UNSW Embryology

Placenta Histology

© Dr Mark Hill (2011)

Acknowledgements

Introduction

Placenta stucture can be observed by studying its histology. First, lets look at the gross structure of the embryo, its membranes and the developing placenta.

The embryo can be seen enclosed in the amniotic membrane with tthe umbilical cord to the left. Within the cord the placental blood vessels can be seen branching into finer vessels before they enter the fetal side of the main placental structure. The fetal side of the placenta is relatively smooth and is continuous with the choriononic membrane. To the far left of the image, placental villi can be seen radiating out from placenta facing towards the maternal side. Note also the small yolk sac (bottom centre) covered in a fine network of anastomosing vitelline blood vessels.	Placental (umbilical) Cord cross-section. Note that post-parturition significant changes in blood vessel calibre and the volume occupied by Whatron's jelly.	Fetal anchoring Villi of Placenta with Maternal Decidua. Now look at histological sections through both the placenta and cord.

Page Links: Introduction | Human Placental Cord | Placental Vein | Placental Artery | Placental Allantois | Human Placental Cord 3 months | Computer Activities | Placental Membrane | Wharton's jelly | Anchoring Villi | Placental Histology Links | Blood flow through the Embryo | Terms | References | About Notes

Human Placental Cord

Placental (umbilical) cord in cross-section following parturition. Note the changes which occur at parturition involving shutting down of placental circulation; smooth muscle contraction of vessel walls and expansion of surrounding Wharton's jelly. This section close to the fetus also shows the allantois.     Note - an earlier version of this online image had transposed the labelling of vein and artery, the current version is now correct.

Human Placental Vein

Placental vein (umbilical) in cross-section following parturition. Note the spaces in Wharton's jelly surrounding the vein that show the significant decrease in vein size that has occurred.

Human Placental Artery

Placental (umbilical) Artery in cross-section following parturition. Note the surrounding Wharton's jelly and the folded intima of the artery. Approximately 25% of all infants born with only one umbilical artery have associated congenital anomalies.

Human Placental Allantois

	The allantois is a diverticulum from the hindgut which extends from the superior end of the developing bladder into the adjacent placental cord. Image is a cross-section of the allantois within the placental cord. The inner layer is the endoderm surrounded by extraembryonic mesoderm.
	Placenta (Umbilicus) showing the relationship of allantois to the bladder and cloaca (common urogenital sinus). Within the fetus this will degenerate to form a ligament-like remnant, the urachus.

Wharton's jelly

Wharton's jelly is a placental cord (umbilical cord) gelatinous connective tissue composed of myofibroblast-like stromal cells, collagen fibers, and proteoglycans. It is seen at parturition when it increases in volume (myxomatous, connective tissue embedded in mucus) to assist closure of placental blood vessels. Matrix cells from Wharton's jelly have recently been identified as a potential source of stem cells. This placental cord substance is named after Thomas Wharton (1614-1673) an English physician and anatomist who first described it.

Human Placental Cord (approx 3 month)

	Umbilical Vein Umbilical Artery
Human placenta with paired umbilical arteries, and a single umbilical vein. (Gomori Trichrome stain)

 

Placental Membrane

Placental Membrane (3 month)

Fetal Side Villi generally cut in cross-section showing trophoblastic shell with mesenchyme core. Shell (dark border) consists of an outer variable thickness of syncitiotrophoblasts with and inner layer of cytotrophoblast cells. Note region closest to maternal side shows large syncitiotrophoblast clumps (dark purple regions). Core (pale interior, space is artefact) consists of mesenchymal cells forming an undifferentiated connective tissue with fetal blood vessels coursing through this region.

Maternal Side Notice the decidual reaction (cellular changes) and fibrin deposits (bright magenta/red) close maternal surface on right of image lumenal space of endometrial glands with precipitated contents. Compare the thickness of the endometrial layer with that seen during the menstrual cycle.

Anchoring Villi

Image shows early placenta interface between the fetal villi and the maternal decidua. The villi are anchored to the maternal decidua by a column of trophoblastic cells at the tip of each villi. The remainder of each villi is covered in a trophoblast shell consisting of a continuous layer of cytoptrophoblast cells and a discontinuous layer of syncitiotrophoblast cells. Other villi (not shown) are floating, in that they do not anchor to the decidua and "float" in the maternal blood-filled lacunae (space).

Placenta Histology Links

Note that class computers have the Histology programs "Fabric of Life" and "Neocortex" which both contain labelled images of placenta and female reproductive system (these are not available on the WWW).

Virtual Slidebox of Histology - Female genital tract

Blue Histology - Female Reproductive System

Boston University Histology - Female Reproductive System

University of Ottawa - Histology - Placenta

Blood flow through the Embryo

Maternal Blood | -> umbilical vein -> liver -> anastomosis -> sinus venosus -> atria ventricles-> truncus arteriosus -> aortic sac -> aortic arches-> dorsal aorta-> pair of umbilical arteries | Maternal Blood

This is shown in the image stage 13/14 pig G6 section.

Development Overview

Placental Blood vessels - form initially in the connecting stalk (then umbilical cord).

• anastomose in chorioni
• extend maternally toward chorionic villi
• extend embryonically to the sinus venosus and dorsal aorta

Arteries

• paired
• carry deoxygenated blood (from dorsal aorta) and waste products to the placental villi

Veins

• paired initially then only left at end of embryonic period
• carry oxygenated blood to the embryo (sinus venosus)

Glossary of Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Old Glossary

Placenta Development Terms

• anastomose
• angiogenesis development of new vessels from already existing vessels, this process is secondary to vasculogenesis which is the initial formation of first blood vessels by differentiation of pluripotent mesenchymal cells (extraembryonic mesoderm).
• angioblasts form clusters or blood islands on surface of yolk sac.
• capsularis
• chorionic sac fetal membrane that surrounds the developing embryo.
• cord knotting umbilical cord knotting occurs in 1%, prevents the passage of placental blood. pseudoknots also occur usually with no effect.
• cotyledons maternal side cobblestone appearance, originally placental septa formed grooves is covered with maternal decidua basalis.
• cytotrophoblast extraembryonic cells of trophoblastic shell surrounding embryo, contribute to villi and placental membranes.
• decidua basalis reaction occurs in maternal endometrium at site of, and following, blastocyst implantation. Seen as a deposition of glycogen and proliferation of blood vessels. (see also decidualization)

• decidualization process by which uterine stromal cells differentiate in response to both steroid hormones and embryonic signals into large epitheliod decidual cells. This process is essential for the progress of implantation and establishing fetal-maternal communication.

• endocrine function of placenta:
  • Human chorionic gonadotrophin (hCG) like leutenizing hormone, supports corpus luteum
  • Human chorionic somatommotropin (hCS) or placental lactogen, stimulate mammary development
  • Human chorionic thyrotropin (hCT)
  • Human chorionic corticotropin (hCACTH)
  • progesterone and estrogens support maternal endometrium
  • relaxin- role in parturition, softens ligaments
• fetal drug addiction - occurs when drugs used maternally cross the placental barrier and can establish addiction in the unborn fetus.
• fetal erythroblastosis - (Haemolytic Disease of the Newborn), an immune problem from fetus Rh+ /maternal Rh-, leakage from fetus causes anti-Rh antibodies, which is then dangerous for a 2nd child.
• frondosum -
• haemocytoblasts - (hemangioblast) stem cells for embryonic blood cell formation, often appearing as a "cluster" or "island".
• Haemolytic Disease of the Newborn - see fetal erythroblastosis.
• Hofbauer cells - placental villi macrophages of mesenchymal origin with potentially additional functions (vasculogenesis/angiogenesis, villi remodeling, regulation of stromal water content) to their macrophage role.
• Human chorionic gonadotrophin - (hCG) like leutenizing hormone, supports corpus luteum
• Human chorionic somatommotropin - (hCS) or placental lactogen stimulate mammary development
• Human chorionic thyrotropin - (hCT) placental derived hormone equivilant to thyroid
• Human chorionic corticotropin - (hCACTH) placental derived hormone equivilant to corticotropin (ACTH) from the pituitary.
• maternal antibodies - antibodies from the mother's immune system that are capable of crossing placental barrier. They can provide immune protection to the embryo, but may also participate in immune disease (fetal erythroblastosis).
• maternal sinusoids - placental spaces around chorionic villi that are filled with maternal blood. Closest maternal/fetal exchange site.
• Nitabuch's layer (fibrinoid layer) layer formed at maternal/fetal interface during placentation and is thought to act to prevent excessively deep conceptus implantation. Fibrin-type fibrinoid (maternal blood-clot product) and matrix-type fibrinoid (secreted by invasive extravillous trophoblast cells).
• placenta- (Gk. plakuos= flat cake) describes its typical mature discoid shape (20cm diameter and 3 cm thick at term,weighs 500-600 gm).
• placenta accreta - abnormal, adherence with absence of decidua basalis.
• placental arteries - paired, carry deoxygenated blood and waste from the embryo (dorsal aorta->internal iliacs->PA)
• placental blood- blood found within the placental vessels. Obviously part of the fetal blood, but can be collected at birth for theraputic use containing blood stem cells (see cord blood banks).
• placental blood vessels - form initially in the connecting stalk (then umbilical cord), anastomose in chorioni and extend maternally toward chorionic villi, extend embryonically to the sinus venosus and dorsal aorta.
• placental layers - 4 layers separate maternal and fetal blood: syncitiotrophoblast, cytotrophoblast, villi connective tissue, and fetal capillary endothelium.
• placenta percreta - abnormal, villi penetrate myometrium.
• placenta previa - placenta overlies internal os of uterus, abnormal bleeding, may require cesarian delivery.
• placental veins - paired initially then usually only one left at end of embryonic period, carry oxygenated blood to the embryo (sinus venosus)
• primary villi - develop week 2, consist of trophoblastic shell cells both syncitiotrophoblasts and cytotrophoblasts. Form finger-like extensions into the maternal endometrium.
• protein hormone - usually a protein distributed in the blood that binds to membrane receptors on target cells in different tissues. Do not easliy cross placental barrier.
• relaxin - hormone.
• secondary villi - develop week 3, extraembryonic mesoderm grows into villi, initially covers entire surface of chorionic sac.
• sinus venosus - cavity into which all major embryonic paired veins supply (vitelline, placental, cardinal)
• syncitiotrophoblast - extraembryonic cells of trophoblastic shell surrounding embryo, outside the cytotrophoblast layer, involved with implantation of the blastocyst by eroding extracellular matrix surrounding maternal endometrial cells at site of implantation, also contribute to villi. (dark staining, multinucleated)
• tertiary villi - develop week 4, mesenchyme within secondary villi differentiates into blood vessels and cells, forms arteriocapillary network, fuse with placental vessels developing in connecting stalk.
• trophoblast -
• umbilical cord - fetal attachment cord 1-2 cm diameter, 30-90cm long, covered with amniotic attached to chorionic plate, umbilical vessels (artery, vein) branch into chorionic vessels. Vessels anastomose within the placenta.
• vasculogenesis formation of first blood vessels by differentiation of pluripotent mesenchymal cells (extraembryonic mesoderm) followed by angiogenesis which is the development of new vessels from already existing vessels.
• villi - initially outgrowth of the trophoblastic shell which involve other tissues with development. Develop in sequence (primary, secondary, tertiary) with mature villi being stem por branched type.
• virus - small infectious agent able to cross placental barrier. Can infect embryo and cause developmental abnormalities. (e.g. cytomegalovirus, rubella, measles)
• vitelline- Blood vessels cover entire surface of yolk sac and connect to embryo through yolk stalk
  • Vitelline Arteries - arises from dorsal aorta and contribute to adult GIT arteries.
  • Vitelline Veins - empties into sinus venosus and contribute to the adult portal system.
• waste products - products of cellular metabolism and cellular debris, e.g.- urea, uric acid, bilirubin
• Wharton's jelly placental cord (umbilical cord) gelatinous connective tissue composed of myofibroblast-like stromal cells, collagen fibers, and proteoglycans. Increases in volume (myxomatous, connective tissue embedded in mucus) at parturition to assist closure of placental blood vessels. Matrix cells from Wharton's jelly have recently been identified as a potential source of stem cells. This placental cord substance is named after Thomas Wharton (1614-1673) an English physician and anatomist who first described it.

Quick Links

Placenta Pages:

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- All Systems Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Renal Genital Musculoskeletal Integumentary

Movies ----------------- Movies Quicktime Flash Audio Week 1 Week 2 Week 3 Stage 13 Stage 22 Neural Heart Gastrointestinal Renal Genital Other Embryos

Other Embryos ----------------- All Animals Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G