Placenta and Placentation: Definition, Types and Functions
What is Placenta?
Placenta is a temporary organ formed by the association of extraembryonic membranes of the fetus and the maternal endometrium for physiological exchange of materials.
What is Placentation?
Placentation is the process of formation and attachment of the fetal placenta to the uterine wall.
Types of Placenta
Based on embryonic tissues involved, placenta can be classified into two major types.
Choriovitelline (Yolk-Sac) Placenta
The yolk sac enlarges and fuses with the chorion while the allantois remains small.
Examples: Macropus, Didelphys and many marsupials.
Chorio-Allantoic Placenta
The allantois develops extensively and fuses with the chorion, supplying chorionic circulation.
Examples: Most eutherian mammals.
Classification Based on Degree of Intimacy
Non-Deciduous Placenta
Found in pigs, cattle and horses.
Deciduous Placenta
Found in cats, dogs, rodents and primates.
Contra-Deciduate Placenta
Found in Parameles, Talpa and some primitive mammals.
Classification Based on Distribution of Villi
- Diffuse Placenta – Pig, Horse, Cetaceans
- Cotyledonary Placenta – Cattle, Sheep, Deer
- Zonary Placenta – Dog, Cat, Seal
- Discoidal Placenta – Rat, Mouse, Rabbit
- Metadiscoidal Placenta – Human, Monkey
Histological Classification of Placenta
| Type | Barriers | Examples |
|---|---|---|
| Epitheliochorial | 6 | Pig, Horse |
| Syndesmochorial | 5 | Ungulates |
| Endotheliochorial | 4 | Carnivores |
| Haemochorial | 3 | Primates |
| Haemoendothelial | 1 | Mouse, Rat, Rabbit |
Functions of Placenta
- Anchorage of embryo
- Nutritional function
- Respiratory function
- Excretory function
- Storage function
- Enzymatic function
- Endocrine function
- Immunological function
- Barrier function
Destructive Effects of Placenta
- Pathogens may cross placental barrier.
- Certain drugs can harm fetal development.
- Thalidomide is a classic teratogenic example.
Frequently Asked Questions
What is placenta?
A temporary organ connecting fetus and mother for exchange of nutrients and gases.
What is placentation?
The formation and attachment of placenta to the uterine wall.
What are the major types of placenta?
Choriovitelline placenta and Chorio-allantoic placenta.