Placenta
General
Anatomy & histology-placenta & umbilical cord
Author: Mandolin S. Ziadie, M.D.
Last author update: 1 June 2011
Last staff update: 3 May 2023 (update in progress)
Copyright: 2002-2023, PathologyOutlines.com, Inc.
PubMed Search: Placenta histology [title]
Table of Contents
Definition / general | Essential features | Terminology | Diagrams / tables | Clinical features | Gross description | Gross images | Basal plate (maternal surface) | Chorionic plate (fetal surface) | Placental villi | Cells | Umbilical cordCite this page: Ziadie MS. Anatomy & histology-placenta & umbilical cord. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/placentanormalhistology.html. Accessed May 30th, 2023.
Definition / general
- Placenta: fetomaternal organ involved in nutrition, waste elimination and gas exchange between the developing fetus and mother
- Umbilical cord: anatomic tubular structure that physically connects the developing intrauterine fetus to the placenta, which is anchored to the maternal uterine wall
Essential features
- Umbilical cord: functions as the conduit by which oxygenated, nutrient rich blood from the mother (via the intervening placenta) reaches the fetus; returns deoxygenated, nutrient poor blood from the fetus back to the mother
Terminology
- Umbilicus (or navel) is the attachment site of the umbilical cord to the fetus
- Two umbilical arteries in the umbilical cord return deoxygenated blood to the mother
- One umbilical vein in the umbilical cord carries oxygenated blood to the fetus
- Wharton jelly is the mucopolysaccharide rich gelatinous substance that provides turgor regulation to the umbilical cord and in which the two umbilical arteries and single umbilical vein are embedded
Diagrams / tables
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Placenta
Clinical features
Umbilical cord
- Derived from the allantois and yolk sac during the fifth week of fetal development
- Fetoplacental circulation (connection) develops when allantoic vessels establish continuity with the developing villi of the placenta
- Allantoic duct remnant is seen in about 15% of umbilical cords
- Omphalomesenteric duct remnant is seen in about 1.5% of umbilical cords
- Two umbilical veins are initially present but one atrophies during the second month of pregnancy
- Two umbilical arteries, in the vast majority of deliveries (96%), anastomose with 1.5 cm of the placental insertion site
Gross description
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Parts of placenta
- Disc: composed of fetal portion (chorionic plate) and maternal portion (basal plate decidua)
- Divided into cotyledons from primary stem villi and lobules from secondary stem villi
- Average size at term: 22 cm diameter, 2.0 - 2.5 cm thickness, 470 g
Membranes: composed of amnion, exocoelomic space, chorion and decidual capsularis
- Usually insert directly onto the placental edge
- Amnion: innermost covering of amniotic cavity; composed of flat epithelial cells that rest on a thin basement membrane with underlying thin band of loose connective tissue; may show squamous metaplasia, especially near cord or in pregnancies complicated by oligohydramnios
- Exocoelomic space: potential space between the amnion and chorion which is usually obliterated but allows the membranes to slide against each other
- Chorion: connective tissue membrane containing fetal vessels, internal to amnion, external to villi
- Membranous chorion (chorion laeve): formed by collapse of the intervillous space during development; composed of mononuclear, sometimes vacuolated, trophoblasts and scattered atrophic chorionic villi
- Decidual chorion (chorion frondosum): located in the placenta proper
Umbilical cord:
- Twisted cable that connects the fetus to the placenta and carries the two umbilical arteries and a single umbilical vein
- Vessels branch out over the fetal surface to form the villous tree
- Average size: 55 - 60 cm length and 2.0 - 2.5 cm diameter in a term gestation
- Pearly white on gross examination, bordering on semitranslucent on closer inspection (due to the gelatinous nature of Wharton jelly)
- Usually coiled in a counterclockwise direction, approximately 0.1 - 0.2 coils per centimeter of length
- Insertion on the placenta is normally centrally located by midgestation but may become more eccentric as gestation proceeds
Gross images
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Basal plate (maternal surface)
Chorionic plate (fetal surface)
Basal plate (maternal surface)
- Insertion site of the anchoring villi into maternal endometrium
- Invasive intermediate trophoblasts are seen here
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Basal plate (maternal surface)
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
- 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
Cells
Cytotrophoblast:
Intermediate implantation site and extravillous (X cells) trophoblasts:
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Intermediate villous trophoblast:
Syncytiotrophoblast:
Hofbauer cells:
- 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
- Microscopic (histologic) description: 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 Ki67 (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
- Microscopic (histologic) description: 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)
- Microscopic (histologic) images:
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Intermediate trophoblast cells
- 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 Ki67
Intermediate villous trophoblast:
- Form the inner layer of the villous trophoblastic mantle
- Microscopic (histologic) description: 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), Ki67 (3 - 10%; > 90%)
- Negative stains: EMA, hCG, hPL (may be weak), PLAP and HNK-1
- Additional references: Am J Surg Pathol 2002;26:914
Syncytiotrophoblast:
- Form the outer layer of the villous trophoblastic mantle
- Synthesize and secrete hCG, hPL
- Microscopic (histologic) description: 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, Ki67, MEL-CAM and PLAP
- Electron microscopy description: vacuoles are due to dilated endoplasmic reticulum and lacunae from plasma membrane infoldings
Hofbauer cells:
- Fetal macrophages located in villous stroma
- Microscopic (histologic) description: round to ovoid cells with eccentric nuclei and granular cytoplasm
Umbilical cord
- Two umbilical arteries have slightly thicker, double layered muscular walls without an elastic layer
- Single umbilical vein has a thinner muscular wall containing a subintimal elastic layer
- Intervening Wharton jelly contains loose ground substance, a fine network of microfibrils and scattered nucleated cells (predominantly macrophages, myofibroblasts and mast cells)
- Umbilical cord is surfaced by a single layer of amnion, which is continuous with the surface of the placenta and the fetal skin
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Normal three vessel umbilical cord
