Placenta

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Contributed by Marta Cohen, M.D.

Chorionic plate (fetal surface)

• Covered by a layer of amnion and chorion
• Accumulation of fibrin beneath the chorionic plate is a common finding at term and is visible as tan-white plaques
• Primary stem villi protrude perpendicularly downward from the chorionic plate
• Microscopically, the chorionic plate and stem villi, both primary and secondary, are composed of dense collagenized connective tissue surrounding thick walled arteries and veins

Basal plate (maternal surface)

• Consists of fibrin, extravillous trophoblast cells, decidua basalis and maternal blood vessels
• Endometrial spiral arteries undergo physiological transformation characterized by lumen dilation, extravillous trophoblast invasion of the endothelial lining and fibrinoid replacement of the muscular and elastic tissue of the arterial wall (J Perinat Med 2006;34:447)
• Therefore, maternal spiral arteries transform from high resistance, low capacitance to low resistance, high capacitance vessels
• Maternal veins generally run parallel to the basal plate and are less conspicuous
• Contour of the basal plate is coarsely folded, forming clefts known as placental septa that protrude into the parenchyma and can extend as far as the fetal surface
• Insertion site of the anchoring villi into maternal endometrium
• Invasive intermediate trophoblasts are seen here

Membranes

• Composed of amnion, intermediate or spongy layer (reticular zone that represents remnants of the coelomic cavity), chorion laeve and chorion frondosum
• Membrane layers adhere together but can be peeled apart with minimal effort
• Amnion: innermost covering of amniotic cavity that is closest to the fetus; thin, avascular layer composed of cuboidal to low columnar epithelial cells that rest on a thin basement membrane with underlying thin band of loose connective tissue (spindled to stellate mesenchymal cells)
  • May show squamous metaplasia (plaques of keratinizing, stratified squamous epithelium), especially near umbilical cord insertion
  • Present as small gray nodules of 3 - 4 mm in diameter and should be distinguished from amnion nodosum
  • Squamous metaplasia is a normal variant (seen in nearly 50% of term placentas), while amnion nodosum is pathological and is strongly associated with longstanding oligohydramnios
• Intermediate or spongy layer: potential space between the amnion and chorion, which results from incomplete fusion of amnionic and chorionic mesoderm during early pregnancy; this intermediate layer is composed of loosely arranged bundles of collagen fibers with a few scattered fibroblasts, separated by a communicating system of clefts and 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 the collapse of the intervillous space during development; composed of mononuclear, sometimes vacuolated, extravillous trophoblast and scattered atrophic chorionic villi
• Decidual chorion (chorion frondosum): located in the placenta proper; it eventually fuses with the parietal decidua, thereby obliterating the uterine cavity (Front Bioeng Biotechnol 2020;8:610544)

Umbilical cord

• 2 umbilical arteries have slightly thicker, double layered muscular walls without an elastic layer (Fetal Pediatr Pathol 2005;24:297)
• 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
• A single umbilical artery is present in ~1% of cases
• Vessels branch out over the fetal surface to form the villous tree

Placental villi

• Comprised of capillaries, fetal macrophages (Hofbauer cells) and perivascular fibroblasts without associated collagen; the surrounding trophoblast bilayer consists of an outer syncytiotrophoblast layer and inner cytotrophoblast layer
• Site of gas exchange, metabolic and endocrine activities
• Dual blood supply from both the fetal and maternal circulations
• Villous trees are made up of ~40 major primary stem villi, each measuring 0.2 - 1.5 mm in diameter, which protrude perpendicularly downward from the chorionic plate; contain arteries, veins, fibrous stroma and capillaries
• Primary villi separate into 4 - 8 secondary stem villi, which travel parallel to the chorionic plate
• These secondary stem villi further divide into tertiary stem villi, which form the placental lobules and insert into the basal plate
• Villi types evolve during pregnancy: immature intermediate villi are dominant in early pregnancy; mature intermediate branch into terminal villi represent the main exchange area of the third trimester placenta

Cytotrophoblast cells

• Present in early gestation; differentiates into villous or extravillous trophoblast and forms syncytiotrophoblasts by fusing on villous surface
• During early pregnancy, this layer is nearly complete but later in gestation it becomes discontinuous and is inconspicuous in term placenta
• Cuboidal, polyhedral or ovoid cells with well demarcated cell borders and large, lightly staining nuclei containing finely dispersed chromatin
• Cytoplasm is usually clear to slightly granular and somewhat basophilic
• Mitotic figures are occasionally found

Extravillous (X cells) or intermediate trophoblast cells

• Old literature described extravillous trophoblast as intermediate trophoblast, as the morphology appeared intermediate between cytotrophoblast and syncytiotrophoblast
• 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 (Cell Mol Life Sci 2019;76:3479)
• Form trophoblastic shell
• Secrete parathyroid hormone (PTH) related protein
• Located in the basal plate, septa and chorion laeve; 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; may resemble adjacent decidua
• In myometrium, they are more spindled and resemble adjacent smooth muscle cells; may fuse to become multinucleated cells

Intraplacental trophoblast islands

• Form the inner layer of the villous trophoblastic mantle and play an important role in maintaining the structural integrity of the villi
• Serve as a source from which implantation site and chorionic type intermediate trophoblast cells are derived
• Larger than cytotrophoblasts, mononucleate, polygonal, abundant pale to clear cytoplasm, distinct cell borders and single nuclei
• Frequently, the islands contain villous remnants with perivillous fibrin

Syncytiotrophoblast cells

• Single, continuous, normally uninterrupted layer that extends over the surfaces of all villous trees and chorionic villi as well as over parts of the inner surfaces of chorionic and basal plates
• Synthesize and secrete hCG, hPL
• Multinucleated giant cells with abundant homogeneous to finely granular, somewhat basophilic cytoplasm, often with multiple intracytoplasmic vacuoles and small dense pyknotic nuclei with moderately dense chromatin

Hofbauer cells

• Fetal macrophages located in villous stroma
• 10 - 35 μm in diameter
• Present from 18 days postconception and throughout gestation
• 4 subtypes according to differences in the expression of major histocompatibility complex (MHC) type II complement receptors, lectins, lipopolysaccharide coreceptor (CD14) and CD68 (Anal Quant Cytopathol Histpathol 2013;35:283)
• Round to ovoid cells with eccentric nuclei and granular cytoplasm with many vacuoles containing flocculent material and occasional osmiophilic bodies
• Their most important function is the immunological defense of the fetus and the prevention of vertical transmission of pathogens

Contributed by Marta Cohen, M.D.

Placenta product of a concealed pregnancy is submitted for assessment of the gestational age. The trimmed placental weight is 51 grams. The histology shows a prevalence of intermediate immature villi with presence of extravillous (intermediate) trophoblast. What is the most likely gestational age of the placenta?

1. 12 weeks
2. 26 weeks
3. 36 weeks
4. 40 weeks
5. 42 weeks

A. 12 weeks gestation. Immature intermediate villi are dominant in early pregnancy (first trimester). The presence of extravillous trophoblast invades the uterus during implantation. Answers B - E are incorrect because these features would likely not be present by the gestational age listed.

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Reference: Anatomy & histology-placenta & umbilical cord

In which placental compartment is the Wharton jelly present?

1. Basal plate
2. Chorionic plate
3. Membranes
4. Umbilical cord
5. Villi

D. Umbilical cord. Intervening Wharton jelly contains loose ground substance, a fine network of microfibrils and scattered nucleated cells (predominantly macrophages, myofibroblasts and mast cells). Answers A - C and E are incorrect because Wharton jelly is not present in these placental compartments.

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Reference: Anatomy & histology-placenta & umbilical cord