Placenta

General

Anatomy & histology-placenta & umbilical cord



Topic Completed: 1 June 2011

Minor changes: 5 October 2021

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PubMed Search: Placenta histology [title]

Mandolin S. Ziadie, M.D.
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Cite this page: Ziadie MS. Anatomy & histology-placenta & umbilical cord. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/placentanormalhistology.html. Accessed December 8th, 2021.
Definition / general
  • Placenta: fetomaternal organ involved in nutrition, waste elimination and gas exchange between the developing fetus and mother Anatomy & histology-placenta & umbilical cord
  • 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
    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:
  • 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

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