General characteristics and components
The placenta is a temporary (provisional) organ structurally consisting of two inextricably linked components belonging to different organisms:
- Fetal part. Formed by fetal structures—the villous chorion and the amnion closely adhered to it.
- Maternal part. Represented by the basal decidua (decidua basalis)—a modified area of the uterine endometrium.
The key functional role of the placenta is to ensure oxygenation of the fetal blood (the respiratory function is realized here rather than in the fetal lung tissue), as well as continuous metabolism. At the same time, placental circulation is arranged so that maternal and fetal blood never mix directly. A unique histological feature of the organ is the complete absence of innervation and lymph drainage—there are neither nerves nor lymphatic vessels in the placenta.
Principle of histological classification
The classification of mammalian placentas is based on the structure of the blood-placental (hematochorial) barrier. This barrier separates the maternal and fetal bloodflows.
The type of placenta is determined by how strongly the chorionic villi destroy the uterine tissues upon implantation. The deeper the invasion, the thinner the maternal part of the barrier becomes, up to its complete disappearance.
Four types of placentas
Mammals exhibit four main types of placental structure, sequentially reflecting the degree of destruction of maternal tissues:
- Epitheliochorial placenta (semiplacenta). Chorionic villi contact the intact epithelium of the uterine glands. The blood-placental barrier here is the thickest and most substantial, as it includes absolutely all tissue layers from both the maternal and fetal sides. Found in horses, pigs, and cetaceans.
- Syndesmochorial placenta. Villi partially destroy the uterine epithelium and contact the underlying connective tissue directly. On the maternal side, the epithelium and its basement membrane are excluded from the barrier. Characteristic of ruminants (cows, sheep, deer).
- Endotheliochorial placenta. Villi destroy not only the epithelium but also the uterine connective tissue, growing deep into the endometrial vessels. Contact occurs directly with the endothelium of maternal vessels. The maternal part of the barrier is thinned down to a single layer—the endothelium of microvessels. Representatives: carnivores (cats, dogs, seals, walruses).
- Hemochorial placenta. Villi destroy even the walls of endometrial blood vessels. Maternal blood is poured 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.
Briefly: answers to topical 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.
Anatomical components of the placental barrier in the structure of villi:
- Syncytiotrophoblast;
- Cytotrophoblast;
- Layer of mesenchymal cells — villous stroma;
- Endothelium of the fetal capillary.
What functions, besides respiratory and trophic ones, does the placenta perform?
In addition to 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 on the surface of chorionic villi to prevent fetal rejection.
To remember the types of placentas as the barrier thins, use the acronym ESEG: Epitheliochorial (entire epithelium) → Syndesmochorial (contact with connective tissue/syndesmosis) → Endotheliochorial (only endothelium remains) → Hemochorial (contact directly with blood/hemo).
Class and exam questions
Does maternal and fetal blood mix?
No. Placental circulation is arranged so that gas exchange and metabolism occur through the placental barrier without direct contact and mixing of the two blood flows.
Are there nerves in the placenta?
No, a critical feature of the placenta is the complete absence of innervation and lymph drainage (lymphatic vessels).
Which type does the human placenta belong to?
Humans have a hemochorial placenta. In it, chorionic villi destroy the walls of uterine vessels and are directly bathed by maternal blood poured into the lacunae.
What is the blood-placental barrier?
This is a complex of tissues separating maternal and fetal blood. In humans (with the hemochorial type), it consists exclusively of fetal tissues, since maternal tissues at the site of contact are completely destroyed.
What else to study on this topic
- Histological structure of the villous chorion and amnion
- Structure and functions of decidua basalis (basal decidua)
- Cellular structure of the human blood-placental barrier (syncytiotrophoblast, cytotrophoblast)
- Features of placental circulation: lacunae and fetal vessels
- Dynamics of placental formation until the end of the 3rd month of embryogenesis
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