In this lecture on germ cell development,
we?re going to follow the process of fertilization
and implantation into the uterine lining.
Now after ovulation has occurred,
the oocyte does not immediately enter the oviduct
but is in actually in the peritoneal cavity for a limited period of time.
Generally, the oviduct?s fimbriae, the finger-like extensions of its end
will move towards the portion of the ovary that is released in the egg
and sweep the follicular fluid and the oocyte into the oviduct.
There we?ll move into the infundibulum and eventually to the ampulla
which is where most fertilization takes place.
On the other end,
approximately 1% of the spermatozoa that are entering into the vagina
can actually make it to the cervix and then to the uterus,
but thereafter, contractions of the uterus
and the oviduct will propel them towards the ampulla
where fertilization most commonly is going to occur.
But as the egg is released from the ovary, the corpus luteum will form
and allow a pregnancy be maintained should fertilization happen.
Now after the spermatozoa have been in the uterine environment
for a couple of hours, they become capacitated and the acrosome reaction occurs
which allows the cap on the spermatozoa, the acrosome,
to maneuver through the dense cloud of cells around the secondary oocyte
so as the cumulus oophorous cells have turned into the corona radiata,
they?ve maintained and protected the egg,
but now the spermatozoa are able to migrate through
and reach the zona pellucida,
that glycoprotein coating that?s deep to the corona radiata
and just on the outside of the egg.
As a single spermatozoa migrates and makes its way through that zona pellucida
it will fuse with the cytoplasm of the oocyte and it will move into the oocyte
and at that point fertilization has occurred.
And at this time, the egg does everything in its power
to prevent more than one sperm from entering the egg,
so there?s an immediate reaction where cortical granules in the cytoplasm of the egg
release and make the zona pellucida absolutely impermeable to further sperm.
Polyspermy will result in multiple problems such as aneuploidy,
too many copies of the chromosomes present,
so the egg does everything in its power to prevent that from happening.
Once the spermatozoa has entered the oocyte,
it completes the process of meiosis two and moves through metaphase
into the stage where it has a mature haploid nucleus
that can combine with the haploid nucleus
that was brought into it by the spermatozoa.
The male and female pronuclei within the cytoplasm of the egg
will replicate their DNA and then fuse.
This fused nucleus will then serve as the first nucleus
of this brand new and unique individual and the chromosomes will associate
and begin the process of getting ready for cell division
and will then proceed through mitosis as we?ve seen before,
so through metaphase, anaphase, telophase,
producing instead of one gigantic zygote, a multiple celled morula.
The first division creates a 2-celled zygote after roughly 30 hours
and we move on to the 4-celled stage after roughly 40 hours.
In this process, we don?t actually enlarge as an organism
but rather the cytoplasm that?s in existence within the egg
gets subdivided more and more finally.
The cells in the new zygote compact bringing them
a little bit further away from the zona pellucida
and protecting them from the outside environment.
And after three days, we have roughly 16 cells taking up the space
that was taken up by a single egg just a bit earlier.
At this point, we have a group of cells that are on the inside
separated from the zona pellucida
called the inner cell mass and the group on the outside,
conveniently called the outer cell mass.
After four days, the zona pellucida has degraded to the point
where it allows fluid from outside to permeate into the morula.
This creates a cavity known as the blastocyst cavity
and at this point we call the entire assembly a blastocyt.
The blastocyt have a group of cells on the inside that segregate off to one pole
with the cavity intervening on the other side.
These groups of cells are called the embryoblast and will give rise to all the tissues
and all the structures of the embryo, and thereafter the fetus.
The cells on the outside are called the trophoblast
and they will become the supporting tissues
for the developing embryo and fetus such as the placenta.
After about four days, the fertilized egg and then zygote,
then morula, then blastocyst has migrated into the uterus and is ready to implant.
At this point it starts producing human chorionic gonadotropin, hCG,
which is going to keep progesterone levels high
and maintain the follicular region of the uterus.
So it?s gonna keep the uterine lining thick
and able to support the growing embryo
and eventually, enough progesterone will be produced by the placenta
that we will no longer need the corpus luteum in the egg to do that job.
Because human chorionic gonadotropin is the first new hormonal signal
signaling that we have an implanted fetus,
it is what you use in pregnancy tests so pregnancy tests generally measure
the levels of human chorionic gonadotropin in the urine.
Eventually, the placenta will take over
and we have a cell sustaining embryo and fetus
that will maintain its environment and allow itself to develop further
up until the 9th month period when delivery occurs.