VASCULAR ANATOMY OF THE HUMAN PLACENTA AND ITS SIGNIFICANCE FOR PLACENTAL PATHOLOGY

TRANSFER of substances between the maternal and fetal bloodstreams is generally recognized as a major function of the human placenta. It follows that accurate knowledge of the anatomical and physiological relationships between the maternal and fetal placental circulations is essential both for an understanding of normal placental function and for the interpretation of pathological changes. Surprisingly, however, despite extensive research there is still doubt and disagreement over many of the basic details. Much of the relevant anatomical and physiological literature has recently been reviewed by Boyd and Hamilton (1967) and Dawes (1968). In recent years investigation of the fetal vascular tree by a variety of injection and dissection techniques has shown that there are about 200 primary villous stems arising from the chorionic plate and supplied by primary branches of the fetal vessels. Each of these subdivides to form secondary and tertiary stems from which the terminal villi arise, so that each primary villous stem gives rise to a separate fetal cotyledon or lobule. There are 50-60 major lobules which are attached to the decidua by a ring of villous stems modified for this purpose; the villi are packed round the periphery leaving the centre of each lobule relatively empty. Section of a delivered placenta parallel to the maternal surface shows a number of cavities (Crawford, 1962) which Gruenwald (1966) identifies as the hollow centres of the fetal lobules. The large cotyledons seen on viewing the placenta from the maternal aspect represent aggregates of fetal lobules as described above, and are separated by fissures where many of the venous openings of the intervillous space are sited (Wilkin, 1965) and from which the septa extend towards the chorionic plate. The number of maternal spiral arterial openings into the intervillous space is unknown and estimates given vary from about 50 to nearly 500 (Boyd and Hamilton, 1967). It has been suggested recently that the blood entering the intervillous space passes to the centre of the fetal lobules (Freese, 1966, 1968) and that the maternal blood may even have a haemodynamic moulding influence in forming the central space of the lobule (Reynolds, 1966). Gruenwald (1966) has shown that the lobular pattern of the placenta can be recognized in histological sections, but claims on the basis of placental reconstructions that the spiral arteries of the human placenta open into the intervillous space between the lobules. Boyd and Hamilton (1967) are unable to confirm the existence of hollow-centred lobules. Panigel et al. (1967), however, using in vitro radiographic techniques, have been able to study the flow through both maternal and fetal circulations of isolated cotyledons and confirm the spiral arterial inflow to the centre of the lobule. The nature of the intervillous space itself is open to doubt as some workers claim that it is a capillary space (Hormann, 1958; Freese, 1966) whereas others maintain that the villi are too widely separated for a true capillary space to exist (Boyd and Hamilton, 1967). Radioangiography in both the rhesus monkey (Ramsey et al., 1963) and man (Bore11 et al., 1958) shows that maternal blood entering the intervillous space rises high towards the chorionic plate before lateral dispersion takes place. Proponents of the “physiological” concept of placental circulation based on the above finding claim that it is the high pressure of the maternal blood entering the intervillous space which prevents short circuiting to the adjacent venous exits. The possibility that the structure of the feta1 lobule might play some part in determining the