Thrombosis Research (2004) 114, 397 — 407 intl.elsevierhealth.com/journals/thre Growth and function of the normal human placenta Neil M. Gudea,b,*, Claire T. Robertsc, Bill Kalionisa,b, Roger G. Kingd a Department of Perinatal Medicine, Royal Women’s Hospital, 132 Grattan Street, Carlton, VIC 3053, Australia b Department of Obstetrics and Gynaecology, University of Melbourne, Parkville, VIC 3052, Australia c Department of Obstetrics and Gynaecology, University of Adelaide, Adelaide, SA 5005, Australia d Department of Pharmacology, Monash University, Wellington Road, Clayton, VIC 3800, Australia Received 28 May 2004; received in revised form 17 June 2004; accepted 23 June 2004 Available onlne 30 July 2004 KEYWORDS Placenta; Fetus; Growth; Function; Trophoblast; Transport; Chorionic villus; Decidua; Endometrium Abstract The placenta is the highly specialised organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure the exchange of nutrients and waste products between the maternal and fetal circulatory systems operates at maximal efficiency. The main functional units of the placenta are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. After implantation, trophoblast cells proliferate and differentiate along two pathways described as villous and extravillous. Non-migratory, villous cytotrophoblast cells fuse to form the multinucleated syncytiotrophoblast, which forms the outer epithelial layer of the chorionic villi. It is at the terminal branches of the chorionic villi that the majority of fetal/maternal exchange occurs. Extravillous trophoblast cells migrate into the decidua and remodel uterine arteries. This facilitates blood flow to the placenta via dilated, compliant vessels, unresponsive to maternal vasomotor control. The placenta acts to provide oxygen and nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functional changes occur that accommodate the increasing metabolic demands of the developing fetus throughout gestation. D 2004 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +61 3 9344 2637; fax: +61 3 9347 2472. E-mail address: neil.gude@rwh.org.au (N.M. Gude). 0049-3848/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.thromres.2004.06.038 398 Introduction The placenta is the highly specialised organ of pregnancy that, along with the fetal membranes and amniotic fluid, support the normal growth and development of the fetus. Changes in placental development and function have dramatic effects on the fetus and its ability to cope with the intrauterine environment. Implantation and the formation of the placenta is a highly coordinated process involving interaction between maternal and embryonic cells. Trophoblast cell invasion of uterine tissues and remodelling of uterine spiral arterial walls ensures that the developing feto-placental unit receives the necessary supply of blood and that efficient transfer of nutrients and gases and the removal of wastes can take place. Different types of placentation are categorised according to the number and types of layers between the maternal and fetal circulations [1]. The human placenta is a haemochorial villous organ, whereby maternal blood comes into direct contact with placental trophoblast cells and allows an intimate relationship between the developing embryo and its supply of nutrients. This review describes the basic structural arrangements of the mature human placenta and fetal membranes and gives an overview of the processes of implantation, decidualisation and placentation that result in their formation. Finally, the main functions of the placenta are discussed under the headings of transport and metabolism, protection and endocrine function. N.M. Gude et al. back through endometrial veins. Oxygen-deficient fetal blood passes via two umbilical arteries and the branched chorionic arteries to the extensive arterio—capillary—venous system within the chorionic villi (Fig. 1A). The well-oxygenated fetal blood in the capillaries returns to the fetus via the various chorionic veins and the single umbilical vein [3]. The placental membrane The term placental membrane (sometimes called the placental barrier) refers to the layers of cells that separate the maternal blood in the intervillous space and the fetal blood in the vasculature in the core of the villi [4]. Initially, the placental membrane is made up of four layers, the maternal facing syncytiotrophoblast, a layer of cytotrophoblast cells, connective tissue of the villus and the endothelium lining the fetal capillaries (Fig. 1B). By approximately 20 weeks, however, the cytotrophoblast cell layer of many villi becomes attenuated and disappears. Subsequently, in most of the chorionic villi, the membrane consists of three layers and, in some areas, becomes extremely thin such that the syncytiotrophoblast comes in direct contact with the fetal capillary endothelium (Fig. 1C). Thus, in these positions, the maternal blood and fetal blood come into very close proximity (as little as 2—4 Am). The fetal membranes The structure of the mature placenta The utero-placental unit is composed of both fetal tissue derived from the chorionic sac and maternal tissue derived from the endometrium. In the mature placenta, the fetal aspect is called the chorionic plate. This region carries the fetal chorionic blood vessels, which are branching radials from the umbilical vessels. The maternal aspect of the placenta is called the basal plate. In between these two regions is the intervillous space, which contains the main functional units of the placenta, extensively branched and closely packed villous structures containing fetal blood vessels. It is at the terminal regions of these chorionic villi that the large majority of maternal—fetal exchange occurs [2]. The intervillous space is completely lined with a multinucleated syncytium called the syncytiotrophoblast. Circulating maternal blood enters this space via spiral endometrial arteries, bathes the villi and drains The fetal membranes contain the fetus throughout the pregnancy and eventually undergo programmed rupture during the first stage of labour. They consist of the fetal-facing amnion and maternal-facing chorion [5]. The amnion comprises five distinct layers. The innermost layer is the amniotic epithelium, which is in direct contact with the amniotic fluid on one side and a basement membrane on the other. The other layers consist of the compact layer, the fibroblast layer and the spongy or intermediate layer. The chorion consists of the reticular layer, a basement membrane and the trophoblast cell region, which at term firmly adheres to the maternal decidual tissue. Like the placenta, the fetal membranes play an integral role in fetal development and progression of pregnancy. In addition to autocrine regulatory activities, the membranes secrete substances both into the amniotic fluid, affecting amniotic fluid homeostasis, and towards the uterus, where they may influence maternal cellular physiology. The Growth and function of the normal human placenta 399 Figure 1 A representative drawing of the fetal placental circulation (A), in which the dotted line shows the position from which a drawing of a section through the chorionic villus at approximately 10 weeks (B) is taken. A section through the chorionic villus at full term is also shown (C). membranes also play a protective role for the fetus against infection ascending the reproductive tract. Implantation The period for uterine receptivity for implantation is relatively short. Physiological preparation of the endometrium is modulated by cyclic secretion of 17h-estradiol and progesterone. These hormones regulate growth factors, cytokines and adhesion molecules that alter the endometrial surface and open the implantation window [3]. Other substances, such as fibronectin, close the window several days later. Prior to attachment to the endometrial epithelium, the zona pellucida surrounding the blastocyst is lost. Immediately after attachment, the trophoblast cell layer of the blastocyst proliferates rapidly and differentiates into an inner cytotrophoblastic layer and an outer multinucleated syncytiotrophoblastic mass. The syncytiotrophoblast extends into the endometrial epithelium and invades the connective tissue [6]. The blastocyst sinks beneath the endometrial surface, which is gradually repaired. Nourishment is obtained from the eroded maternal tissues and lacunar networks form within the syncytiotropho- blast [7]. Maternal blood moves in and out of these networks, thus establishing the uteroplacental circulation. Extensions of proliferating cytotrophoblast cells evaginate into the syncytiotrophoblast in various places. These extensions are the first stage in the development of the chorionic villi of the placenta [4]. The decidual reaction Decidualisation of the endometrial stroma occurs as part of the normal menstrual cycle; however, in the event of pregnancy, decidual changes become more extensive. Glycogen and lipids accumulate in the cytoplasm of the cells causing them to enlarge and take on the appearance of the pale-staining decidual cells. The cellular and vascular changes of the endometrium as the blastocyst implants is referred to as the decidual reaction. The function of the decidua, however, is not certain. Rather than facilitating implantation and trophoblast migration, it has been suggested that the main role of the decidua is to restrain the inherently invasive trophoblast and control its migration [1]. This may be achieved by conversion of the motile invasive trophoblast cells into the static placental bed giant cells. 400 N.M. Gude et al. Trophoblast development After successful implantation and initiation of placentation, trophoblast cells undergo extensive proliferation and differentiation. There are two main pathways by which trophoblast differentiation may occur, that is, villous and extravillous (Fig. 2). By days 13 to 14 of pregnancy, cytotrophoblast cells penetrate the layer of syncytiotrophoblast surrounding the early conceptus to form columns of extravillous cytotrophoblast cells. These contiguous cells form the cytotrophoblastic shell that is at the interface of the feto-maternal compartments [6]. Extravillous trophoblast cells invade the decidua and migrate so that they penetrate and remodel maternal blood vessels in the uterine decidua (endovascular trophoblast). This process produces dilated, compliant uterine arterioles that are unresponsive to maternal vasomotor control. Thus, the maternal blood supply to the placenta is promoted by this process and is, by term, about 30% of the mother’s cardiac output, which itself has increased by 30—40% [8]. Trophoblast cells do not invade the decidual veins. Figure 2 Nevertheless, syncytial knots that detach from the chorionic villi into the intervillous space are deported into the maternal circulation via these veins [1]. Extravillous cytotrophoblast cells also invade interstitially (interstitial trophoblast). These invasive cells promote the circumferential expansion of the placental site and recruitment of maternal arterioles; allowing subsequent expansion of the villous region of the placenta below [6]. The full thickness of the uterine mucosa to the decidual— myometrial border has been extensively colonised by 8 weeks of pregnancy. Interstitial trophoblast cells become multinucleated and more rounded and form placental bed giant cells as they move deeper into the decidua [1]. These cells are regarded as the terminally differentiated end-point of the extravillous pathway (Fig. 2). Extravillous cytotrophoblast cells at the tips of anchoring villi rapidly proliferate to form the cytotrophoblast cell columns. Cells distal in the column subsequently switch from an epithelial to a mesenchymal cell type, facilitating their migration into, and invasion of, the decidua and its vascula- The pathways of trophoblast differentiation and function. Growth and function of the normal human placenta ture. This phenotypic switch is essential for migration of the cells [9]. Production and/or secretion of type IV collagenase, matrix metalloproteinases, hglucuronidase, aminopeptidases, cathepsin B, urokinase-type plasminogen activator (uPA), uPA receptor and laminin by the extravillous cytotrophoblast cells enables their infiltration into the decidua by promoting degradation of extracellular matrix [10]. Insulin-like growth factor-II (IGF-II) may also be involved in this process, as it is abundantly expressed at the invading front [11] and induces cytotrophoblast migration in culture [12]. Counterbalancing the degradative activity that promotes invasion of the decidual extracellular matrix, extravillous cytotrophoblast cells also secrete plasminogen activator inhibitor (PAI) and tissue inhibitors of matrix metalloproteinases (TIMPs). These secretory activities appear to be regulated by autocrine and/or paracrine actions of growth factors, particularly, transforming growth factor (TGF)-h [13]. Villous (non-migratory) cytotrophoblast cells proliferate, differentiate and fuse to form the outer epithelial layer of the chorionic villi, the syncytiotrophobast (Fig. 2). The primary villi are formed by evaginations of syncytiotrophoblast with a cytotrophoblast core. Fetal mesenchyme grows into the cytotrophoblast to form the secondary villi and by the third week of gestation fetal capillaries develop within the villous mesenchyme forming the tertiary villi [6]. Initially, chorionic villi are found on the surface of the entire chorion but as the conceptus grows the decidua on the uterine luminal pole (decidua capsularis) atrophies, as does the villi apposed to it. This leaves the definitive placental villi in a discoid region [6]. As gestation progresses the chorionic villi grow and arborise (see below). Exchange takes place in the most part through the terminal villi that project into the intervillous space. It is the tips of villi that are attached to the basal plate (anchoring villi) that give rise to the invasive cytotrophoblast cell columns [6]. Paradoxically, during the first trimester of human pregnancy, the placenta differentiates and grows under a low oxygen environment. This is because uterine spiral arterioles are dpluggedT by endovascular cytotrophoblast cells and uterine blood flow to the conceptus is limited [14]. It is not until about 10—12 weeks of pregnancy that maternal blood begins to flow from the maternal spiral arteries into the intervillous space [15]. At this time, oxygen tension rises from b20 mm Hg (8 weeks) to N50 mm Hg at 12 weeks [16]. Exposure to a relatively hypoxic environment is thought to be 401 an important regulator of cytotrophoblast function in early pregnancy. Indeed, it has been shown in vitro that proliferation and differentiation of human extravillous cytotrophoblasts is regulated by oxygen tension [17]. Evidence suggests that limited blood flow in first trimester is essential to pregnancy success. Assessment by Doppler ultrasound demonstrated that women who had premature onset of blood flow to the intervillous space had a higher incidence of miscarriage [18]. Blood vessel formation The microvascular fetal capillary networks within the terminal villi are the culmination of the vascular tree of the placenta that originates with the macrovascular arteries and vein of the umbilical cord. The umbilical cord envelops a pair of arteries that carry deoxygenated blood and waste products to the placental villi, and a vein that carries oxygenated blood to the fetus. Without proper vascularisation, optimal growth and function of the placenta is not possible. Vascularisation of the embryo and placenta commences at about 21 days post-conception, when villi initially undergo vasculogenesis and blood vessels are formed [19]. In the following phase, branching angiogenesis predominates. This involves the formation of new branches from preexisting vessels and results in increased capillary density. During this period, there is a corresponding rise in end-diastolic blood flow velocity, most likely due to decreased fetoplacental vascular impedance and increased fetal blood pressure [20]. The steadily increasing metabolic demands of the growing fetus are matched by the increased blood flow and growth of the placenta [21]. The final phase of villous blood vessel growth occurs around the beginning of the third trimester (24—26 weeks). This period is characterised by the longitudinal growth of the capillaries, the characteristic looping and coiling of the capillaries and the formation of the terminal villi [22]. During this phase, non-branching angiogenesis predominates and exponential formation of the terminal villi ensues [2]. Terminal villus formation dramatically increases the surface area to volume ratio and, as described above, it is these villi that are the primary sites of gas and nutrient exchange [2]. Increased blood flow within the placenta is provided by the formation and growth of the placental vascular bed and regulated by changes in the dimensions and spatial arrangements of the vessels [23]. The important relationship between villous vascular- 402 isation and optimal embryonic/fetal growth is made evident by the association of common pregnancy disorders that involve poor fetal growth (e.g. fetal growth restriction and preeclampsia) and impaired fetal placental vascularisation [24]. Placental function The main functions of the placenta can be categorised under the headings of transport and metabolism, protection and endocrine. The placenta acts to provide oxygen, water, carbohydrates, amino acids, lipids, vitamins, minerals and other nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. It can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. It also releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functions change during gestation, and there are many species differences in placental function. However, unless otherwise stated, it is specifically human placental function that is discussed below. Transport and metabolism Much of what is known about human placental transport is derived from studies of term placenta. However, there is increasing evidence that placental transport in early pregnancy may differ from that at term in many respects [25]. An important factor that may alter the expression of particular transporter proteins is oxygen tension and blood flow to the intervillous space. As described above, maternal blood flow to the intervillous space is only established from 10 to 12 weeks of gestation. During the first trimester prior to the onset of maternal blood flow to the intervillous space, nutrition is histiotrophic with trophoblast phagocytosis of endometrial glandular secretions including glycogen and a variety of glycoproteins [26]. After 10—12 weeks, maternal blood is in contact with the terminal villi of the placenta, and transfer of respiratory gases, nutrients and waste products occurs across the placental membrane. The placenta is not just a simple conduit for gases, nutrients and waste products, since it requires oxygen and nutrients for itself, and it produces metabolic products. N.M. Gude et al. Transfer of respiratory gases The placental membrane is highly permeable to respiratory gases. Thus rapid diffusion of oxygen can readily occur from maternal to fetal blood, and of carbon dioxide from fetal to maternal blood. Because diffusion of respiratory gases occurs so readily, a rate-limiting factor for their transfer can be blood flow to and from the site of transfer. Interestingly, fetal haemoglobin has a higher affinity for oxygen and a lower affinity for carbon dioxide than maternal haemoglobin. This will therefore favour transfer of oxygen to the fetus and carbon dioxide to the mother. Transport and metabolism of carbohydrates Glucose is the main carbohydrate transported across the placenta from mother to fetus. It is a primary source of energy for the fetus, and it can also partake in a number of anabolic processes. As the fetus is capable of very little gluconeogenesis, this glucose must be derived from the maternal circulation. Transport of glucose across the placenta is generally via protein-mediated facilitated diffusion, and a number of glucose transporters (GLUTs) are involved. Uptake of maternal glucose occurs initially across the microvillous membrane of the syncytiotrophoblast. Once inside the syncytiotrophoblast cytoplasm, glucose can be transported out of the syncytiotrophoblast, e.g. via the basement membrane towards the fetal capillary endothelial cells, which also have membrane-located glucose transporters. However, the rate-limiting step for maternal—fetal glucose transfer is thought to be within the syncytiotrophoblast [27]. In the syncytiotrophoblast, glucose can be converted into glucose-6phosphate or placental glycogen. Glucose-6-phosphate, in turn, can be utilised via aerobic or anaerobic respiration or via the pentose phosphate pathway. Human placental glucose transport is stereo-selective (for d-glucose), and placental transfer of fructose occurs at a much lower rate than that of glucose. The facilitative glucose carrier GLUT1 has been located at term both in the maternal blood-facing and fetal capillary-facing membranes of the placental tissues, and is thought to be responsible for a major component of glucose transport across term placenta [28]. At term, GLUT3 is localised to the endothelial cells lining the fetal capillaries and is thought to be important for regulating glucose levels between these cells and fetal blood [28]. GLUT4, an insulin-responsive glucose transporter, is present in placental stromal cells and may be Growth and function of the normal human placenta important for transporting glucose and conversion to glycogen in these cells in response to insulin in the fetal circulation [29]. GLUT8 has been found to be expressed in human placenta at term, but may be less important in early pregnancy [30]. At term, immunohistochemistry has shown that GLUT12 staining was virtually completely absent from the syncytiotrophoblast and was found predominantly in villous vessel smooth muscle cells and villous stromal cells [31]. There are differences in cellular distribution of GLUTs between first trimester and term placentas that suggests differences in function in early and late pregnancy. Table 1 summarises these differences. While GLUT1 may be important for maternal—fetal glucose transport throughout pregnancy, GLUTs 3, 4 and 12 may only be important for this function in first trimester. The human placenta produces large amounts of lactate, and lactate is also transported by the placenta. The human fetus may also be a net lactate producer, and the placenta can play a role in removing lactate from the fetus [34]. Transport and metabolism of amino acids Amino acids are required by the fetus for protein synthesis, but they can also be metabolised by the fetus. There are over 20 amino acids found in plasma, some of which are not synthesised by human tissues (bessentialQ amino acids), and some of which can be synthesised, e.g. from glycolytic and citric acid cycle intermediates. Transport of amino acids to the fetus during pregnancy occurs via the microvillous and basal membranes of the syncytiotrophoblast. The ratio of most amino acids in fetal compared with maternal plasma is generally greater than 1 (typically between 1 and 4) [35], indicating active, i.e. energy-requiring, transport of amino acids from mother to fetus. There are many isoforms of amino acid transporters, and several families to which these isoforms belong. The expression of different isoforms can vary between and within different tissues. Table 1 403 Amino acid transporters can be categorised as heterodimeric or monomeric. Heterodimeric transporters consist of a heavy chain linked to one of a series of at least seven light chains. A number of heterodimeric amino acid transporters are thought to be expressed in placenta [36], including isoforms of the following transporter families: system L (that can transport a number of neutral amino acids); system y+L (that can mediate the exchange of cationic amino acids for sodium and neutral amino acids); and system b0,+ (that can mediate sodiumindependent transport of cationic amino acids and cystine). As with heterodimeric amino acid transporters, monomeric transporters belong to a number of families, each with a variety of isoforms. The main families found to be expressed in human placenta [36] include the following: system y+ (a high capacity sodium-independent transporter of cationic amino acids); system XAG (a transporter of the anionic amino acids glutamate and aspartate that is thought to utilise ionic gradients for active transport); system ASC (a sodium-dependent transporter of short chain neutral amino acids such as alanine, serine and cysteine); and system A (a sodiumdependent transporter of neutral amino acids). Transport and metabolism of lipids Lipids include free fatty acids, triacylglycerols, phospholipids, glycolipids, sphingolipids, cholesterol, cholesterol esters, fat-soluble vitamins, and a variety of other compounds. Many lipids are bound to proteins within plasma. For example, free fatty acids bind to serum albumin, whereas phospholipids, cholesterol and triacylglycerol form a number of different types of lipoprotein complexes. The maternal surface of the placenta contains lipoprotein lipase, which can release free fatty acids from the lipoprotein complexes circulating in maternal plasma. Both free fatty acids and glycerol (but not triacylglycerols) can readily cross the membranes of the placental syncytiotrophoblast. They can do so by simple diffusion, as they Distribution of GLUT isoforms in human placenta in first trimester and term GLUT isoform First trimester Term References GLUT1 Syncytiotrophoblast, villous cytotrophoblast, vascular smooth muscle, endothelium, stromal cells Extravillous cytotrophoblast, villous cytotrophoblast Syncytiotrophoblast Syncytiotrophoblast, extravillous cytotrophoblast, villous cytotrophoblast Syncytiotrophoblast, villous cytotrophoblast, vascular smooth muscle, endothelium, stromal cells Endothelium [27,28] Stromal cells Vascular smooth muscle, stromal cells [27,29,33] [27,31] GLUT3 GLUT4 GLUT12 [27,28,32] 404 are lipophilic. However, they can also cross via the action of membrane-bound and cytosolic fatty acid binding proteins [37]. These proteins are important in determining the direction and amount of net flux of fatty acids. The placenta is able to preferentially transport long chain polyunsaturated fatty acids [38], and the fetal blood is enriched in these compounds compared with maternal blood. Once fatty acids reach the cytoplasm of the placental trophoblast, they can bind to cytosolic binding proteins, they can be transported out of the trophoblast, or alternatively they can be oxidised or esterified. Placental microsomes contain the enzymes necessary for the synthesis of glycolipids from glycerol-3-phosphate, free fatty acids and other precursors [39]. Also, cultured human trophoblast has been shown to readily synthesise oleic, palmitic and palmitoleic acids, and to have a limited capacity for synthesis of stearic, myristic and lauric acids [40]. Although the placenta can synthesise cholesterol, under normal circumstances cholesterol is derived from maternal blood via an interaction of circulating low-density lipoproteins (LDLs) with LDL receptors on the microvillous membrane of the syncytiotrophoblast, followed by internalisation of LDLs by receptormediated endocytosis. The liver and biliary system are responsible for the biotransformation and elimination of bile acids, biliary pigments and many lipid-soluble exogenous compounds. However, the fetal liver is immature, and the placenta subsumes many of the roles of the adult liver. For example, the placental trophoblast contains enzymes and transporting proteins that are involved in the handling of bile acids, biliary pigments and xenobiotics [41]. Transfer of water, inorganic ions, minerals and vitamins Water transfer across the placenta is dependent upon hydrostatic and osmotic pressure. It is presumed to move across the placenta passively, and its transfer may be facilitated by a water channelforming integral protein expressed in the trophoblast [42]. Sodium and chloride levels in fetal and maternal blood are similar, whereas potassium, calcium and phosphate levels are higher in fetal blood [43]. Potassium, magnesium, calcium and phosphate are all transported across the placenta actively, whereas at least in some species sodium and chloride transfer may occur passively [42]. The situation is quite complex, however, as there are many active ion-transporting systems in the pla- N.M. Gude et al. centa, including Na/K ATPase, Ca ATPase, Na/H exchangers [44], and many others. Also, ion transport can be affected by proteins such as sodiumdependent amino acid transporters discussed above. Vitamins are transferred from the maternal to the fetal circulations, as are many minerals. For example, iron dissociates from transferrin at the placental interface, and is transported across the placenta. Endocrine functions of the placenta The placenta is devoid of nerves, and therefore any communication between it and the mother and/or fetus would normally occur via blood-borne substances. Substances are also produced by the placenta that can play a localised role, e.g. in the uterus or within the placenta itself. Endocrine, paracrine and/or autocrine factors that are produced by the placenta include oestrogens (produced in conjunction with the fetal adrenal gland and possibly fetal liver), progesterone, chorionic gonadotrophin, placental lactogen, placental growth hormone, a number of growth factors (including epidermal growth factor, insulinlike growth factors I and II, platelet-derived growth factor), cytokines, chemokines, eicosanoids and related compounds, vasoactive autacoids, pregnancy-associated proteins of placental origin, corticotrophin-releasing hormone, gonadotrophinreleasing hormone, thyrotrophin-releasing hormone and many others. Some of these are briefly discussed below. Progesterone is produced by the human placenta and is released into both maternal and fetal circulations. Progesterone inhibits uterine contraction. It suppresses oestrus and release of luteinizing hormone from the pituitary gland. The corpus luteum also produces progesterone, but by about the 9th week of pregnancy it has atrophied, and then the placenta is responsible for the production of most of the circulating progesterone. At around this time, the placenta also becomes the main source of circulating oestrogens, which include oestrone, oestradiol and oestriol. Oestrogens act as specialised growth hormones for the mother’s reproductive organs, including breasts, uterus, cervix and vagina [39]. Conjugation of oestrogens (e.g. with sulphate or glucuronide) occurs within the fetal circulation, and may help to protect the fetus from high levels of free oestrogens. Human chorionic gonadotrophin (hCG) is a dimeric glycoprotein that is produced by the trophoblast and secreted predominantly into the mater- Growth and function of the normal human placenta nal circulation. It is produced mainly in early pregnancy with peak levels at about 8 weeks, falling to low levels from about 12 weeks, but with a rise again in late pregnancy. It may help prolong the life of the corpus luteum in early pregnancy. Cytotrophoblast cell fusion and the functional differentiation of villous trophoblast are stimulated by hCG, as well as by estradiol and glucocorticoids [45]. Human placental lactogen has homology with both human growth hormone and prolactin. It is synthesised by the syncytiotrophoblast and released into both maternal and fetal circulations. In the fetus, human placental lactogen acts to modulate embryonic development, regulate intermediary metabolism and stimulate the production of insulin-like growth factors, insulin, adrenocortical hormones and pulmonary surfactant [46]. It may also be involved in angiogensis [47]. Placental growth hormone differs from pituitary growth hormone by 13 amino acids. It is secreted by the placenta into the maternal circulation and may play a role in maternal adjustment to pregnancy, control of maternal insulin-like growth factor I (IGF-I) levels, and placental development via an autocrine or paracrine mechanism [48]. Both human placental growth hormone and human placental lactogen act to stimulate maternal IGF production and modulate intermediary metabolism, resulting in an increase in the availability of glucose and amino acids to the fetus [46]. Insulin-like growth factors I and II (IGF-I and -II) are produced by fetal tissues and play an important role in feto-placental growth throughout gestation. Both Igf1 and 2 genes are expressed in human placenta, and the role of IGF-I and -II in fetoplacental and fetal growth have been recently reviewed [49]. The placenta produces small amounts of chorionic thyrotropin and corticotropin that are released into the maternal blood stream and may help modulate maternal metabolism and other physiological functions. The placenta and extraplacental membranes produce a large number of cytokines, chemokines, eicosanoids and related factors, and some of these may be involved in parturition [50]. Eicosanoids may also be involved in control of blood flow in the placenta, along with many other locally produced autacoids [51]. Indeed, there are numerous vasoactive autacoids that are produced by the placenta including endothelins [52], adrenomedullin [53], nitric oxide [54] and many others. There are many pregnancy-associated proteins of placental origin, and not all of these have been well studied. One that has been studied is pregnancy-associated plasma protein A (PAPP-A), which 405 is produced by the placenta and belongs to the metzincin superfamily of metalloproteinases. It is an insulin-like growth factor binding protein-4 (IGFBP-4) proteinase, and its levels may be reduced in first trimester when a fetus with Down’s syndrome is present [55]. The placenta produces large amounts of acetylcholine [56]. Although the functions of placental acetylcholine are not clear, it has been postulated that non-neuronal acetylcholine may play a role in cell proliferation, differentiation, organization of the cytoskeleton and the cell—cell contact, cell migration and immune functions [57]. Protective functions of the placenta The placenta can act to protect the fetus from certain xenobiotics that could be circulating in maternal blood. Many small xenobiotic molecules can cross the placenta by simple diffusion via transcellular or paracellular routes. Alternatively, some xenobiotics can be transported across the placenta by one or more of the large number of placental transport systems, many of which are not completely specific for the endogenous transported molecule(s). However, there are a number of protective features of the human placenta, which can help reduce placental transfer of potentially toxic substances. These features include export pumps in the maternal-facing membrane of the syncytiotrophoblast, including multidrug resistance protein 1 (MDR1), several members of the multidrug resistance-associated protein (MRP) family, placenta-specific ATP-binding cassette proteins (ABCP), breast cancer resistance protein (BCRP) and mitoxantrone resistance-associated protein (MXR) [41]. In addition, the placenta contains a number of cytochrome P450 enzymes that can metabolise drugs and other xenobiotics, together with other phase I and phase II xenobiotic-metabolising enzymes [58]. However, although the placenta can help reduce the exposure of the fetus to some xenobiotic substances, there are many that can cross the placenta and have teratogenic effects, including alcohol, thalidomide, many anticonvulsants, lithium, warfarin, isotretinoin and numerous others. Although most proteins do not readily cross the placenta, some are transported across the placenta by pinocytosis, including maternal antibodies mainly of the immunoglobulin G class. Such antibodies help provide passive immunity in the newborn baby. There has been considerable debate about how the trophoblast tissue of the human placenta resists immunological rejection from the maternal immune 406 system present in the adjacent uterine decidua, but no consensus has emerged [59]. The placenta generally forms a barrier against transmission of many bacteria from mother to fetus. However, some bacteria, some protozoa, and a number of viruses can be transmitted across the placenta. For example, although the majority of human immunodeficiency virus (HIV) infection is transmitted from mother to baby around the time of birth, it is estimated that in about 1.5—2% of pregnancies in HIV-positive mothers, transplacental HIV transfer may occur, e.g. via HIV binding to lectins expressed by the placenta with subsequent viral absorption [60]. Other viruses that can infect the fetus include cytomegalovirus, rubella, polio, varicella, variola and coxsackie viruses. The bacterium that causes syphilis can also be transmitted across the placenta, as can the protozoal parasite that causes toxoplasmosis. It has also been postulated that viral infection of trophoblast may be related to poor pregnancy outcomes [61]. In summary, the placenta is the physical and functional connection between the mother and the developing embryo/fetus. 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