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Home , Amnion, Chorion, Chorionic villi, Decidua, Trophoblast, Umbilical cord

Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from POSTGRAD. MED. J. (1962), 38, 193

THE FOETAL MEMBRANES A Review of the Anatomy of Normal and and Some Aspects of Their Function GORDON BOURNE, F.R.C.S., M.R.C.O.G. Obstetric and Gyncecological Surgeon, St. Bartholomew's Hospital, London, E.C.I

THE foetal membranes of the human are composed stance that enters or leaves the amniotic compart- of the entire amnion and the non-placental ment must pass through tissue of foetal origin. An chorion. The amnion is the innermost of the two understanding of the anatomy, physiology and human fcetal membranes and, as such, is in contact pathology of the foetal membranes has therefore with the contents of the , namely the become important if and its compli- , the foetus and the . cations are to be better understood. The chorionic membrane, which is attached to the outer surface of the amniotic membrane, The Structure of the Amnion separates the amnion from the and the It is now generally accepted that the human maternal . amnion is not merely an epithelial lining for the Throughout pregnancy the human foetus is uterine contents, but that it is a complicated tissue surrounded by amniotic fluid which is enclosed constructed histologically of several different within the sac formed the faetal membranes. Careful of the amnion by layers. study reveals that by copyright. It is an important fact that both the amnion and it is composed of five layers (Bourne, 1960). chorion are composed entirely of foetal tissue. The These are shown in a semi-diagrammatic manner close proximity of these structures to the maternal in Fig. i. The face of this illustration shows the organism, as they line the inner surface of the layers as they appear in cut vertical sections, uterus, may influence their structure or behaviour, whilst the treads of the steps show how they are but their chromosomal sex remains unaltered by seen when membrane preparations of the tissue this close association (Bourne, 1962; Klinger, are examined. Confusion has arisen over the use I957). It is now accepted that the fluid environ- of ' inner' and ' outer' in relation to the amnion ment of the foetus should be as a medium and the term ' inner ' refers to that regarded chorion; part, http://pmj.bmj.com/ which is constantly being renewed or replaced, or layer, nearest to the amniotic cavity and the and perhaps, altered in composition. It is not, term ' outer' refers to that part, or layer, nearest therefore, a static liquid but, similar to the other to the . fluid compartments of the body, the liquor amnii The amnion, which is normally o.oz to 0.5 has a circulation for which the term 'Amniotic cm. in thickness, consists of five layers. These are, Fluid Circulation' has been evolved. According to from within outwards:- Plentl (I957), from work carried out by the use of I. . deuterium, the exchange of water is about 450 2. Basement Membrane. on September 26, 2021 by guest. Protected ml. per hour at term and there are differential 3. Compact Layer. rates of transfer of sodium and potassium ions. 4. Fibroblast Layer. The exact sites of origin of amniotic fluid are un- 5. Spongy Layer. known nor is there any definite evidence to show I. The Epthelium is the innermost layer and is how such large amounts of fluid are conveyed in contact with the amniotic fluid. It consists of a from the amniotic sac to the mother or to account single layer of cells which are usually cuboidal for the passage of various ions at different rates. but may be columnar over the or flattened Undoubtedly some of the fluid passes into the into pavement cells over isolated areas of the foetus and thence to the mother via the fcetal remainder of the amnion. The apex or inner circulation and placenta, but it is very probable that surface of the cells is slightly convex in shape. some of the amniotic fluid exchange occurs via Small evaginations of the cell membrane protrude the foetal membranes into either the placenta itself into the amniotic fluid from the free surface as or the uterus direct. microvilli to form a brush border (Fig. 2). The Theoretically, the foetus is in complete charge well-defined intercellular membrane is con- of its own environment since any fluid or sub- densed at the apex to form terminal bars. A2 Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from 194 POSTGRADUATE MEDICAL JOURNAL April 1962

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FIG. i.-Composite diagram to illustrate the appearances of the layers of reflected amnion and chorion when examined by both membrane preparations and routine cut sections.

FIG. 2.-Amnion Epithelium. The upper, or free, margin of a portion of two columnar-shapedby copyright. epithelial cells from the placental amnion at term. Microvilli (M.V.) cover the surface of the cells. The nuclei (N) are irregular in outline. Inter-cellular canals (I.C.C.) ex- tending from the free surface are dilated in places to form vacuolar-like spaces. Many smaller vacuoles are present in the cell cyto- * f. plasm. x 6,500. http://pmj.bmj.com/ EI .

The base or outer edge of the cells is in contact deeper in the cytoplasm. Projecting into some of with the basement membrane. At high magni- the vacuoles are a variable number of microvilli, on September 26, 2021 by guest. Protected fication the basal region of the cell is complex generally less per unit area than those seen on and irregular in outline forming irregularly- the upper surface of the cells. shaped basal processes of various sizes (Fig. 3). When viewed from above, as in membrane These basal processes are in intimate contact with preparations, the cells are seen to be polygonal in the basement membrane to which they are densely shape, appearing as an irregularly arranged adherent. mosaic. Along the lateral aspect, the two membranes of The epithelial cells normally contain a single adjacent cells delimit a series of irregularly shaped nucleus which is irregular in outline and sometimes vacuoles (Fig. 2). Some of these vacuoles simply fenestrated. The cytoplasm at high magnifi- appear to extend into adjacent ones; others are fication is dense and granular and-contains many connected by fine channels formed by the close vacuoles which vary both in size and content apposition of the two cell membranes. Whilst the (Fig. 2). Mitochondria are scarce and small. A majority of these vacuoles are located in the Golgi apparatus has been described by some regions between adjacent cells (that is, hollowed workers but has not been seen by others. Studies out into the side of the cells) a few are seen lying of the amniotic epithelial cells at high resolution Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from April I962 BOURNE: The Fcetal Membranes 195

FIG. 3.-Amnion Epithelium. The base of an epithelial cell (E.C.) showing the basal processes which form a ~.2. iF3 ii complex arrangement with the base- ment membrane (B.M.). Intercellular canals (I.C.C.) extend upwards be- :i· g - tween the cells. The compact layer ;r En;- of (C.L.) the amnion lies at the bottom of the photograph. X 8,500.

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and magnification demonstrate that comparatively layer is almost completely devoid of cells and large areas of the cytoplasm contain numerous consists of a complex network of reticular fibres. paired parallel lines and circular elements (Fig. The outline of the mesh alters as tension is 4) (Bourne and Lacy, I960). These observations applied to it. This layer, which is probably the are interpreted as showing the presence of mem- strongest of the amniotic layers, is rarely thickened branes arranged mainly in the form of fine canals by cedema, and it to resist, to some appears extent, http://pmj.bmj.com/ with some occurring as parallel sheets. The penetration by leucocytes when the amnion is membranes are each about 60 A thick, the dis- inflamed. tance between then (canal lumen) is about Ioo A. 4. The Fibroblast Layer. This is the thickest Examination of representative parts of the cell layer of the amnion. It is composed of a loose surface, where it borders the lateral and basal fibroblast network embedded in a mass of reticulin. system of vacuoles referred to above, reveals many The cells occasionally show phagocytic activity. circular apertures (about 75-Ioo A internal 5. The Spongy Layer. The tissue of the extra-

diameter) which communicate with the intra- embryonic is compressed between the on September 26, 2021 by guest. Protected cellular channels and canals. The evidence amnion and the chorion to form the spongy suggests, therefore, that amniotic epithelial cells layer. Its wavy bundles of reticulin, bathed in contain an extensive system of canals and channels mucin, render routine staining difficult, but these which communicate directly with the extra- bundles are seen by phase microscopy as branch- cellular ' space '. ing fibres having triangular shaped nodes at the 2. The Basement Membrane. This is a thin junctions. A few isolated fibroblasts are present layer composed of a network of reticular fibres in this layer. It frequently becomes edematous and is well marked over both the placental and the and, as such, accounts for the increase in thickness reflected parts of the amnion. The superficial or which often occurs in the amnion. This layer inner aspect of this layer has a complex relation- permits the amnion to slide upon the underlying ship with the epithelial cells. Short, blunt pro- chorion which is firmly adherent to the maternal cesses from the bases of the epithelial cells inter- decidua (Fig. 5). digitate with similar processes that arise from the (Hofbauer Cells). These were basement membrane. described in the placenta by Hofbauer in 1905, 3. The Compact Layer. This relatively dense since when they have frequently been known as Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from i96 POSTGRADUATE MEDICAL JOURNAL April I962

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"'". 8->; ·6?r ..ti: i-· FIG. 4.-Micrograph of the num- erous membranes found in various regions of the cytoplasm of amniotic epithelial cells. The membranes are arranged mainly in the form of canals which appear to open on the cell surface and communicate with the extracellular space. X 100,000. by copyright. http://pmj.bmj.com/ on September 26, 2021 by guest. Protected

FIG. 5.-Human amnion and cho- rion. A vertical histological section through amnion (above) and chorion (below). Compare with Fig. i, A = epithelium, ... ]. . 3fs ti 1 !::,!:. F72:-.: a ...... la C = compact layer, D = fibro- blast layer, E = spongy layer, | , * e , | | || G = reticular layer, I= , OV = obliterated villus. Note the fine reticular tissue connecting the spongy layer with its adjacent structures. Hgematoxylin-eosin .'^ .'' ...... x 90. Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from April I962 BOURNE: The FIetal Membranes 197 Hofbauer cells, though they had been previously Supply described and are probably the cells discussed by The only blood vessels present in the foetal Muller in I847. They are single nucleated macro- membranes are those in the region of the placenta phages varying from 15 to 35 ,J in diameter and itself. The umbilical vessels, on reaching the having a foamy or vacuolated cytoplasm. Their placenta, spread out and radiate over its surface, single nucleus is eccentrically situated and often travelling within the substance of the reticular reniform in shape. The cellular outline varies, layer of the chorion. Although this layer contains being usually oval or round, but, not infrequently, the foetal vessels, it does not receive a capillary pseudopodia are present. Hofbauer cells are blood supply from them, either over the placenta normally present in the fibroblast and spongy or in the reflected part of the membrane. The layers of the amnion and in the cellular, reticular amnion has no blood supply of its own, nor does it and trophoblast layers of the chorion. They are contain blood vessels even over the placenta. occasionally seen in the compact layer of the amnion, especially if they are actively phagocytic. Amniotic Fluid Exchange In vitro, these cells readily take up dye which they From the above description and from the experi- concentrate in their vacuoles and in vivo they mental work that has so far been performed it commonly contain relatively large quantities of seems reasonable to conclude that the amnion and . chorion do influence the fluid environment of the foetus. The structure of the amniotic epi- The Chorion thelium indicates that it is physiologically capable The chorion being the outermost of the two of taking an active part in some of the amniotic foetal membranes is in contact with the amnion on fluid exchange mechanism. its inner aspect and the maternal decidua on its So far as can be determined on a purely morpho- outer aspect. The placenta is composed of logical basis, it seems likely that some of the chorion and is formed by the hypertrophied villi amniotic fluid which passes from the foetus to of the chorion frondosum. The the and the mother, accompanying differential by copyright. over the remainder of the chorion (chorion laeve) rate of turnover of various ions, may be brought atrophy and may be recognized in histological about by the amnion. As a working hypothesis it sections as obliterated or ghost villi (Fig. 5). The has been suggested that the fluid enters the non-placental chorion consists of four layers. amnion via (i) the opening between adjacent cells, These are, from within outward: where it passes into the lateral system of vacuoles 6. Cellular Layer. and (ii) the upper surface, the area of which is 7. Reticular Layer. greatly increased by the presence of microvilli. 8. Pseudo-basement Membrane. Within the cells the fluid presumably enters the of fine canals and channels. These 9. Trophoblast. complex system http://pmj.bmj.com/ 6. Cellular Layer. This is a thin layer consisting communicate with the lateral and basal vacuoles of an interlacing fibroblast network. It is fre- and hence with the extracellular space; this quently imperfect or completely absent from the results in an enormous increase in surface area. chorion when examined at term, but is more Within the vacuoles there is presumably some easily recognized in early pregnancy. mixing of any solution which may have entered 7. Reticular Layer. This forms the majority via the two principal pathways. Assuming that of the thickness of the chorion and consists of a some ions travel more readily along one route reticular network, the fibres of which tend to be than the other, then this could offer an explanation on September 26, 2021 by guest. Protected parallel. Nodes are present on the fibres at those (although much over-simplified) of the differential places where branching occurs. A few fibroblasts rate of turnover of different ions. Having passed are present, together with many Hofbauer cells. through the epithelial layer the fluid must permeate 8. Pseudo-basement Membrane. This forms a the layers. Since these layers basement membrane for the trophoblast. It is a consist mainly of networks of fibres there is no layer of dense argyrophil connective tissue that is objection to this on morphological grounds. firmly adherent to the reticular layer above and which sends anchoring and branching fibres Transfer of Meconium down into the trophoblast. Meconium is passed by the foetus at times of 9. Trophoblast. The deepest layer of the stress, especially when it is subjected to anoxia, chorion consists of from two to io layers of tropho- and the presence of meconium in the amniotic blast cells in contact, on their deeper aspect, with fluid is accepted by most obstetricians as an maternal decidua. This layer contains the obli- indication of anoxia or hypoxia, although the terated chorionic villi, one of which is shown exact level at which this occurs is uncertain. diagrammatically in Fig. i. Meconium absorbed by the amnion and chorion, Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from 198 POSTGRADUATE MEDICAL JOURNAL April I962

:.~im~:.~:-~. !~i~'i~?:~i~1ii~:~~~~~~~~~~~~~~*;-~.ii'-~.!,!~,~?,::~~..~..... FIG. 6.-Meconium-stained membrane. Dense in- clusions of dark brownish-green meconium are present in the degenerating epithelial layer. Meconium is also present in the cytoplasm of three fibroblasts in the amniotic connective tissue. Formalin fixed paraffin embedded tissue. Very light eosin stain. x 435.

stains the membranes green. Histological sections imply, is indicated by the fact that Jeffcoate and of such meconium-stained membranes reveals Scott collected 14 cases in less than two years. The that meconium may be present in the epithelium paucity of the recorded material probably resultsby copyright. and the Hofbauer cells of the amnion (Fig. 6). In from inadequate examination of the amniotic deeply stained membranes the meconium penetrates membrane immediately after delivery. It is also not only the reticular and trophoblast layers of possible that the small nodules are not seen, or the chorion but also into the decidua, where it may that their significance is not appreciated, so that be found in cells adjacent to maternal capillaries the secundines are destroyed before the clinical (Fig. 7). This passage of meconium, from amniotic condition of renal agenesis is recognized. cavity to decidua, is further proof of the movement Macroscopic Appearance. The lesions are small, of material from the foetus to the mother across the circular nodules situated on the foetal surface of membranes. the amniotic membrane and the epithelium of thehttp://pmj.bmj.com/ umbilical cord. They vary in colour from white Amnion Nodosum to dark grey, or red, but are mostly pale grey with This is a pathological condition of the amnion a faint yellowish tinge. The junction of the in which the fcetal surface of the amniotic mem- umbilical cord with the placenta is the centre of brane is studded with multiple greyish nodules. their area of distribution and it is around this The term 'Amnion Nodosum' was coined by point that the nodules are most frequently found.

Landing (1950) when he reported eight cases, Whilst the nodules are commonest over the on September 26, 2021 by guest. Protected five of which suffered from a major congenital placental amnion they also occur on the juxta- abnormality of the renal tract and three, who were placental amnion, although they are found less stillborn, were not examined by autopsy, although frequently in areas further from the umbilical cord. two suffered from multiple external abnormalities. The nodules vary in size from microscopic to Recently Jeffcoate and Scott (1959), in a critical 3 or 4 millimetres in diameter and, especially near review of and , the placental-cord junction they frequently reported their findings in 14 cases and came coalesce to form irregularly shaped plaques but, to the conclusion that the lesion is associated individually, each nodule has a circular outline primarily with a shortage of liquor, rather than of almost geometric precision (Fig. 8). A charac- directly with the renal defect. The author has teristic feature of the nodules is that they can be seen amnion nodosum five times and on each picked off the underlying amnion leaving a semi- occasion it was associated with oligohydramnios translucent saucer-shaped depression. The and renal agenesis. nodules are composed of a soft, waxy but granular That the incidence of amnion nodosum is material which is friable and may be crushed much greater than the 43 reported cases would easily. Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from April 1962 BOURNE: The Foetal Membranes 1

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Ci-::j·=.::::I::::::·:·:.·::::li.il. :'':·::'::::: i . B.jilllJ::::·.::ili...j.:::::.i:::i::-'··::.:i:i::::::inG::::ii::::-.·C;..:::::.* :·-.II:::-:::: :'iiQiii.:::. ''..'''"''''" iiiiiiiii. :::::'::"::.::: :r:-ii iiil.lil· ···- I. ,:::: ;:I:.·:::: ;::I.;::I:.·.:I:::i:::::-:I:n:lj.,.n::. 8auuss.se.p-saa.P-.::.I: o .:::2::i f X ril:. ::::·-·:.::::::::::-j::::::::.::. ;·;' .i s.iii. iiii'jiiliiii.. i.... ".r·. ."" Ii ·.·:·:·'·:I:::;:::::: ·;:; '::::::'I:I:I:i:i:i::·:·:·:; ;iS -'''''"";:':':' "'::·. :Iii: .,:i:iliii2iii Iraiiii:'i"i r·:·:·:·: ':':' .-:·:·- :18;·:· :; ::i r: :: ·::·-.: ::-j::i i:::-;:;j:::::.i::ij :I:·-·. -."~a"-""s..":::.sa..:; ilili: i: li:i.l·'llllli:i:..L8i:li:i...g ::x .iiXiiilp Bii.iiiii. .f%:;" i- FIG. 7.-Meconium-stained chorion. The reticular I.J.i·' layer of the chorion (R) above and the trophoblast FIG. 8.-Amnion Nodosum. A portion of amnion and decidua below. The shrunken and nodosum from above and trans- (T) (D) photographed http://pmj.bmj.com/ disintegrating stromal cells lie within empty clefts illuminated from below. The nodules are circular in the reticular tissue. The small round dark in- except where they are distorted by pressure from clusions are meconium (M). Note how the neighbouring nodes. Occasionally they coalesce to meconium has penetrated the membrane and lies form oval or irregularly-shaped areas. Note that within the substance of the maternal decidua. most of the nodes appear to be umbilicated and that Htematoxylin and Eosin x 400. they are surrounded by a relatively translucent area. An area from which the nodules have been removed is present and demonstrates how the Microscopic Examination. Many more nodules lesions may be ' picked off' without disturbing the are present than can normally be seen by the underlying amnion or the neighbouring nodules. on September 26, 2021 by guest. Protected naked eye. Histological sections reveal many tiny Fresh specimen x 25. nodes which may be as small as o.I millimetres in diameter. They consist of masses of keratinized Aetiology. Most of the reported instances of squamous cells embedded in an amorphous amnion nodosum have been in in acidophil matrix. The nodes lie directly upon the which there has been very little or no amniotic connective tissue of the amnion, forming a minute fluid. Any abnormality that prevents the secretion saucer-shaped depression within the superficial of foetal urine may be associated with absence of layers of the amnion. At the site of their attach- amniotic fluid. Renal agenesis is the lesion most ment the amniotic epithelium has, for the most frequently associated with amnion nodosum and, part, disappeared though occasionally a few cells in fact, has been present in nearly every recorded remain. The structure of the nodes in vertical instance of amnion nodosum. Professor Jeffcoate section gives the appearance of crude and in- and J. S. Scott (1959) consider, from an analysis efficient whorl formation the centre of which, in of their 14 cases, that amnion nodosum is not a the smaller lesions, is near the centre of the developmental error of genetic origin associated nodule (Fig. 9). specifically with renal agenesis. Their conclusion Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from 200 POSTGRADUATE MEDICAL JOURNAL April I962

FIG. 9.-Amnion Nodosum. A typical :: ~~~~~ ~ ·:·'..:': .:%:'·:":. 3' -'· "...... :.':' 't"'·?.'-', nodule of amnion nodosum show- ing attempts at whorl formation. The lesion is not attached to the amnion, part of whose epithelium persists at the edges, beneath it...... ":~...... A.. .:"' ,..:.*' This particular node gives the "~liiii:ii'l!'*i'"'~'~'~'~::i:::r~ e~,sEJiE~~e~E:~:~::-:-i.::,F::i i i ======'~:.--~. ~:,::~":~:l" impression of having been de- ,...,.,. posited upon the amnion as a ~~.~~::·.:.,:... pre-formed nodule. The remainder of the amnion and the chorion .,:--~. (C) are normal in this instance, :.&~:?- ,.. except for extensive deposition of fibrin in the trophoblast. T = Trophoblast. D = Decidua. B.V. = Maternal blood vessel. Haema- toxylin and Eosin x 75.

····:· /:·:·: by copyright. FIG. io.-Necrosis of the cervical amnion in a patient whose membranes ruptured spontaneously at 32 weeks' gestation. The epithelium (E) is reduced to a continuous narrow bandet of eosinophilic amorphous tissue in which the remnants of three nuclei are visible. The remaining layers form a relatively continuous pale-staining strip of reticular tissue containing no cells. Compare 6i::::·::::: ·:·i:· :':: with Fig. 5. Haematoxylin and Eosin x 390. ·:·.:: :::.:·:· ·:"i: ii::::i: http://pmj.bmj.com/ :;.ii:·i.:: .::i:iii.i..l.i...(11.8.8.dlti:ai.i..f ::"`i:'::::··::·".:ii: ····i :r:·:·i:·:·::.ii.::re... . i::;·· ii: ::::i:..I··:·: i!ii', .:I··· i·:i ;··. :·:· :':'.:· ::::' ·· ':·:: .;:·:::e· ::·.:: ·i·: ;: .·· ·*·: :·::. .* '··;Pi.:.t.B.i.it.;.t....tiifJ..Blj..l ·;··;·:::.;;:;. ·;.·:i· i:·li'3F.i...a.L$TB3%.,sL.-i·:·· i·.. ";:· :··· :.:;::: on September 26, 2021 by guest. Protected is supported by finding one instance of amnion the amnion producing secondary degenerative nodosum in the presence of a normal renal tract, changes in the amniotic epithelium. although the remaining 13 had renal agenesis. They consider that the lesion is actually Premature Rupture of the Membranes caused by, or associated with, oligohydramnios The aetiology of premature labour is in- rather than being the direct result of congenital adequately understood and many investigations renal abnormality. have been undertaken in attempts to find the cause The exact mechanism whereby the nodules are or causes. Unfortunately there has not been formed is uncertain. Squamous cells, normally agreement regarding the classification of the shed from the fietal skin, float freely within the aetiological factors involved, with the result that amniotic fluid, unless or until, they are swallowed correlation of published findings is very difficult. by the ftetus. When a shortage of liquor prevents Most authors stress the importance of premature free movement of these cells they coalesce into rupture of the faetal membranes as a causative tiny rounded coagula which become adherent to factor in the onset of premature labour. A search Postgrad Med J: first published as 10.1136/pgmj.38.438.193 on 1 April 1962. Downloaded from April 1962 BOURNE: The Fcetal Membranes 201

through the copious literature shows that pre- membranes rupture at 32 weeks, or at term, in a mature is thought to be a patient whose cervix is shut tightly and not yet major aetiological factor in from 6% to 60% of all taken up, whilst in another patient of similar cases of prematurity although some authors do not maturity they need to be artificially ruptured at even include it amongst the possible causes. full dilatation of the cervix? It is this apparent The author found that premature rupture of incompatibility that has led many investigators membranes, as a major causative factor, occurred to consider that such accidents are the result of an in 34% of a series of I,ooo premature infants inherent weakness of the membranes themselves. delivered at the City of London Maternity Examination of the tensile strength of the mem- Hospital and the North Middlesex Hospital. In branes has failed to demonstrate any significant a number of these cases there were both maternal difference between the strength of those mem- and foetal factors which contributed to the onset branes that rupture prematurely and those that of labour, such as pre-, multiple preg- do not. It is impossible for these estimations of nancy, anaemia, foetal abnormality and malnu- tensile strength to be undertaken upon the trition, etc. but, even so, in 34% the membranes precise area of rupture, partly because it cannot be ruptured before the onset of true labour and must exactly defined but also because it has already therefore be considered to be a major factor in ruptured. its onset. A method has been devised (Bourne, 1962) by One of the main functions of the amnion and which the membranes presenting at the internal chorion is to maintain the fluid environment cervical os can be stained in situ and then studied upon which the foetus depends for its survival in histologically after delivery. This means that the utero. There is little correlation between their amnion and chorion can be studied at their exact premature rupture and other known factors, site of rupture. Using a satisfactory series of although the association between the young controls it has been demonstrated that about 90% primigravida and the under privileged patient has of the membranes that rupture prematurely, been noted; so also has its recurrence in sub- between the 3oth and 34th week of pregnancy, sequent pregnancies of a patient so afflicted in have previously undergone extensive degenerative by copyright. an earlier pregnancy. There is, however, still no changes at their precise site of rupture over the definite link with any known clinical condition to internal cervical os (Bourne, 1962). account for the majority of instances. Such atrophic and degenerative changes in the The incompetent cervix is now associated with amnion and chorion (Fig. io) overlying the a syndrome that includes painless rupture of internal cervical os would almost certainly change the membranes in the second trimester of preg- both the function and strength of the membranes nancy. In some patients the membranes may in this area. It seems reasonable to suppose that rupture when the cervix is only one finger dilated, such changes would weaken their tensile strength whilst in others the bulging membranes may fill and hence lead to their premature rupture. The http://pmj.bmj.com/ the vagina before they finally break. Similarly, reason for the necrosis of this particular area of patients who abort during this period may rupture the membranes is unknown and work is proceed- their membranes early, whilst others deliver the ing in an attempt to isolate the atiological factors. foetus within a complete sac. These apparent These findings await confirmation from other inconsistencies are not entirely explained by the workers, but they do, perhaps, provide a new and protection which the well-placed presenting part interesting approach to the is to to the foremembranes. lem of ever-increasing prob- supposed give prematurity. on September 26, 2021 by guest. Protected Similar conditions may be found at any sub- Figs. I, 2, 3, 4, 5, 6, 9 and io are reproduced from 'The Human Amnion and Chonon' by kind permission of Lloyd-Luke sequent stage of pregnancy. Why should the Books) Ltd. (Medical REFERENCES BOURNE, G. L. (1962): The Human Amnion and Chorion. London: Lloyd-Luke (Medical Books). (1960): The Microscopic Anatomy of the Human Amnion and Chorion, Amer. J. Obstet. Gynec., 79, 1070. , and LACY, D. (1960): Ultrastructure of Human Amnion and Its Possible Relation to the Circulation of Amniotic Fluid, Nature (Lond.), I86, 952. HOFBAUER, J. (1905): Grundziige einer Biologie der menschlichen Plazenta. Leipzig: W. Braumiiller. JEFFCOATE, T. N. A., and SCOTT, J. S. (1959): Polyhydramnios and Oligohydramnios, Canad. med. Ass. J., 8o, 77. KLINGER, H. P. (I957): Sex Chromatin in Fetal and Maternal Portions of the Human Placenta, Acta. anat. 30, 37I. (Basel), LANDING, B. H. (1950): Amnion Nodosum: A Lesion of the Placenta apparently associated with Deficient Secretion of the Fetal Urine, Amer. J. Obstet. Gynec., 60, 1339. MOLLER, H. (1847): Abhandlung fiber den Bau der Molen. Wiirzburg: Bonitas-Bauer. PLENTL, A. A. (I957): In Gestation: Transactions of the Fourth Conference. Ed. C. A. Villee. New York: Macy Jr. Foundation, p. 71. Josiah