You've watched the module on the menstrual cycle and now we're going to build.
We're going to talk about what happens during the process of conception.
Now every good conception begins with fertilization and every fertilization begins with ovulation.
If you'll remember, ovulation occurs when the ova actually erupts
from the follicle and moves into the fallopian tube.
Now, think about the edges of the fallopian tube,
they look like little fingers and they are sort of beckoning
that ovum to move into the fallopian tube which is great because on the other side,
when we think about the uterus and the cervix,
we have sperms swimming up through the vagina, through the cervix,
into the uterus and out to the fallopian tubes and that's where they're going to meet.
Now typically fertilization occurs at the outer one third of the fallopian tubes
and that's what you see here, so after fertilization occurs,
then the cells are going to go through rapid division.
We have a 2-cell stage, a 4-cell stage and a 6-cell stage.
This is all part of cleavage which we'll talk about in just a second.
Now the cells are moving out of the fallopian tubes into the actual uterus
and it becomes a morula, which is again a bigger collection of cells;
and from that we have an early blastocyst that is formed
and then a late blastocyst, and the blastocyst is what's going to bury itself
into the endometrium and actually become our fetus.
Okay, lets break down the stages in a little more detail so that it's clear.
So you can see right here that the sperm is going to enter into the ovum,
that is actually the moment when fertilization occurs,
and then the cells are going to split and the cells becomes then the zygote,
this happens day 0 to 14.
Think back to our picture of the fertilization and by the time the cells reach the endometrium
and the inside of the uterine cavity we have the formation of the morula and the blastocyst.
So, by the time we get to the end of that eighth week, we're going to have a fetus.
We're going to talk about that in just a second,
but I want to break down what's going on inside the blastocyst.
So if you look at this picture what you'll see is an inner mass of cells
that are forming at the top and then you'll see the trophoblastic cells.
Now these trophoblastic cells will eventually become the placenta.
And then inside the blastocyst cavity, we're going to have the formation
of what is actually going to be the fetus.
So let's break this down just a little bit more and think about the embryonic primary germ layers
and how they're formed and what they're eventually going to become.
The ectoderm is going to give rise to the central and peripheral nervous system.
We're going to think about the sensory pieces that are involved in the eye and the ear and the nose.
Think about the skin and the nails, the mammary glands,
the hypophysis, subcutaneous glands and the enamel of the teeth.
Now we're going to move to the mesoderm.
Now the mesoderm actually becomes all the connective tissue.
Think about cartilage and bone and muscles.
The heart walls, the blood and the lymph vessels, the kidneys.
Think about our sexual organs, ovaries and testicles; the spleen, the adrenal cortices,
that's what's going to happen from the formation of the mesoderm.
And then, finally the endoderm,
and this becomes the epithelial lining in the GI tract and the respiratory tract.
When we think about the liver and the thymus and the thyroid and the lining of the bladder,
this is all comes from the endoderm. Okay, let's look at stage three
and let's think about the formation of the fetus and the development.
And as you can see as we progress through the weeks,
the baby actually begins to look more like a baby that we're familiar with.
So as we get to 10 weeks, we have something
that actually looks like the beginning of our Clitus the fetus.
One of the questions I'm often ask by my students is, how do you get twins?
Where do they come from? So let's talk about that.
So twins can be formed either from two separate eggs
or one egg that actually cleaves a little bit later in the process.
Let's first talk about a situation where there are two separate eggs.
Now, ideally during the menstrual cycle, we only release one egg at a time, but sometimes,
more than one egg will be released.
So if more than one egg is released it's possible that we can have twins,
and because we have two separate eggs, then we have what's called dizygous twins, two zygotes.
Two totally separate eggs and that's how we have fraternal twins
that look sometimes completely different,
or they may look alike just because they're siblings, but it's two completely different eggs.
We have two options for dizygous twins.
We can have a situation where the placenta are separate or a situation
where the placenta are fused together, but it's still two separate conceptus.
They are called dichorionic, diamniotic which means we have two chorion and two amnion,
so two separate sacs. They can be far apart or together and fused by the placenta.
What about identical twins?
Identical twins are actually formed from one single zygote, monozygous, one zygote.
Now, depending on when the splitting happens,
can have a pretty profound impact on the ultimate outcome of the pregnancy.
We can have early splitting which happens day 4 through 8, or we can have later splitting,
which happens after day 13.
We talked about the chorion, the outer part of the sac, and the amnion
which is the inner part of the sac.
Now the reason why this is important,
because those two babies can either be in the same sac or they can be in two separate sacs.
And why does that matter?
Well, there's an umbilical cord in there and I don't know about you,
but when I see two siblings together, sometimes they fight.
So if we have two separate sacs, we have a monochorionic and a diamniotic sac,
they actually are in separate areas so the cords don't get mixed up
and we're not likely to have a cord accident from just people tripping over cords.
If, however, we have monochorionic, monoamniotic sac,
both babies are in the very same sac together with two umbilical cords floating around,
and the potential for them to get tangled up is very high.
So if we have a situation where we know we have a monozygous twin formation
then we're going to be really vigilant about watching that baby, watching those babies,
because the chances of having a cord accident and losing those babies is going to be pretty high.
Now let's get to that very last category.
Now we're still talking about babies that are in the same sac, monochorionic, monoamniotic.
If that splitting happens later than 13 days, then sometimes the babies don't completely split
and they actually developed formed together or fused together
and that's how conjoined twins are formed.
So that's twin formation, now you know.