placenta

Placental Development Notes

Fertilization

  • Occurs usually in the ampulla of the fallopian tube.

  • Spermatozoa contact the following layers:

    • Corona radiata

    • Zone pellucida

  • Penetration of the sperm into the membrane allows the sperm DNA to release, triggering meiosis II to form a zygote.

  • The zygote remains in the fallopian tubes for approximately 3-5 days, undergoing mitotic division. This division occurs without an increase in overall size, termed cleavage.

  • The resultant cell cluster is referred to as a morula, which later develops into a blastocyst.

What is a Blastocyst?

  • Structure: Two key components - Trophoblast (outer) and Inner cell mass (inner).

    • Trophoblast:

    • Will form the placenta and chorion.

    • Produces human chorionic gonadotropin (HCG), which helps maintain the endometrium by ensuring continued production of progesterone.

    • Inner cell mass:

    • Will develop into the fetus, umbilical cord, and amnion.

The Placenta

  • At term, the average dimensions are approximately 20 cm in diameter, 2.5 cm thick, and weighs around 600 g.

Functions of the Placenta

  • Respiration: Facilitates gas exchange (Oxygen and Carbon Dioxide) between mother and fetus.

  • Nutrition: Delivers essential nutrients from maternal blood to the fetus.

  • Excretion: Removes waste products from the fetal blood into the maternal circulation.

  • Protection: Acts as a barrier against pathogens and maintains a stable environment for the fetus.

  • Hormone Production: Produces hormones necessary for pregnancy maintenance and fetal development.

Implantation

  • Formation of the placenta begins from embryonic trophoblast cells.

  • The initial trophoblast cells are referred to as the cytotrophoblast.

  • Cytotrophoblast differentiates into syncytiotrophoblast, which invades the uterine lining to allow the embryo to embed by approximately day 10 of development.

  • As syncytiotrophoblast expands, small cavities or spaces begin to form within, leading to an event called quiescence, where the uterus is relaxed.

Placental Development

  • The blastocyst develops projections known as trophoblastic fingers that spread over its surface, termed developing chorionic villi.

  • The embryo is suspended within the chorionic sac during this phase.

Chorionic Villi Development

  • Early in the 3rd week, embryonic mesenchyme invades the primary chorionic villus to form secondary chorionic villi.

  • Some mesenchyme cells differentiate to form fetal blood capillaries.

  • By day 20, a functional arteriocapillary venous network is connected to embryonic heart vessels.

  • By the end of the 3rd week, blood circulation through the chorionic villi begins, and the exchange of substances between maternal and fetal circulation systems commences.

Cytotrophoblast Shell

  • Cells proliferate and extend through the syncytiotrophoblast to form a cytotrophoblast shell.

  • This shell attaches the chorion to the maternal endometrium by anchoring villi, facilitating the exchange between maternal and fetal circulation.

Later Placental Development

  • Once implantation occurs, the endometrium becomes known as decidua.

  • Three regions are identified within the decidua:

    1. Decidua basalis: Located beneath the embryo, it provides the maternal component of the placenta.

    2. Decidua capsularis: Overlies the embryo.

    3. Decidua vera/parietalis: The remaining uterine lining.

  • As the embryo grows, the decidua capsularis expands into the uterine cavity and fuses with decidua vera.

  • The entire surface of the chorionic sac is covered with villi until week 8. As the sac grows, the decidua capillaries become compressed, reducing blood supply and leading to the formation of the chorionic membrane.

  • Villi penetration into decidua erodes maternal blood vessel walls, allowing blood pools to form; however, there is no direct contact between maternal and fetal blood.

  • The placenta is fully formed by 10 weeks of gestation.

Formation of Placental Sinuses

  1. Lacunae Formation (Days 9–10):

    • The syncytiotrophoblast begins forming small cavities or lacunae filled with tissue fluid and maternal blood.

  2. Maternal Capillary Erosion (Days 11–13):

    • Syncytiotrophoblast invades deeper into uterine lining, eroding maternal capillaries, allowing maternal blood to flow into the lacunae.

  3. Lacunae Merge into Sinuses (Days 15–17):

    • Lacunae connect and expand into a network of blood-filled spaces known as maternal sinuses. This area becomes the intervillous space, bathing fetal chorionic villi in maternal blood.

  4. Establishment of Circulation:

    • By around day 17, both maternal and fetal blood vessels function effectively, allowing for continuous maternal blood flow to the developing placenta.

Maternal Surface of the Placenta

  • Contains approximately 20 cotyledons (lobes) separated by sulci (grooves).

  • Each cotyledon comprises lobules, each containing a single villus with branches, characterized by a dark red color due to the presence of maternal blood.

Fetal Surface of the Placenta

  • Features visible umbilical vein and arteries spreading out from the umbilical cord insertion.

  • The amnion is the inner membrane derived from the inner cell mass, characterized as:

    • Single-layered

    • Smooth

    • Tough and translucent

    • Covers the outer surface of the umbilical cord and produces amniotic fluid.

Amniotic Membranes

  • Chorion:

    • The outer membrane that lines the uterus, derived from the trophoblast of the early embryo, connects continuously with the edges of the placenta.

  • Amnion:

    • The membrane closest to the fetus, smooth, transparent, and stronger than the chorion, covering both the fetal surface of the placenta and the umbilical cord.

Amniotic Fluid

  • Continuously produced by the amnion; recycled through fetal swallowing and urine.

  • Functions of amniotic fluid include:

    • Cushions the fetus against pressure from uterine walls.

    • Allows for fetal growth and movement.

    • Protects from external noise.

    • Helps in regulating temperature around the fetus.

Umbilical Cord

  • Connects the placenta to the fetus, comprising:

    • 2 arteries

    • 1 vein

    • Contains Wharton's jelly for cushioning.

Placental Circulation

  • Placental villi provide a vast surface area for the exchange of substances between maternal and fetal circulations.

  • Maternal blood enters the intervillous space through 80-100 uterine spiral arteries, leading to pooling of blood.

  • Maternal blood does not come into direct contact with fetal blood, ensuring the exchange of O2, nutrients, and waste products occurs via diffusion. It is essential that maternal and fetal blood remains separate.

Third Stage of Labour

  • Defined as the period from the birth of the baby to the expulsion of the placenta (NICE 2014).

Medication Crossing the Placenta

  • There are implications for various substances crossing the placental barrier, including:

    • Illegal substances may lead to withdrawal symptoms in the fetus.

    • Opioids: Risk of dependence.

    • Antidepressants: Potential effects on fetal development.

    • Antibiotics: Consideration of safety and effects during pregnancy.