PLACENTA CIRCUMVALLATA. A THEORY OF FORMATION INCLUDING RELATIONSHIP TO NORMAL PLAC]1NTA, TO PLACENTA MARGINATA AND TO PLACENTA MEMBRANACEA. PRELIMINARY REPORT* BY RICHARD TORPIN,
(From the Department of Obstetrics ancl Gynecology, Rush Medica! College, of the University of Chicago)
study has been spent on placentation in relation to anomalous M UCH location or site such as results in various types of ectopic pregnancy, tubal, ovarian, abdominal, etc. Practically no thought has been given to the subject of the area over which placentation may take place or the chang·ing relation of the size of the placenta to that of the uterine cavity as pregnancy advances. This is remarkable because of the complications which may arise from having too small or too large an area over which the placenta extends in the early months of pregnancy. This subject deals entirely with too large an area. The first question to arise will be, is there a possibility of having too great an area of placentation. We know of one condition in which this is definitely proved; i. e., placenta membranacea where the fetus at term is entirely surrounded by aetive placenta except at one small area over which stretch the membranes composed of amnion, chorion laeve and decidua capsularis. This condition represents almost the extreme area over which the placenta could occur and must originate in the early months of pregnancy following which the placenta is distended as the gestation sac enlarges. On the other hand, placentation usually extends over merely a small area of one wall or side of the uterine cavity (it has been stated one-fourth of total area of uterine cavity) so that as the cavity enlarges the area covered grows gradually into that of a normal full-term placenta without undue stress or strain. Placentation occasionally must extend over an area larger than normal in the early months because in placenta membranacea it extends over almost all the cavity. What happens to the excess placenta when short of producing a membranaceous placenta; in other words what becomes of the excess marginal tissue? While we may see a full-term placenta extend over nearly all the uterine cavity, we never see one which extends over only half. Something must occur to reduce this, else we would see not infrequently a hollowed out hemispherical placenta at term, one never described to my knowledge. Consider now what must occur in such a condition. As the fetus enlarges it bulges more and more into the membranes because they are more distensible than the complicated fixed structures in the placenta. Now as the nonplacental portion of the uterine muscle is stretched more than that over the placenta there arises a tension at the margin of the *Read at a meeting of the Chicago Gynecological Society, May 15, 1931.
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placenta caused by the tendency for the muscle to stretch off of the relatively rigid placenta. This constant tension aided probably by BraxtonHicks' contractions ruptures the fine marginal decidual vessels, causing ultimate atrophy of the associated villi which gradually retract into a fibrous ring always at the margin of the placenta. 'l'his tension continues until the placenta is reduced to the proper size to care for the optimal nourishment of the fetus. Such a placenta at term is known as placenta marginata. Let us now suppose that early placentation extends over more than half the cavity of the uterus. If the placenta covers not too large an area of the uterine cavity the tension of the growing fetus on the membranes is such that it protrudes from the placental cavity into the more distensible cavity surrounded by the membranes. Now as tension on the muscle wall over this area increases, there is a tendency for the muscles to slip off the margin of the placenta causing a so-called "infarct" ri11g as above described. This fibrous ring cannot expand. As the fetus enlarges it gradually herniates from the relati\'ely rigid nondistensible cavity surrounded by the placenta into the freer distensible cavity of the expansible membranes. The tension on the muscle covering this latter cavity increases and the pull of the muscle tending to slip off the margin of the placenta continues. Consequently there is a slow separation of the margin of the placenta as the ring traverses back over the emerging fetus. This separation takes place through the decidua so that a layer of the latter covers the anterior surface of the ''infarct'' ring as is found in all specimens and was ever a source of considerable difficulty in explaining its presence in this position. As the extraplacental cavity is enlarged by the growth of the fetus. the ring with its veil is gradually approximated to the placenta ana is found in various stages of agglutinaUon and in various stag·es of obliteration oi its component parts depending on the pressure and the time elements. Such a placenta at term is known as placenta circumvallata. The veil in the best preserved specimens then is romposed as follows from its maternal to its fetal surface: ( 1) amnion, ( 2) chorion Irondosum atrophied into socalled "infarct" ring or white necrosis of villi, ( 3) a thin layer of decidua (torn loose from the decidua basalis at the margin), ( 4) a thin layer of decidua capsularis, ( 5) chorion laeve, ( 6) amnion. The ring, the annulus fibrosus, is composed of the ''infarcted'' atrophic and often obliterated chorion frondosnm with a thin layer of decidua basalis from the uterine wall external to the remaining active placenta. If the muscle tension is greater on one side of the herniating cavity as the fetus emerges from the "placental cup" into the cavity surrounded only by the membranes, there is formed an eccentric ring or simply a ring fold on one margin only, the muscle tension having been greater here, more "infarction" occurs at this point. Such results are frequently seen in partial placenta cireumvallata and in partial placenta marginata or in combinations of the two.
PLACRH'l'A. t.mt.&BRANACEA Fig. 1.--Schematic chart illustrating development of the placenta in the four theoretically possible types of extent of area of early placentati6n. In all figures the uterine wan is represented by outside unbroken line; decidua by fine dotted line; dead chroionic villi of chorion laeve or of the "infarct ring" by heavy black dots ; active chorionic villi by small circles and amnion by inner unbroken line. 1 .• S, 9, 13 represent the early stages of the four types, respectively; while 4, 8, 12, 16 represent the terminal stages. S to 8 and 9 to 12 outline the theoretical reduction in the relative size of the placenta as gestation advances with formation of the "infarct rings" as labeled.
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If the extent of the early placenta is such that the fetus cannot make enough pressure on the small surfac·e of the membranes to grow out of its placental inclosure it must needs distend it which apparently occurs in placenta membranacea. Probably most of these feti do not survive. The idea so far embodied may be stated in various ways. The four forms noted, normal placenta, placenta marginata. placenta eireumvallata and placenta membranacea are the results respeetively of the clt>gree of extent of area of early plarentation. Placenta marginata and placenta circumvallata are merely the result of atrophy of the marginal chorionic villi covered by a thin layer of decidua basalis from which the muscle and outer layer of decidua have been pulled off, giving rise to the annulus fibrosus which has caused difficulty in former theories. It ma,v also be stated thus: placenta marginata and placenta circumvallata are the results of the effort of naturr to convert execss placental tissue into passive membranes. They each contain the same elements and ordinarily the line of demarcation is a; still farther would result in uterointerstitial pregnancy (Klehs). Indeed it is possible that ns placenta membramwea is very rare, it may be the end-product of an interstitial pregnancy implanted close to the uterine cavity but still dt>ep enough for a good maternal blood supply to all sides of the ovum. The chorion frondosum and subsequently placenta develops over the whole ovum :md then the further growth of the gestation sac gmclually takes pltwe into the uterine cavity. If the theory represents the facts a stuJy of a ~eries of midterm pregnant uteri, Porro operation or neeropsy specimens hardened before mutilation should show a variation in relative area of placentation; nbout 85 per cent extenning over possibly one-fourth of the uterine cavity; about H per eent should have placenta extending over approximately one-half of the uterine cavity and about l per cent or 1.5 per cent with the placenta extending over about three-fourths of the uterine cavity. This is about the ratio of' occurrenee of normal placenta, plltcPnta marginata :md placenta circumvallata given in Williams' comprehensive review of the Huhject. Placentn ll1('11lbranacea is very rare, only a few eases having he('n reported. Facts pertinent to this theory, photomicrographs, illustrative plates, review of literature, de .. will appear in n st>parate paper. For their interest and many valuable suggestions I wish to thank Dr. Carey Culbertson, Associate Professor of Obstetrics and Gynecology, Rush Medical College, and Dr. George W. Bartelmez, Professor of Anatomy, University of Chicago.
(For discussion, see pa.ge 611.)