The placenta is a transient foetal organ that develops from the blastocyst immediately after implantation and continues to develop throughout the pregnancy. This organ is essential for the exchange of nutrients, gaseous waste, and waste products between the physically separate maternal and foetal circulations, and it is also a significant endocrine organ responsible for the production of hormones that regulate both maternal and foetal physiology during pregnancy. The placenta connects to the foetus through the umbilical cord, while on the other side, it attaches to the maternal uterus in a species-dependent manner, depending on the species. When the placenta is removed from the uterus following birth, a thin layer of maternal decidual (endometrial) tissue is expelled with it (this is frequently referred to as the ‘maternal component’ of the placenta, which is inaccurate). In addition to being a distinguishing property of placental mammals, placentas can also be found in marsupials and some non-mammals, however at differing developmental stages.
Mammalian placentas are thought to have initially appeared approximately 150 million to 200 million years ago. It has been hypothesised that the protein syncytin, which is located in the outer barrier of the placenta (the syncytiotrophoblast) between mother and foetus, was derived from an ancient retrovirus, which was essentially a “good” virus that assisted in the transition from egg-laying to live birth.
Structure
Placental mammals, such as humans, have a chorioallantoic placenta, which is formed from the chorion and allantois throughout the pregnancy process. Among humans, the placenta measures approximately 22 cm (9 inches) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the thickest portion in the middle and thinnest portions at the edges of the placenta. On average, it weighs about 500 grammes (about) (just over 1 lb). A dark reddish-blue or crimson tint can be seen on its surface. For the foetus to be connected to the mother, an umbilical cord measuring around 55–60 cm (22–24 inches) in length is used. This cord contains two and one umbilical arteries as well as one umbilical vein. The umbilical cord is inserted into the chorionic plate during pregnancy (has an eccentric attachment). Placental vessels branch out across the surface of the placenta, where they further divide to form a network of vessels that is covered by a thin layer of cells. The creation of villous tree structures is the result of this process. Villous tree structures, which are found on the maternal side of the tree, are organised into lobules known as cotyledons. The placenta in humans is typically shaped like a disc, but the size of the placenta varies greatly between mammalian species.
The placenta can occasionally take on a shape in which it is divided into multiple different pieces that are linked together by blood arteries. The sections, which are referred to as lobes, might be two, three, four, or more in number. Bilobed/bilobular/bipartite placentas are one type of placenta, whereas others are classified as trilobed/trilobular/tripartite placentas, and so on. It is referred to as a succenturiate placenta when the main lobe and auxiliary lobe are readily distinguishable from one another. During labour, the blood veins linking the lobes can get in the way of the foetal presentation, which is known as vasa previa (obstructed foetal presentation).
Function
The placental function includes gas exchange, metabolic transfer, hormone secretion, and foetal protection. Nutrient and drug transfer through the placenta can occur through passive diffusion, facilitated diffusion, active transport, and pinocytosis.
Except for neuromuscular blocking medicines, almost all anaesthetic medications pass through the placenta with little difficulty.
Formation
Towards the end of pregnancy, the placenta is the largest foetal organ, and the umbilical circulation gets at least 40% of the biventricular cardiac output, which is significant. As a result, it should come as no surprise that the development of the placenta and the development of the foetal heart are expected to be closely linked in terms of hemodynamics. The development of the placenta is rapid and occurs well ahead of the development of the foetus. At the end of the first trimester of pregnancy, the placenta undergoes significant remodelling, and its vasculature is capable of responding to changes in external conditions as well as fluctuations in the blood supply received from the mother. There are two components of the placental membranes to consider: the secondary yolk sac and the chorioallantoic placenta. The secondary yolk sac is a sac that develops after the placenta is formed. Condensations in the mesenchyme at around 17 days post-conception (p.c.) give birth to endothelial and erythroid precursors, making the yolk sac the earliest of the extraembryonic membranes to be vascularized. In the first 24 days of pregnancy, a network of blood arteries is formed, with the vitelline vein draining into the sinus venosus from the area of the developing liver where it originates. Early pregnancy failures are frequently characterised by improper development of the yolk sac, which is thought to be a result of defective foetal growth and development. Vasculogenesis occurs in the villous mesenchyme of the chorioallantoic placenta at a similarly early stage to that observed in the chorioallantoic placenta. For the majority of the first trimester, nucleated erythrocytes fill the lumens of the placental capillaries, and end-diastolic flow is nonexistent in the umbilical artery circulation, suggesting a strong resistance to blood flow. Resistance in the umbilico-placental circulation begins to decrease at approximately 12–14 weeks of pregnancy. During a typical early pregnancy, the placental capillary network is malleable, and significant remodelling occurs in response to changes in local oxygen content, and in particular to oxidative stress, during the pregnancy. Smooth muscle cells surrounding the placental arteries become dedifferentiated and proliferative in pregnancies complicated by preeclampsia and/or foetal growth limitation, which results in the development of preeclampsia and/or foetal growth restriction. This alteration is connected with greater umbilical resistance as assessed by Doppler ultrasound, and it is expected to have a significant impact on the growing heart through the afterload it produces. As a result, both the umbilical and the maternal placental circulations may have an impact on the development of the heart during pregnancy.
Conclusion
In mammals, the placenta is critical for the development of the embryo in utero. When it comes to humans, abnormal placental development is at the root of frequent pregnancy illnesses such as pre-eclampsia and foetal growth limitation. Because of the wide range of placental types found in different mammals, animal models have only been of limited value in understanding human placental development until recently. In recent years, new technologies for researching human placental development, such as 3D organoids, stem cell culture systems, and single-cell RNA sequencing, have provided important new insights into the area of human placental development. In this paper, we discuss the morphological, molecular, and functional features of human placental formation, with a particular emphasis on the trophoblast, which is the placenta’s defining cell.