cytotrophoblast

**Origin and timeline** - The trophoblast is the outer cell layer of the blastocyst (about day 5-6). It differentiates after implantation (about day 6-8) into two layers: - **Cytotrophoblast (inner layer):** distinct, mononuclear cells with clear cell borders. It is mitotically active and acts as the stem cell population. - **Syncytiotrophoblast (outer layer):** a multinucleated syncytium with no cell borders and no mitotic activity. It is formed by fusion of cytotrophoblast cells. - Both layers are derived from the trophoblast, which is trophectoderm-derived (not inner cell mass). The syncytiotrophoblast is what invades the endometrium.

**Functions** 1. **Stem cell for syncytiotrophoblast.** Cytotrophoblast cells divide and fuse to replace and expand the syncytiotrophoblast. This is mediated by syncytin (an endogenous retroviral envelope protein). 2. **Primary chorionic villi.** Cytotrophoblast pushes into the syncytiotrophoblast around day 13, forming primary villi. These become secondary villi (mesenchyme core) and tertiary villi (fetal vessels). 3. **Extravillous trophoblast (EVT).** Cytotrophoblast cells at the tips of anchoring villi form the cytotrophoblastic shell and invade the decidua and inner myometrium. - They remodel the maternal spiral arteries into low-resistance, high-capacity vessels by replacing the endothelium and muscular wall. - This ensures adequate uteroplacental blood flow. 4. **Placental barrier.** In early pregnancy, the barrier has several layers: syncytiotrophoblast, cytotrophoblast, villous stroma, and fetal capillary endothelium. The cytotrophoblast layer becomes discontinuous and largely disappears in the second and third trimesters, which thins the barrier. This is relevant to transplacental passage of IgG and pathogens such as TORCH organisms.

**Hormone production** - **hCG** is made by the *syncytiotrophoblast*, not the cytotrophoblast. This is a classic exam trap. - The cytotrophoblast makes hCG-regulating factors such as GnRH. Syncytiotrophoblast makes hCG, hPL, progesterone, and estrogens. - hCG maintains the corpus luteum until the placenta takes over progesterone production (about weeks 8-10, the luteal-placental shift).

**Clinical correlations** - **Preeclampsia:** shallow cytotrophoblast (EVT) invasion causes incomplete spiral artery remodeling. This leads to placental hypoperfusion and ischemia, release of antiangiogenic factors (sFlt-1, soluble endoglin), endothelial dysfunction, hypertension, proteinuria, and end-organ damage. Treatment is delivery of the placenta. Low-dose aspirin is used for prevention in high-risk patients. - **Placenta accreta spectrum (accreta, increta, percreta):** excessive trophoblast invasion due to a defective decidua basalis, usually after prior cesarean section or uterine surgery. It causes severe postpartum hemorrhage when the placenta fails to separate. - **Hydatidiform mole and gestational trophoblastic disease:** hyperplasia of both cytotrophoblast and syncytiotrophoblast leads to very high hCG, a "snowstorm" appearance on ultrasound, and a uterus that is large for dates. - **Complete mole:** 46,XX (paternal origin, empty egg with duplicated sperm), diffuse swelling, no fetal parts, higher risk of choriocarcinoma. - **Partial mole:** 69,XXY (triploid), focal swelling, fetal parts present. - **Choriocarcinoma:** malignant proliferation of both cytotrophoblast and syncytiotrophoblast with no villi. It is hemorrhagic and has very high hCG. It metastasizes hematogenously to the lung ("cannonball" metastases). It is highly responsive to methotrexate or actinomycin D. - **Placental site trophoblastic tumor and epithelioid trophoblastic tumor:** arise from intermediate (extravillous) trophoblast. They produce human placental lactogen (hPL), have low hCG, and respond poorly to chemotherapy (treated with hysterectomy).

**High-yield mnemonic** "**C**ytotrophoblast = **C**ells (distinct borders), **C**ytotrophoblast = **C**ytotrophoblast makes syncytium; **S**yncytiotrophoblast = **S**ecretes hCG and **S**ays 'invade'." (Simplified: Cytotrophoblast = inner, mononuclear, dividing. Syncytiotrophoblast = outer, multinucleated, secretes hCG.)