The placenta is the interface between mother and fetus, responsible for gas exchange, nutrient transport, hormone production, and immune modulation. Histological examination of the placenta is essential for understanding pregnancy complications and fetal outcomes.
Gross Examination
The delivered placenta is examined fresh or fixed. Gross assessment includes: weight (normal term placenta: 450-600 g), shape (round/oval, accessory lobes), cord insertion (central, eccentric, marginal, velamentous — velamentous insertion carries risk of vasa previa), cord length (normal 40-70 cm; abnormally short or long cords are associated with complications). Maternal surface — completeness of decidual separation, retroplacental hematomas. Fetal surface — transparency of the amnion, meconium staining (green discoloration indicates fetal distress). The placenta is sectioned at 1-2 cm intervals; representative sections are taken from the center, margin, cord, and membranes.
Normal Term Placental Histology
The chorionic villi are the functional units. Term villi are terminal villi — small, branching structures with abundant capillaries, thin vasculosyncytial membranes (VSM — the fusion of trophoblast and capillary endothelium for gas exchange), and syncytial knots (aggregates of syncytiotrophoblast nuclei). The intervillous space contains maternal blood. The trophoblast has two layers: syncytiotrophoblast (outer, multinucleated, continuous — site of hormone production and nutrient transport) and cytotrophoblast (inner, discontinuous at term — stem cell layer). The villous stroma contains fetal capillaries, Hofbauer cells (fetal macrophages), and fibroblasts.
The umbilical cord contains two arteries and one vein (three vessels) embedded in Wharton jelly (mucoid connective tissue rich in proteoglycans). Single umbilical artery occurs in 0.5-1% of deliveries and is associated with congenital anomalies.
Villous Maturation
Villous maturation is assessed relative to gestational age. Preterm placenta — larger, edematous villi with prominent cytotrophoblast and fewer syncytial knots. Term placenta — smaller, more branched villi with thin VSM and abundant syncytial knots. Post-term placenta — increased syncytial knots, villous agglutination (clumping), and fibrinoid deposition. Accelerated maturation (small, hypermature villi for gestational age) is associated with maternal vascular malperfusion (preeclampsia, IUGR). Delayed maturation (persistence of immature-appearing villi) is associated with fetal anomalies and some metabolic disorders.
Placental Pathology
Maternal vascular malperfusion (MVM) — lesions resulting from defective spiral artery remodeling: decidual arteriopathy (atherosis, fibrinoid necrosis), placental infarction (ischemic necrosis of villi), and distal villous hypoplasia (sparse, thin terminal villi). MVM is associated with preeclampsia, intrauterine growth restriction (IUGR), and preterm birth.
Fetal vascular malperfusion (FVM) — lesions from fetal circulatory obstruction: avascular villi (loss of fetal capillaries), villous stromal-vascular karyorrhexis (nuclear debris in villous stroma), and thrombotic occlusion of fetal vessels. FVM is associated with fetal thrombophilia, diabetes, and umbilical cord obstruction.
Acute chorioamnionitis — maternal inflammatory response to ascending infection: neutrophils in the chorion and amnion, progressing to funisitis (neutrophils in the umbilical cord vessels). Associated with preterm labor, premature rupture of membranes, and neonatal sepsis.
Chronic villitis — lymphocytic infiltration of villi, often due to infectious (CMV, toxoplasma, syphilis, rubella, HSV, Zika) or immunologic (chronic villitis of unknown etiology — CVUE) causes. CVUE is associated with IUGR and may recur in subsequent pregnancies.
Placental Examination in Special Circumstances
Twin placentas — chorionicity determines risk: monochorionic (one placenta, one chorion) has higher complication rates than dichorionic (two placentae or one fused placenta with two chorions). Twin-to-twin transfusion syndrome occurs only in monochorionic placentas.
Molar pregnancy — complete hydatidiform mole shows swollen, avascular villi with trophoblast hyperplasia (46,XX, all paternal). Partial hydatidiform mole shows focal villous swelling with scalloped borders and trophoblastic inclusions (69,XXX or 69,XXY). IHC for p57 (paternally imprinted gene) is negative in complete mole, positive in partial mole and normal placenta.
Quality Assurance
Placental examination should be performed for all abnormal pregnancies. Standardized reporting templates (Amsterdam Placental Workshop Group Consensus) ensure completeness. Key elements include: macroscopic description, microscopic description of villous maturation, maternal vascular pathology, fetal vascular pathology, inflammation, and specific diagnoses. Quality assurance includes correlation with clinical outcomes and regular review of diagnostic criteria.
