The placenta is made of both fetal and maternal tissue, composed primarily of specialized cells called trophoblasts, a network of blood vessels, connective tissue rich in collagen, and a surprisingly large amount of water. At delivery, it weighs roughly 500 to 620 grams and is about 87% water by weight, with only 13% being solid tissue. Despite its temporary existence, it is the first and largest organ a fetus develops, functioning as lungs, liver, kidneys, and hormone factory all at once while the fetal organs are still forming.
Fetal and Maternal Tissue Combined
The placenta is unique because it contains tissue from two genetically different individuals. The bulk of the organ originates from the fetus. After a fertilized egg implants in the uterine wall, its outer layer of cells begins rapidly dividing and differentiating into the structures that will become the placenta. The maternal contribution comes from the uterine lining, which transforms into a specialized tissue called the decidua. This decidual tissue forms the base of the placenta where it anchors to the uterine wall.
Humans have what’s called a hemochorial placenta, the most invasive type found in mammals. The embryo embeds completely into the uterine wall, and fetal cells erode into the mother’s blood vessels so that maternal blood directly bathes the placental surface. This is different from other mammals like horses, where the placenta simply sits against the uterine lining without penetrating it.
The Cells That Do the Work
Two cell types form the functional surface of the placenta: cytotrophoblasts and syncytiotrophoblasts. Cytotrophoblasts are the progenitor cells. They either multiply to maintain the cell population or fuse together into a continuous layer called the syncytiotrophoblast. This fused layer is a single, unbroken sheet of cellular material that covers the entire surface of the placenta’s branching tree-like structures. It has no gaps between cells, which makes it an effective barrier.
The syncytiotrophoblast is where most of the action happens. It controls the exchange of oxygen, carbon dioxide, nutrients, and waste between mother and fetus. It is also the primary site where the placenta manufactures hormones. Cytotrophoblasts play a different but equally critical role early in pregnancy: they invade the uterine wall and physically remodel the mother’s spiral arteries, widening them to increase blood flow to the placenta. When this remodeling doesn’t happen properly, it can lead to complications like preeclampsia and restricted fetal growth.
A Branching Tree of Villi
The internal architecture of the placenta looks like a dense, branching tree. The branches are called chorionic villi, and they develop in stages. Primary villi are simple finger-like projections of trophoblast cells that push into the maternal tissue. Secondary villi form when connective tissue grows into these projections. Tertiary villi appear when blood vessels develop inside them, creating a network of tiny capillaries carrying fetal blood.
These villi come in several types, each with a different job. Anchoring villi (also called stem villi) attach the placenta to the uterine wall. Branched villi extend from the sides of stem villi and are the primary sites of nutrient and gas exchange. They float freely in spaces filled with maternal blood. Terminal villi, which dominate in the third trimester, form as passive bulges created by the coiling of fetal capillaries. In certain spots, the barrier between maternal and fetal blood thins to just 1 to 2 microns, roughly one-fiftieth the width of a human hair, making exchange extremely efficient.
The Scaffolding: Collagen and Connective Tissue
Holding everything together is an extracellular matrix, essentially a scaffold of structural proteins. The primary building material is type I collagen, which forms large, cross-banded fibers often bundled together for strength. Type III collagen weaves around these bundles as thinner, beaded fibers that create a mesh-like wrapping. Types V and VI collagen appear as fine filaments that connect and coat the larger fibers, reinforcing the structure.
Two other proteins play important roles. Fibronectin is present throughout the connective tissue core of each villus, both as free filaments and as a coating on collagen fibers. It also circulates in both the maternal and fetal blood within the placenta. Laminin, along with type IV collagen, forms the basement membranes, the thin sheets that underlie the trophoblast cell layers and surround blood vessels. Together, these proteins create a flexible yet durable framework that supports millions of villi while allowing the organ to grow and adapt throughout pregnancy.
A Hormone-Producing Powerhouse
The placenta is one of the most active hormone-producing organs in the human body. It synthesizes both protein and steroid hormones. The most well-known is human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests. It signals the ovaries to keep producing progesterone in early pregnancy until the placenta takes over that job itself.
Other protein hormones include placental lactogen, which alters the mother’s metabolism to ensure the fetus gets enough glucose, and chorionic thyrotropin, which influences thyroid function. On the steroid side, the placenta produces large quantities of progesterone, which maintains the uterine lining and prevents contractions, along with estrogens that support fetal development. It also produces relaxin during the first trimester and stress-related hormones similar to the ones made by the brain. Additional signaling molecules called tachykinins are also manufactured, contributing to blood flow regulation.
The Circulatory System Inside
The placenta contains two completely separate blood supplies that never mix. Fetal blood flows through the umbilical arteries into the capillary networks inside each villus, then returns to the fetus through the umbilical vein. Maternal blood, meanwhile, flows from the remodeled spiral arteries into the open intervillous spaces where it surrounds the villi.
The volume of maternal blood passing through the placenta increases dramatically over the course of pregnancy, from about 45 milliliters per minute in the non-pregnant uterus to around 750 milliliters per minute at term. That means roughly three-quarters of a liter of blood washes over the villi every 60 seconds, delivering oxygen and nutrients while carrying away carbon dioxide and waste.
How It Develops Over Nine Months
Placental development begins within the first two weeks after fertilization, when the outer cells of the implanting embryo start differentiating into trophoblasts. During the first 13 weeks, the placenta operates in a relatively low-oxygen environment while it establishes its structure and begins remodeling the maternal blood supply. This low-oxygen period is actually important for normal development.
As pregnancy progresses, the villi branch into increasingly dense networks. The placenta grows heavier as well, gaining roughly 60 grams between weeks 37 and 42. By the time of delivery, placentas from vaginal births weigh an average of 545 grams, while those from cesarean deliveries average 621 grams (the difference likely reflects the blood that drains during labor contractions). The organ is disc-shaped, typically 15 to 20 centimeters across, and attached to the fetus by the umbilical cord.
At its core, the placenta is a temporary organ built from a remarkably complex mix of living cells, structural proteins, blood vessels, and hormones, all organized to keep two separate circulatory systems in close contact without ever letting them touch.