blood. Peripheral to the secondary villi is a zone composed 

 largely of cytotrophoblast comprising cell columns (d) and the trophoblastic shell (c). The 

 latter is poorly demarcated from the surrounding decidua basalis (a) and decidua capsularis 

 (e). X 30. (Rock and Hertig, 1948.) 



