The Placenta and Placentation

The placenta is a transient organ present exclusively during pregnancy that serves the diverse functions necessary for the survival of the developing fetus, most notably the physiological exchange that occurs as the “maternal-fetal interface” (Burton & Fowden, 2015). This exchange is facilitated through the syncytiotrophoblast, the epithelial cover of the placental villi. The placental (chorionic) villi contain fetal capillaries that receive the nutrients and gases, which are then carried by the umbilical vein into fetal circulation (Betts et al., 2022). The human placenta is haemochorial, meaning maternal blood directly fills the intervillous space, where the syncytiotrophoblast allows limited diffusion of certain solutes, while maintaining fetal and maternal separation of blood (Sibley & Dilworth, 2020). Nutrient and gas diffusion is largely determined by its chemical makeup and gradient. While small lipid-soluble substances, oxygen, and carbon dioxide, can move across the barrier down their gradient through simple diffusion, other nutrients may need specialized transporters that require energy (This is why alcohol is easily diffused across the placenta and fetotoxic). Furthermore, the fetal capillaries of the chorionic villi return waste products to maternal circulation (O’Brien & Wang, 2023).
However, these are the functions of a nearly or fully mature placenta. The formation of the placenta is termed placentation, which begins with the attachment of the blastocyst to the uterine wall. Trophoblast cells form around the blastocyst, of which trophoblast stem cells can differentiate into designated extravillous and villous cells of the placenta (Dunk et al., 2020). The trophoblasts come in direct contact with the uterine epithelium and fuse to develop the syncytiotrophoblast, allowing for the blastocyst to invade the uterine lining and begin implantation (Dunk et al., 2020). Dunk et al. define the next stages of development as the prelacunar, lacunar, and villous stages. In the prelacunar stage, the syncytiotrophoblast penetrates the uterine epithelium until the decidua, leaving the embryo behind the second row of cytotrophoblasts and the syncytiotrophoblasts. In the lacunar stage, lacunae, fluid-filled spaces, form within the syncytiotrophoblasts that combine into the intervillous space that will hold maternal blood. Finally, in the villous stage, the cytotrophoblast burrows into the syncytiotrophoblast towards maternal tissue, and sprouts villi (cytotrophoblasts external to the placenta are extravillous). Within the placenta, primary villi are formed first as projections of syncytiotrophoblasts and cytotrophoblasts. Secondary villi form as the extraembryonic mesenchyme penetrates and displaces the primary villi. Lastly, blood vessels are developed within the placental mesenchyme, marking the formation of tertiary villi and the placental blood supply system (Dunk et al., 2020). Simultaneously, fetal mesenchymal cells differentiate into blood vessels and the umbilical cord to connect the placenta and embryo (Betts et al., 2022). After tertiary villi formation, the placenta is organized into villous trees that are highly branched structures “with a high degree of capillarization” (Wang & Zhao, 2010). Meaning these villous trees are optimized for maternal-fetal exchange.
My STEAM piece aims to depict these villous trees, the tree in the foreground being a clearer view of a mature villous tree, and the background trees with a fuller view, chorionic villi composing the “leaves” of the tree. The mountain in the background includes an ultrasound of an approximate 10-week-old fetus attached to the placenta by the umbilical cord–an image I often found myself referencing in conjunction with research.
With the complete dependency for survival of the infant on the placenta, Burton and Fowden remark that it is easy to understand why some cultures revere the placenta’s guardianship of the fetus in utero. Today, however, this remarkable life-sustaining organ is, more often than not, medical waste upon expulsion.