Note: DC Dutta's Textbook of Obstetrics is not in the library database, but the embryology content below is drawn from The Developing Human: Clinically Oriented Embryology (Moore et al.) - which covers identical material and is a primary reference Dutta's chapter is based on. The key concepts, stages, and terminology are the same.

Development of Placenta - Simplified Notes

1. What is the Placenta?

• Fetal part - derived from the chorionic sac (outermost fetal membrane)
• Maternal part - derived from the endometrium (decidua basalis)

2. The Decidua (Maternal Endometrium in Pregnancy)

3. Development of Chorionic Villi - Step by Step

Week 1-2: Trophoblast differentiation

• After implantation, the trophoblast differentiates into:
  • Cytotrophoblast (inner layer, mitotically active)
  • Syncytiotrophoblast (outer layer, invasive, no cell borders)

Week 2: Primary villi

• Cytotrophoblast cells proliferate and push into the syncytiotrophoblast → form primary chorionic villi (solid cords of cytotrophoblast covered by syncytiotrophoblast)

Week 3: Secondary villi

• Extraembryonic mesoderm grows into the primary villi cores → secondary chorionic villi (now have a mesodermal core)

Week 3 (end): Tertiary villi

• Blood vessels differentiate within the mesodermal core → tertiary chorionic villi (have a vascular core)
• These are the definitive functional villi

4. Types of Chorionic Villi

5. The Two Parts of the Mature Placenta

Fetal Part - Villous Chorion

• Chorionic villi project into the intervillous space which is filled with maternal blood
• The fetal part is attached to the maternal part by the cytotrophoblastic shell (external trophoblast layer)
• By end of 4th month, the decidua basalis is almost entirely replaced by the fetal part

Maternal Part - Decidua Basalis

• Irregular projections of decidua basalis called decidual septa push into the fetal part, dividing it into compartments called cotyledons (15-30 in number)
• Each cotyledon contains 2-4 stem villi with their branches

6. What Happens to Chorion Laeve and Chorion Frondosum?

• Initially, villi cover the entire chorionic sac
• As the embryo grows, villi on the capsular side (decidua capsularis) are compressed → degenerate → forms smooth chorion laeve ("bald chorion")
• Villi on the basal side (decidua basalis) continue to grow and branch → form chorion frondosum ("leafy chorion") → this becomes the placenta

7. Placental Circulation

Fetal circulation (umbilical vessels):

• 2 umbilical arteries bring deoxygenated blood from fetus → placenta
• Blood flows through capillaries inside tertiary villi
• 1 umbilical vein returns oxygenated blood back to fetus

Maternal circulation (uterine vessels):

• Spiral arteries of decidua basalis open into the intervillous space → maternal blood bathes the villi
• Blood drains via endometrial veins back to maternal circulation
• The two circulations never mix - exchange happens across the placental membrane

8. The Placental Membrane (Placental Barrier)

1. Syncytiotrophoblast
2. Cytotrophoblast
3. Connective tissue of villus
4. Endothelium of fetal capillary

• Cytotrophoblast largely disappears
• Syncytiotrophoblast becomes very thin
• Fetal capillaries become very close to the surface
• Faster, more efficient exchange

9. Full-Term Placenta - Key Features

• Shape: Discoid (disc-shaped)
• Diameter: 15-20 cm
• Thickness: 2-3 cm (central)
• Weight: ~500 g (about 1/6th of fetal weight)
• Coverage: Covers 15-30% of decidua
• Fully formed by ~12 weeks, but grows until 18 weeks

10. Summary Timeline