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Placenta: General Characteristics and Classification

Placenta

For medical students2 min readUpdated 2026-10-10

The placenta is a unique extraembryonic organ that fully develops by the end of the third month of gestation. It assumes the function of the lungs, providing a vital connection for gas exchange and nutrition between the maternal and fetal organisms without direct mixing of their blood.

Maturation timeFully formed by the end of the 3rd month of development

Organ partsFetal part (chorion + amnion) and maternal part (decidua basalis)

CirculationMetabolic exchange occurs without direct mixing of maternal and fetal blood

OxygenationFetal blood oxygenation occurs in the placenta, not in the lungs

Key featuresNerves and lymphatic vessels are completely absent in placental tissue

General Characteristics and Components

The placenta is a temporary (provisional) organ structurally composed of two inextricably linked components belonging to different organisms:

Fetal component. Formed by fetal structures — the villous chorion and the closely adherent amnion.
Maternal component. Represented by the basal decidua (decidua basalis) — a modified region of the uterine endometrium.

The key functional role of the placenta is to provide fetal blood oxygenation (respiration occurs here rather than in the fetal lung tissue) alongside continuous metabolic exchange. Placental circulation is organized so that maternal and fetal blood never mix directly. A unique histological feature of the organ is the complete absence of innervation and lymphatic drainage — the placenta contains neither nerves nor lymphatic vessels.

Principle of Histological Classification

The classification of mammalian placentas is based on the structure of the blood-placental (hemochorial) barrier. This barrier separates the maternal and fetal bloodstreams.

The type of placenta is determined by the extent to which chorionic villi erode uterine tissues during implantation. Deeper invasion results in a thinner maternal portion of the barrier, up to its complete disappearance.

Four Types of Placentas

Mammals exhibit four main types of placental structure, reflecting the sequential degree of maternal tissue erosion:

Epitheliochorial placenta (semiplacenta). Chorionic villi contact the intact epithelium of uterine glands. The placental barrier here is the thickest and most substantial, as it includes absolutely all tissue layers from both maternal and fetal sides. Found in horses, pigs, and cetaceans.
Syndesmochorial placenta. Villi partially erode the uterine epithelium and contact the underlying connective tissue directly. The maternal epithelium and its basement membrane are excluded from the barrier. Characteristic of ruminants (cows, sheep, deer).
Endotheliochorial placenta. Villi erode not only the epithelium but also the uterine connective tissue, growing deep to the endometrial vessels. Contact occurs directly with the endothelium of maternal vessels. The maternal portion of the barrier is thinned down to a single layer — the microvascular endothelium. Representatives: carnivores (cats, dogs, seals, walruses).
Hemochorial placenta. Villi even erode the walls of endometrial blood vessels. Maternal blood spills into special lacunae, directly bathing the villi. Maternal tissues are completely excluded from the barrier; it is formed exclusively by fetal tissues. This type is characteristic of humans, as well as primates, rodents, and lagomorphs.

Mnemonic

To remember the types of placentas in order of decreasing barrier thickness, use the mnemonic ESEH: Epitheliochorial (entire epithelium) → Syndesmochorial (connective tissue/syndesmosis contact) → Endotheliochorial (only endothelium remains) → Hemochorial (direct blood/hemo contact).

Frequently asked questions

What specific histological layers make up the human blood-placental barrier?

The human blood-placental barrier is formed exclusively by fetal tissues: maternal tissues are completely excluded from the barrier composition.

Anatomical components of the placental barrier within the villous structure:

Syncytiotrophoblast;
Cytotrophoblast;
Layer of mesenchymal cells (villous stroma);
Endothelium of the fetal capillary.

What functions, besides respiratory and trophic, does the placenta perform?

Beyond respiratory and trophic functions, the placenta performs barrier (protective), endocrine, and immune functions.

Barrier function — selectively protects the fetus from adverse factors and acts as a barrier between independent organisms.
Endocrine function — the placenta acts as a temporary endocrine organ, duplicating the functions of the pituitary gland and ovaries to maintain pregnancy.
Immune function — includes the suppression of immune responses at the surface of chorionic villi to prevent fetal rejection.

Do maternal and fetal blood mix?

No. Placental circulation is designed so that gas exchange and metabolism occur across the blood-placental barrier without direct contact or mixing of the two bloodstreams.

Are there nerves in the placenta?

No, a crucial feature of the placenta is the complete absence of innervation and lymphatic drainage (lymphatic vessels).

What type of placenta do humans have?

Humans have a hemochorial placenta. In it, chorionic villi erode the walls of uterine vessels and are directly bathed by maternal blood pooled in lacunae.

What is the blood-placental barrier?

It is the complex of tissues separating maternal and fetal blood. In humans (with the hemochorial type), it consists exclusively of fetal tissues, as maternal tissues at the site of contact are completely destroyed.

Go deeper

Histological structure of the villous chorion and amnion
Structure and functions of the decidua basalis
Cellular structure of the human blood-placental barrier (syncytiotrophoblast, cytotrophoblast)
Features of placental circulation: lacunae and fetal vessels
Dynamics of placental formation up to the end of the 3rd month of embryogenesis