Human fertilization and early development | High school biology | Khan Academy

• [Instructor] What we're gonna do with this video is talk about fertilization and development in human beings, or at least early development in human beings.

And this right over here is an actual image of fertilization about to happen or happening. So this right over here is a sperm cell, one of the many sperm cells that will come from a male human being, and this right over here is the egg cell that is inside the female human being that it is actually fertilizing.

Fertilization is the sperm cell coming together with this egg cell, and this act, this is the conception of what has the potential to turn into a real organism, in this case, a human being. So let's just first appreciate the scale here. Just appreciate how small these things are. A distance of about that would be approximately one-fiftieth of a millimeter. So this is happening on a very, very, very small scale.

Now another thing that you should appreciate and something like this occurred at the beginning of every one of us is that this is one of maybe several hundred million sperm cells are in competition to be the one that gets to that egg cell. And so you should feel proud of yourself, or at least half of yourself. You won a pretty big race! Think about it, there's 300 million people in the United States, that half of you that was from your father that won a race with the several hundred million of the other sperm cells from your father to get to be that DNA, that combination of DNA from your father that will then fuse with the combination of DNA that's from your mother in this egg cell.

And so how is that DNA packaged? Well, it's packaged in chromosomes. Chromosomes are just strips of DNA, and a full set of human DNA, in most of the cells in your body, you have 46 chromosomes and it's really 23 pairs of two. In each pair, you have one from your mom and one from your dad. But it turns out that in these sex cells, and we call these gametes right over here—let me write this down—these are gametes. So a sperm cell is the gamete that comes from your father, and an egg cell, or an ovum, is the gamete that comes from your mother.

Gametes have half the number of chromosomes. Each of these have 23, not 23 pairs, they have 23 chromosomes. And so when the sperm and the egg come together when the egg is fertilized, then all of a sudden, you once again have 46 chromosomes or 23 pairs.

But what does it look like as soon as fertilization has occurred? Well, this right over here is a picture of a zygote, which is this fertilized—you could view it as a fertilized egg. It's now starting to have all 46 chromosomes, and what's interesting here is you can see the nuclei from the two cells, they haven't completely fused yet. These right here, these are called pronuclei, but they will eventually fuse, and that, when you think about your first day as a cell, that's when the genetic makeup that you got from your father and your mother came together to be the genetic makeup that would eventually give all the information or most of the information necessary to construct you.

Now, as soon as this happens, you start having cell division, so the zygote, this zygote right over here, through the process of mitosis, will then split into two, and then split into four, and you have about a cell division or two every day or so. And so after about two, three days, you have about eight, then 16 cells. Those 16 cells are called morula, and then eventually, that keeps splitting, and after about five to nine days, you have something that looks like this, and you can't really appreciate it, but there's two to three hundred cells here.

There's two to three hundred cells in this sphere, and it's actually hollowed out on the inside. You can't fully see it right over here, but this thing is called a blastocyst. And then, and notice it's still roughly on the same scale; it's a little bit bigger than that original egg cell that was fertilized, but then this will continue to split and split and split.

And just to appreciate the entire stages that we're talking about, so when we talk about fertilization, we're talking about right over here. So all these pictures that I'm showing you are right in this phase, and then if you go maybe three, four, five, six, seven days, maybe in that first week, you get to this blastocyst stage, and so we are right over here on our timeline.

Now one thing that you might be intrigued by is why am I starting this at week two? Well, it turns out that there is also a gestational age, and gestational age is something that doctors and scientists will use as a measure of how far along this embryo is and eventually the embryo or the fetus is.

Gestational age, in particular, is measured from the first day of the mother's last menstrual cycle, which can be two weeks before the actual moment of conception, which I am showing right over here. So that's just a little technical thing that is interesting, and why you see this roughly two-week shift.

But then we get into this phase where people would consider it to be an embryo. Near the end of the embryo stage, you might have the potential human being to look something like this. This would be about seven weeks after conception, and you could see even here it is quite small. This is about one centimeter in length, so about the size of a blueberry.

But you hardly see something that's starting to look like a human being, or at least a mammal. One of the fascinating things about developmental biology is that if you look at the animal kingdom, especially things that are closely related to human beings, you will see even at this stage things look quite similar. But you already start to see things that you'd recognize. You can see arms right over here, you can see kind of an early ear, you can see an early eye, nose, even ribs it looks like, and so this is already beginning to resemble a human being, but it is quite small.

And if you look at the development of other mammals or even things that aren't mammals, even things like fish, you see things that look not too different from this at this stage. But then once you get into—you're going into the 10th, 11th, and 12th weeks, then people will call this a fetus.

So right now, this is an embryo. So this was an egg being fertilized, zygote, zygote goes to morula, goes to blastocyst, you're an embryo, then from an embryo, you go into being a fetus.

And so this is an image of a fetus being connected to the placenta, and the placenta is really the interface with the mother's body. And you can see even here it is quite small; depending on your screen size, this might be about the same scale. On my screen right now, this is about five centimeters in actuality, so this is about the size of this fetus.

And the boundary between embryo and fetus isn't super well-defined; that's why you see this kind of transition point, but most people would consider it to be once you get into about the 12th week, you're definitely a fetus.

Now some interesting things right over here as you see the entire cycle all the way until you get to the 36th through the 42nd week, which is when most, when what is considered kind of full term, when a baby is ready to come out. But you start to see over here this notion of viability, which is roughly a time where there's a decent chance that if the baby were to come out of the womb, they would survive on their own.

And this is just an interesting stat that there's roughly a 50% survival rate at around the sixth month. But the general big idea here, and it really is mind-boggling, all of us start in this almost grand race of sperm between hundreds of millions of sperm cells to be the one to fertilize your mother's egg.

And then that one cell, that zygote that has the genetic information from both your mother and father, now that that egg has been fertilized, it then keeps replicating until it turns into a fairly complex organism that is capable of making videos and reflecting about how it developed.