At some point we've all asked, Where do babies come from? Your
parents may have sat you down at one point to have a little talk
about the birds and the bees, and your school teachers might
have explained the concepts of cell division, meiosis and
childbirth. However, this still begs the question, what happens
in between?
How did we get from one cell to the billions of cells that make
up each of us? How did our organ systems form? These questions
are probably what led you to click on this video, and if so,
you're in luck. In this video, we'll talk about embryology or
the changes that occur between conception and formation of an
embryo, which is a developing human eight weeks after
fertilization. But before we get ahead of ourselves, let's start
at the beginning with fertilization. fertilization
occurs as sex cells produced by your parents fused together to
produce a zygote. These sex cells also called gametes are
made in the gonads, which are specialized body structures used
in reproduction. The male gonads or the testes, while the female
gonads are the ovaries. Gonads produce gametes following a
special division called meiosis. Meiosis is really important for
two reasons. First, it scrambles your parents' genetic information
or DNA to increase diversity and prevent genetic diseases.
Second, it divides the information in half so that it
can be combined with another gamete later on. In males,
gametes are called sperm, while in females they're called ova,
or eggs. Now we've got mom and dad's gametes, but since humans
usually have two copies of their DNA in each cell, the sperm and
the egg need to combine their genetic material to get a full
set. This is where fertilization comes in. Following intercourse,
millions of sperm are deposited into the female reproductive
tract and raced towards the ovum. However, only around 100
of these make it through the female reproductive tract and
reach the egg, and usually only one sperm can fertilize each
egg. Ultimately, the sperm does this by eating through the
protective layers surrounding the ovum, fusing to release the
genetic material and producing zygote. Once fertilized, the
zygote wastes no time and starts dividing, all while moving
through the mother's reproductive tract towards the
uterus. In only a few days, the zygote has divided into a mass
of 12 cells called the morula. The cells keep dividing, and by
the fifth day after fertilization, there are 32
cells. Despite the cells dividing so quickly, they're
still within the egg's protective layers, so things are getting
pretty cramped. Once it's too cramped, the cell mass finally
bursts out of this container to form an expanded structure
called the blastocyst, which continues dividing. On the inside
of the blastocyst is a fluid filled cavity called the blastocele,
which will expand to accommodate the entire
developing embryo. Day five is also important for another
reason. Until now, we've only been considering the zygote and
its descendants cells, called blastomeres. These are a special
type of cell designated as totipotent, or all-powerful, because
they can form any type of cell. However, these cells are still
undecided as to their future. Just as you might decide to play
a sport, join a club or volunteer in your community,
these cells must take on a specialized role in embryonic
growth. Cells do this by making decisions that commit them to a
particular role, while distancing them from others. To
form a blastocyst, blastomeres have to specialize into
pluripotent, or many-powered, cells. While these pluripotent
cells still have a huge potential in what they can form,
they're slightly more specialized than before. Now,
you might be wondering, how do the cells know what they want to
be? Well, just as your family or friends might influence your
decisions, cells also commit to different roles based on signals
from their surroundings, although in cells, these
signals come in the form of chemicals. The first decision
cells make is whether they want to be part of the embryo. While
some cells might stick around to form the embryo, becoming cells
of the inner cell mass, others might form structures outside
the embryo that will support it as it grows. This fate decision
depends on two molecules called CDx2 and Oct4. Fancy
names, I know, but their function is pretty simple.
Associating with CDx2 causes a cell to form the external
structures, while Oct4 allows cells to say pluripotent.
As you might guess, CDx2 and Oct4 are mutually exclusive,
meaning that we can only have one or the other. By day seven,
the blastocyst finally reaches the uterus and implants into
its wall, where the CDx2-expressing cells will play a
very important role in forming the placenta, an interface for
the exchange of materials between the developing human and
their mother. Although they've made their first decision, the
cells still have many more important choices to make, as
they eventually give rise to the body's tissues. The inner cell
mass continues to divide, forming a hollow structure
called the amniotic cavity, which is enclosed by a layer of
cells called the amnion. Formation of this cavity causes
part of the inner cell mass nearest to the blastocele to
separate into a flat layer of cells, called the embryonic disc.
The embryonic disc is composed of two major cell types, each
expressing one of two chemical signals NANOG and GATA6. Cells
expressing the same signal come together and will each form
different tissues. Cells expressing NANOG will form
pluripotent epiblast cells, and those expressing GATA6 form
the hypoblast cells. While the hypoblast mostly makes
structures outside the embryo, the epiblast undergoes a
defining step in embryogenesis, called gastrulation. During
gastrulation, epiblast cells specialize into one of three
types, which will divide our two layered embryonic disk into
three layers, called germ layers, these germ layers are
extremely important because they will eventually form many body
structures. gastrulation begins 14 days after fertilization at
the tail end of the disk, where some cells of the epiglottis
come together and form a thickened line called the
primitive streak. This streak helps to define some major body
axes of the embryo and positions relative to those axes, such as
cranial, or towards the head, caudal, or towards the tail end,
left, and right. Think of these axes like the axes on the graph,
which help us to plot different points based on their placement
along each axis. At the cranial end of the embryonic disc, the
primitive streak expands to form a structure called the primitive
node, which contains a round indent, called the primitive
pit. The indentation spreads back along the primitive streak
towards the caudal end to form a primitive groove. Once this
primitive groove is formed, some epiblast cells move into the
groove, going onto the epiblast layer entirely, and then
spreading outwards. Depending on the extent of their migration
during gastrulation, different cells give rise to different
germ layers. Cells that displaced the hypoblast form the
endoderm, cells that don't migrate at all form the
ectoderm, and cells that migrate between these two layers form
the mesoderm. Together, these three layers are the beginning
of the embryo. Each germ layer gives rise to a variety of
organs and tissues through organogenesis, which occurs
between days 14 and 60, post-fertilization, the endoderm can
turn into several structures, including the lungs, the
bladder, a bunch of different glands and ducts, your tonsils,
and the lining of the digestive tract. The mesoderm gives rise
to the middle layer of skin called the dermis, the heart and
the rest of the cardiovascular system, the muscles that make us
big and strong, our bones and our kidneys. The ectoderm gives
rise to the top layer of our skin called the epidermis, tooth
enamel, parts of our eye, the outer ear, the nasal cavity, and
a large part of our nervous system. And with that, we have a
three centimetre long, 2.5 gram alien-looking creature.
Obviously, our embryo still has quite a lot of growing to do,
but luckily, it has a lot of time to do it, 38 weeks in fact.
During this time that embryos organs will develop and the
organism, now called a fetus, will continue to grow. A pretty
interesting change is the buildup of fatty tissue that
gives nutrients and insulates the fetus after it's born. This
fat, especially in the cheeks, can also help the baby to eat by
increasing the negative pressure it's able to generate in its
mouth. So when you see a chubby baby, remember that their chub
has got a function outside of adding to their cuteness.
When all is said and done, the fetus will be about 50
centimetres long, 3300 grams in mass, and able to survive outside
its mother. Now the baby is ready to join the outside world.