Placentation: Study with Video Lessons, Practice Problems & Examples

The placenta is a vital temporary organ formed from maternal and fetal tissues, facilitating oxygen, nutrient, and waste exchange during pregnancy. It produces essential hormones like HCG, estrogen, and progesterone, supporting gestation. Placentation begins during implantation, creating lacunae filled with maternal blood, and develops chorionic villi for efficient exchange. By week 12, the placenta is fully formed, with the maternal endometrium contributing to its structure, ensuring the fetus receives necessary nutrients and oxygen while removing waste products.

The Placenta

In this video, we're going to be talking about the placenta. So the placenta is the site of oxygen exchange, as well as nutrient and waste transfer between a mother and a conceptus, and it is a temporary organ that is derived from both maternal and fetal tissues and it is delivered after birth. So the placenta is going to be producing hormones to support the pregnancy all throughout the entire gestational period. And so one of the first hormones that it begins to produce is HCG like we already talked about. So, like we mentioned, the production of this begins almost immediately around days like 10 to 12 post-conception when that syncytiotrophoblast begins to release HCG and then we can detect that on a modern pregnancy test. Now, the placenta will also be producing estrogen and progesterone, even placental lactogen, as well as a hormone called relaxin. And the production of these hormones will begin a little bit more slowly and their levels will fluctuate throughout the pregnancy. And we're going to have a whole video talking about the effect that these hormones have on the mother. So for now just know that the placenta is producing them. Alright. And the final term we're going to go over is placentation. So placentation is just the process of forming the placenta and, like we talked about, it begins during the implantation phase and it continues throughout the entire fetal period. By the time a placenta is birthed, around 40 weeks, it's going to be between 1 to 2 pounds, so they get quite large. So what you see here is an image and this is how it would look if a baby and a placenta were birthed without the umbilical cord being cut. You can see we have the baby still attached to the placenta via the umbilical cord there. So we'll go ahead and label our umbilical cord and our placenta, and the placenta has a lot of blood vessels so it tends to be a similar color to, like, a liver where it's just, like, very, very, like, a rich deep red color. Alright. So that is our little intro to the placenta, and I will see you guys in our next video. Bye bye.

Placentation Example 1

This example asks us which of the following statements best describes the role of the placenta during pregnancy? So let's run through them and see what we have. So option A reads that the placenta encapsulates the developing fetus, holding it in place within the uterine cavity, and that is incorrect. The placenta basically grows up against the uterine wall. It does not encapsulate the fetus the way that the amniotic sac would, for example. So option A is incorrect.

Option B says that the placenta facilitates the exchange of nutrients and gases between the mother and the conceptus, and that is correct. That is a great definition of what a placenta does, so our answer is going to be B.

Looking at option C, I can tell that it's incorrect because it refers to the placenta as a permanent organ, and it's not. The placenta is a temporary organ that is birthed at the end of pregnancy. And then looking at option D, it basically says that the placenta functions during the first and second trimesters and then slowly dissolves within the uterine cavity, and that is also incorrect. The placenta will stay fully intact throughout the entire fetal period; it does not begin dissolving at any point.

So our answer here is going to be B, and I will see you guys in the next one. Bye bye.

Which of the following hormones is NOT produced by the placenta at any point during pregnancy?

• A. Human chorionic gonadotropin.
• B. Relaxin.
• C. Melatonin.
• D. Estrogen.

Placentation – Weeks 2-3

In this video, we're going to go over placentation in weeks 2-3, so kind of the earliest stages of placentation. So, do you guys remember how during implantation the syncytiotrophoblast is going to be releasing those digestive enzymes and kind of breaking down the endometrium a little bit? Well, that process creates pools of maternal blood within the endometrium. And those pools of maternal blood are known as lacunae or intervillous spaces. And as all this is happening and development is continuing, cells from the embryonic disc are going to proliferate and create a layer of extraembryonic mesoderm. So if you look down at our image here, we're seeing a conceptus that's approximately, like, you know, 2 and a half weeks or so. You can see we're fully implanted within the uterine wall and we have our little embryonic disc over here and you can see we have that blue layer and that yellow layer and the mesoderm is kind of building up in the middle, so all these little red cells that are kind of building up and forming that mesoderm. Then we know that the term extraembryonic just means outside of the embryo. Right? So extraembryonic mesoderm are just mesodermal cells that form on the outside of the embryo there. So all of these, like, reddish-pink cells are just mesodermal cells that are forming around the embryo essentially. And then just like we saw before, all of this yellow is our syncytiotrophoblast, just like we've seen in previous lessons. That's kind of eroding that endometrium. And you can see how around that syncytiotrophoblast we have all of these pools of maternal blood forming, so that is the lacunae. Alright. Now, over time the extraembryonic membrane and the syncytiotrophoblast are going to become a structure called the chorion. And you can kind of think of the chorion as like a pre-placental structure that basically just surrounds the conceptus and it will go on to become the outer layer of the embryonic sac. So if you look at our image here, we are now pretty much calling the extraembryonic mesoderm as well as that syncytiotrophoblast that all of this is now considered the chorion and that, of course, is going all the way around the conceptus there. So that is our chorion. And then the chorion is going to go on to develop projections called chorionic villi, and those are going to essentially enter the lacunae and they are going to be the main site of exchange between the mother and the conceptus. So nutrient exchange, gas exchange, waste exchange, that's going to be happening within these chorionic villi. So if you look at our image here, basically, these little extensions that are coming out of the chorion, those are the chorionic villi. And we are going to see much more mature versions of those coming up in our next video as we talk about placentation, a little bit later in development. So I will see you guys there. Bye-bye.

Placentation Example 2

Okay. So which structure is primarily responsible for nutrient and gas exchange between the mother and the fetus? That is, of course, going to be a placental structure because that is where all exchange between the mother and the conceptus takes place. So just knowing that, we can already eliminate c and d, and so we're left with the chorion and the chorionic villi. The chorion is going to be kind of the outer layer of the embryonic sac. It's basically going to be surrounding the conceptus, but the chorion itself is not going to be the structure that allows for the exchange that happens between the mother and the conceptus. That is going to be specifically those chorionic villi, which extend into those pools of maternal blood and that is going to allow for that exchange of things like nutrients, waste, and gas. So our answer here is b, the chorionic villi.

The chorion is made up of the:

• A. Syncytiotrophoblast and lacunae.
• B. Ectoderm and syncytiotrophoblast.
• C. Extraembryonic mesoderm and syncytiotrophoblast.
• D. Extraembryonic endoderm and syncytiotrophoblast.

Placentation – Weeks 4-12

Okay, let's finish up talking about placentation going through weeks 4 through 12 of development. So, around week 4, the chorionic villi are going to develop and extend, forming blood vessels that will connect to the umbilical arteries and veins. And then, from weeks 5 through 12, the endometrium is undergoing massive changes at a cellular level to accommodate the pregnancy and placentation, although we're not going to cover all of that in detail. There are two main things that you should probably know.

First, the endometrium that is beneath the embryo or away from the lumen is going to contribute to the placenta. This will essentially become the maternal portion of the placenta. The endometrium that is toward the lumen or the cavity of the uterus will go on to surround the embryo and the amniotic sac. If you look down here at our images, we are showing weeks 4 to 12 of development. Focusing on week 4 first, just to orient you, we are still fully implanted within the uterine wall there. The lumen or the cavity of the uterus would be in this direction and the uterine wall, toward the myometrium, is over in this direction. You can see we have our little embryo inside of the amnion and a little early umbilical cord forming there with these umbilical blood vessels, which are the two umbilical arteries and the umbilical vein.

You can see how we have these chorionic villi, which now have these intricate networks of blood vessels within them, nestled within these intervillous spaces filled with maternal blood all throughout there. We also have our maternal blood supply depicted here. To give you a closer view, we have this image here. What we're seeing is the umbilical cord right there, the umbilical blood vessels, and how those are connecting to the blood vessels within that chorionic villi. The chorionic villi are, of course, embedded in the intervillous space full of maternal blood, with the maternal blood vessels supplying blood to that space. Essentially, all of this is the portion of the endometrium that will be contributing to our placenta, forming the maternal portion of the placenta. Meanwhile, the chorionic villi form the fetal portion of the placenta, jutting into that maternal portion.

This will be the main site of all the exchanges between the mother and the conceptus, such as nutrient delivery, waste removal, and gas exchange, which will occur within those chorionic villi. Another important note is that you can probably see quite easily that the chorionic villi on this side, toward the lumen, are much smaller than those on the side toward the uterine wall. This is because these villi are getting somewhat flattened. As the embryo grows and the embryonic sac pushes out, they're getting compressed against the sac and this endometrial tissue.

Over time, these will essentially flatten out, which we can see clearly here in our week 12 image, showing a cross-section of a fully-formed placenta. By week 12, our placenta should be fully formed and should be acting as the conceptus' main source of all of its necessities like nutrients, gas, and oxygen exchange, and waste removal. You can see how this side of the placenta has basically flattened out completely. We are looking at that here; this orangey peach-colored layer and then we have the endometrial tissue here in pink that is completely surrounding the embryo and the amniotic sac, extending all the way out and around the conceptus.

Alright, that is placentation. I will see you guys in our next video. Bye-bye.

Placentation Example 3

So the placenta should be fully functional as the fetus' sole source of nutrient, gas, and waste exchange by what timeframe? We would expect to see the placenta be fully functional by about 12 weeks post-conception. So our answer here is going to be 12 weeks.

From 6 to 8 weeks, we are still seeing some development of the placenta, growth of the chorionic villi, and development of those blood vessels within the chorionic villi, but by about 12 weeks, we should see a good maturation of that organ, to the point where it can act as the sole source of exchange between the mother and the conceptus. So our answer here is 12 weeks.

Here’s what students ask on this topic:

What is the role of the placenta during pregnancy?

The placenta is a crucial temporary organ formed from both maternal and fetal tissues. It facilitates the exchange of oxygen, nutrients, and waste products between the mother and the fetus. Additionally, the placenta produces essential hormones such as human chorionic gonadotropin (HCG), estrogen, progesterone, placental lactogen, and relaxin, which support the pregnancy throughout the gestational period. By the time of birth, the placenta weighs between 1 to 2 pounds and is delivered after the baby. Its rich network of blood vessels ensures efficient nutrient and gas exchange, vital for fetal development.

How does placentation begin and develop during pregnancy?

Placentation begins during the implantation phase, around days 10 to 12 post-conception, when the syncytiotrophoblast releases digestive enzymes that create pools of maternal blood called lacunae. Cells from the embryonic disc proliferate to form the extraembryonic mesoderm, which, along with the syncytiotrophoblast, becomes the chorion. The chorion develops chorionic villi that extend into the lacunae, facilitating nutrient, gas, and waste exchange. By week 12, the placenta is fully formed, with the maternal endometrium contributing to its structure, ensuring the fetus receives necessary nutrients and oxygen while removing waste products.

What hormones does the placenta produce, and what are their functions?

The placenta produces several hormones essential for maintaining pregnancy. Human chorionic gonadotropin (HCG) is produced almost immediately after conception and can be detected by pregnancy tests. Estrogen and progesterone are produced to support the uterine lining and prevent contractions. Placental lactogen helps regulate the mother's metabolism to ensure a steady supply of nutrients to the fetus. Relaxin helps relax the ligaments in the pelvis and softens and widens the cervix in preparation for childbirth. These hormones play critical roles in supporting the pregnancy and preparing the body for labor and delivery.

What are chorionic villi, and what is their function in placentation?

Chorionic villi are finger-like projections that develop from the chorion, the outer layer of the embryonic sac. These villi extend into the lacunae, which are pools of maternal blood within the endometrium. The primary function of chorionic villi is to facilitate the exchange of nutrients, gases, and waste products between the mother and the fetus. They contain blood vessels that connect to the umbilical arteries and veins, ensuring efficient transfer of oxygen and nutrients to the fetus while removing waste products. This exchange is vital for the fetus's growth and development throughout pregnancy.

How does the maternal endometrium contribute to the formation of the placenta?

The maternal endometrium undergoes significant changes to accommodate pregnancy and placentation. The endometrium beneath the embryo, away from the uterine lumen, contributes to the maternal portion of the placenta. This area becomes rich in blood vessels and forms the intervillous spaces filled with maternal blood. These spaces interact with the chorionic villi, which are part of the fetal portion of the placenta. The endometrium towards the uterine lumen surrounds the embryo and the amniotic sac, providing structural support. By week 12, the placenta is fully formed, with the maternal endometrium playing a crucial role in nutrient and gas exchange.