Trophoblast

Chorionic villi are part of a normal pregnancy. They are derived from the conceptus, i.e. NOT maternal.

Tumours arising from the trophoblast are dealt with in gestational trophoblastic disease.

Chorionic villi 101

• Maternal blood is around villi.
• Fetal blood (nucleated (fetal) RBCs) in the villi.

Basic histology

• Outer layer of villus / closer to mother : syncytiotrophoblasts.
• Inner layer of villus / closer to fetus : cytotrophoblasts.

Chorionic villi 102

• Syncytiotrophoblasts (eosinophilic cytoplasm with vacuoles (contain hCG), multiple hyperchromatic nuclei).
  • Large + many irreg. or lobular hyperchromatic nuclei.
  • Eosinophilic vacuolated cytoplasm (contains hCG).
  • Closest to mom - covers cytotrophoblast.
• Cytotrophoblasts (polygonal shape, distinct borders, clear cytoplasm, in cords, single nucleus).
  • Polygonal shaped cells in cords/masses.
  • Distinct cell borders.
  • Covered by syncytiobrophoblast - closer to fetus than syncytiotrophoblasts.
  • Clear cytoplasm.
  • Single uniform nucleus.

Intermediate trophoblast

General

• Abbreviated: IT.

• Thought to be the cell of origin for:
  • Exaggerated placental site (EPS).
  • Placental site nodule (PSN).
  • Placental site trophoblastic tumour (PSTT).
  • Epithelioid trophoblastic tumor (ETT).

Subtypes

Three subtypes of IT are recognized:

1. Implanation site IT.
2. Villous IT.
3. Chorionic-type IT.

Microscopic

Features:

• Large cells.
• Eosinophilic cytoplasm.
  • May be multi-nucleated.

IHC

Features:

• hPL (human placental lactogen) +ve.
• beta-hCG -ve.
• Keratin +ve.
• EMA +ve in 2nd & 3rd trimester.

Notes:

• Cytotrophoblast and syncytiotrophoblast - keratin +ve.

Age of villi

Young villi

• Do not have blood vessels.
• No syncytial knots.

Old villi

• Blood vessels present and at the periphery of villus - contain nucleated (fetal) RBCs.
• Syncytial knots.

Trophoblast table

Based on Shih et al.:

	Cytotrophoblast	Syncytiotrophoblast	Villous intermediate trophoblast	Implanation site intermediate trophoblast	Chorionic-type intermediate trophoblast
Nuclear features	round, small	multinucleated	polyhedral	pleomorphic +/-multinucleation (occasional), +/-nucleoli	round / polyhedral
Cell borders	well-defined	poorly defined (?)	well-defined	moderately-defined	poorly defined (?)
Cytoplasm	minimal, clear to granular	abundant, vacuolated	abundant & eosinophilic to clear	abundant & eosinophilic	abundant & eosinophilic
Location	villi - deep to syncytiotrophoblast	villi - superficial to cytotrophoblast - nearest to maternal blood	villi	implanatation site	chorion
Function	stem cell	produces hormones	???	anchor placenta	???
IHC	beta-hCG ???	beta-hCG	???	inhibin+, hPL+, beta-hCG-	???
Image				[1]

hPL = human placental lactogen.

Notes:

• Keratin is positive in all trophoblastic tissue and negative in decidual tissue.

Villi and age - table

Gestational age	Nucleated RBCs	Cytotrophoblast / syncytiotrophoblast	Villi size	Vessels	Other
First trimester	present	both can be identified	large	few; location central
Second trimester	rare	cytotrophoblast difficult to identify; syncytial knots uncommon	intermediate	intermediate; location: central/periphery
Third trimester	absent	cytotrophoblast indistinct; syncytial knots common	small	abundant; location: periphery of villi	focal calcifications

See also

• Endometrium.
• Products of conception.
• Ectopic pregnancy.
• Choriocarcinoma.
• Arias-Stella reaction - benign atypical changes associated with chorionic tissue.