syncytiotrophoblast

**Origin and structure** - At implantation (~day 6-7 after fertilization), the blastocyst's trophoblast differentiates into two layers. The inner layer is the cytotrophoblast, which is mononucleated and proliferative (stem-cell-like). The outer layer is the syncytiotrophoblast, which is multinucleated, has no distinct cell borders, and does not divide. It forms by fusion of cytotrophoblast cells (mediated by syncytin proteins from endogenous retroviral genes, e.g., syncytin-1). - The syncytiotrophoblast is terminally differentiated, so it cannot undergo mitosis. It must be continually replenished by cytotrophoblast fusion. - The syncytiotrophoblast, cytotrophoblast, and extraembryonic mesoderm make up the chorion. In mature chorionic villi, the syncytiotrophoblast is the outermost layer in direct contact with maternal blood in the intervillous space.

**Functions** 1. **Implantation/invasion:** It secretes proteolytic enzymes that erode the endometrial stroma and maternal vessels. This establishes lacunae that become the intervillous space (uteroplacental circulation). 2. **Hormone production:** - **hCG:** an α/β heterodimer (α subunit shared with LH, FSH, TSH; β is unique and is the basis of pregnancy tests). It rescues the corpus luteum so it continues to make progesterone until the placenta takes over (~weeks 8-10). It is detectable in maternal blood ~1 week after conception (about day 8 after fertilization) and in urine slightly later. It peaks at ~10 weeks, then declines. - hPL (human placental lactogen/chorionic somatomammotropin): causes maternal insulin resistance and increased free fatty acids for fetal glucose use (gestational diabetes link). - Progesterone and estrogens (estriol, which requires fetal adrenal/liver precursors, so it is part of the fetoplacental unit; used in the quad screen). - CRH (placental), which is relevant to the timing of parturition. 3. **Exchange barrier:** It handles nutrient, gas, and waste exchange. Layers of the placental barrier: syncytiotrophoblast → (cytotrophoblast, which is largely absent later in pregnancy) → villous connective tissue → fetal capillary endothelium. It also mediates transport of maternal IgG (FcRn-mediated transcytosis). 4. **Immune tolerance:** It lacks MHC class I and II expression, which protects against maternal T-cell attack. Extravillous trophoblasts express HLA-G, which inhibits NK cells.

**Clinical correlations** - **Pregnancy testing and abnormal hCG:** - Elevated: multiple gestation, molar pregnancy, choriocarcinoma, Down syndrome (trisomy 21). - Decreased: ectopic pregnancy (slower-than-expected rise), trisomy 18/Edwards syndrome, impending miscarriage. - **Hydatidiform mole:** Both complete and partial moles show trophoblastic hyperplasia with very high hCG. The complete mole is 46,XX (paternal origin, empty egg; "snowstorm" appearance on ultrasound, no fetal parts, high risk of choriocarcinoma). The partial mole is triploid (69,XXY; fetal parts present). Possible complications are hyperemesis, early preeclampsia, hyperthyroidism (hCG's α subunit is similar to TSH), and theca lutein cysts. - **Choriocarcinoma:** A malignancy of both syncytiotrophoblast and cytotrophoblast. It is hCG-secreting, hematogenous (lung metastases, "cannonball"), and has no villi. It is very chemosensitive (methotrexate/actinomycin D). In males, it can occur as a germ cell tumor of the testis. - **Preeclampsia:** Characterized by inadequate remodeling of spiral arteries by the extravillous trophoblast, causing placental ischemia, release of anti-angiogenic factors (sFlt-1) from syncytiotrophoblast, and endothelial dysfunction. This presents with hypertension and proteinuria after 20 weeks, and cure is delivery of the placenta. - **Placenta accreta spectrum:** Abnormally deep trophoblast invasion due to a defective decidua basalis (e.g., placenta previa, prior C-section). - **Congenital infections:** Pathogens that cross the placenta (the TORCH organisms, plus Parvovirus B19, syphilis, Listeria) cross the syncytiotrophoblast layer.

**High-yield mnemonic/pearls** - "Syncytio = Secretes hCG" and "Cyto = Cell division (stem cells)". - Syncytiotrophoblast: multinucleated, outer, makes hCG. Cytotrophoblast: mononuclear, inner, proliferative. - The syncytiotrophoblast is the earliest source of hCG (the β-hCG assay is the basis of pregnancy tests).