How Does the Placenta Form and Function?
The placenta is a developing organ in the uterus during pregnancy. This structure provides oxygen and nutrients to a growing baby. It also cleans the baby's blood of waste products. The placenta attaches to the uterine wall and gives rise to the baby's umbilical cord.
It also produces a variety of pregnancy-related hormones, including lactogen, oestrogen and progesterone. Placentas are a distinguishing feature of placental mammals, but they are also found in marsupials and some non-mammals at various stages of development.
What is Placenta?
Placentation is a Greek term that means "flat cake."
The term "placenta" comes from the "human placenta", which is a flat, round mass that resembles a pancake.
In its broadest sense, this term refers to any region in a viviparous organism where maternal and embryonic tissues of any type are closely apposed and serve as a site for physiological exchange between parent and embryo.
It is a temporary organ.
The placenta performs the functions of the embryo's lung, intestine, kidney and endocrine glands.
It allows for the exchange of substances between maternal and foetal blood.
Structure of Placenta
Both maternal and embryonic tissue make up the placenta.
The chorion is the embryonic component of the placenta. It is composed of trophoblasts i.e. the cells that form the blastocyst's outer cell layer.
The placenta is foetal tissue that is embedded in the uterine wall and perfused with maternal blood via the uterine spiral arteries.
The decidua beneath the embryo is known as the decidua basalis, and it forms the maternal face of the placenta.
The placenta is divided into three layers:
The amnion is the name given to the innermost placental layer that surrounds the foetus.
The allantois is the placenta's middle layer (derived from the embryonic hindgut); blood vessels from the umbilicus pass through this membrane.
The chorion, the placenta's outermost layer, comes into contact with the endometrium. It is made up of two layers of cells: inner cytotrophoblast and outer syncytiotrophoblast.
Structure of Human Placenta
Development of Placenta
The placenta begins to develop very early in pregnancy, around week 4, with the implantation of the blastocyst into the maternal endometrium.
The blastocyst's outer layer develops into the trophoblast, which forms the placenta's outer layer.
The outer layer is further subdivided into two layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer.
The syncytiotrophoblast is a continuous cell layer with multiple nuclei that covers the surface of the placenta. It develops from the differentiation and fusion of the underlying cytotrophoblast cells, a process that occurs throughout placental development.
As a result, the syncytiotrophoblast (also known as syncytium) contributes to the placental barrier function.
After about 12 weeks of pregnancy, the chorionic villi and uterine tissue interdigitate and form the placenta, a structural and functional unit between the developing embryo (foetus) and the maternal body.
By the end of the first trimester of pregnancy, week 14, the maternal blood supply to the placenta is complete.
Placental Development
Functions of Placenta
The placenta acts as an important endocrine gland during pregnancy.
Placental hormones include- oestrogen, progesterone, human chorionic gonadotropin(hCG) and human placental lactogen (hPL).
Placental hormones are required for pregnancy establishment and maintenance as well as for foetal growth.
It stores glycogen for the foetus prior to the formation of the liver.
It allows selective diffusion, preventing the passage of harmful materials from maternal blood to foetal blood.
Fun Facts about Placenta
It is the only disposable organ i.e. it naturally expels itself once its function is completed. Each pregnancy produces a new placenta to perfectly support the baby.
Many women consume their placenta after birth. Evidence suggests that mothers who eat their placentas after birth experience a variety of benefits including more balanced hormones, increased energy and milk supply, less postpartum bleeding and replenished iron stores.
The placenta has the unusual ability to grow and infiltrate the mother's body without being attacked by her immune system. The placenta's ability to evade the immune system is tightly controlled, and it knows when to stop infiltrating so as to not cause any harm to the mother.
The placenta is also known as the "tree of life", partly because of its life-sustaining function but also because its veins resemble those of a tree.
Conclusion
The placenta is the lifeline that connects a mother and her child. It is in the organ that the link between them is first established. Scientists believe that the placenta is the least understood human organ, despite being one of the most important organs in the body. This article gives insight into the structure known as the placenta which only develops during the pregnancy. It also talks about the different layers of the placenta, the role of the placenta, its developmental stages and its important functions.
FAQs on Placenta: Key to Fetal Development
1. What is a placenta and what are its primary functions?
The placenta is a temporary organ that develops in the uterus during pregnancy. It forms a vital connection between the mother and the developing foetus. Its primary functions are to:
Nutrition: Facilitate the transfer of nutrients like glucose, amino acids, and vitamins from the mother's blood to the foetus.
Respiration: Allow the exchange of oxygen from the mother to the foetus and carbon dioxide from the foetus to the mother.
Excretion: Remove metabolic waste products, such as urea, from the foetal blood into the mother's bloodstream for disposal.
Endocrine Function: Produce essential hormones to maintain pregnancy.
Immunity: Transfer maternal antibodies (like IgG) to the foetus, providing passive immunity.
2. Can you explain the basic structure of the human placenta?
The human placenta has two main parts: the foetal part (Chorion) and the maternal part (Decidua basalis). The foetal side is covered by the amnion and features finger-like projections called chorionic villi. These villi extend into the maternal uterine wall, where they are bathed in maternal blood. This intricate arrangement creates a large surface area for the exchange of substances, without allowing direct mixing of maternal and foetal blood. The entire structure is connected to the foetus via the umbilical cord.
3. Why is the placenta considered a temporary endocrine gland?
The placenta is called a temporary endocrine gland because, for the duration of pregnancy, it produces and secretes several crucial hormones that are vital for maintaining the pregnancy and supporting foetal development. Key hormones produced include:
Human Chorionic Gonadotropin (hCG): This is the hormone detected in pregnancy tests. It maintains the corpus luteum, which continues to produce progesterone in early pregnancy. Learn more about the Full Form of HCG.
Human Placental Lactogen (hPL): This hormone helps prepare the mammary glands for lactation and affects the mother's metabolism. You can read about the Full Form of HPL here.
Progesterone and Estrogens: These hormones are critical for maintaining the uterine lining and preventing new ovulatory cycles.
Relaxin: Secreted in later stages, it helps relax the pelvic ligaments in preparation for childbirth.
4. What happens to the placenta after the baby is born?
After the baby is born, the placenta is also delivered from the uterus. This is the third and final stage of labour, often called the placental stage or 'afterbirth'. Uterine contractions continue, causing the placenta to detach from the uterine wall. It is then expelled through the vagina. It is important for the entire placenta to be delivered to prevent postpartum complications like bleeding or infection.
5. What are the main types of placental complications a student should know about?
While there are several complications, three important ones mentioned in biology are:
Placenta Previa: This occurs when the placenta partially or completely covers the cervix, the opening of the uterus. This can cause severe bleeding during pregnancy or delivery.
Placental Abruption: This is a serious condition where the placenta separates from the wall of the uterus before birth, which can deprive the foetus of oxygen and nutrients.
Placenta Accreta: This happens when the placenta's blood vessels grow too deeply into the uterine wall, making it difficult to detach after childbirth and potentially causing major blood loss.
6. How does the position of the placenta, like anterior vs. posterior, impact a pregnancy?
The position of the placenta refers to where it attaches to the uterine wall. An anterior placenta is on the front wall (near the abdomen), while a posterior placenta is on the back wall (near the spine). Generally, both positions are considered normal and do not affect the health of the baby or the pregnancy. The main difference is that with an anterior placenta, the mother might feel the baby's kicks a little later or less strongly, as the placenta acts as a cushion. You can explore the Difference Between Anterior and Posterior Placenta for a detailed comparison.
7. How does the placenta act as a protective barrier while also preventing the mother's immune system from rejecting the foetus?
The placenta performs a remarkable dual role. It forms the placental barrier, which prevents the direct mixing of maternal and foetal blood, protecting the foetus from certain infections and blood type incompatibilities. Simultaneously, it prevents immune rejection by the mother's body, which would otherwise recognise the foetus (containing paternal genes) as foreign. It does this through specialised cells on its surface that do not trigger a strong maternal immune response. Furthermore, it selectively allows beneficial substances to pass, such as maternal antibodies (IgG), which provide the foetus with crucial passive immunity.
8. Is there a difference between the placenta in humans and placentation in flowering plants?
Yes, the terms refer to completely different structures and functions, which is a key distinction in biology.
In humans (mammals), the placenta is a complex temporary organ for nourishing a developing foetus.
In flowering plants, placentation refers to the arrangement of ovules inside the ovary. The 'placenta' in a plant is the specific tissue within the ovary to which the ovules (which become seeds after fertilization) are attached. You can see examples of this in the NCERT Solutions for Class 11 Biology Chapter 5.
