So, in the last lecture we had seen how blastocyst was formed
and today we will discuss how this blastocyst is implanted into the uterus. Hello,
I am Dr Aizaz from medicovisual.com welcome to this visual lecture.
At about day 5 or day 6 after fertilization the zona pellucida degenerates, Right,
and that is called hatching of the blastocyst and as the blastocyst hatches out of zona pellucida
something is exposed on the surface of the cytotrophoblast and these are selectin molecules.
So, these selectin molecules are exposed and they bind with their receptors on Endometrium. Right,
so, in this way these Selectins, due to presence of these selectin molecules the
endometrium of uterus sort of captures the embryo that was floating Into, into the uterus. Right,
meanwhile the endometrium during this time period is well prepared for Implantation. How come? First
you can see the glands they become more deep, specialized and more tortuous and they start
secreting, they start pouring out uterine milk that, that provides nutrition to the embryo.
Secondly, the, the endometrium becomes more vascular, more the tissue,
endometrial tissue become more edematous and thirdly these cells, the endometrial cells
they become filled with glycogen and fat and as they become filled with glycogen
and fat they become fatty tissues not fatty tissues they become polyhedral shaped. Right,
so, these cuboidal cells as they become filled with fat and glycogen they become Polyhedral.
Right, these cells as they become filled with glycogen and fat and they become polyhedral now
they are called decidual cells and this, this process, this reaction it becomes more marked
and more pronounced as the implantation occur. With the touch of that embryo with endometrium
all these things, this reaction, this reaction it becomes more pronounced especially at the site of
implantation and then this pronounced reaction it spreads generally to the whole of the endometrium
and what is this called? This is called decidual reaction. Now a million dollar question here.
The question is that does decidual reaction occur each month during the secretary phase of menstrual
cycle or it will only occur if implantation will be there? yes? what do you think?
I’m not sure. Huh. That's a big controversy among embryologists
although this decidual-like reaction, this decidual reaction-like reaction
occurs each month during the secretary phase under the action of progesterone
but this reaction it becomes more marked and more pronounced only and only when this, this,
this embryo will touch the endometrium. Right, so most embryologists think that
as the embryo touches the endometrium and try to implant, burrow inside
then as the reaction becomes more pronounced that should really be called decidual reaction.
Right, but some other embryologists they think that this decidual like reaction because it
is occurring each month whether or not embryo comes, whether or not fertilization happens so
they say that each month decidual reaction occurs. So, it's a controversial thing.
Then there are some awesome embryologists who have really ended up this debate
what they say is that during each month in the mid secretary phase of menstrual cycle the decidual
like changes that occur that should be called pre decidual reaction and if the embryo comes and it,
it tries to implant in the, in the endometrium then it really is decidual reaction. So,
what they say is that pre decidual reaction occurs each month but decidual reaction only occurs when
embryo tried to implant itself into the Endometrium. Right, I will share reference
to the books that discusses pre decidual reaction in the description. Right, so let's move Forward.
So, initial loose attachment of embryo with the endometrium it is mediated by selectin
and then and then there is a tight attachment that embryo it really integrates in the endometrial
tissue by these integrins and what is this F? F is for? Well! I am not Sigmund Freud to say this for
embryo and mother relationship. F is for Fibronectin. So, this real tight
attachment between mother's endometrium and embryo is mediated by fibronectin and integrin
molecules. So, after this integration of embryo with the endometrium what happens is that these
trophoblastic cells they proliferate rapidly and they form new trophoblastic cells at the embryonic
pole. Right, at the site of attachment with the endometrium. Now, as they proliferate some of
these trophoblastic cells they fuse with each other and they lose their cell membrane, right,
and now they act as a single entity. Right, and this single entity which is,
which, which in English we call syncytium, this single entity is now called Syncytiotrophoblast
and these cells with a clear-cut cellular boundary or clear-cut cell membrane they are called
Cytotrophoblasts. Cyto means “cell” and trophoblast means as they are "concerned with
Nutrition” trophic, trophic mean nutrition so they are called Cytotrophoblast. So,
now trophoblast has been divided into syncytiotrophoblast and cytotrophoblast. So,
what this syncytiotrophoblast will now do is that since cytotrophoblast has excellent invasive
potential it, it can invade the endometrial tissue of mother by releasing some proteosomal enzymes
and some special factors that will cause apoptosis of cells of the endometrium. Right, and in this
way this embryo is embedded into the endometrial tissue. Right, and this is called implantation.
At the end of first week there is superficial implantation it is slightly embedded
into the endometrium and later on in the week two the embryo is more deeply embedded into the
endometrium. And remember as it is embedding into the endometrium you know these decidual
cells that were filled with the glycogen and fat so syncytiotrophoblast it gets nutrition by eating
up the glycogen and fat out of these cells. As the cells are eroded it gets nutrition from the,
from the glycogen and fat. Right, now as the embryo is embedded into the endometrium,
this endometrium now should not be shed off. As you know during the menstruation the functional
layer of endometrium is shed off each month and now because embryo is embedded into it,
it should not be shed off otherwise there will be loss of pregnancy and that should not happen.
So, this, this baby, this future baby should send a message to corpus luteum
and that message is in the form of HCG, human chorionic gonadotropin,
this HCG is released into maternal blood and it, as it goes to the ovary through blood
here it will stimulate the corpus luteum. Corpus luteum will become happy and it will proliferate
and it will start releasing lots of pro-gestational hormone. What is that?
Progesterone. Progesterone is pro-gestational hormone. As the level of progesterone continue
to rise this wall will not shed off. Right, and this corpus luteum it is now converted into a
happy corpus luteum, corpus luteum graviditatis or you can simply call it corpus luteum of pregnancy.
It remains there for about two months and it will continue to secrete progesterone
and after that it will be degenerated and placenta will be formed and that placenta will secrete its
own Progesterone. Right, another thing that HCG will do is that it will induce nausea
in the pregnant woman. Most of the pregnant women especially in first trimester they experience
nausea and why they experience nausea? The reason is that the culprit is HCG. Hundreds of years back
when there was no pregnancy test the elder ladies and mother of the family they used to see that
okay this woman is nauseating she must be expecting a baby. So, they start caring for
that woman. Right, so that is the significant of, significance of HCG. Another thing that it start
releasing is that EPF early pregnancy factor. So, at this time embryo start producing and secreting
early pregnancy factor. Interesting thing is that this early pregnancy factor it can be detected
in the serum of mother as early as about 36 to 48 hours after fertilization.
How come? Actually what happens that when the embryo is about 2 cell or 4 cell stage
it start producing some unknown factors and these unknown factors stimulate the
ovary of mother to produce EPF, early pregnancy factor, it is not itself producing EPF initially.
Right, it stimulates the ovary of women to produce EPF and later on it start producing its own EPF
early pregnancy factor. Both EPF and HCG, they have immunosuppressive Function.
You know this embryo it has its own unique genome which is different from the genetic makeup of,
of the mother. So, the maternal leukocytes, maternal immune system may reject this embryo
but EPF HCG and some other factors they prevent this rejection. Right, HCG can be detected in
serum of mother at about sixth or seventh day after fertilization. Some of the HCG it from the
mother's serum it also spills into urine and that is the basis of urinary pregnancy test.
Normally the embryo should implant at upper or upper middle part of body of uterus
at its anterior or posterior wall, usually at posterior wall.
Right, if it implants at any other location that is called ectopic
pregnancy. That is ectopic Implantation. That is implantation that is at an abnormal site.
What could be that abnormal sites? The most common is tubal pregnancy. It may implant in any part of
the uterine tube but most common is implantation at ampulla of uterus, ample of uterine tube.
Actually, as we have already discussed that the fertilized ovum I mean zygote it divides and
as it divides, as it undergo, as it undergoes cleavage it has to move toward the uterine cavity
and this movement is mediated by the ciliary action of cilia present in the uterine tube. So,
if there is some problem with the cilia the fertilized ovum, the zygote it may remain
at this point, at ampulla and it will continue to grow at this point and ultimately it may threaten
to rupture this uterine tube and that is an emergency situation. Right, sometimes what happens
that it may go back, the fertilized egg it may go back into, into the infundibulum of tube or maybe
even into the ovary or sometime it may even fall into abdomen and that is called abdominal ectopic
pregnancy. The tubal pregnancy as I have told you it is most common type of ectopic pregnancy.
Sometimes what happens that secondary oocyte it may fail to come into the ampulla
and the sperm may come here in the infundibulum or even it, it may go into the ovary to fertilize the
Secondary oocyte and sometimes even the zygote may fall back into the ovary. It
may also implant at cervix or in the interstitium of body of uterus. So,
this was about implantation and ectopic pregnancy. Thank you so much for watching this video.