Overview
This lecture traces the formation of the extraembryonic membranes and placenta from implantation through to term: the decidual reaction, formation of the amniotic and chorionic sacs, differentiation of the trophoblast into primary, secondary and tertiary chorionic villi, resulting placental structure and function, chorion frondosum versus smooth chorion, placental anomalies, amniotic fluid and its functions, umbilical cord anatomy, the mechanisms and complications of twin pregnancies, and the two main methods of prenatal diagnosis.
Decidual reaction
- After implantation the endometrium is called the decidua.
- Decidual cells contain lipids and glycogen.
- They secrete prolactin and IGF (“uterine milk”) for early nutrition of the conceptus.
- They secrete protease inhibitors that modulate the degrading action of the trophoblast.
- The decidua has three regions: basal, capsular and parietal. The capsular and parietal layers fuse by 5 months.
Implantation and formation of the amniotic and chorionic sacs
Sequence from the blastocyst diagrams:
- Blastocyst: an outer cell mass (OCM) surrounds the blastocyst cavity, with the inner cell mass (ICM) at one pole.
- The ICM pole differentiates into an outer syncytiotrophoblast and an inner cytotrophoblast.
- The blastocyst implants into the endometrium, gaining access to maternal vessels and glands.
- An amniotic cavity forms at the former ICM pole; a yolk sac occupies the rest of the original blastocyst cavity.
Further structures formed at this stage:
- Trophoblastic lacunae fill with maternal blood.
- The hypoblast forms the exocoelomic (Heuser’s) membrane and lines the primitive yolk sac (extracoelomic cavity); amnioblasts and epiblast line the amniotic cavity.
- Extraembryonic mesoderm and extraembryonic coelom appear between the trophoblast and the amnion/yolk sac complex.
- A coagulation plug seals the point of entry into the endometrial epithelium.
As development proceeds, the embryo lies within the amniotic cavity, and the chorionic cavity (bounded by the chorion) surrounds it; the yolk sac initially occupies the chorionic cavity. At a more developed stage, the embryo hangs within the amniotic cavity, suspended by the connecting stalk/umbilical cord; villous stems of the chorion frondosum project into the vascularised decidua basalis; the decidua capsularis and parietalis surround the rest of the sac; the myometrium forms the outer uterine wall; a mucus plug seals the cervical canal.
One diagram (paired blue/yellow 3D shapes, showing the amniotic sac apparently sitting atop the yolk sac/chorionic sac) is unlabelled; its exact meaning is not made explicit by the slide.
The four extraembryonic membranes are the amnion, chorion, yolk sac and allantois.
Differentiation of the trophoblast into chorionic villi
- The trophoblast invades the endometrium as chorionic villi.
- Lacunar spaces develop within the trophoblast and become confluent, forming the intervillous space, which fills with maternal blood from ruptured endometrial vessels.
Three stages of villus maturation:
- Primary villi: syncytiotrophoblast and cytotrophoblast outgrowths only.
- Secondary villi: same trophoblast layers, now surrounding a mesodermal core.
- Tertiary villi: same layers, now with blood vessels (villous capillaries) inside the mesoderm core.
The slide immediately following the villus-maturation diagrams rendered as a solid black image with no extractable text; its content could not be transcribed.
Placenta: structure and functions
Structure:
- Maternal side: adherent to the decidua, rough, forms cotyledons.
- Fetal side: connective tissue containing fetal vessels, smooth, lined by amnion.
- The endometrium dips between cotyledons to form septa, anchoring the placenta and conveying maternal blood vessels.
- Cross-section, maternal to fetal: myometrium, decidua basalis (divided by placental septa), spiral arteries and cytotrophoblastic shell delivering maternal blood into the intervillous space, anchoring villi, branching stem villi carrying fetal circulation, amnion, smooth chorion, decidua parietalis.
- On the delivered placenta: the maternal side is lobulated, dark red/purple, divided into cotyledons; the fetal side is smoother and pale, with a branching vessel network radiating from the umbilical cord insertion point.
Functions:
- Exchange of nutrition, gas and metabolites.
- Transfer of substances: antibodies (maternal IgG, from 14 weeks), drugs (e.g. thalidomide, alcohol), pathogens (e.g. Listeria, HIV).
- Endocrine function: produces progesterone, oestrogen, hCG and hCS (human placental lactogen, hPL).
Chorion frondosum versus smooth chorion
- First two months: villi present all over the chorion.
- From the third month: chorion in contact with the decidua basalis develops extensively into the Chorion frondosum (villous chorion); chorion in contact with the decidua capsularis atrophies into the smooth chorion (chorion laeve).
Placental anomalies
- Membranous/diffuse placenta.
- Accessory placenta.
- Placental abruption: premature separation of the placenta from the uterine wall.
- Placenta previa: placenta implants low in the uterus, covering or near the internal cervical os (contrasted with a normal placenta implanted at the fundus/upper wall). [slide does not elaborate further on mechanism or presentation]
Amniotic fluid
- Amniotic fluid is the immediate surface contact for the growing fetus.
- Sources: amnion, maternal and fetal blood filtrates, fetal lung secretions, fetal urine.
- Volume increases across gestation: 10 ml by 8 weeks, 30 ml by 10 weeks, 350 ml by 20 weeks, 1 litre by 37 weeks. Abnormally low volume is oligohydramnios; abnormally high is polyhydramnios.
Functions:
- Buoyancy for the growing fetus.
- Shock absorption/protection.
- Symmetrical growth of the embryo.
- Prevents adhesion of the amnion to the embryo.
- Free fetal movement.
- Maintains body temperature.
- Contains pluripotent stem cells.
Umbilical cord
- Made up of Wharton’s jelly, lined by the amniotic membrane.
- Length: 45-50 cm.
- Contains two umbilical arteries and one (left) umbilical vein, plus a remnant of the allantois.
Twins and multiple pregnancies
Dizygotic twins (dichorionic-diamnionic):
- Two separate zygotes each undergo cleavage and form their own blastocyst, implant separately, and each develops its own chorionic sac, amnion and placenta (two chorions, two amnions, separate vascular supplies).
- Variant: if the two zygotes implant close together, the chorionic sacs and placentas are fused at the margin but remain separate (still two chorions and two amnions).
Monozygotic twins, monochorionic-diamnionic:
- A single zygote forms two embryoblasts within one blastocyst, which implants as a single conceptus, producing one placenta shared by two amniotic sacs (one chorion, two amnions).
- Anastomoses can form between the placental vessels, creating arteriovenous communications between the two fetal circulations; this is the mechanism of Twin-to-twin transfusion syndrome.
Monozygotic twins, monochorionic-monoamnionic:
- A single blastocyst divides within one chorionic sac and one amniotic sac (one chorion, one amnion shared).
- Possible outcomes: (A) separate twins sharing a single placenta, chorion and amnion; (B) conjoined twins, from incomplete separation of the embryonic disc; (C) parasitic twin, from highly asymmetric/incomplete division.
- Because both membranes are shared, these twins are at risk of umbilical cord entanglement.
Prenatal diagnosis
- Amniocentesis: performed at 14-15 weeks of pregnancy; approximately 15-20 ml of amniotic fluid is aspirated transabdominally under ultrasound guidance.
- Chorionic villus sampling: performed at 10-12 weeks of pregnancy, earlier than amniocentesis; a catheter is passed transcervically under ultrasound guidance to sample chorionic villus tissue.
Self-test
- Describe the changes that occur in the endometrium after implantation, including the secretions of decidual cells and their functions.
- List the three regions of the decidua and state when the capsular and parietal layers fuse.
- Describe the sequence of events from the blastocyst stage to the formation of the amniotic cavity and yolk sac.
- Which cell layer forms the exocoelomic (Heuser’s) membrane and lines the primitive yolk sac?
- List the four extraembryonic membranes.
- Describe the steps of chorionic villus maturation from primary to tertiary villus, stating the distinguishing feature of each.
- Explain how the intervillous space forms and what fills it.
- Distinguish the maternal side of the placenta from the fetal side.
- Describe the three main functions of the placenta, giving an example of each type of substance transferred.
- Distinguish chorion frondosum from smooth chorion (chorion laeve) in terms of location and development.
- Distinguish placental abruption from placenta previa.
- List the sources of amniotic fluid.
- What is the approximate volume of amniotic fluid at 20 weeks and at 37 weeks, and what terms describe abnormally low or high volumes?
- List the functions of amniotic fluid.
- Describe the structure and contents of the umbilical cord.
- Distinguish the chorionicity and amnionicity of dizygotic twins from monozygotic monochorionic-diamnionic twins.
- Explain the mechanism of twin-to-twin transfusion syndrome.
- Describe the three possible outcomes when division occurs within a single chorionic and amniotic sac (monochorionic-monoamnionic twins), and the risk associated with this type of twinning.
- Distinguish amniocentesis from chorionic villus sampling in terms of timing and technique.
- A twin pregnancy is found to be monochorionic-diamnionic. Explain why this places the fetuses at risk of twin-to-twin transfusion syndrome, and why a dichorionic-diamnionic twin pregnancy would not carry this risk.
Answers
Reveal answers
- After implantation the endometrium is called the decidua; decidual cells contain lipids and glycogen and secrete prolactin and IGF (uterine milk) for early nutrition of the conceptus, and protease inhibitors that modulate the degrading action of the trophoblast.
- Basal, capsular and parietal regions; the capsular and parietal layers fuse by 5 months.
- The outer cell mass surrounds the blastocyst cavity with the inner cell mass (ICM) at one pole; the ICM pole differentiates into an outer syncytiotrophoblast and inner cytotrophoblast; the blastocyst implants into the endometrium, gaining access to maternal vessels and glands; an amniotic cavity then forms at the former ICM pole while a yolk sac forms from the remainder of the original blastocyst cavity.
- The hypoblast.
- Amnion, chorion, yolk sac and allantois.
- Primary villus: outgrowth of syncytiotrophoblast and cytotrophoblast only; secondary villus: same trophoblast layers now surrounding a mesodermal core; tertiary villus: same layers, now containing blood vessels within the mesoderm core.
- Lacunar spaces develop within the trophoblast and become confluent, forming the intervillous space, which fills with maternal blood from ruptured endometrial vessels.
- Maternal side: adherent to the decidua, rough, forms cotyledons. Fetal side: connective tissue with fetal vessels, smooth, lined by amnion.
- Exchange of nutrition, gas and metabolites; transfer of substances (e.g. maternal IgG antibodies from 14 weeks, drugs such as thalidomide and alcohol, pathogens such as Listeria and HIV); endocrine function (progesterone, oestrogen, hCG, hCS/hPL).
- Chorion frondosum is the villous chorion that develops extensively where it contacts the decidua basalis from the third month; smooth chorion (chorion laeve) is the atrophied chorion where it contacts the decidua capsularis.
- Placental abruption is premature separation of a normally sited placenta from the uterine wall; placenta previa is a placenta implanted low in the uterus, covering or near the internal cervical os.
- Amnion, maternal and fetal blood filtrates, fetal lung secretions, fetal urine.
- About 350 ml at 20 weeks and about 1 litre at 37 weeks; too little is oligohydramnios, too much is polyhydramnios.
- Buoyancy, shock absorption/protection, allows symmetrical embryo growth, prevents amnion-embryo adhesions, allows free fetal movement, maintains body temperature, contains pluripotent stem cells.
- Made up of Wharton’s jelly, lined by the amniotic membrane; 45-50 cm long; contains two umbilical arteries and one (left) umbilical vein, plus a remnant of the allantois.
- Dizygotic twins are dichorionic-diamnionic (two chorions, two amnions, from two separate zygotes/blastocysts implanting separately); monozygotic monochorionic-diamnionic twins share a single placenta and chorion but each has its own amnion (two embryoblasts within one blastocyst).
- In monochorionic twins sharing one placenta, anastomoses can form between the placental vessels, creating arteriovenous communications between the two fetal circulations, allowing unbalanced blood flow between the twins.
- (A) Separate twins sharing a single placenta, chorion and amnion; (B) conjoined twins, from incomplete separation of the embryonic disc; (C) parasitic twin, from highly asymmetric/incomplete division. Because both membranes are shared, there is a risk of umbilical cord entanglement.
- Amniocentesis is performed at 14-15 weeks, aspirating about 15-20 ml of amniotic fluid transabdominally under ultrasound guidance; chorionic villus sampling is performed earlier, at 10-12 weeks, via a transcervical catheter under ultrasound guidance.
- Monochorionic-diamnionic twins share one placenta, so vascular anastomoses can form between the two fetal circulations, risking unbalanced blood flow (twin-to-twin transfusion syndrome); dichorionic-diamnionic twins have entirely separate placentas and vascular supplies, so no such shared-circulation risk exists.