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Normal histology

Reviewers: Mandolin Ziadie, M.D. (see Reviewers page)
Revised: 10 October 2011, last major update June 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.

Basal Plate (Maternal Surface)

● Insertion site of the anchoring villi into maternal endometrium
● Invasive intermediate trophoblasts are seen here

Chorionic Plate (Fetal Surface)

● Stem villi arising from umbilical vessels surrounded by connective tissue and fibrin
● Covered by a layer of amnion and chorion

Placental villi

● Capillaries, fetal macrophages (Hofbauer cells) and fibroblasts surrounded by trophoblast and syncytiotrophoblast layers
● Site of gas exchange and waste elimination
● The primary stem villi give off secondary stem villi which divide into tertiary stem villi, which form the placental lobules and insert into the basal plate
● They then give off branches to form the terminal villous units


● Present in early gestation; differentiates into villous or extravillous trophoblast (see below) and forms syncytiotrophoblasts by fusing on villous surface
● They are inconspicuous in term placenta
Micro: small, round mononuclear cells with distinct cell border, minimal clear or eosinophilic cytoplasm and single vesicular nuclei
Positive stains (early placenta): AE1/AE3 (keratin) and Ki-67 (25-50%)
Negative stains (early placenta): EMA, hCG, HLA-G, HNK-1, HPL, inhibin-alpha, Mel-CAM (CD146) and PLAP

Intermediate implantation site and extravillous (X-cells) trophoblasts:
● Infiltrate decidua and myometrium of placental site, invade and replace spiral arteries of the basal plate to establish maternal-fetal circulation and keep vessels patent
● Form trophoblastic shell
● Secrete PTH-related protein
Micro: located in the basal plate, septa and chorion lavae; morphology varies by location
In the basal plate, they are enlarged polyhedral to spindle cells with abundant amphophilic and vacuolated cytoplasmic and large, hyperchromatic nuclei and may resemble adjacent decidua
In myometrium are more spindled and resemble adjacent smooth muscle cells; may fuse to become multinucleated cells (Am J Surg Pathol 1992;16:1226)
Positive stains: cytokeratin (Mod Pathol 1990;3:282), hCG in multinucleated cells, HLA-G, hPL, Mel-CAM and PLAP (weak)
Negative stains: EMA (usually), HNK-1 and Ki-67
Micro images: Intermediate trophoblast cells

Intermediate villous trophoblast:
● Form the inner layer of the villous trophoblastic mantle
Micro: larger than cytotrophoblasts, polygonal, abundant clear or eosinophilic cytoplasm, distinct cell borders and single nuclei
Positive stains: cytokeratin, EMA, HLA-G, hPL and MEL-CAM (in cells with eosinophilic cytoplasm; towards distal end only), PLAP (in clear cells), Ki-67 (3-10%; >90%)
Negative stains: EMA, hCG, hPL (may be weak), PLAP and HNK-1
Additional References: Am J Surg Pathol 2002;26:914

● Form the outer layer of the villous trophoblastic mantle
● Synthesize and secrete hCG, hPL
Micro: multinucleated giant cells with abundant eosinophilic or basophilic cytoplasm, often with multiple intracytoplasmic vacuoles and dense pyknotic nuclei
Positive stains: hCG, hPL and inhibin-alpha
Negative stains: HLA-G, Ki-67, Mel-CAM and PLAP
EM description: vacuoles are due to dilated endoplasmic reticulum and lacunae from plasma membrane infoldings

Hofbauer cells:
● Fetal macrophages located in villous stroma
Micro: round-to-ovoid cells with eccentric nuclei and granular cytoplasm

End of Placenta > Normal histology

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