Implantation
How the blastocyst invades the uterine wall
Implantation begins on day 6–7 when the blastocyst, having shed the zona pellucida, contacts the uterine endometrium. The trophoblast cells initiate an invasion that is unique in biology: the embryo literally buries itself inside the mother's tissue, eroding decidual cells and spiral arteries to establish its own blood supply.
The trophoblast differentiates into two layers: cytotrophoblast (inner, mitotically active, stem cell-like trophoblast) and syncytiotrophoblast (outer layer formed by fusion of cytotrophoblasts, multinucleated, non-dividing, invasive). The syncytiotrophoblast secretes hCG (human chorionic gonadotropin) — the hormone detected by home pregnancy tests — from the moment of implantation.
💡 Placental Hormones and Pregnancy Tests
The placenta functions as its own endocrine organ, producing a series of hormones that maintain pregnancy and prepare the mother for birth and lactation:
hCG (human chorionic gonadotropin): Secreted from implantation onward. Maintains the corpus luteum → keeps progesterone production up → prevents menstruation and maintains the pregnancy. Peaks at week 10, then declines (which often correlates with resolution of morning sickness). hCG is the hormone detected by home pregnancy tests — present in urine within days of implantation. Also used as a tumor marker for gestational trophoblastic disease and testicular germ cell tumors.
Progesterone: Initially from the corpus luteum (maintained by hCG), then produced by the placenta itself from week 8 onward (the luteo-placental shift). Maintains uterine quiescence, prevents premature contractions, and prepares the breast for lactation.
hPL (human placental lactogen): Causes maternal insulin resistance → more glucose available for the fetus. Contributes to gestational diabetes in susceptible women.
Sync
Syncytiotrophoblast — the invasive outer layer
The syncytiotrophoblast is a giant multinucleated cell (syncytium) formed by fusion of cytotrophoblasts. It is the invasive leading edge of the placenta that erodes the decidua and maternal spiral arteries. It also secretes all placental hormones: hCG (maintains the corpus luteum and progesterone production in the first trimester), progesterone (after week 8, when the placenta takes over from the corpus luteum), estrogens, hPL (human placental lactogen — metabolic hormone), and CRH (corticotropin-releasing hormone — involved in timing of parturition).
The syncytiotrophoblast is also the layer that directly contacts maternal blood in the intervillous space — it is exposed to the full immune system of the mother but is not rejected. This immune tolerance depends on the expression of non-classical MHC molecules (HLA-G, HLA-E, HLA-C) that suppress NK cell and T cell activity at the maternal-fetal interface.
Memory trick: Syncytio = syncytium = fused cells = one giant invasive cell layer. Makes all the hormones, invades the uterus, and hides from mom's immune system.
Villi
Chorionic villi — the functional exchange units
By week 3–4, the trophoblast organizes into chorionic villi — finger-like projections that dramatically increase surface area for exchange. Primary villi (trophoblast only) → secondary villi (mesenchymal core added) → tertiary villi (fetal capillaries form inside). By term, the combined surface area of all chorionic villi is approximately 14 square meters — the size of a studio apartment floor.
The placenta is hemochorial in humans — maternal blood directly bathes the chorionic villi in the intervillous space. This is the most invasive type of placentation and allows for very efficient exchange, but also means the fetus is one thin layer away from the mother's immune system.
Memory trick: Villi = massive surface area = efficient exchange. Hemochorial = maternal blood directly contacts villus surface. The most invasive, most efficient type.
Exch
The placental barrier and fetal-maternal exchange
The 'placental barrier' is not a true barrier in the pharmacological sense — it is a selectively permeable membrane that allows passage of gases, nutrients, and waste products while restricting large proteins and cells. What crosses and what doesn't:
Freely crosses: O₂ and CO₂ (simple diffusion), glucose (facilitated diffusion via GLUT transporters), amino acids (active transport), fatty acids and fat-soluble vitamins, IgG antibodies (Fc receptor-mediated transport — the basis of maternal passive immunity to the fetus), and most lipid-soluble drugs including alcohol, nicotine, cocaine, and most medications.
Does not cross (normally): Red blood cells, most proteins, IgM (too large), maternal cells (though small numbers do cross and can establish fetal microchimerism), and most bacteria (but viruses including HIV, CMV, rubella, Zika can cross).
The fetal circulation runs opposite to maternal flow in the intervillous space (countercurrent exchange) — maximizing extraction of O₂ and nutrients.
Memory trick: Lipid-soluble = crosses. Hydrophilic/large protein = blocked. IgG crosses (Fc receptors) → baby gets mom's immunity. Red blood cells do NOT cross.
🔬 Clinical Scenario — Placental Disorders
Disorders of placentation are among the most important causes of maternal and fetal morbidity:
A
Preeclampsia — shallow trophoblast invasion. Normally, extravillous trophoblasts invade the spiral arteries and replace their muscular walls with trophoblast tissue → vessels become wide, low-resistance conduits to the placenta. In preeclampsia, this remodeling fails → spiral arteries remain narrow and high-resistance → placental ischemia → placenta releases antiangiogenic factors (sFlt-1/sVEGFR-1) into maternal circulation → maternal endothelial dysfunction → hypertension, proteinuria, edema. The only definitive treatment is delivery. Affects 2–8% of pregnancies and is a leading cause of maternal mortality worldwide.
B
Placenta previa — placenta covering the cervical os. When the placenta implants over or near the internal cervical os, it can obstruct delivery and cause painless vaginal bleeding (as the lower uterine segment stretches and the placenta shears). Diagnosed by ultrasound. Requires cesarean delivery. Associated with previous uterine surgery (prior C-section scars alter uterine anatomy) and multiple pregnancies.
C
Gestational trophoblastic disease (GTD). Abnormal proliferation of trophoblastic tissue: complete hydatidiform mole (fertilization of an empty egg by one or two sperm → all genetic material is paternal → abnormal placental tissue, no fetus → classic 'snowstorm' appearance on ultrasound, very high hCG). Can progress to choriocarcinoma — a malignant trophoblastic tumor with high metastatic potential but extraordinary sensitivity to chemotherapy (methotrexate, actinomycin D). GTD is one of the few solid tumors that can be cured with chemotherapy even after widespread metastasis.
D
Fetal programming — the Barker hypothesis. David Barker proposed that the intrauterine nutritional environment programs the fetus's metabolism for life. Fetuses exposed to nutritional restriction develop a 'thrifty phenotype' — metabolically programmed to conserve calories. If they are then born into nutritional abundance, they are at dramatically increased risk of obesity, type 2 diabetes, and cardiovascular disease as adults. This developmental programming operates through epigenetic changes (DNA methylation, histone modification) that are established in utero and persist throughout life.
📌 Exam Application
Placentation questions appear in developmental biology, obstetrics, and physiology:
1. Trophoblast layers: Cytotrophoblast (mitotically active, stem-like) → fuses to form Syncytiotrophoblast (invasive, hormone-secreting, contacts maternal blood).
2. Placental hormones: hCG (maintains corpus luteum, detected by pregnancy test, peaks week 10). Progesterone (prevents contractions). hPL (causes maternal insulin resistance → gestational diabetes). Know all four hormones and their functions.
3. What crosses the placental barrier: O₂, CO₂, glucose, amino acids, IgG, lipid-soluble drugs, alcohol, viruses. Does NOT cross: red blood cells, IgM, most bacteria.
4. Preeclampsia: Shallow trophoblast invasion → ischemic placenta → sFlt-1 released → maternal endothelial dysfunction → HTN + proteinuria. Treatment = delivery.
5. hCG as tumor marker: Elevated in gestational trophoblastic disease and testicular germ cell tumors.
⚠️ The Most Commonly Confused Placentation Facts
IgG crosses the placenta — IgM does not. IgG is actively transported across the syncytiotrophoblast via neonatal Fc receptors (FcRn). This is how babies acquire passive immunity to infections their mothers have been exposed to. IgM is too large to cross. This distinction is clinically important: maternal IgG anti-Rh antibodies cross and cause hemolytic disease of the newborn (HDN); maternal IgM anti-A and anti-B antibodies do not cross the placenta.
The corpus luteum maintains the pregnancy for the first 8 weeks — then the placenta takes over. hCG (from the syncytiotrophoblast) stimulates the corpus luteum to keep producing progesterone. At week 8 (the luteo-placental shift), the placenta takes over progesterone production and the corpus luteum is no longer needed. If the luteo-placental shift fails → pregnancy loss at week 8–10.
Preeclampsia is caused by SHALLOW trophoblast invasion (failure of spiral artery remodeling), not by excessive invasion. The shallow invasion is the initial problem → placental ischemia → sFlt-1 secretion → systemic maternal endothelial dysfunction. Students sometimes confuse the cause (shallow invasion) with the consequence (maternal hypertension and proteinuria).
✓ Quick Self-Test
1. What is the difference between cytotrophoblast and syncytiotrophoblast?
2. What is hCG, when is it produced, and what is its function?
3. What molecules freely cross the placental barrier and which are restricted?
4. What is preeclampsia and what is its underlying developmental cause?
5. What is the luteo-placental shift?
Answers:
1. Cytotrophoblast: inner, mitotically active, stem-like trophoblast cells. Syncytiotrophoblast: outer layer formed by fusion of cytotrophoblasts into a multinucleated syncytium; invasive, secretes all placental hormones (hCG, progesterone, hPL, estrogens), and directly contacts maternal blood in the intervillous space.
2. hCG (human chorionic gonadotropin) is produced by the syncytiotrophoblast from the time of implantation. It maintains the corpus luteum, which continues to produce progesterone during the first trimester — preventing menstruation and maintaining the pregnancy. It is the hormone detected by home pregnancy tests and peaks at week 10.
3. Freely crosses: O₂, CO₂ (simple diffusion), glucose (GLUT transporters), amino acids (active transport), IgG antibodies (Fc receptor-mediated), lipid-soluble drugs, alcohol, most viruses. Restricted: red blood cells, IgM (too large), most bacteria, large hydrophilic proteins.
4. Preeclampsia results from failure of extravillous trophoblasts to adequately remodel uterine spiral arteries → arteries remain narrow and high-resistance → placental ischemia → placenta secretes antiangiogenic factors (sFlt-1) → maternal endothelial dysfunction → hypertension, proteinuria, edema. Clinically presents after 20 weeks of gestation.
5. The luteo-placental shift (around week 8) is the transition from corpus luteum to placenta as the primary source of progesterone. Before week 8, hCG maintains the corpus luteum to produce progesterone. After week 8, the placenta produces progesterone directly and the corpus luteum is no longer needed.