The Human Placenta

The human placenta is a hemochorial, villous type where the maternal blood enters the intervillous space via the spiral arteries and flows directly around the terminal villi of the fetal circulation without any intervening maternal vessel wall. The surface area available for exchange gradually increases throughout pregnancy until it reaches around 10-15 m2 in the last trimester (Figure 1). The nature of the exchangeable surface of the placenta also changes throughout gestation with the mature intermediate villi appearing towards the end of the second trimester and the terminal villi, which represent the main site of feto-maternal exchange, appearing a few weeks later. The rate of fetal blood delivery to the placenta (umbilical flow) also changes markedly during pregnancy and is approximately linearly related to fetal weight, and hence the fetal nutrient requirement, throughout gestation (Figure 1).

Anatomically, the human placenta is a large structure typically weighing around half a kilogram. However, its physical bulk belies the flimsy nature of the separation between the maternal and fetal

Figure 1 Changes with gestational age in placental exchangeable surface area and umbilical blood flow (A), accretion of fat and protein in the fetus (B), and the components of fetal energy requirements (C). (Reproduced with permission from: Sutton MS, Theard MA, Bhatia SJ, Plappert T, Saltzman DH, and Doubilet P (1990) Changes in placental blood flow in the normal human fetus with gestational age. Pediatric Research 28: 383-387;Widdowson EM (1968) Growth and composition of the fetus and newborn. In: Assali NS (ed.) The Biology of Gestation, pp. 1-49 New York: Academic Press; Sparks JW (1984) Human intrauterine growth and nutrient accretion. Seminars in Perinatology 8: 74-93.)

Figure 1 Changes with gestational age in placental exchangeable surface area and umbilical blood flow (A), accretion of fat and protein in the fetus (B), and the components of fetal energy requirements (C). (Reproduced with permission from: Sutton MS, Theard MA, Bhatia SJ, Plappert T, Saltzman DH, and Doubilet P (1990) Changes in placental blood flow in the normal human fetus with gestational age. Pediatric Research 28: 383-387;Widdowson EM (1968) Growth and composition of the fetus and newborn. In: Assali NS (ed.) The Biology of Gestation, pp. 1-49 New York: Academic Press; Sparks JW (1984) Human intrauterine growth and nutrient accretion. Seminars in Perinatology 8: 74-93.)

Placenta

Placenta

Figure 2 Nutrient exchanges between the maternal circulation, placenta, and fetus.

circulations, which consists of only two cell layers; the syncytiotrophoblast and the capillary endothelium. The endothelium allows the passage of nutrients through pores within the interendothelial cleft and therefore is not a significant barrier to nutrient passage. The effective barrier between the maternal and fetal circulation is provided by a thin trophoblastic cover in the form of a syncytium (a tissue in which the cytoplasm of constituent cells is continuous), known as the syncytiotrophoblast. Between 10 weeks and term the thickness of the villous trophoblast falls from around 10 mm to 4 mm and the overall materno-fetal diffusion distance from 40 mm to 5 mm. Any substance crossing between the maternal and fetal circulation has to pass though this barrier, which consists of two membranes: the micovillous membrane (MVM) facing the maternal blood and the basal membrane (BM) facing the fetal blood. The surface area of the maternal-facing MVM is around 5-6 times that of the fetal-facing BM. There are other cell types and structures within the placenta, such as maternal myometrium and decidua, connective tissue, Hofbauer cells, and persisting cytotrophoblast cells, which contribute to the metabolic activity and nutrient requirements of the placenta but which are not thought to be significant barriers to transport.

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