DEVELOPMENT OF PLACENTA

Development of the Placenta

The placenta forms from both fetal and maternal contributions, allowing nutrient, gas, and waste exchange.

1. Implantation of the blastocyst

  • Around day 6–7 after fertilization, the blastocyst adheres to the endometrium.

  • The trophoblast (outer layer of blastocyst) differentiates into:

    1. Cytotrophoblast – inner cellular layer

    2. Syncytiotrophoblast – outer multinucleated layer that invades maternal endometrium

2. Development of Chorionic Villi

  1. Initial implantation

    • The syncytiotrophoblast is the first layer to invade the endometrium, breaking down maternal tissue and forming lacunae (maternal blood spaces).

    • The cytotrophoblast follows behind, forming proliferating cellular columns.

  2. Primary villi

    • Composition: Cytotrophoblast core surrounded by syncytiotrophoblast

    • Function: Structural beginnings of the villous tree

    • Key point: No mesoderm yet

  3. Secondary villi

    • Composition: Cytotrophoblast + syncytiotrophoblast invaded by extraembryonic mesoderm

    • Function: Provides a scaffold for future fetal blood vessels

  4. Tertiary villi

    • Composition: Secondary villi + fetal blood vessels (capillaries) develop in the mesoderm core

    • Function: Now capable of maternal-fetal exchange (oxygen, nutrients, wastes)

Summary Table

Stage

Composition

Key Feature

Function

Primary villi

Cytotrophoblast core + syncytiotrophoblast

No mesoderm

Structural beginnings

Secondary villi

Cytotrophoblast + syncytiotrophoblast + extraembryonic mesoderm

Scaffold for vessels

Support future circulation

Tertiary villi

Secondary villi + fetal blood vessels

Functional fetal circulation

Exchange with maternal blood

✅ Key point

  • Syncytiotrophoblast first, then cytotrophoblast, then mesoderm, then fetal vessels → primary → secondary → tertiary villi.

  • Tertiary villi = functional placental villi capable of exchange.

3. Development of maternal contribution

  • The **syncytiotrophoblast erodes maternal spiral arteries and endometrial tissue

  • This forms lacunae (maternal blood spaces) that bathe the villi, enabling maternal-fetal exchange

4. Formation of placenta proper

  • Around the 4th month the placenta becomes fully functional:

    • Fetal side: chorionic villi anchored to the chorionic plate

    • Maternal side: decidua basalis (maternal portion)

  • Umbilical cord forms, containing:

    • 2 umbilical arteries → fetal blood to placenta

    • 1 umbilical vein → oxygenated blood to fetus

5. Structure of the mature placenta

Part

Origin

Function

Chorionic plate

Fetal (chorion)

Anchors villi; umbilical cord attachment

Chorionic villi

Fetal (trophoblast + mesoderm)

Gas, nutrient, waste exchange

Intervillous space

Maternal (decidua basalis)

Maternal blood bathes villi

Basal plate

Maternal (decidua basalis)

Separates placenta from uterus; attachment to myometrium

Key points

  1. Fetal contribution: chorionic villi (cytotrophoblast + syncytiotrophoblast + mesoderm + fetal vessels)

  2. Maternal contribution: decidua basalis

  3. Exchange occurs across: trophoblast layers + fetal capillary endothelium + maternal blood in lacunae

  4. Umbilical cord connects fetus to placenta

Mnemonic for fetal vs maternal parts

  • Fetal = Chorionic villi → Cytotrophoblast + Syncytiotrophoblast (Fetal = CS)

  • Maternal = Decidua basalis (Maternal = D)

One-line exam answer

The placenta develops from the trophoblast (fetal) invading the decidua basalis (maternal), forming chorionic villi bathed in maternal blood, establishing a site for nutrient, gas, and waste exchange.