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Course: High school biology > Unit 4
Lesson 5: Fertilization and developmentHuman fertilization and early development
Human fertilization and development begins with the race of hundreds of millions of sperm cells to fertilize an egg. The victorious sperm and egg unite, creating a zygote with genetic information from both parents. This single cell replicates, progressing through stages of a morula, blastocyst, embryo, and eventually a fetus. This complex process unfolds over approximately 36 to 42 weeks, resulting in a fully developed human being. Created by Sal Khan.
Want to join the conversation?
- Sal said something about it potentially forming a human life, but I know a lot of people especially in certain faiths like mine believe life is made in that moment. Scientifically when is it no longer potential life but actual life? When the egg attaches to the uterus?(11 votes)
- Depends on what do you mean by life?
Scientifically a cell is the smallest unit that is considered alive, it's the basic unit of life. So sperm cells are alive, and eggs are alive also. They are both living entities.
So, it's not really a "potential life" because it's already alive even before the conception, I suppose, it's an organism (zygote is considered an organism) that is a potential "human person".
When does it turn into a "human person" though? Well, that's a heated subject that many people feel strongly about, but scientifically a fetus is gaining personhood at around 20-24 weeks. Before this point it seems it lacks the necessary parts that we would consider would make it a person.
Yes, it may look like a baby, just like dolls do. Yes, it may have a heartbeat, just like hearts do. Yes, it may respond to stimuli, just like many living things do. But does it make it a person? Definitely not.
I sympathize with people who disagree with me, I really do, because I think many of them do it so out of the urge to be noble and kind. And for the longest time I was skeptical about my position. But now I feel confident in this stance, and I believe it is the most solid one out of the various ones I've been trying to adopt.
With this stance I am able to answer whatever hypotheticals can be presented to me with ease and coherency. It almost feels like math. I am open to changing my mind, if a sufficient challenge to this view is presented.(4 votes)
- If 1 sperm cell fertilizes the egg cell, what happens to the other thousands of sperm cells?(5 votes)
- They most likely die to to lack of ATP being produced.(11 votes)
- What if more than 1 sperm fertilizes an egg cell? Is that how twins were created?(4 votes)
- Nope. One sperm wins the race. Twins are when the resulting fertilized egg divides, or when there was more than one egg present to attract sperm.(6 votes)
- When a fertilized egg is dividing. That doesn't seem like the simple Mitosis that was described in the last unit. In the Mitosis videos it seemed like it was describing replication of a single cell.
Is this type of division still Mitosis or is it something else?(4 votes)- No it is still normal mitosis. Once the sperm cell fuses with the egg cell, it becomes one cell, known as the zygote. This zygote (fertilised cell) then divides just like any other cell going through mitosis.(4 votes)
- What if two sperms fertilization with one egg?(5 votes)
- If you are asking if two sperms can go in to the same fertilized egg then no the woman would need two individual fertilized eggs(2 votes)
- About how many weeks you are 1cm(4 votes)
- So when one sperm cell finds an egg and fertilizes it, then the baby comes and all that, but what about twins/triplets, etc.? Is it just 2 or more sperm cells fertilizing egg cells at the same time?(3 votes)
- Twins or multiples can result from either one fertilized egg splitting into two or more embryos (identical twins) or from multiple eggs being fertilized by separate sperm cells (fraternal twins or multiples).-hope this helps!(3 votes)
- If ya'll ever feel sad, just remember that you were the fastest sperm. Unless you're a twin, triplet, etc. If that's the case, I don't know what to tell you. Just love your sibling(s), I guess.(3 votes)
- The Kaplan biology textbook state that "the embryo becomes a solid mass of cells known as morula, which undergoes blastulation to form blastocyst"? As compared to this lecture video that states that embryo comes after blastocyst. Which is correct?(3 votes)
- The Kaplan biology textbook is mistaken. It should have said "the fertilized egg becomes a solid mass of cells known as a morula, which undergoes blastulation to form a blastocyst." The fertilized egg rapidly divides. The blastocyst implants in the uterine wall. It is not until the cells begin to differentiate--becoming distinct cell lines such as brain cells or skin cells or liver cells--that the blastocyst becomes an embryo.(2 votes)
- if1 sperm cell fertilizes the egg cell,what happens to the other thousands of the sperm cells(2 votes)
Video transcript
- [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 be appreciate and something like this occurred at the beginning of every one 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 you 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 pair, 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 pair. 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 form 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, it will then split into two,
and then split into four, and you have about a cell
division or two everyday 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 set 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 long this embryo is and eventually the embryo or the fetus is and 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 and 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 and 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, 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's 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.