chorionic villi

Chorionic villi develop from the trophoblast (cytotrophoblast and syncytiotrophoblast layers) surrounding the blastocyst after implantation. They invade the maternal decidua, anchoring the placenta and creating a large surface area for maternoplacental exchange. Each villus contains a core of fetal mesenchyme with fetal capillaries, surrounded by cytotrophoblast (inner, mitotically active layer) and syncytiotrophoblast (outer, hormone-secreting, invasive layer that produces hCG, hPL, estrogen, and progesterone). Maternal blood fills the intervillous space, bathing the villi (hemochorial placentation), allowing exchange of oxygen, nutrients, and waste without direct mixing of maternal and fetal blood.

Clinically, chorionic villi are sampled via chorionic villus sampling (CVS), typically performed at 10-13 weeks gestation, to obtain fetal genetic material for karyotyping and diagnosis of chromosomal abnormalities (e.g., trisomy 21, 18, 13) or single-gene disorders. CVS carries a small risk of limb defects if performed too early (<10 weeks) and a slightly higher miscarriage risk than amniocentesis, but it allows earlier diagnosis.

Pathologically, complete hydatidiform moles show diffuse hydropic (edematous) swelling of chorionic villi with trophoblastic proliferation and absent fetal tissue (46,XX from paternal duplication), while partial moles show focal villous swelling with identifiable fetal parts (69,XXY triploidy). Chorionic villi are also central to understanding placental abruption, preeclampsia (abnormal trophoblastic invasion and spiral artery remodeling), and choriocarcinoma (malignant proliferation of trophoblastic tissue).