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Placenta

Placental development

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

Implantation
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● The blastocyst implants on postovulation day 6-7; by day 10, ovum is implanted in stroma
● The trophoblasts proliferate and erode the maternal capillaries and venules to form the intervillous space
● Extraembryonic mesoderm grows into the primary villi and capillary formation occurs
● By 5-6 weeks, the villous vessels are formed
● At 8 weeks, they contain nucleated red blood cells (nRBC) which diminish to 10% by 10 weeks and are absent at 12 weeks

First Trimester
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Implantation and Arterial Plugs:
● The extravillous intermediate trophoblasts invade the endometrium while endovascular trophoblasts grow into arteries and form cellular plugs

Villous morphology:
● Early mesenchymal villi are large (170 microns) with scant connective tissue, few Hofbauer cells, no thick-walled vessels
● They have a complete outer layer of syncytiotrophoblast and an inner cytotrophoblast layer
● During mid-trimester, they mature to immature intermediate villi which have loose stroma with many Hofbauer cells and a complete trophoblastic coat
● Then transform to stem villi, which have denser stroma and thick-walled vessels
● This process continues through the second trimester

Micro images:

Chorionic villi covered by cytotrophoblast and syncytiotrophoblast

Second Trimester
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Implantation and Vascular Remodeling:
● The invading trophoblasts extend into the myometrium while endovascular trophoblasts invade the arterial walls and destroy their endothelium and media, replacing them with fibrinoid material, creating a low-pressure circulation

Villous morphology:
● Mesenchymal villi give rise to mature intermediate villi, from which terminal villi sprout
● Mature intermediate villi are large with loose stroma, capillaries, arterioles and venules
● Terminal villi appear near the end of the trimester and are much smaller (70 microns) with denser stroma surrounded primarily by syncytiotrophoblasts and a thin cytotrophoblast layer that may have syncytial knots
● Vessels are numerous (3-5 capillaries per villous) and in contact with the trophoblastic coat

Micro images:

"Syncytial knots" and pink fibrin

Third Trimester
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Villous morphology:
● Mature intermediate and terminal villi are now more prevalent and smaller than second trimester with thin stroma
● More have syncytial knots (approximately 30%) and vasculosyncytial membranes (fused fetal capillaries with syncytiotrophoblasts)
● Trophoblastic inclusions are common

Micro images:

       
Vascularized chorionic villi and hCG immunostain

Mean placental weight by gestational age
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● Prior to 28 weeks: 253 grams
● 28-32 weeks: 314 grams
● 33-36 weeks: 391 grams
● 37-40 weeks: 456 grams
● >40 weeks: 496 weeks

End of Placenta > Placental development

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