Early Embryonic Development - Online Tutor, Practice Problems & Exam Prep

Cleavage occurs around days 2-3 post-conception, involving rapid mitotic divisions that produce genetically identical cells called blastomeres. This process increases cell number while decreasing cell size, enhancing nutrient absorption due to a higher surface-to-volume ratio. By day 3, the embryo, referred to as a morula, enters the uterus. Around days 4-5, the zona pellucida dissolves, forming a blastocyst with two cell types: the embryoblast, which develops into the embryo, and the trophoblast, which forms the placenta, essential for nourishing the developing conceptus.

Cleavage: Days 2-3

In this video, we're going to be talking about cleavage, which happens around days 2-3 after conception. So, cleavage is a series of rapid mitotic divisions, and because these are mitotic divisions they're going to be producing genetically identical cells called blastomeres. So the blastomeres are the main output here, and what's happening during cleavage is that the cell number is increasing. However, cell size is actually getting progressively smaller and there is a reason for both of these things. We want the cell number to be increasing because having a whole bunch of cells is going to act as a nice solid foundation as the development of that conceptus continues. So, if we had just one huge giant cell, that's going to make it a lot harder to build up that conceptus. Imagine you're trying to build a house and you were just given one huge slab of granite. Tricky, right? But if you're given a big old pile of little bricks that you can move around however you need to, much easier process. Same idea applies here. So having a bunch of little cells is going to make that process of building up the conceptus much more efficient. And then cell size becomes smaller because the smaller size gives the cells a higher surface-to-volume ratio, which allows for more efficient absorption of nutrients and oxygen to keep those cells nice and healthy. That's what's happening there.

And during cleavage, our embryo is going to be contained in the zona pellucida, which at this point is acting kind of like a little eggshell, if you can think of it that way. You can see we have the zona pellucida pictured in our image in purple surrounding our little developing conceptus there. So here in our image, if we look at day 2 when cleavage is taking place, we are still moving through the uterine tube. We are still pretty much in the ampulla of that tube and we're going to begin day 2 at a 2-cell stage. So we've got 2 little cells encased in that zone of pellucida there. And by the end of day 2, we're going to be at about 4 cells still traveling in that uterine tube. And then at day 3, we're going to begin day 3 at about the 8-cell stage still in the uterine tube. And then by the end of day 3, we're going to be at a 16-cell stage and a very exciting event. We have usually, by this point, left or are leaving the uterine tube and we are entering the lumen or the cavity of that uterus. And at this stage, the conceptus is sometimes referred to as a morula which translates to little mulberry. How stinking cute is that? So our little mulberry is entering the uterus, very exciting, and the big takeaway here is that the process of cleavage is just a series of rapid mitotic divisions and it's going to be producing blastomeres, a whole bunch of little tiny cells that will act as the building block of our conceptus moving forward. Alright, I'll see you guys in the next one. Bye bye!

Early Embryonic Development Example 1

Okay. So this one asks us, what is the primary outcome of cleavage in embryonic development? So remember, cleavage is going to be a series of rapid mitotic divisions that produce a whole bunch of little cells called blastomeres. So based on that our answer is going to be C, the production of blastomeres is the primary outcome of cleavage. Looking at these other answer choices, the zona pellucida is formed before conception even takes place, so that's definitely not an outcome of cleavage. B reads the formation of body organs, but we are a long way off from that. So B is out. And then D reads the differentiation of primary germ layers. But those germ layers don't even exist yet, let alone are they getting differentiated. So, D is definitely out and our answer again is C. I'll see you in the next one.

Which of the following is a benefit of cells getting smaller as cleavage occurs?

• A. Smaller cells are more resilient.
• B. Their surface area to volume ratio increases, allowing more efficient uptake of nutrients.
• C. Increased storage capacity for cellular organelles.
• D. None of the above

Blastocyst Formation: Days 4-5

In this video, we're going to be going over blastocyst formation. So around days 4 to 5, our conceptus is going to essentially hatch from that zona pellucida. Up until now, that zona pellucida has been acting mostly like an eggshell surrounding our conceptus, but it's going to begin to dissolve, and the conceptus will emerge from that structure and we now refer to the conceptus as a blastocyst. By this point, the blastocyst will have developed two distinct cell types. First, we have the embryoblast, which is the inner cluster of cells, and the embryoblast will go on to form the embryo. The embryoblast becomes the embryo. You can see here we have this little cluster of blue cells in our little blastocyst, and that is our embryoblast. And then next, we also have the trophoblast cells, which will make up the outer layer of the blastocyst, surrounding both a fluid-filled cavity and the embryoblast, and our trophoblast will go on to form the placenta. If you look at our little blastocyst here, you can see these orange cells going around the perimeter; that is the trophoblast. One way I remember that the trophoblast becomes the placenta is by seeing the word "trough," and I think of those wooden troughs that cows and horses eat from. So I think a trough is something that nourishes. In terms of pregnancy, the nourishing structure is going to be the placenta. So, if we look at our little timeline here, like I mentioned, around days 4 to 5, that zona pellucida is degrading. You can see here how it's dissolving a little bit, and at this point, our blastocyst is still free-floating within the uterine cavity. Then as you can see around day 6 or 7, we're going to be making contact with the uterine wall, and that will allow the process of implantation to begin. Our next few videos will actually be about that process, so I will see you guys there. Bye bye.

Early Embryonic Development Example 2

So for this one, we're going to be looking at these answer choices and determining which is the correct sequence of development stages that occur during cleavage. Cleavage is going to begin with our fertilized egg, which we know is called a zygote. Based on that, we can already narrow our answer choices down to c or d. From there, we're going to have cleavage take place, that series of rapid mitotic divisions that produce blastomeres. The blastomeres will eventually form a 16-cell morula, our little mulberry, and then that little mulberry will eventually turn into a blastocyst. Based on that, our answer is going to be d. We're going to start off with a zygote, we will then have blastomeres, a morula, and then a blastocyst. And there you go.

A hollow ball of cells created by cleavage is called a(n)?

• A. Blastocyst.
• B. Morula.
• C. Trophoblast.
• D. Embryoblast.

Here’s what students ask on this topic:

What is cleavage in early embryonic development?

Cleavage is a series of rapid mitotic divisions that occur around days 2-3 after conception. These divisions produce genetically identical cells called blastomeres. During cleavage, the number of cells increases while their size decreases, enhancing nutrient absorption due to a higher surface-to-volume ratio. This process is crucial for building a solid foundation for the developing conceptus. By the end of day 3, the embryo, now referred to as a morula, enters the uterus.

What are blastomeres and their role in early embryonic development?

Blastomeres are the cells produced during the cleavage stage of early embryonic development. These cells are genetically identical and result from rapid mitotic divisions. The primary role of blastomeres is to increase the cell number, providing a solid foundation for the developing conceptus. Their smaller size, due to continuous division, allows for a higher surface-to-volume ratio, which enhances the absorption of nutrients and oxygen, essential for healthy cell function.

What happens during the formation of the blastocyst?

Blastocyst formation occurs around days 4-5 post-conception when the zona pellucida dissolves, allowing the conceptus to emerge. The blastocyst consists of two distinct cell types: the embryoblast, which forms the embryo, and the trophoblast, which forms the placenta. The blastocyst is free-floating in the uterine cavity until around day 6-7, when it makes contact with the uterine wall, initiating the process of implantation.

What is the role of the trophoblast in early embryonic development?

The trophoblast is the outer layer of cells in the blastocyst, formed around days 4-5 post-conception. Its primary role is to develop into the placenta, which is essential for nourishing the developing conceptus. The trophoblast surrounds both the fluid-filled cavity and the embryoblast, providing structural support and facilitating nutrient exchange between the mother and the embryo.

How does the zona pellucida function during early embryonic development?

The zona pellucida is a protective glycoprotein layer surrounding the conceptus during the early stages of development. It acts like an eggshell, maintaining the integrity of the developing cells during cleavage. Around days 4-5, the zona pellucida dissolves, allowing the conceptus to emerge as a blastocyst. This dissolution is crucial for the subsequent implantation of the blastocyst into the uterine wall.