During pregnancy, the placenta serves as a temporary organ that links the baby to the uterus. The placenta develops immediately after conception and connects to the uterine wall. The umbilical cord connects the baby to the placenta. The placenta and umbilical cord work together to keep the baby alive in the uterus.
What is a Placenta?
The placenta is a developing organ in the uterus during pregnancy. A developing infant gets oxygen and nutrients from this structure. It also cleans the baby's blood of waste materials. The placenta adheres to the uterine wall and gives rise to the baby's umbilical cord. The organ is normally linked to the uterus's top, side, front, or rear. In rare situations, the placenta may attach to the lower part of the uterus. This is referred to as a low-lying placenta (placenta previa).
Examples of Placenta
Human, bovine, horse, and canine placentas, for example, differ greatly at both the macroscopic and microscopic levels. The capacity of these species' placentas to give maternal immunoglobulins to the fetus is also different.
Functions of Placenta
The placenta acts as a lifeline between the mother and the fetus, ensuring that the fetus obtains the nutrients needed to survive from the mother's body. At the same time, it acts as a barrier, shielding the fetus from a variety of maternal illnesses. The placenta serves the following functions-
Because the fetus cannot feed, breathe, or expel waste while in the womb, the placenta effectively acts as the fetus's organ system.
It aids the fetus in obtaining adequate nutrition.
It collects waste for processing from the fetus through the mother's body.
It enables gas exchange, ensuring that the fetus obtains enough oxygen.
It generates hormones that keep the mother's body prepared for pregnancy.
Some microorganisms that might cause illness are filtered away by the placenta.
The antibodies are passed from the mother to the fetus via the placenta, giving some immunological protection.
The placenta helps regulate the fetus' bodily temperature.
Types of Placenta
The placenta can be classified into the following categories based on several criteria-
Classification Based on the Degree of Association Between Fetal and Maternal Tissue (EUTHERIAN MAMMALS)
The following table gives details about the types of placenta based on the different criteria-
Classification of Placenta
Description
Examples
Contra-Deciduate Placenta
A slightly modified kind of deciduate placenta, termed contra-deciduate placenta, exists in Parameles and Talpa (mole). In such circumstances, not only maternal tissue is lost but also the fetal component of the placenta, which is absorbed in situ by maternal leucocytes.
This sort of mammal (mole) includes Parameles and Talpa.
Deciduous Placenta
A placenta in which the chorionic villi are firmly lodged in the uterine endometrium, causing copious blood to leak from the uterine wall following villi removal.
Found in all primates, including Humans.
Non-Deciduous Placenta
Chorionic villi are loosely linked with the uterine endometrium in this placenta, so that the villi may be readily removed from the endometrium without any blood loss.
Pig, cow, goat, and so forth
Classification Based on the Distribution of Villi or Chorion
The following table gives details about the types of placenta based on the different criteria-
Classification of Placenta
Description
Examples
Cotyledonary Placenta
Chorionic villi are present in groups or patches in this placenta, while the remainder of the chorion's surface is smooth. Villi patches emerge as cotyledons. As a result, the cotyledonary placenta was coined.
Ruminants (such as a cow or a camel) are an example.
Diffuse Placenta
The villi are numerous and consistently scattered across the whole surface of the chorion, giving it a diffuse look. This type of placenta is known as a diffuse placenta.
Ungulates (pig, horse, etc.)
Discoidal Placenta
The villi of this placenta are limited to a circular disc or plate on the blastocyst's dorsal surface.
Insectivores, bats, and rodents.
Meta Discoidal Placenta
Primates have a specific sort of discoidal placenta in which villi are initially distributed but eventually become limited to one or two discs.
The Mon Discoidal placenta is seen in humans and contains a single disc-shaped villous region.
Discoidal Placenta: The monkey and ape placentas have two disc-shaped villous areas.
Zonary Placenta
Chorionic villi are located in a specific zone, like a band, in a semicircular way in this placenta. The term was chosen since the villi are present in a certain zone.
Carnivores (such as cats and dogs) are an example.
Classification Based on the Extent of Embryonic Tissue Involvement
The following table gives details about the types of placenta based on the different criteria-
Classification of Placenta
Description
Examples
Chorion-Vitelline Placenta
(yolk sac placenta)
The chorion-vitelline placenta is an association formed by the embryonic chorion, vitelline circulation of the yolk sac, and uterine epithelium.
Didelphys sp. and Macropus sp. are two examples.
Chorioallantoic Placenta
The chorioallantois placenta is a complex structure formed by the chorion and allantois of the embryo and the endometrium of the uterus, via which nutrients and other necessary materials from the maternal blood pass on to embryonic circulation.
For example, all uterine mammals.
Classification Based on the Extent to which Fetal and Maternal Tissue are Involved
The following table gives details about the types of placenta based on the different criteria-
Uterine Tissue
Epithelium
Connective Tissue outside Blood Vessel
Endothelium of Blood Vessel
Maternal Blood
Fetal Tissue
Chorionic Epithelium
Connective Tissue
Endothelium of Chorionic Blood Vessel
Fetal Blood
All of these tissues contribute to the creation of the placenta.
Purpose of Placenta
The placenta serves the following purposes-
Purposes
Details
The placenta is an endocrine gland that secretes hormones including estrogen, progesterone, and human chorionic gonadotropin (HCG).
Barrier
The placenta is an effective barrier (defensive wall), enabling useful aerials to reach the bloodstream. Toxic compounds from cigarettes, as well as addictive opioids like heroin, can pass through the placenta. As a result, pregnant women should avoid smoking and drug use. Viruses and bacteria can both travel through the placenta.
Excretion
Nitrogenous wastes such as urea are excreted by the mother's kidneys after flowing from the fetal blood into maternal blood via the placenta.
Nutrition
Food elements are delivered from the mother's blood to the fetal blood through the placenta.
Protein Digestion
It occurs when the trophoblast of the placenta breaks down proteins before delivering them to the fetal circulation.
Respiration
The placenta carries oxygen from the maternal blood to the fetal blood and carbon dioxide from the fetal blood to the maternal blood during respiration.
Storage
Before the infant's liver develops, the placenta stores glycogen, fat, and other resources for the newborn.
Histological Types of Placenta
The following table gives details about the types of placenta histologically-
Histological Types
Description
Examples
Epithelia-Chorial Placenta
The epithelio-chorial placenta is a loose interaction between the chorionic villi of the fetus and the uterine epithelium. The uterine epithelium folds to form pockets, and chorionic villi reside within these pockets. So six barriers exist between maternal and fetal blood.
Pigs, horses, marsupials, and other animals are examples.
Syndesmochorial Placenta
The uterine epithelium vanishes, and the fetus' chorion comes into direct contact with the connective tissue of the mother's uterine wall. Between maternal and foetal blood, there are five tissue barriers.
Sheep, giraffes, deer, and other animals are examples.
Endothelia-Chorial Placenta
The chorion of the foetus comes into direct contact with the endothelium of the uterine capillaries in this form of the placenta. Because the uterine epithelium and connective tissue both vanish, there are four tissue layers between the maternal and foetal blood.
Examples include a dog, a cat, and a fox.
Haemon-Chorial Placenta
The endothelium of uterine blood vessels is absent in this form of the placenta; therefore, the chorionic epithelium is bathed directly in maternal blood. There are just three tissue layer barriers. The chorionic villi are surrounded by gaps (sinuses) devoid of endothelial lining, into which maternal blood enters via the uterine arteries and travels to the uterine vein.
Examples include humans, insectivores (moles and shrews), and chiropterans (bats).
Haemon-Endothelial Placenta
Haemon-endothelial placenta - In this form of the placenta, the chorionic epithelium and connective tissue of the foetus are destroyed, causing the endothelium of the foetal blood vessel to come into direct contact with maternal blood and serve as the only barrier between the foetal and maternal blood.
Rodents (mice, rats, guinea pigs, and rabbits) are one example.
Common Types of Placenta
The placenta connects and develops wherever the fertilized egg implants in the uterus. The placenta can be implanted in various sites; however, the following are the most common-
Anterior Placenta: The placenta attaches to the front wall of the uterus, while the foetus develops behind it.
Fundal Placenta: When the placenta connects to the top wall of the uterus, this is known as a fundal placenta.
Left/Right Lateral Placenta: The placenta clings to the uterine wall on either the left or right side.
Posterior Placenta: The placenta develops in the rear of the uterine wall, where the fertilized egg is connected.
Points to Remember
A placenta is a round organ located in the uterus of a pregnant mammal.
During pregnancy, it nourishes the foetus through the umbilical cord.
From the chorion's outer surface, several finger-like projections known as chorionic villi develop into uterine tissue.
The placenta acts as a barrier, shielding the foetus from a variety of maternal illnesses.
The placenta begins to develop when a fertilized egg called a blastocyst implants in the maternal endometrium.
The placenta excretes foetal waste products. It travels to the mother's bloodstream.
The placenta releases the HCG (human chorionic gonadotropin) hormone during pregnancy, which pauses the mammal's menstrual cycle.
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