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[Histology](https://durgical.com/en/histology/) · [Reproductive system](https://durgical.com/en/histology/#a-reproductive)

# Placenta

- Organ

The placenta links fetus and mother: its villi, bathed in maternal blood, swap oxygen, nutrients and waste.

Updated 2026-11-01

## Key points

- Syncytiotrophoblast: the outer multinucleated layer of the villus; it makes hCG and, later, progesterone.
- Cytotrophoblast: the inner layer of single cells that feeds the syncytium; it thins out in the second half of pregnancy.
- Preeclampsia: the trophoblast fails to remodel the maternal spiral arteries and the placenta is poorly perfused.
- Hydatidiform mole: swollen villi with trophoblast proliferation and very high hCG.
- Placenta accreta: the decidua basalis is missing and the villi stick to or invade the myometrium.

## What it is

The placenta has a fetal part, the chorionic villi, and a maternal part, the decidua basalis. The villi float in the intervillous space, filled with maternal blood, and are covered by syncytiotrophoblast outside and cytotrophoblast inside. Fetal and maternal blood do not mix: exchange crosses the placental barrier. The placenta is also endocrine and makes hCG, progesterone and estrogens.

## Structure

- Early: large villi with both trophoblast layers clearly visible and loose stroma.
- At term: many small villi, with fetal capillaries pressed against a thin syncytium and syncytial knots.
- Villous stroma with fetal macrophages (Hofbauer cells).
- Decidua basalis: large, pale decidual cells on the maternal side.

## Function

- Exchange of gases, glucose, amino acids, iron and waste without mixing the bloods.
- Production of hCG, which keeps the corpus luteum going early, and later of its own progesterone and estrogens.
- Passage of maternal IgG antibodies to the fetus; drugs, alcohol, nicotine and some microbes can also cross.

## Placental barrier: from maternal to fetal blood

| Layer | What it is |
| --- | --- |
| Syncytiotrophoblast | Continuous multinucleated layer touching maternal blood; transport and hormones |
| Cytotrophoblast | Trophoblast stem cells; discontinuous at term |
| Trophoblast basement membrane | Support for the villous epithelium |
| Villous stroma | Connective tissue with Hofbauer cells and fetal vessels |
| Fetal capillary endothelium | The last layer before fetal blood |

## Clinical notes

- Placenta previa: low implantation covering the cervical os; painless bleeding in the third trimester.
- Placental abruption: separation before birth; painful bleeding and fetal risk.
- Congenital syphilis, a public health priority in Colombia, crosses the placenta; an infected placenta can be large and pale.

## Memory aid

The placenta is a mangrove: the fetus's roots (villi) bathe in a lake of maternal blood, and the water on the two sides never mixes.

## Test yourself

Think of the answer, then tap the question to see it.

## Which trophoblast layer makes hCG?

The syncytiotrophoblast, the outer multinucleated layer of the villi.

## Do fetal and maternal blood mix in the placenta?

No. Maternal blood bathes the villi in the intervillous space and exchange crosses the placental barrier.

## Which placental failure underlies preeclampsia?

Poor remodelling of the maternal spiral arteries by the trophoblast, leaving the placenta under-perfused.

## What are Hofbauer cells?

Fetal macrophages in the stroma of the chorionic villi.

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## Related

- [Uterus and the endometrial cycle](https://durgical.com/en/histology/uterus-and-the-endometrial-cycle/)
- [Ovary: follicles, ovulation and corpus luteum](https://durgical.com/en/histology/ovary-follicles-ovulation-and-corpus-luteum/)

## Related conditions

- [Preeclampsia](https://durgical.com/en/conditions/preeclampsia/)
- [Molar pregnancy (hydatidiform mole)](https://durgical.com/en/conditions/molar-pregnancy/)
- [Placenta praevia](https://durgical.com/en/conditions/placenta-praevia/)
- [Placental abruption](https://durgical.com/en/conditions/placental-abruption/)

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: [28.2 Embryonic Development](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-2-embryonic-development) (OpenStax, Rice University, 2022); [27.2 Anatomy and Physiology of the Ovarian Reproductive System](https://openstax.org/books/anatomy-and-physiology-2e/pages/27-2-anatomy-and-physiology-of-the-ovarian-reproductive-system) (OpenStax, Rice University, 2022).

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Representative model, for educational purposes only.

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