Placenta: Definition, Structure, Types and Functions

The placenta is the connective tissue that connects the fetus to the mother. It aids in the transfer of nutrients and oxygen to the fetus and is also in charge of collecting carbon dioxide and trash from the fetus. The placenta is disc-shaped and can grow to be up to 22 cm long. It is only found in mammals and is densely packed with blood vessels. 

Read More: Human Reproductive System

Key terms: Placenta, umbilical cord, Fetus, Nutrients, Mammals, circulatory organ, Anti-bodies, Excretion, Blood, Blood vessels

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What is Placenta?

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The placenta is an organ that is in charge of feeding and protecting the fetus throughout pregnancy. It is distinct as it is a transient organ; it develops with the fetus throughout pregnancy and then is discharged with the fetus at the time of birth.

• The placenta is sometimes known as the "afterbirth" because it is discharged via the vagina after the fetus is delivered.
• The placenta is a transient circulatory organ found in mammals that connects the fetus to the mother's uterus during pregnancy.
• It is found only in the case of mammals.
• The umbilical cord connects the fetus to the placenta, allowing materials to be transferred.
• The placenta is normally linked to the uterus at the top, bottom, side, front, or back.
• In severe situations, the placenta becomes connected to the lower section of the uterus, resulting in placenta previa.

Placenta

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Structure of Placenta

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The placenta is a structure that forms a strong bond between the fetus and the mother.

• A number of finger-like projections known as chorionic villi grow into uterine tissue from the chorion's outer surface.
• These villi pierce the mother's uterine wall and create the placenta.
• The placenta is a connective tissue that connects the fetal membrane to the inner uterine wall.
• As a result, the placenta is regarded as both embryonic as well as maternal.
• The developing embryo absorbs oxygen and nutrients from the mother via the placenta while eliminating carbon dioxide and nitrogenous waste.

Structure of Placenta

Read More: Female Reproductive System

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Types of Placenta

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The various types of the placenta can be classified as:

Depending on the extent of Embryonic Tissue Involvement

On the basis of the involvement of the embyonic tissue placenta can be classified as – 

1. Yolk-Sac Placenta: When the midgut extension of the splanchnopleure surrounding the yolk merges with the extraembryonic somatopleure to make embryonic contact with the uterine wall, this is referred to as a Yolk-Sac Placenta. Mustelus is an example of the same.
2. Chorio-Allantok Placenta: Hindgut’s allantoic evagination joins the extraembryonic somatopleure in order to further make contact with the uterine tissue. Eutherian mammals and a lizard, Chalcides, are two examples of organisms with Chorio-Allantok Placenta.

Depending on the Villi Distribution

1. Diffused Placenta: The villi are numerous and evenly scattered throughout the chorion. Ungulates and Cetacea are two examples.
2. Cotyledonary Placenta: The villi combine to form tiny tufts in specific areas of the placenta. Ruminants are an example.
3. Zonary Placenta: The villi are limited to an annular zone on the chorion in zonary placentas. Examples: Carnivora (Pinnipedia).
4. Discoidal Placenta: As seen in iruro-dents and insectivores, the villi become limited to a discoidal region. The placenta in apes and humans is metadiscoidal.

Depending on the Villi's Relationship with the Uterine Wall

1. Deciduate Placenta: The villi become associated with the uterine wall's mucous membrane intimately. It comes out with the embryo at delivery.
2. Placenta indeciduate or adeciduate: The villi are detached from the uterus before the birth as they are loosely connected with the uterine walls.

Depending on the Degree of Foetal and Maternal Tissue Involvement

1. Epitheliochorial Placenta: The uterine epithelium stays in simple apposition with the embryonic chorion.
   Pig and horse are two examples of organisms with the epitheliochorial placenta.
2. Haemochorial Placenta: The glandular epithelium, capillary endothelium, and endometrium vanish and the flowing maternal blood drenches the chorion.
   For example in humans.
3. Haemoendothelial placenta: As in haemochorial placentas, the glan­dular epithelium, endothelium, and endometrium of the maternal blood capillaries all vanish in the case of haemoendothelial placentas to.
   The outer layer of the blastocyst, known as the trophoblastic epithelium of the fetus, therefore, declines with the disappearance of these maternal structures.
   The fetal endothelium hence separates the fetal and maternal circulating bloodstreams. Rodents are examples of organisms with haemoendothelial placenta.
4. Syndesmochorial Placenta: The epithelium of the uterus fades, and as a result, the chorion directly comes into contact with the endometrium or glandular epithelium of the uterus.
   Sheep are a good example of organisms with syndesmochorial placenta.
5. Endotheliochorial or Vasochorial Placenta: Both the endometrium and the glandular epithelium vanish, and the chorion comes into close contact with the uterine capillary endothelium.
   Dogs and cats are two examples.

Read More: Stages of gamete development

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Function of the Placenta

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The placenta serves as a lifeline between the mother and the fetus, ensuring that the fetus receives the nutrients it requires from the mother's body in order to survive. At the same time, it works as a barrier, protecting the fetus from several maternal infections. The placenta performs the following functions:

• It allows for gas exchange, ensuring that the fetus receives enough oxygen.
• It helps the fetus acquire enough nourishment.
• Placenta aids in the regulation of the fetus' body temperature.
• It removes waste for the purpose of processing from the fetus through the mother's body.
• Placenta filters out some germs that could cause infection
• Through the placenta, the antibodies are transferred from the mother to the fetus, providing some immunological protection.
• It produces hormones that keep the mother's body in a state of readiness for pregnancy.
• As the fetus is unable to feed, breathe, or expel waste while within the womb, the placenta essentially serves the role of numerous organ systems for the fetus.

Function of the Placenta

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Purpose of the Placenta

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The placenta has the following purpose– 

1. Nutrition: Food ingredients are transferred from the mother's blood to the fetal blood via the placenta.
2. Storage: The placenta stores glycogen, fat, and other nutrients for the baby before the liver develops.
3. The placenta acts as an endocrine gland, secreting hormones such as estrogen, progesterone, and human chorionic gonadotropin (HCG).
4. Protein digestion: The placenta's trophoblast breakdown proteins before delivering them into the fetal circulation.
5. Excretion: Nitrogenous wastes like urea flow from the fetal blood into maternal blood via the placenta and are filtered out by the mother's kidneys.
6. Respiration: The placenta transports oxygen from the maternal blood to the fetal blood and carbon dioxide from the fetal blood to the maternal blood.
7. Barrier: The placenta acts as an efficient barrier (defensive wall), allowing useful aerials to enter the blood. Toxic chemicals like nicotine from cigarettes and addictive narcotics like heroin can pass through the placenta. As a result, pregnant women should abstain from smoking and using drugs. Viruses and bacteria are capable of passing through the placenta.

Also Read:

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Things to Remember

• A placenta is a circular organ found in a pregnant mammal's uterus. It feeds the fetus via the umbilical cord during pregnancy.
• During pregnancy, the placenta releases the HCG (human chorionic gonadotropin) hormone, which stops the mammal's menstrual Cycle.
• The placenta excretes waste materials from the fetus. It spreads to the maternal blood.
• When a fertilized egg termed a blastocyst implants in the maternal endometrium, the placenta begins to form. 
• A number of finger-like projections known as chorionic villi grow into uterine tissue from the chorion's outer surface
• The placenta works as a barrier, protecting the fetus from several maternal infections.

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Previous Year Questions

1. Gonads develop from embryonic…..[NEET 1990]
2. If for some reason, the vasa efferentia in the human reproductive system get blocked, the gametes will not be transported from… [NEET 2011]
3. About which day in a normal human menstrual cycle does rapid secretion of LHLH (popularly called LHLH surge) normally occurs?...[NEET 2011]
4. Which of the following hormones is not a secretion product of human placenta?​...[NEET 2004]
5. Foetal ejection reflex in human female is induced by….​[NEET 2009]
6. Which one of the following is the correct matching of the events occurring during menstrual cycle?….​[NEET 2009]
7. Hormones secreted by the placenta to maintain pregnancy are….​[NEET 2018]
8. The difference between spermiogenesis and spermiation is….​[NEET 2018]
9. Sertoli cells are regulated by the pituitary hormone known as….​[NEET 2006]
10. After ovulation Graafian follicle regresses into​...[NEET 1999]
11. Bartholin's are situated….​​[NEET 2003]
12. Blastopore is….​[NEET 1992]
13. Capacitation occurs in….​[NEET 2017]
14. Capacitation refers to changes in the….[NEET 2015]
15. Coelom derived from blastocoel is known as….[NEET 1994]
16. During cleavage, what is true about cells?….​[NEET 1991]
17. Ectopic pregnancies are referred to as...[NEET 2015]
18. Egg is liberated from ovary in….​[NEET 1989]
19. Eye lens is formed from….​[NEET 1992]
20. Extrusion of second polar body from egg nucleus occurs….​[NEET 1993]