Statistics from Altmetric.com
This work focuses on the very early development of the human placenta, its macroscopic and microscopic anatomy and structural organisation and the cells that make up the placenta proper.
A brief introduction to the development of the human placenta is given, followed by a description of the structural characteristics of a delivered term placenta.
EARLY STAGES OF PLACENTAL DEVELOPMENT
During human development, between the stages of the morula and blastocyst (days 4–5 post-conception), the trophoblast is the first cell lineage to differentiate. After establishment of the trophoblast, the blastocyst consists of an inner cell mass that is surrounded by a single layer of mononucleated trophoblasts. This outer layer surrounds not only the embryoblast, but also the blastocoel, the blastocyst cavity. Later during pregnancy, the trophoblast gives rise to larger parts of the placenta and fetal membranes, while the inner cell mass gives rise to the embryo and umbilical cord as well as the placental mesenchyme. The latter is derived from extraembryonic mesoderm outgrowing from the early embryo. At about days 6–7 post-conception, the blastocyst hatches from the zona pellucida and attaches to the uterine epithelium; at that stage formation of the placenta begins.1
The localisation of the inner cell mass defines the part of the trophoblast cover that makes the final attachment of the blastocyst to the uterine epithelium (fig 1). Only those trophoblasts overlying the inner cell mass (referred to as polar trophoblast) seem to be able to finally lead to implantation.2 Rotation of the blastocyst at that stage may even lead to failure of pregnancy due to reduced contact of the polar trophoblast to the uterine epithelium (fig 1). It has been described that varying the orientation of the blastocyst at the time of attachment and implantation results in abnormalities of umbilical cord insertion into the chorionic plate.3 It has been further described that in pregnancies arising from in vitro fertilisation techniques, a higher rate of abnormal placental shapes as well as eccentric umbilical cord insertions occur.4 It may be speculated that the timing and normal interaction between maternal and embryonic cells regulating implantation is altered in these cases.
As soon as the blastocyst has firmly attached to the uterine epithelium, the polar trophoblast undergoes the next differentiation step, syncytial fusion of mononucleated cells to generate the first oligonucleated syncytiotrophoblast (fig 1). At that stage of development the syncytiotrophoblast displays an invasive phenotype, and only by means of this syncytiotrophoblast is the blastocyst able to penetrate the uterine epithelium.2 During the next few days the early embryo embeds itself into the decidual stroma with the syncytiotrophoblast forming a complete mantle surrounding the conceptus. The remaining mononucleated trophoblasts are now referred to as cytotrophoblast, which are found in the second row without contacting maternal tissues. The cytotrophoblasts act as stem cells, which rapidly divide and subsequently fuse with the syncytiotrophoblast, resulting in a continuous expansion of the latter.5 Thus at that stage of development the syncytiotrophoblast is the only embryonic tissue coming into direct contact with maternal cells and fluids, which has been hypothesised as a means to reduce rejection of the embryo.
Eight days after conception, fluid-filled spaces occur within the syncytiotrophoblast and soon coalesce to form larger lacunae. The remaining syncytiotrophoblastic masses between the lacunae are termed trabeculae and are of great importance for the further development of the villous trees of the placenta. As soon as the lacunae have developed, the three fundamental zones of the placenta can be defined: the early chorionic plate facing the embryo; the lacunar system together with the trabeculae developing into the intervillous space and the villous trees; and the primitive basal plate in contact with the maternal endometrium.
During implantation the invading syncytiotrophoblast penetrates into the interstitium of the endometrium and comes into contact with maternal capillaries and the superficial venous plexus of the endometrium. Erosion of these small vessels leads to the presence of first maternal blood cells within the lacunae of the syncytiotrophoblast.6 7 The appearance of first maternal blood cells in these lacunae has been equated with the onset of the maternal circulation in the placenta. However, as was pointed out more than 50 years ago, at that stage of placental development the number of maternal erythrocytes within the lacunae is extremely low, and arterial connections with the lacunar system of the syncytiotrophoblast cannot be found at this stage of placental development.6 8 Rather, the maternal blood flow within the lacunae should be described as a slow flow of venous blood.
At about day 12 post-conception, implantation may be considered to be finalised. The embryo and its surrounding tissues are completely embedded within the endometrium. The syncytiotrophoblast displays a developmental gradient: it is thicker with better developed lacunae underneath the embryonic pole, the site of first invasion. The more distal parts towards the abembryonic pole are thinner, with smaller lacunae and less developed trabeculae. At that time extraembryonic mesodermal cells derived from the primitive streak have begun to migrate on top of the inner surface of the cytotrophoblast cells.9 10 The combination of extraembryonic mesoderm and cytotrophoblast is termed chorion.
Starting on day 12 post-conception, cytotrophoblasts of the chorionic plate penetrate into the syncytiotrophoblastic mass of the trabeculae, follow their course and reach the maternal side of the placenta by day 15. This is the first time an embryonic cell or tissue other than the syncytiotrophoblast comes into direct contact with maternal tissues. Thus, only at week 5 post-menstruation the first cytotrophoblasts leave the placenta proper and differentiate into extravillous cytotrophoblasts. From the primitive basal plate they (now termed interstitial (extravillous) trophoblast) further invade the endometrial stroma between glands and capillaries. A subset of these cells (endovascular trophoblast) reaches and invades the walls of spiral arteries from the interstitium, finally entering the lumen of these vessels.11 12 This physiological transformation of spiral arteries involves the destruction of the arterial muscular wall and the replacement of the endothelium by trophoblast.12
At about day 13 post-conception the trabeculae begin to develop first side branches, which may simply be syncytiotrophoblast protrusions (syncytial sprouts) or which already contain a core filled with cytotrophoblasts. These purely trophoblastic structures are called primary villi, which now protrude into the intervillous space, hitherto called lacunae.
Shortly after, the extraembryonic mesodermal cells of the chorionic plate follow the cytotrophoblasts and also penetrate into the trabeculae. The mesodermal cells do not reach the maternal side of the trabeculae but rather stop earlier, leaving the more distal parts of the trabeculae filled with cytotrophoblasts only. These parts of the trabeculae are referred to as trophoblastic cell columns, which serve as the proliferating source of the extravillous trophoblast and which diminish throughout gestation. The mesodermal cells penetrate into the primary villi as well, giving them a mesenchymal core and transforming them into secondary villi.
Within the mesoderm of secondary villi, haematopoietic progenitor cells develop and start to differentiate. At about day 20 post-conception, first placental blood cells and endothelial cells develop independent of the vascular system of the embryo proper.13 14 The development of first placental vessels transforms the respective villi into tertiary villi.
This classification of villous development only reflects the very basic stages of development of new villi. This process can principally be found throughout gestation. But since tertiary villi accumulate throughout gestation, the relative number of newly forming primary and secondary villi in a term placenta is extremely low.15
MACROSCOPIC ANATOMY OF THE DELIVERED PLACENTA
The full-term human placenta is a circular discoidal organ with a diameter of about 22 cm, a central thickness of 2.5 cm, and an average weight of 470 g (fig 2). There is considerable variation from placenta to placenta, which strongly depends on the mode of delivery. Especially when planning a morphometric analysis of the placenta, factors such as when and where the umbilical cord has been clamped are critical because loss of maternal and/or fetal blood clearly affects the dimensions of the placenta.16
Fetal surface of the placenta
The chorionic plate represents the fetal surface of the placenta, which in turn is covered by the amnion. The amnion is composed of a single layered epithelium and the amnionic mesenchyme, an avascular connective tissue. The amnionic mesenchyme is only weakly attached to the chorionic mesenchyme and can easily be removed from the delivered placenta.
The umbilical cord mostly inserts in a slightly eccentric position into the chorionic plate. The chorionic mesenchyme contains the chorionic vessels that are continuous with the vessels of the umbilical cord. Deriving from the two umbilical arteries the chorionic arteries branch in a centrifugal pattern into their final branches, which supply the villous trees. The chorionic veins are direct continuations of the veins of the villous trees and usually cross the chorionic arteries underneath. The chorionic veins give rise to the single umbilical vein.
Maternal surface of the placenta
The basal plate represents the maternal surface of the placenta. It is an artificial surface, which emerged from the separation of the placenta from the uterine wall during delivery. The basal plate is a colourful mixture of fetal extravillous trophoblasts and all kinds of maternal cells of the uterine decidua, including decidual stroma cells, natural killer cells, macrophages and other immune cells. The basal plate also contains large amounts of extracellular matrix, fibrinoid and blood clots.
A system of flat grooves or deeper clefts subdivides the basal plate into 10–40 slightly elevated regions called lobes. Inside the placenta, the grooves correspond to the placental septa, which only trace the lobar borders as irregular pillars or short sails.
The lobes that are visible on the maternal surface of the placenta show a good correspondence with the position of the villous trees arising from the chorionic plate into the intervillous space. In a full-term placenta, 60–70 villous trees (or fetal lobules) arise from the chorionic plate. Thus, each maternal lobe is occupied by one to four fetal lobules.2 17 The occurrence of a single villous tree occupying a single lobe was defined as placentone.18
At the placental margin chorionic and basal plates merge and form the smooth chorion, the fetal membranes or the chorion laeve. The chorion laeve is composed of three layers: the amnion with its epithelium and mesenchyme; the chorion with a layer of mesenchyme and a layer of extravillous trophoblast; and the decidua capsularis.
MICROSCOPIC ANATOMY OF THE DELIVERED PLACENTA
The fetal lobules (villous tree) arise from the chorionic plate by a thick villous stem, which in the central region of the placenta is derived from a trabecula during very early placental development. The branches of the stems continue branching, leading to a large number of stem villi generations and further branches, finally ending as freely floating villi in the intervillous space. A few branches may reach and contact the basal plate as anchoring villi, which contain the trophoblastic cell columns. The freely floating villi have been divided into five types of villi on the basis of their calibre, stromal characteristics, vessel structure and appearance during pregnancy (fig 3).2 19–21
Mesenchymal villi (100–250 µm in diameter) are the forerunners of the intermediate villi and can be found predominantly in the earliest stages of pregnancy.22 23 Up to six weeks post-menstruation, mesenchymal villi are the only villous type present in the developing placenta. Their stromal core is only weakly organised and contains a large number of mesenchymal cells and developing vessels, the latter sometimes still lacking a vessel lumen. Mesenchymal villi persist until delivery, but due to their ongoing differentiation into intermediate villi, their number becomes extremely low after the first trimester of pregnancy.
Immature intermediate villi
Developing from differentiating mesenchymal villi, immature intermediate villi (100–400 µm in diameter) are large, bulbous villi that dominate the villous trees between weeks 8 and 22 of pregnancy.20 They further develop into stem villi by fibrosation of the stroma from the centre to the periphery. Immature intermediate villi possess a highly characteristic stroma. The mesenchymal stroma cells display long processes that link together to form matrix-free channels oriented parallel to the long axis of these villi. These stromal channels contain large numbers of placental macrophages (Hofbauer cells) that seem to be able to move along and cross between these channels. Immature intermediate villi only contain smaller arterioles and venules and capillaries. After mid-gestation the number of immature intermediate villi decreases in parallel to the mesenchymal villi and only a few can be found at term. If a larger number of such villi can be found in a term placenta, it is important to note that they are recognised as immature intermediate villi rather than being wrongly interpreted as “oedematous” villi.
Stem villi derive from differentiation of immature intermediate villi and are the villi with the largest diameter (100–3000 µm in diameter). They serve to give mechanical support to the villous tree.2 20 Their villous core is characterised by centrally located arteries and veins in a dense fibrous stroma.24 25 Capillaries are rare, and thus it is speculated that this villous type plays only a small part in materno-fetal exchange. Rather, their physiological significance—besides their role to mechanically stabilise the villous trees—lies in the fact that their vascular system is surrounded by a perivascular contractile sheath.26 27 Vessel media and sheath act together as a functional myofibroelastic unit, which contributes to support tensile and/or contracting forces within the stem villus blood vessels.
Mature intermediate villi
Starting at about mid-gestation, long slender mature intermediate villi (80–120 µm in diameter) differentiate from mesenchymal villi that emerge from stem villi.21 The gently curving mature intermediate villi give rise to terminal villi at intervals. Their villous core consists of a loose stroma with a few small peripheral vessels and capillaries. All vessels present in a villous cross section occupy maximally half the villous cross sectional area.
Terminal villi are the final branches of the villous trees. They have a length of up to 100 µm and a diameter of about 80 µm, and originate from mature intermediate villi.2 20 One of their most important features is their high degree of capillarisation. In a cross section, more than 50% of the overall villous cross sectional area is occupied by vessels. Together with their partly extremely thin placental barrier, this makes them the physiologically most important components of the villous tree of a human placenta. In terminal villi capillaries often dilate into sinusoids, which are covered by a vasculo-syncytial membrane (separating maternal and fetal circulations) with a thickness of 0.5–2.0 µm.28 This vasculo-syncytial membrane consists of the syncytiotrophoblast and the endothelium of the capillary, separated by a joint basement membrane.
Basic villous structures
From the time of the early villous stages until delivery, the placental villi are covered by an epithelium-like layer, the villous trophoblast. This layer rests on a basement membrane, which separates it from the stromal core of the villi (fig 4).
The mononucleated villous cytotrophoblasts (Langhans cells) always stay in direct contact with the basement membrane by their basal surface, while their apical surface always stays in contact with the overlying syncytiotrophoblast. As soon as these cells lose contact to the basement membrane during invasion, they differentiate into the extravillous phenotype. Furthermore, if villous cytotrophoblasts lose their contact to the syncytiotrophoblast due to damage of the syncytial layer, they transform into extravillous trophoblasts. In this situation maternal blood clotting results in the deposition of fibrin-type fibrinoid on the surface of these cells separating them again from direct contact to maternal blood.29 30 Thus even in villous tissues, extravillous trophoblasts can be found.
In placental specimens from the first trimester of pregnancy, the villous cytotrophoblast is present as a complete cell layer below the multinucleated syncytial layer. Thus at that stage of pregnancy all villi are covered by a two-layered trophoblast epithelium. As pregnancy progresses, the cytotrophoblasts seem to reduce in number since at term they only contribute about 15% to the total volume of the villous trophoblast.31 Stereological studies have clearly shown that the total number of cytotrophoblasts steadily increases during pregnancy from about 1×109 cytotrophoblast nuclei at 13–16 weeks of gestation to about 6×109 at 37–41 weeks of gestation.31 32 Due to steady proliferation of cytotrophoblast stem cells throughout pregnancy, the pool of cytotrophoblasts increases and is able to maintain the second layer, the syncytiotrophoblast.33 The reason for the seeming reduction in the number of cytotrophoblasts is the rapid expansion of the villous surface leading to a separation of the single cytotrophoblasts.
Undifferentiated cytotrophoblasts display a cuboidal shape with a cytoplasm that contains only few organelles.34 35 Differentiation after leaving the cell cycle results in the formation of intermediate cells, which display a morphological appearance between the undifferentiated state and the syncytiotrophoblast.34 35 The cytoplasm of these intermediate cells contains large numbers of mitochondria and free ribosomes together with high amounts of rough endoplasmic reticulum and mRNA.36 37 These highly active cells display activities of enzymes involved in aerobic and anaerobic glycolysis.38 39 The activity of these cells has also been demonstrated by the incorporation of high amounts of 3H-uridine in vitro.40 41
The highly differentiated cytotrophoblasts display a concentration of organelles, proteins and mRNA that is much higher than that of the overlying syncytiotrophoblast.38 These cells will soon fuse with the syncytiotrophoblast and will become an integral part of this syncytial layer, incorporating all the organelles, proteins and nucleic acids into this layer.
The syncytiotrophoblast is a multinucleated and polar layer with a basal membrane in contact with cytotrophoblasts or the basement membrane, and a microvillous apical membrane in direct contact with maternal blood. There is a single syncytiotrophoblast in a single placenta, which covers all villous trees and also parts of the chorionic and basal plates towards the intervillous space. It is a continuous layer without lateral cell borders and, depending on the site, contains variable concentrations of organelles. The microvilli on the entire surface of the syncytiotrophoblast amplify the surface of this syncytial layer about seven-fold. Underneath the microvilli there is a dense network of actin filaments, microtubules and microfilaments.42
The syncytial cytoplasm contains a varying number of organelles, ribosomes, pinocytotic vesicles and dense bodies.34 43 The highly differentiated syncytiotrophoblast does not show any proliferative activity in any of its nuclei, which also show a reduced rate of transcriptional activity.41 Thus the maintenance of this syncytial layer is completely dependent on the incorporation of cytotrophoblasts throughout gestation.44
Trophoblast nuclei incorporated into the syncytiotrophoblast by syncytial fusion undergo morphological changes during their stay within this layer. They start as large and ovoid nuclei rich in euchromatin and develop into denser and smaller nuclei during the next 3–4 weeks. Finally they display an annular chromatin aggregation pointing to late apoptotic events in parts of the syncytiotrophoblast.30 31 45 Such late apoptotic nuclei are packed in so-called syncytial knots, which are shed from the apical membrane of the syncytiotrophoblast into the maternal circulation.31 45–47
During normal pregnancy syncytial knots containing multiple nuclei can be detected in maternal uterine vein blood and in maternal pulmonary vessels.48–50 Due to their size these corpuscular fragments of the syncytiotrophoblast cannot pass the lungs and thus cannot be detected in maternal peripheral or arterial blood.48 51
Connective tissue cells of the villous core
All villi are composed of a villous trophoblast epithelium separated by a basement membrane from the mesenchymal core of the villi. This core is built by fixed and free connective tissue cells (macrophages) and blood vessels.
Fixed connective tissue cells
During early placentation the villous core is mostly filled with mesenchymal cells that have the potential to differentiate into a variety of other cell types such as endothelial cells and blood cells, macrophages, myofibroblasts, smooth muscle cells and of course fibroblasts. All of these cell types can be found in different combinations in the villous stroma, depending on stage of gestation and localisation in a specific villous type. The fibroblasts secrete typical matrix proteins such as collagen types I and III, as well as proteoglycans such as hyaluronic acid.52
Placental macrophages (Hofbauer cells)
Macrophages can be found in the villous stroma starting at week 5 post-menstruation. The origin of these cells, which are also referred to as Hofbauer cells, is: (1) from progenitor cells within the population of mesenchymal cells in the villous stroma14 53; and (2) from penetration of embryonic/fetal bone marrow-derived monocytes into the villous stroma and differentiation into macrophages once the blood flow between embryo and placenta is established.38
A non-fenestrated endothelium lines the placental vasculature throughout gestation with junctional complexes to link neighbouring cells and to reduce paracellular transport. Larger molecules cross the endothelial cells by means of vesicular transport.54
Capillaries and sinusoids within terminal villi are surrounded by a basement membrane without any further supporting cells such as pericytes.14 Arteries and arterioles within stem and intermediate villi possess a media with smooth muscle cells but missing elastic laminae. The luminal diameter of placental vessels has to be controlled by paracrine and autocrine factors because there is no neural innervation in the placenta.55 The endothelium of venules and veins has recently been shown to be composed of a more immature endothelial cell phenotype compared to placental arteries.56
Fibrinoid is a homogeneous material that preferably binds acid stains and can be found in paraffin sections of placentas of all stages of pregnancy. Fibrinoid can be found at the intervillous surface of the chorionic and basal plates, encasing or partly covering placental villi, or in the depth of the basal plate. During the last decade fibrinoid was investigated regarding origin and composition resulting in a new classification of two subtypes, fibrin-type fibrinoid and matrix-type fibrinoid.29 57–59
Fibrin-type fibrinoid contains fibrinogen and fibrin and is characterised by a dense meshwork of fibrin fibres.38 60 61 Fibrin-type fibrinoid is typically void of any cell, especially extravillous trophoblasts. It is derived from clotting of maternal blood combined with cellular degeneration, which may contribute to it as well. Thus it is always related to maternal blood and the intervillous space.
On the other hand, matrix-type fibrinoid is found in deeper layers of fibrinoid, never in direct contact with maternal blood. Matrix-type fibrinoid is a secretion product of extravillous trophoblasts containing all kinds of extracellular matrix proteins such as oncofetal fibronectin, tenascin, collagen IV, laminin and cellular fibronectins as well as heparane sulphate and vitronectin, fibrillin and merosin.57 59 62–65
Competing interests: None.
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.