Placenta and Placentation: Definition, Types and Functions
Complete Guide • Human Reproduction • Placenta • Placentation • NEET & Board Notes
Key Takeaways
- Understand the structure and functions of the placenta.
- Learn the process of placentation in mammals.
- Study different types and classifications of placenta.
- Understand fetal-maternal exchange and placental adaptations.
- Important for Class 11 Biology, NEET, CUET and university entrance examinations.
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| Placenta and Placentation |
Introduction
The placenta is a unique temporary organ that develops during pregnancy and establishes a close physiological connection between the mother and the developing fetus. It plays a vital role in supplying nutrients, oxygen and hormones while removing metabolic wastes from the fetal circulation.
The formation of the placenta, known as placentation, is one of the most remarkable adaptations in mammals. Proper placental development ensures normal fetal growth, successful pregnancy and healthy embryonic development.
Quick Fact
The human placenta functions as the lungs, kidneys, digestive system and endocrine gland of the fetus until birth.
What is Placenta?
The placenta is a temporary vascular organ formed through the close association of the chorion of the fetus and the endometrium of the uterus. It serves as the principal site for physiological exchange between maternal and fetal blood without direct mixing of the two circulations.
Besides nutrient and gas exchange, the placenta also functions as an endocrine organ by producing hormones essential for maintaining pregnancy.
Definition
Placenta is a temporary feto-maternal organ formed by fetal extraembryonic membranes and maternal uterine tissues for exchange of nutrients, gases, wastes and hormones.
What is Placentation?
Placentation is the biological process through which the developing fetal membranes attach to the uterine wall and form the placenta. It begins shortly after implantation and continues throughout early embryonic development.
The efficiency of placentation directly influences fetal nutrition, growth and survival during pregnancy.
Importance of Placentation
- Anchors the embryo to the uterus.
- Establishes maternal-fetal circulation.
- Facilitates nutrient and oxygen exchange.
- Supports fetal growth.
- Maintains pregnancy through hormone secretion.
Types of Placenta
Based on the embryonic membranes participating in placental formation, placenta is broadly classified into two major types.
| Type | Main Embryonic Membranes |
|---|---|
| Choriovitelline Placenta | Chorion + Yolk Sac |
| Chorio-Allantoic Placenta | Chorion + Allantois |
Choriovitelline (Yolk Sac) Placenta
In this type of placenta, the enlarged yolk sac fuses with the chorion while the allantois remains poorly developed. It is considered the primitive type of mammalian placenta and is characteristic of many marsupials.
Characteristics
- Formed by fusion of chorion and yolk sac.
- Allantois remains poorly developed.
- Less efficient than chorio-allantoic placenta.
- Found mainly in marsupials.
- Examples: Macropus, Didelphys.
Chorio-Allantoic Placenta
The chorio-allantoic placenta develops when the allantois enlarges and fuses with the chorion, producing an extensive vascular network for efficient exchange between maternal and fetal circulations.
This is the most advanced and common type of placenta found in eutherian (placental) mammals, including humans.
Characteristics
- Formed by fusion of chorion and allantois.
- Highly vascular.
- Efficient transport of nutrients and gases.
- Supports long intrauterine development.
- Found in most placental mammals including humans.
Classification of Placenta Based on Degree of Intimacy
Depending on the amount of maternal tissue shed during birth and the degree of attachment between fetal and maternal tissues, the placenta is classified into three major types.
| Type | Characteristics | Examples |
|---|---|---|
| Non-Deciduous (Adeciduate) | Very little maternal tissue is lost during parturition. | Pig, Horse, Cattle |
| Deciduous | Large amount of maternal tissue is shed during birth. | Human, Dog, Cat, Primates |
| Contra-Deciduate | Both fetal and maternal tissues are extensively damaged. | Parameles, Talpa |
Classification Based on Distribution of Chorionic Villi
The placenta can also be classified according to the distribution pattern of chorionic villi over the chorion.
| Type | Distribution of Villi | Examples |
|---|---|---|
| Diffuse Placenta | Villi distributed over the entire chorion. | Pig, Horse, Cetaceans |
| Cotyledonary Placenta | Villi grouped into cotyledons. | Cattle, Sheep, Deer |
| Zonary Placenta | Villi arranged in a belt-like zone. | Dog, Cat, Seal |
| Discoidal Placenta | Villi restricted to a single disc. | Rat, Rabbit, Mouse |
| Metadiscoidal Placenta | Large discoidal placenta with deep implantation. | Human, Monkey |
Histological Classification of Placenta
Histological classification is based on the number of tissue layers separating maternal and fetal blood.
| Placental Type | Maternal-Fetal Barriers | Examples |
|---|---|---|
| Epitheliochorial | 6 Layers | Pig, Horse |
| Syndesmochorial | 5 Layers | Sheep, Cow |
| Endotheliochorial | 4 Layers | Dog, Cat |
| Haemochorial | 3 Layers | Human, Monkey |
| Haemoendothelial | 1 Layer | Rat, Rabbit, Mouse |
Functions of Placenta
The placenta performs numerous physiological functions that are essential for fetal growth and successful pregnancy.
Major Functions of Placenta
- Provides firm attachment of the embryo to the uterus.
- Transfers nutrients from mother to fetus.
- Supplies oxygen and removes carbon dioxide.
- Removes fetal metabolic wastes.
- Stores glycogen, iron and other nutrients.
- Produces enzymes required for fetal development.
- Acts as an endocrine gland by secreting hCG, progesterone, estrogen and hPL.
- Transfers maternal antibodies to provide passive immunity.
- Acts as a selective barrier between maternal and fetal blood.
Destructive Effects of Placenta
Although the placenta acts as an effective protective barrier, it cannot completely prevent the transfer of certain harmful substances from the mother to the fetus.
Potential Harmful Effects
- Some viruses and bacteria can cross the placental barrier.
- Certain drugs may interfere with fetal development.
- Alcohol and nicotine may cause congenital abnormalities.
- Thalidomide is a classic teratogenic drug causing limb defects.
- Radiation exposure during pregnancy may damage the developing embryo.
Placenta Classification Flow Chart
Placenta
⬇
Based on Embryonic Membranes
⬇
Choriovitelline │ Chorio-Allantoic
⬇
Based on Degree of Intimacy
⬇
Non-Deciduous │ Deciduous │ Contra-Deciduate
⬇
Based on Chorionic Villi Distribution
⬇
Diffuse │ Cotyledonary │ Zonary │ Discoidal │ Metadiscoidal
Memory Tricks
- 👶 Placenta = Mother + Fetus Connection.
- 🥚 Choriovitelline = Chorion + Yolk Sac.
- 🌳 Chorio-Allantoic = Chorion + Allantois.
- ❤️ Human Placenta = Haemochorial + Metadiscoidal.
- 🍼 Placenta = Nutrition + Respiration + Excretion + Hormones.
Quick Revision
- ✔ Placenta is a temporary feto-maternal organ.
- ✔ Placentation is the process of placental formation.
- ✔ Two major placental types: Choriovitelline and Chorio-Allantoic.
- ✔ Human placenta is deciduous, haemochorial and metadiscoidal.
- ✔ Placenta secretes hCG, progesterone, estrogen and hPL.
- ✔ Placenta transfers nutrients, oxygen and antibodies.
Competitive Examination Points
- Frequently asked in NEET Biology.
- Human placenta = Haemochorial + Metadiscoidal.
- hCG maintains the corpus luteum during early pregnancy.
- Placenta is both an exchange organ and an endocrine gland.
- Thalidomide is an important teratogenic drug asked in competitive exams.
Frequently Asked Questions (FAQs)
1. What is placenta?
The placenta is a temporary vascular organ formed between the mother and the fetus. It facilitates the exchange of nutrients, oxygen, hormones and metabolic wastes while maintaining separation between maternal and fetal blood.
2. What is placentation?
Placentation is the process by which the fetal membranes attach to the uterine wall and develop into a functional placenta during pregnancy.
3. What are the two major types of placenta?
- Choriovitelline (Yolk Sac) Placenta
- Chorio-Allantoic Placenta
4. Which type of placenta is found in humans?
Humans possess a chorio-allantoic, haemochorial, deciduous and metadiscoidal placenta.
5. What hormones are secreted by the placenta?
The placenta secretes several important hormones including Human Chorionic Gonadotropin (hCG), Progesterone, Estrogen and Human Placental Lactogen (hPL).
6. What are the major functions of the placenta?
The placenta performs nutritional, respiratory, excretory, endocrine, immunological, storage and protective functions that support fetal growth throughout pregnancy.
7. Can harmful substances cross the placenta?
Yes. Certain viruses, drugs, alcohol, nicotine and radiation can cross the placental barrier and may cause congenital abnormalities or developmental defects in the fetus.
8. Why is the placenta called a temporary organ?
The placenta develops only during pregnancy and is expelled from the uterus after childbirth. Therefore, it functions as a temporary organ.
Conclusion
The placenta is one of the most specialized organs in mammalian reproduction. It forms the vital connection between the mother and the developing fetus, ensuring continuous exchange of nutrients, gases, hormones and metabolic wastes while protecting the fetus from many harmful substances.
A thorough understanding of placental structure, placentation, classifications and physiological functions is essential for students preparing for Class 11 Biology, NEET, CUET, Nursing Entrance, B.Sc., MBBS and other competitive examinations.
Final Revision
- ✔ Placenta is a temporary feto-maternal organ.
- ✔ Human placenta is chorio-allantoic.
- ✔ Human placenta is haemochorial and metadiscoidal.
- ✔ Placentation begins after implantation.
- ✔ Placenta performs nutrition, respiration, excretion and endocrine functions.
- ✔ hCG maintains the corpus luteum during early pregnancy.
- ✔ Placenta transfers maternal antibodies to the fetus.
- ✔ Certain drugs and pathogens can cross the placental barrier.
Common Mistakes Students Make
- ❌ Confusing placenta with placentation.
- ❌ Forgetting that human placenta is haemochorial.
- ❌ Mixing up choriovitelline and chorio-allantoic placenta.
- ❌ Assuming maternal and fetal blood mix directly.
- ❌ Forgetting placental hormone names.
- ❌ Confusing deciduous and non-deciduous placenta.
- ❌ Ignoring the endocrine function of the placenta.
More Topics
References
- NCERT Biology Class XI
- NCERT Biology Class XII
- Campbell Biology
- Langman's Medical Embryology
- Gilbert – Developmental Biology
- Human Embryology by Inderbir Singh
- Modern Reproductive Biology Textbooks