Embryonic Cavities & The Placenta Notes

Embryonic Cavities & The Placenta

Introduction and Tips

  • The presentation will cover embryonic membranes and cavities, as well as the placenta and related conditions.

Learning Outcomes Part 1: Embryonic Membranes and Cavities

  • LO 1: Describe the development of the umbilical region with the consequent separation of the intra- and extra-embryonic coelomic cavities.

  • LO 2: Summarise the events from blastocyst implantation and trophoblast invasion of the uterine endometrium (decidua basalis) until the obliteration of the uterine cavity.

Umbilical Region and Coelomic Cavities Development

Week 1

  • Blastocyst attaches to the endometrium.

    • The blastocyst adheres to the endometrial lining of the uterus, initiating the process of implantation. This occurs approximately 6 days after fertilization.

  • Trophoblast differentiation: The outer layer of the blastocyst begins to differentiate into two layers:

    • Inner layer of cytotrophoblast:

    • The inner cellular layer that contributes to the formation of the chorion.

    • Outer layer of syncytiotrophoblast:

    • A multinucleated layer that erodes into the endometrium, facilitating implantation and forming lacunae that will eventually contain maternal blood.

Week 2

  • Embryonic disc forms: The inner cell mass differentiates into two primary layers:

    • Primitive ectoderm (epiblast) produces amniotic epithelium:

    • The epiblast gives rise to the amnion, which encloses the amniotic cavity.

    • Primitive endoderm (hypoblast) produces umbilical vesicle epithelium:

    • The hypoblast contributes to the formation of the yolk sac, which is essential for early nutrient supply and hematopoiesis.

  • Cells form the extraembryonic mesoderm.

    • This layer of mesoderm surrounds the amnion and yolk sac, contributing to the formation of the chorion.

  • The amnion, embryonic disc, and umbilical vesicle form.

    • These structures are critical for providing a protective environment and nutrients to the developing embryo.

  • Lacunae appear in the syncytiotrophoblast.

    • These spaces fill with maternal blood, establishing the initial uteroplacental circulation.

  • Mesoderm increases, forming isolated cavities (extraembryonic coelomic spaces).

    • These cavities eventually coalesce to form the chorionic cavity.

  • Extraembryonic coelomic spaces fuse to form the chorionic cavity.

    • The chorionic cavity surrounds the amniotic cavity and yolk sac, providing space for further development.

  • The chorionic cavity splits the mesoderm into:

    • Splanchnic mesoderm: around the umbilical vesicle:

    • This layer is associated with the yolk sac and contributes to the development of the gut.

    • Somatic mesoderm:

    • This layer is associated with the trophoblast and contributes to the formation of the chorion.

  • Trophoblast layers cover the amniotic epithelium, forming the amniotic membrane.

    • The amnion provides a protective barrier for the developing embryo and produces amniotic fluid.

  • Chorionic membrane forms the wall of the chorionic sac and presents villi.

    • Chorionic villi facilitate nutrient and gas exchange between the maternal and fetal circulations.

Recap: Rule of Twos

  • 2 germ layers: epiblast and hypoblast.

  • 2 trophoblast layers: cytotrophoblast and syncytiotrophoblast.

  • 2 cavities: amniotic and chorionic.

  • 2 membranes: amnion and chorion.

Week 3

  • The hypoblast produces cells which form the primitive (primary) umbilical vesicle.

    • The primary yolk sac is formed by the migration of hypoblast cells, providing early nutritional support.

  • The definitive (secondary) umbilical vesicle is formed from epiblast at the time of gastrulation, determined by the primitive streak.

    • The secondary yolk sac is smaller and more refined, continuing to support the developing embryo.

Week 4

  • The lateral folds fuse ventrally, therefore decreasing communication between intra- and extraembryonic coela.

    • The fusion of lateral folds results in the formation of the body cavities and reduces the connection between the intraembryonic and extraembryonic spaces.

Weeks 5-8

  • Chorion differentiates into:

    • Smooth chorion (chorion laeve) at the abembryonic pole (away from the embryo):

    • This part of the chorion lacks villi and eventually fuses with the amnion.

    • Irregular chorion frondosum at the embryonic pole (where the placenta is developing):

    • This part of the chorion is characterized by numerous villi, which form the fetal part of the placenta.

Week 8

  • Uterine cavity obliteration begins as the embryo and amnion expand.

    • The expanding amnion gradually fills the uterine cavity, leading to the fusion of the amnion and chorion.

Weeks 18-22

  • The embryo floats in the expanded amniotic cavity, which holds approximately 1L of amniotic fluid at term.

    • Amniotic fluid provides a protective cushion, regulates temperature, and allows for fetal movement.

  • Uterine cavity is obliterated.

    • The decidua capsularis fuses with the decidua parietalis, eliminating the uterine cavity.

  • Fusion of decidua capsularis and parietalis occurs.

    • This fusion results in the formation of the amniochorionic membrane.

  • Amnion and chorion fuse to form the amniochorionic membrane.

    • This membrane ruptures during labor, releasing amniotic fluid.

Summaries of Development

  • Week 1

  • Week 2

  • Week 3

  • Week 5

  • Week 8

  • Weeks 18-22

Decidua

  • Modified endometrium of the uterus in a pregnant individual.

    • During pregnancy, the endometrium undergoes changes to support the developing embryo.

  • Three parts, named after their relation to the implantation site:

    • Decidua capsularis: superficial part overlying the embryo.

    • This part of the decidua surrounds the embryo and eventually fuses with the decidua parietalis.

    • Decidua parietalis: represents remaining parts of the decidua.

    • This lines the main cavity of the uterus.

    • Decidua basalis: part deep to the embryo; forms the maternal part of the placenta.

    • This part of the decidua lies between the embryo and the myometrium, contributing to the formation of the placenta.

The Placenta

Learning Objectives Part 2: Placenta

  • LO 3: Summarise the production of syncytiotrophoblast and cytotrophoblast, with regard to placental development and function.

  • LO 4: Describe the development of the placental circulation, including villi structure and function, and the development of the umbilical cord.

  • LO 5: Discuss the functional anatomy of the feto-maternal/intervascular barrier and examples of placental abnormalities.

Placenta Formation

Reminder

  • Trophoblast has 2 layers: the cellular cytotrophoblast and the multinucleate syncytiotrophoblast.

    • These layers play crucial roles in implantation, placental development, and nutrient exchange.

Days 9-11

  • The blastocyst is fully embedded and closed by a fibrin clot plug.

    • The fibrin clot seals the implantation site, preventing maternal blood from entering the blastocyst.

  • Syncytiotrophoblast starts expanding and invades the decidua.

    • This invasion establishes the initial uteroplacental circulation.

  • Lacunae (plugged) start forming at the embryonic pole, eventually allowing for placental circulation when the plug is removed (8-12 weeks).

    • These lacunae fill with maternal blood, providing nutrients to the developing embryo.

  • Primary chorionic villi are developing, made of both cyto- and syncytiotrophoblast.

    • These villi increase the surface area for nutrient and gas exchange.

Week 2 vs. Week 3

  • Week 2: Mesoderm is present.

    • Mesoderm differentiates into blood cells and blood vessels, essential for establishing the placental circulation.

  • Week 3: Mesoderm present with developing blood vessels, floating vs anchoring villi.

    • Floating villi facilitate nutrient and gas exchange, while anchoring villi provide structural support.

Week 4

  • The umbilical cord forms from the stalk, is covered by the amnion, and carries blood via the umbilical artery and vein.

    • The umbilical cord connects the fetus to the placenta, allowing for the transport of nutrients, oxygen, and waste products.

  • Cotyledons and placental septa form:

    • Irregular convex areas separated by placental septa of syncytiotrophoblast.

    • Cotyledons are functional units of the placenta, facilitating nutrient and gas exchange.

    • Consist of 2 or more stem villi and many branch villi.

Intervascular (Placental) Barrier Changes

  • Branch villus.

  • Fetal capillaries at full term.

  • Stroma.

  • Maternal blood spaces.

  • Syncytiotrophoblast.

  • Cytotrophoblast.

  • Fetal blood vessel.

Placenta at Term

  • Discoidal shape.

  • Diameter: 15−2515-25 cm.

  • Weight: 450−600450-600 g.

  • Maternal aspect is divided into 15−2015-20 cotyledons.

    • These cotyledons facilitate nutrient and gas exchange between the maternal and fetal circulations.

  • Fetal and maternal surfaces.

Maternal Blood Flow

  • Maternal blood flows into cotyledons via spiral arteries and back into maternal circulation via endometrial veins.

    • This circulation provides the fetus with essential nutrients and oxygen.

Intervillous Space

  • Surrounds trees of villi; holds 150150 ml of blood.

    • The intervillous space allows for efficient nutrient and gas exchange between the maternal blood and the fetal blood in the villi.

  • Blood vessels in branch villi hold about 5050 ml of blood.

Further Development

  • Maternal side of placenta with cotyledons amnios.

  • Note the difference in placenta size and development at both stages (12 and 18 weeks).

Placental Conditions

  • Placenta abortion: 11 in 100100 pregnancies.

    • Also known as placental abruption, can lead to premature birth or stillbirth.

  • Placenta praevia: 11 in 200200 pregnancies.

    • Can cause severe bleeding during labor and may require a cesarean section.

  • Placenta accreta: 11 in 25002500 deliveries.

    • Most serious, often requiring a hysterectomy after delivery.

Placenta Abortion

  • Placenta peels away from the uterine wall before delivery (partial or complete).

    • This separation disrupts the flow of oxygen and nutrients to the fetus.

  • Can deprive the fetus of oxygen and nutrients.

    • Leading to fetal distress and potential long-term complications.

  • Profuse bleeding in the pregnant person (spiral arteries).

    • This bleeding can be life-threatening for the pregnant person.

  • Premature birth or stillbirth.

  • Happens fairly late in the pregnancy.

Placenta Praevia

  • Can block the exit from the uterus.

    • Preventing vaginal delivery and requiring a cesarean section.

  • Blood vessels connecting abnormally placed placenta to uterus may tear, causing bleeding at labor.

    • This bleeding can be severe and life-threatening.

  • May cause premature birth.

  • Low-lying placenta covering the cervix opening.

Placenta Accreta

  • Rarest but most serious.

    • Often associated with prior cesarean sections or uterine surgeries.

  • May cause excessive bleeding at birth.

    • Requiring blood transfusions and potentially leading to maternal morbidity or mortality.

  • Placenta implants too deeply and firmly into the uterine wall:

    • Increta: invades myometrium.

    • Invades the muscle layer of the uterus.

    • Percreta: invades perimetrium (uterine serosa).

    • Extends through the uterine wall and may invade adjacent organs.

Placenta-Mediated Conditions

  • Pre-eclampsia: partially mediated by the placenta, related to the pregnant individual’s blood pressure.

    • Characterized by high blood pressure and proteinuria, affecting both maternal and fetal health.

  • Diabetic pregnancy: increased risk of malformation.

    • Poor glycemic control can lead to congenital anomalies and other complications.

  • Smoking: inhibits placental growth and small for date baby.

    • Reduces oxygen and nutrient delivery to the fetus, affecting growth and development.

  • Pregnancy at altitude: from low to high causes lack of oxygen and insufficient transport across placenta and baby.

    • Can result in intrauterine growth restriction and other adverse outcomes.

  • Intrauterine growth restriction (IUGR): placenta not working efficiently enough.

    • Fetus does not receive adequate nutrition and oxygen, leading to growth restriction.

Uses of Placental Tissue

  • Placenta serum: supposed to act as a fountain of youth if injected and improve health.

    • No scientific evidence supports these claims and carries potential risks.

  • Placenta cream: younger, cleaner skin.

    • Limited scientific evidence to support these claims.

  • Eating placenta: reported improved mood, energy, and lactation (no strong scientific evidence).

    • Potential risks associated with infection and transmission of pathogens.

  • Placenta pills: slow aging, revitalize skin, support the immune system.

    • No rigorous scientific evidence to support these claims.

Learning Objectives Recap

  • LO 1: Describe the development of the umbilical region with the consequent separation of the intra- and extra-embryonic coelomic cavities.

  • LO 2: Summarise the events from blastocyst implantation and trophoblast invasion of uterine endometrium (decidua basalis) until the obliteration of the uterine cavity.

  • LO 3: Summarise the production of syncytiotrophoblast and cytotrophoblast, with regard to placental development and function.

  • LO 4: Describe the development of the placental circulation, including villi structure and function, and the development of the umbilical cord.

  • LO 5: Discuss the functional anatomy of the feto-maternal/intervascular barrier and examples of placental abnormalities.