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THE FOETAL MEMBRANES a Review of the Anatomy of Normal Amnion and Chorion and Some Aspects of Their Function GORDON BOURNE, F.R.C.S., M.R.C.O.G

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THE FOETAL MEMBRANES a Review of the Anatomy of Normal Amnion and Chorion and Some Aspects of Their Function GORDON BOURNE, F.R.C.S., M.R.C.O.G 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 Amnion and Chorion 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 amniotic sac, namely the become important if pregnancy and its compli- amniotic fluid, the foetus and the umbilical cord. 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 decidua and the It is now generally accepted that the human maternal uterus. 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 myometrium. 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. Epithelium. 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 placenta 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 ~orp~ Layer ~~;F~rSYLayer '0P1 Firbs Lae .:F " / iv- Gr E"Cellular La.er AMCHORION. F earmnt IE= G Membrane CHORIONr r l.J J, Traphoblost 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. '"" :I. ;a" -. # Z by copyright. 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 coelom 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.
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