Implantation - Online Tutor, Practice Problems & Exam Prep

Implantation occurs between days 6 and 12 post-conception, involving the blastocyst embedding into the endometrium. Trophoblast cells differentiate into cytotrophoblast and syncytiotrophoblast, with the latter eroding the endometrium to facilitate nutrient absorption. By days 10-12, the embryoblast forms the bilaminar embryonic disc and amniotic cavity, while the syncytiotrophoblast secretes human chorionic gonadotropin (HCG), detectable in pregnancy tests. This process is crucial for establishing the placenta and supporting early embryonic development.

Implantation: Days 6-9

In this video, we're going to be talking about implantation, focusing specifically on days 6 through 9. So, implantation is the embedding of a blastocyst into the endometrium of the uterine wall, and this will usually begin around day 6 in a typical pregnancy. What happens is that the trophoblast cells, or the outer layer of that blastocyst, will adhere to the endometrium. At this point in the uterine cycle, the endometrium has become receptive to implantation due to a surge in estrogen and progesterone. Then, between days 7 through 9, the trophoblast cells will proliferate rapidly, differentiating into two layers, and those layers are the cytotrophoblast and the syncytiotrophoblast.

Then that syncytiotrophoblast is going to erode the endometrium and project long extensions into that endometrial tissue. It does that by secreting digestive enzymes as well as growth factors, and that is going to help the blastocyst absorb and digest nutrients. We will discuss that in a little more detail when we talk about placentation or the formation of the placenta a little later in the chapter.

To help you remember which of these layers erode the endometrium, we can focus on the prefix, 'syncedio,' which means fused or united. You can kind of think about how this is the layer that will go on to fuse with the endometrium, which, as I just mentioned, will help form the placenta later in development.

So, if you look at our image here, we have our blastocyst, and then this darker pink is the lumen, or the cavity of the uterus, and this lighter pink is the endometrium, or the inner layer of that uterine wall. You can see we have made contact with the endometrium there. And just like before, we have our embryoblast represented in blue. That is the portion that will go on to form the actual embryo. And now you can see we have this yellow portion kind of embedding itself and eroding into that layer of the endometrium there. So that is going to be our syncytiotrophoblast. And then we have this orange kind of inner layer right there, and that is our cytotrophoblast.

Alright. Those are the early stages of implantation, and I will see you guys in our next video. Bye bye.

Implantation Example 1

Alright. This one asks us, in the process of implantation, the blank cells attach to the uterine wall? And this one's actually quite intuitive if you just think about the structure of that blastocyst. Embryoblast. And looking at our other answer choices, the zona pellucida is already gone by this point, so b is out, and the amnion has not been formed yet, so d is also out. So you can imagine as this blastocyst is coming up against that uterine wall, the part of it that's actually going to attach is going to be that outer layer, which we know is the trophoblast. So our answer here is going to be c, and there you have it.

The trophoblast eventually differentiates into two layers. Which of these layers erodes the endometrium and begins projecting extensions into the uterine lining?

• A. The cytotrophoblast.
• B. The syncytiotrophoblast.
• C. The blastocyst.
• D. The morula.

Implantation: Days 10-12

In this video, we're going to be finishing up talking about implantation, focusing on days 10 through 12 post-conception. So, you guys remember how in our blastocyst, we have that outer layer of cells, our trophoblast. We've talked about how it is developing during this time, and then our inner cluster of cells or the embryoblast is going to go on to form the actual embryo, and that structure is doing some development during this period. So around days 10 to 12, the embryoblast is going to actually separate from the trophoblast and go on to form 2 new structures. First up, it's going to form the bilaminar embryonic disc, and bilaminar literally just means 2 layers. So, this is just 2 layers of cells kind of in a disc shape, and I'll show you that in one second. And then, it's also going to go on to form the amniotic cavity, and the amniotic cavity is basically just a gap between the embryonic disc and the trophoblast.

So if you look at our image here, just like before, we are seeing the lumen of the uterus in that darker pink on the right and then we are seeing the uterine wall or the endometrium in that lighter pink on the left. And then we have our little blastocyst, and it has grown a bit since we last saw it. But just like before, this orange is representing our cytotrophoblast, and then this yellow, where you can see it's kind of projecting extensions into that endometrium, is our syncytiotroblast. And then we have our embryoblast here kind of on the inside of that. And you can see in this structure here, we have these 2 distinct layers of cells. We have a blue layer and a yellow layer there, and that is the bilaminar embryonic disc. That is the structure that will go on to actually form the embryo. And then over here in this kind of sky blue, that little pocket right there is our amniotic cavity. So our amniotic cavity, obviously, right now, it's just kind of a little pocket right next to that disc, but that will in time go on to help form that amniotic sac and the amniotic fluid. So it's going to begin to kind of envelop and enclose that embryo a little later in development.

Now around days 10 to 12, the endometrial cells are going to proliferate and actually cover the blastocyst, which will make it fully implanted. So you can see here we have these light pink endometrial cells are now completely covering the blastocyst. So it is literally inside of the uterine wall at that point and at that point, it is fully implanted. And around this time, the syncytiotroblast is going to begin to secrete human chorionic gonadotropin, much better known as HCG, and that is the hormone that we can detect on a modern pregnancy test. So if you think about the timeline happening here, this makes total sense. So if we just draw a quick timeline like week 1, week 2, week 3, and then week 4. So week 4 is when a woman would typically be getting her period. Right? And we have ovulation happen at week 2. Ovulation, of course, leads to conception. And at this point, we are about 12 days or almost 2 weeks post-conception, which would place us right about here. And so by the time a woman is going to be missing her period potentially and knowing that she should take a pregnancy test, then we would have those nice high HCG levels, which would be at a level high enough to be detected by modern tests at that point. Alright. So, that is our kind of final phase of implantation, and I will see you guys in our next video. Bye-bye.

Implantation Example 2

So this one asks us, which of the following structures begins secreting human chorionic gonadotropin, or HCG, the substance detected by most modern pregnancy tests. So I'm going to give you 2 hints. One is that HCG is a hormone and two is that the placenta is going to be the big hormone producer during pregnancy and, hopefully, based on those clues, you're already thinking, well, the trophoblast goes on to form the placenta. So based on that, it has to be either a and b, which are the layers of the trophoblast and you are absolutely correct. Great job. So we are left with our cytotrophoblast and our syncytiotrophoblast and these both do go on to form the placenta, but what happens is our cytotrophoblast will go on to form more of like the structural support of that placenta, while our syncytiotrophoblast is going to not only help with nutrient and oxygen absorption but it's going to be our main hormone producer, including the hormone HCG. So our answer here is going to be b, the syncytiotrophoblast.

The process of implantation is usually complete by approximately:

• A. 1-2 days after fertilization.
• B. 4-5 days after fertilization.
• C. 10-12 days after fertilization.
• D. 20-28 days after fertilization.

Here’s what students ask on this topic:

What is the role of trophoblast cells during implantation?

Trophoblast cells play a crucial role during implantation by facilitating the embedding of the blastocyst into the endometrium. Around day 6 post-conception, these cells adhere to the endometrium, which has become receptive due to a surge in estrogen and progesterone. The trophoblast then differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast. The syncytiotrophoblast erodes the endometrium by secreting digestive enzymes and growth factors, allowing the blastocyst to absorb and digest nutrients. This process is essential for the formation of the placenta and the support of early embryonic development.

How does the syncytiotrophoblast contribute to nutrient absorption during implantation?

The syncytiotrophoblast contributes to nutrient absorption during implantation by eroding the endometrium and projecting long extensions into the endometrial tissue. It secretes digestive enzymes and growth factors that break down the endometrial cells, allowing the blastocyst to absorb and digest nutrients from the maternal tissue. This process is vital for providing the developing embryo with the necessary nutrients and establishing a connection with the maternal blood supply, which will later form the placenta.

What changes occur in the embryoblast between days 10 and 12 post-conception?

Between days 10 and 12 post-conception, the embryoblast undergoes significant changes. It separates from the trophoblast and forms two new structures: the bilaminar embryonic disc and the amniotic cavity. The bilaminar embryonic disc consists of two layers of cells that will eventually develop into the embryo. The amniotic cavity is a gap between the embryonic disc and the trophoblast, which will later form the amniotic sac and amniotic fluid, providing a protective environment for the developing embryo.

How is human chorionic gonadotropin (HCG) related to implantation and pregnancy tests?

Human chorionic gonadotropin (HCG) is a hormone secreted by the syncytiotrophoblast around days 10 to 12 post-conception. HCG plays a crucial role in maintaining the corpus luteum, which produces progesterone to support the endometrium and early pregnancy. HCG levels rise rapidly and can be detected in a woman's urine or blood, making it the basis for modern pregnancy tests. By the time a woman misses her period, HCG levels are typically high enough to be detected, confirming pregnancy.

What is the significance of the bilaminar embryonic disc in early embryonic development?

The bilaminar embryonic disc is significant in early embryonic development as it represents the initial stage of the embryo's formation. Comprising two layers of cells, the epiblast and the hypoblast, this disc will give rise to all the tissues and organs of the developing embryo. The formation of the bilaminar embryonic disc marks the beginning of the complex process of gastrulation, where these two layers will further differentiate into the three primary germ layers: ectoderm, mesoderm, and endoderm.