The mammalian placenta is classified into four types based on the number of tissue layers separating the maternal and fetal blood supplies. These types are essential for understanding the variations in nutrient and waste exchange during pregnancy across different species.
Four Types of Placenta
The placenta types are categorized based on the number of layers between maternal and fetal blood. Here's a breakdown:
| Placenta Type | Description | Examples |
|---|---|---|
| Epitheliochorial | Has all six layers present; this is the least invasive type. Maternal capillaries are separated from the fetal capillaries by the maternal uterine epithelium and the maternal and fetal connective tissue. | Swine, horse |
| Endotheliochorial | Maternal epithelium and connective tissue are absent, only four layers exist, bringing maternal capillaries into direct contact with chorionic epithelium. | Dog, cat |
| Hemochorial | Three layers are present, as maternal blood directly bathes the fetal chorionic tissue. | Humans, primates, rat, rabbit, guinea pig |
| Syndesmochorial | The maternal uterine epithelium is temporarily absent, leading to five layers present. | Sheep, ruminants |
Detailed Explanation
Here's a more in-depth look at each type:
- Epitheliochorial:
- Characterized by six tissue layers separating the maternal and fetal blood.
- This is the most basic type of placental barrier, found in animals like horses and pigs.
- The maternal epithelium remains intact.
- Endotheliochorial:
- Found in animals like dogs and cats.
- The maternal epithelium and connective tissue are absent, resulting in only four layers between maternal and fetal blood.
- This allows for more efficient exchange compared to the epitheliochorial type.
- Hemochorial:
- This is the type of placenta found in humans, primates, rats, rabbits and guinea pigs.
- The maternal blood is in direct contact with the fetal chorion, making it the most intimate interaction and allowing maximum exchange.
- Only three tissue layers are present.
- Syndesmochorial:
- Found in ruminants like sheep.
- The maternal epithelium is temporarily absent, so there are five layers between maternal and fetal blood.
- It exhibits some characteristics of both epitheliochorial and endotheliochorial types.
Reference:
- Pelkonen et al., 2006
