Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43

Summary

This video explores the incredible journey from a single cell zygote to a fully-formed human baby. It delves into the stages of early development, from blastomeres to morula to blastocyst, and the hormonal processes driving these changes. It also highlights the significant anatomical and physiological changes in the mother's body during pregnancy, and the complex hormonal sequence that initiates labor. The video underscores the remarkable coordination between maternal and embryonic tissues, culminating in the birth of a new human being.

Takeaways

• 📏 The human body grows from a tiny zygote, just 2 millimeters, to about 1.7 meters tall, an 850-fold increase.
• 👶 The process of human development starts with the division, differentiation, and development of cells.
• 🔬 After fertilization, a zygote rapidly divides during the cleavage phase, forming a morula and then a blastocyst.
• 🚶 The developing cells move through the fallopian tube to the uterus, where they implant in the endometrial lining.
• 💉 Hormones, especially hCG, play a crucial role in maintaining the pregnancy and supporting the developing embryo.
• 🫀 Pregnancy triggers significant anatomical changes in the mother, including increased blood volume and changes in organ positioning.
• 🍼 The placenta, formed from maternal and embryonic tissues, supports nutrient, hormone, and waste exchange between mother and fetus.
• 📈 As pregnancy progresses, the mother’s body undergoes various adaptations, such as increased flexibility and glucose hoarding for the fetus.
• 🫁 Labor is initiated by hormonal changes, primarily a decrease in progesterone and an increase in estrogen, leading to uterine contractions.
• 🏥 The labor process involves dilation, expulsion, and placental stages, culminating in the delivery of the baby and placenta.

Q & A

What is the initial size of a human zygote?

The initial size of a human zygote is about 2 millimeters, roughly the size of the head of a pin.

What are the three simple instructions that early human cells follow during development?

The three simple instructions are: Divide, Differentiate, and Develop.

What is the cleavage phase in embryonic development?

The cleavage phase is when the zygote's cells cleave or split in two over and over, starting about 24 hours after fertilization.

What is a morula, and when does it form?

A morula is a berry-shaped cluster of cells formed about three days after fertilization. It marks the end of the cleavage stage.

What role do trophoblasts play during early pregnancy?

Trophoblasts form the outer layer of the blastocyst and will develop into the placenta and blood vessels that nourish the embryo.

What hormone does the blastocyst secrete to ensure continued support from the endometrium?

The blastocyst secretes human chorionic gonadotropin (hCG) to maintain the endometrium and prevent menstruation.

What is the function of the placenta during pregnancy?

The placenta provides direct transfer of nutrients, hormones, and wastes between the mother and the developing fetus.

What anatomical changes occur in a mother's body during pregnancy?

A mother's body undergoes changes such as swelling of breasts, expansion of the uterus, increased blood volume, and greater urine production.

What hormonal changes prepare the mother's body for labor?

As labor approaches, progesterone levels decline, and estrogen levels increase, leading to the preparation of the uterus for contractions and the formation of oxytocin receptors.

How does oxytocin contribute to the initiation of labor?

Oxytocin binds to receptors in the uterus, stimulating the release of prostaglandins, which together cause uterine muscles to contract and initiate labor.

What happens during the dilation stage of labor?

During the dilation stage, contractions cause the cervix to thin and dilate to about 10 centimeters in preparation for the baby to be pushed through.

What occurs in the expulsion stage of labor?

The expulsion stage involves the mother pushing the infant head-first through the cervix and out of the vagina.

What is delivered during the placental stage of labor?

During the placental stage, strong contractions dislodge and expel the placenta from the uterine wall, delivering the so-called afterbirth.

What is the role of human placental lactogen (hPL) during pregnancy?

Human placental lactogen (hPL) helps prepare the breasts for lactation and causes the mother's body to hoard glucose for the fetus.

How does increased blood volume during pregnancy affect the mother's cardiovascular system?

The increased blood volume makes the heart work harder, and can cause swelling, varicose veins, and sometimes blurred vision due to fluid retention.

Outlines

00:00

🧬 From Zygote to Embryo

The journey from a single cell, the size of a pinhead, to a fully formed human begins with rapid cellular division and differentiation. The initial stage, known as cleavage, starts 24 hours after fertilization. Cells divide without growing, creating a cluster called a morula, which then transforms into a blastocyst. This blastocyst, containing an inner cell mass that becomes the embryo and trophoblast cells forming the placenta, travels down the fallopian tube to the uterus. Implantation occurs, driven by hormonal changes, setting the stage for further development.

05:00

🤰 Pregnancy: Mother’s Adaptations

After implantation, the blastocyst becomes an embryo, and the mother’s body undergoes significant changes to support the developing fetus. These changes include the expansion of the uterus, swelling of breasts, and increased blood volume by 40%. Hormones like relaxin, hPL, estrogen, and progesterone play crucial roles in these adaptations, loosening joints, preparing the breasts for lactation, and altering metabolism to provide nutrients for the fetus. These physiological changes highlight the profound impact of pregnancy on the mother’s body.

10:01

👶 Labor and Delivery

As pregnancy nears its end, hormonal shifts prepare the body for labor. Progesterone levels drop, allowing estrogen to trigger the uterus to start contractions by releasing oxytocin and prostaglandins. Labor progresses through three stages: dilation, expulsion, and placental delivery. Contractions push the baby through the cervix, and after delivery, the placenta is expelled. This intricate process showcases the coordination between hormones and physical changes necessary for childbirth.

Keywords

💡Zygote

A zygote is the initial cell formed when a sperm cell fertilizes an egg cell. In the video, the zygote is described as starting from the size of the head of a pin and growing into a fully-formed human being, highlighting the remarkable process of human development from a single cell.

💡Cleavage phase

The cleavage phase is the stage of early embryonic development where the zygote undergoes rapid cell divisions without growing in size. This phase is crucial for increasing the number of cells and setting the stage for further differentiation and development.

💡Blastocyst

A blastocyst is a structure formed in the early development of mammals, consisting of an outer layer of cells and an inner cell mass. The blastocyst stage follows the morula stage and is significant for implantation into the uterine wall, marking the beginning of pregnancy.

💡Implantation

Implantation is the process by which the blastocyst attaches to the uterine wall to receive nutrients and develop into an embryo. This critical step, driven by hormonal changes, enables the blastocyst to establish a stable and nourishing environment for further growth.

💡Human chorionic gonadotropin (hCG)

hCG is a hormone produced by the trophoblast cells of the blastocyst. It is crucial for maintaining the corpus luteum and ensuring the continued production of estrogen and progesterone to support pregnancy. This hormone is also the basis for pregnancy tests.

💡Placenta

The placenta is a temporary organ that develops during pregnancy, formed from both maternal and embryonic tissues. It facilitates the exchange of nutrients, oxygen, and waste between the mother and the fetus, playing a vital role in sustaining fetal development.

💡Relaxin

Relaxin is a hormone produced by the placenta that increases the flexibility of joints and ligaments, preparing the mother's body for childbirth. This hormone helps accommodate the growing fetus and eases the delivery process by loosening the pelvic joints.

💡Labor

Labor is the process of childbirth, initiated by a complex interplay of hormonal signals, including oxytocin and prostaglandins, which stimulate uterine contractions. The video details the stages of labor, including dilation, expulsion, and placental stages.

💡Oxytocin

Oxytocin is a hormone that plays a key role in childbirth and lactation. It stimulates uterine contractions during labor and helps with the expulsion of the placenta. In the video, oxytocin is highlighted as a major hormone driving the positive feedback loop that leads to delivery.

💡Positive feedback loop

A positive feedback loop is a process where the output of a system amplifies the system itself. In the context of labor, the release of oxytocin and prostaglandins creates a positive feedback loop, intensifying uterine contractions and facilitating childbirth.

Highlights

You were once the size of the head of a pin.

The average human is about 1.7 meters, which is about 850 times taller than when you started out as a zygote.

A single cell eventually grows into a fully-formed, several-trillion-celled body through the processes of dividing, differentiating, and developing.

The cleavage phase starts about 24 hours after fertilization, where a zygote turns from 1 cell into 16 cells called blastomeres.

About three days after fertilization, the cells form a morula, a little berry-shaped cluster of cells.

The morula transforms into a blastocyst, which contains an outer layer of trophoblast cells and an inner cell mass that will become the embryo.

The blastocyst moves down the fallopian tube toward the uterus and starts the process of implantation around a week after ovulation.

Hormones like estrogen and progesterone from the corpus luteum prepare the endometrium for the blastocyst's implantation.

Human chorionic gonadotropin (hCG) hormone keeps the corpus luteum active, maintaining the production of estrogen and progesterone.

The placenta forms, an organ created by both maternal and embryonic tissues, providing nutrients, hormones, and waste exchange between mother and offspring.

As the fetus develops, the mother's body undergoes significant changes, including increased blood volume by as much as 40 percent.

The hormone relaxin loosens joints and ligaments, increasing flexibility in preparation for childbirth.

The increase in metabolism and blood volume impacts the cardiovascular system, causing swelling and potential varicose veins.

Labor begins with a change in hormone levels, primarily a decline in progesterone and an increase in estrogen.

Oxytocin and prostaglandins stimulate uterine contractions, initiating the labor process through a positive feedback loop.

The stages of labor include dilation, expulsion, and the placental stage, culminating in the delivery of the baby and the placenta.

The process of pregnancy and childbirth is a complex interplay of hormonal signals, anatomical changes, and physiological adaptations.

Crash Course helps make educational videos accessible to everyone, supported by contributions from Patreon patrons.

Transcripts

You were once the size of the head of a pin. Let’s just start out with that.

Two millimeters. At the most.

And today -- I don’t know you, but I’m assuming that you’re a lot larger than that.

Many, many pin-heads from head to toe.

Assuming the average human is about 1.7 meters, it’s safe to say that you’re about 850

times taller than when you started out, as a zygote.

I think we all deserve a round of applause. I’m very proud of us. But how did it happen?

How does a single cell eventually grow into the fully-formed, several-trillion-celled

body we’ve spent all year discussing?

And for that matter, how the heck does a female body nurture, protect, and generally tolerate

growing a whole new person inside of it -- with all the mood swings, sore and swollen parts,

increased blood volume, constant pee breaks, and general body weirdness?

On an emotional level, I have no idea how the mothers -- or the zygotes, for that matter -- endure all of that.

But on a physiological level, like so many aspects of our body’s functions,

pregnancy begins and ends with the same thing: Hormones.

Last week we left off with a sperm and an egg, against all odds, finally getting together to make a zygote.

Now, how that tiny head of a pin gets to be even a fraction of the size you are now -- a

newborn human that’s, say, 50 centimeters long, is complex.

It involves a system of hormonal signals that -- as sophisticated as they are -- get interpreted

by the very earliest human cells into three simple instructions:

Divide. Differentiate. Develop.

The first step, dividing, begins in what’s known as the cleavage phase, when cells cleave,

or split in two, over and over.

It starts about 24 hours after fertilization, when a little zygote turns from 1 cell into

16 cells -- called blastomeres.

The cells divide so quickly that they don’t actually grow between divisions -- they just

create more smaller cells.

This allows each little cell to have more surface area, which helps them take in the

oxygen and nutrients they need from their environment.

And, of course, it also creates more raw materials for building an even larger zygote,

and, eventually, an embryonic human.

Like, you don’t make a car by carving it from a single chunk of metal. You put together

lots of smaller components, each with its own form and function. That’s what these

cells are during cleavage.

About three days after fertilization, these divisions have formed a little berry-shaped

cluster of cells that looks different and complex enough to get a whole new name -- a

morula, from the Latin for “mulberry.”

It’s one of the cuter terms you’ll come across in human physiology, and it also marks

the end of the cleavage stage.

Because: The little cells that make up the morula don’t stay together as a solid mass

-- instead, they start to form a hollow sphere filled with fluid -- a blastocyst.

A blastocyst contains a single outer layer of large, flat trophoblast cells, and inside

a cluster of smaller cells forms, called the inner cell mass.

This mass is what’s going to turn into the embryo, eventually, while the trophoblasts

will go on to form the placenta and blood vessels that will nourish it.

But, keep in mind: During all of these early divisions, the zygote, and then the morula,

are on the move -- they’re headed down the fallopian tube toward the uterus.

So, in addition to manufacturing a ton of tiny pieces and assembling them into something

that’s getting increasingly complex, the whole operation is mobile. It’s kind of

like you’re building a car while you’re driving it.

When the blastocyst reaches the uterus, it just floats around for a few days, soaking

up secretions that are full of vitamins and glycoproteins, looking for a place to call home.

But soon, it’s gonna need permanent nutritional support. So, about a week after ovulation,

it snuggles up to the endometrial layer and starts the process of implantation.

Now, up to this point, we’ve just been talking about the zygote itself, and the changes it’s

been undergoing. And we’re all very proud of it.

But from here on, the changes are best understood not just in terms of the budding, soon-to-be

embryo, but in terms of the whole environment where its changes are taking place.

That environment being: the mother’s body.

Implantation is only possible, after all, thanks to surges of estrogen and progesterone

from the corpus luteum -- the ruptured follicle that we talked about last time.

And together, they prepare the endometrium to receive the blastocyst, allowing the uterine

lining to bind to little proteins on the trophoblasts, holding onto them for the duration of the pregnancy.

If all goes according to plan, implantation takes about five days and finishes up around

twelve days after ovulation, right around when menstruation would otherwise kick in

and slough off the endometrium.

If that happened it would be bad, so the trophoblasts secrete a luteinizing-like hormone called

human chorionic gonadotropin, or hCG.

This hormone bypasses the whole hypothalamic-pituitary-ovarian axis -- if you remember that -- and talks

directly to the corpus luteum, telling it to keep pumping out estrogen and progesterone.

So, the first thing to notice here is that, from this point on, most of what happens to

this little clump of cells is happening because: Hormones.

hCG is basically calling all the shots, triggering the release of other hormones that are crucial

to the blastocyst’s development.

But while hormones are doing the grunt work, you might also notice that the blastocyst

is really in charge now. Essentially, it has taken over hormonal control of the whole uterus.

The release of hormones is that powerful.

In fact, a whole new -- albeit temporary -- organ is forming at this point, too, which is going

to take over the hormone-controlling job -- that’s the placenta.

The placenta is one of the defining and most amazing structures in the mammalian class.

It’s an organ that only appears during pregnancy, and is created by the melding of both maternal

and embryonic tissues.

Together with the umbilical cord, it provides for the direct transfer of nutrients, hormones,

and wastes between mother and offspring.

These elements start to really take shape after implantation is complete, and we enter

the embryonic stage. This is where the blastocyst differentiates into various cell types and

develops into a legit embryo, surrounded by an amniotic sac, and hooked up to the placenta.

And around the end of week eight, this tiny thing is now officially a fetus.

Over the next several months, it rapidly develops organ systems and bones, and grows into what

you see in the delivery room.

But rather than focus on those changes, I want to look more at what profound, and frankly

sometimes bizarre, adaptations the mother is going through.

The most obvious changes are of course the anatomical changes.

Basically, everything’s getting huge.

Her breasts swell and engorge with blood, and her baby bump gains dimension as the normally

fist-sized uterus expands, pushing organs out of the way until it pretty much takes

up the entire abdominal cavity from the diaphragm to the bladder.

The growing placenta is still secreting estrogen and progesterone, but it’s also pumping

out relaxin, a hormone that loosens joints and ligaments to increase flexibility, and

it also releases human placental lactogen, or hPL.

This hormone cocktail does helpful stuff like tell the fetus to grow, and get the breasts

ready to lactate, and it also causes the mother’s body to start hoarding glucose for the fetus to use.

This increase in metabolism, combined with the fact that the kidneys also have to process

waste from the fetus, leads to greater urine production, which every expectant mother is familiar with.

It also has a huge effect on the cardiovascular system, because a pregnant woman’s blood

volume can increase by as much as 40 percent.

40 PERCENT!

Imagine walking around with an extra 2 liter bottle’s worth of blood in your body, and

how much harder your heart would have to work to move it all around.

This increased blood flow and pressure can actually make your gums swell and bleed, and

fluid retention can literally change the shape of your corneas, potentially blurring your vision.

Not only that, but the expanded uterus compresses pelvic blood vessels, affecting veins’ ability

to bring blood up from the lower limbs, resulting in swelling, varicose veins, and if all that

weren’t bad enough, hemorrhoids.

Fortunately, every pregnancy eventually comes to an end, usually about 38 to 40 weeks after

fertilization, if all goes as planned.

But unfortunately, the process of getting the baby out is pretty much a painful mess.

Ask any mother -- giving birth is not for the faint of heart.

The process of preparing the body for labor, and then actually initiating it, begins -- AGAIN!

-- with a change in hormones.

Up until the last few weeks before birth, the placenta has been kicking out equally

high amounts of both progesterone and estrogen.

One of progesterone’s main jobs has been to keep the smooth muscles in the uterus relaxed,

so they can’t contract and stimulate labor too early.

But as the due date nears, the mother undergoes a sudden decline in progesterone.

Now, estrogen takes over. This is partially because the fetus itself is ready to go, and

it starts releasing hormones like cortisol, that tell the placenta to release even more

estrogen to get the uterus ready for birth.

Just like hCG calls the shots around the time of implantation, here estrogen is

barking out all kinds of orders.

For one thing, it prepares the uterus to start receiving new chemical signals, by triggering

its myometrial cells to start making receptors for the hormone oxytocin.

It also triggers the formation of gap junctions between smooth muscle cells in the uterus

-- this will let individual muscle cells contract simultaneously when the time comes.

Then, as labor nears, special cells in the fetus itself start secreting oxytocin, which

binds to all the newly-minted receptors and tells the placenta to release prostaglandins.

Together, both of these hormones -- oxytocin and prostaglandins -- stimulate the uterine

muscles to start contracting.

When the contractions get strong enough to distend the cervix, it stimulates the release

of even more oxytocin and prostaglandins, which keep the contractions rolling in one

big positive feedback loop, initiating labor.

The earliest stage of labor, dilation, is the period from when the first contractions

begin, to when the cervix becomes fully dilated, to about 10 centimeters.

During this time, each new contraction pushes the infant’s head against the cervix, causing

the cervix to thin and dilate.

Once the cervix is fully dilated, the mother should feel urge to push. The resulting expulsion

stage lasts from full dilation through crowning and actual delivery, as the infant is pushed

head-first through the cervix and out the vagina.

But even after the baby is out, the mother still has a placenta to get out. Within about

30 minutes of delivery, strong contractions carry out the placental stage of labor, dislodging

the placenta from the uterine wall to deliver the so-called afterbirth.

Then, finally, the mother can rest, and marvel at how her body just built another tiny body

inside it -- complete with all the complex systems we’ve spent the last year talking about.

And maybe someday that little baby will grow up and get a twinkle in its eye, and combine

alleles with somebody else, and start the whole process over again.

And that is how the human race continues to exist.

Today you learned about the stages of pregnancy, beginning with how a zygote develops into

blastomeres to a morula to a blastocyst and finally to an embryo and a fetus. You also

learned about the amazing anatomical changes that take place in the mother, and the hormonal

sequence of events that lead to labor. After that, all you have to worry about is how you’re

gonna put the little thing through college.

Thank you to our Headmaster of Learning, Linnea Boyev, and thanks to all of our Patreon patrons

whose monthly contributions help make Crash Course possible, not only for themselves,

but for everyone, everywhere. If you like Crash Course and want to help us keep making

videos like this one, you can go to patreon.com/crashcourse.

This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written

by Kathleen Yale, the script was edited by Blake de Pastino, and our consultant is Dr.

Brandon Jackson. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound

designer is Michael Aranda, and the Graphics team is Thought Cafe.