As the implantation of the blastocyst progresses, morphological changes are observed in the embryoblast that are constituted in the formation of the bilaminar embryonic disc. These phenomena occur during the second week of gestation. Hello!
welcome or welcome to my Youtube channel in this video we are going to continue with the phenomena and changes that occur during the second week of gestation If you have missed the video of the first week I leave you here at the top right as a suggestion for you to access more quickly and easily before starting remember how he ended the first week, the blastocyst had begun invaginarse in the maternal endometrium and erodes this endometrium is eroded by the syncytiotrophoblast and is what allows the blastocyst is introduced inside the The outermost layer is formed by the cytotrophoblast and both the cytotrophoblast and the syncytiotrophoblast are part of the trophoblast. Finally , it should be said that between the seventh and eighth day the embryoblast began to differentiate into a hypoblast and an epiblast and we will see this now in more detail At the eighth day of development the blastocyst appears partially submerged in the endometrium mat erno on the one hand, as we have said, the trophoblast is differentiated into two layers in cytotrophoblast as the inner layer of mononuclear cells and in syncytiotrophoblast as the outer multinucleated layer without visible cell limits. It should be clarified that the cytotrophoblast in mitosis migrates outwards to form the syncytiotrophoblast On the other hand, the embryoblast differentiates into two layers into a layer or sheet of cuboid cells known as the hypoblast and into a layer or sheet of tall cylindrical cells called the epiblast, both the hypoblast and the epiblast constitute the bilaminar germinative disc in the epiblast.
small cavity that increases in size and becomes the amniotic cavity The cells of the epiblast that are adjacent to the cytotrophoblast are called amnioblasts and together with the rest of the epiblast line the amniotic cavity By the ninth day of development the blastocyst is implanted at a higher depth in the endometr Maternal io and as a consequence of its immersion on the surface of the epithelium, an occlusion is generated by a fibrin clot In the syncytiotrophoblast there are spaces that are called vacuoles and then these vacuoles merge and constitute large spaces called trophoblastic lagoons On the surface The internal cytotrophoblast begins to form a thin membrane that is known as the exocoelomic or Heuser's membrane, this membrane, together with the hypoblast, generates the lining of the exocoelomic cavity or also called the primitive yolk sac By days 11 and 12 of development, the blastocyst it is totally immersed in the endometrial stroma and the surface epithelium closes completely, causing only a small bulge in the lumen of the uterus Trophoblastic lacunae of the syncytiotrophoblast form a remarkable intercommunicating network and at the same time the syncytium cells penetrate deeper into stroma and erode the lining Endothelial growth of the maternal capillaries causing maternal blood to enter the lagoon system These capillaries that are congested and dilated are known as maternal sinusoids in this way as the trophoblast continues to erode more sinusoids, the maternal blood begins to flow through the trophoblastic system thus establishing the uteroplacental circulation At the same time a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity that form a loose and fine connective tissue, this tissue is called the extra-embryonic mesoderm and eventually occupies the entire the space located between the trophoblast on the outside and the amnion and the exocoelomic membrane on the inside. Soon in the extra-embryonic mesoderm, cavities develop that end up merging into one and create a new space known as the extra-embryonic cavity or chorionic cavity, later this space surrounds the primitive yolk sac and the amniotic cavity except at the point where the germinal disc connects with the trophoblast by means of the fixation pedicle that we will see later as a consequence of the appearance of the chorionic cavity the extraembryonic mesoderm unfolds into two leaves in a sheet that covers the cytotrophoblast and the amnion and is known as extraembryonic somatic mesoderm and another that covers the yolk sac and is called extraembryonic splanchnic mesoderm The maternal endometrium in these days becomes rich in glycogen and lipids and this tissue is also becomes edematous and these changes are known as decidual reaction By day 13, the scar in the superficial epithelium has already disappeared, however, sometimes there is hemorrhage at the implantation site as a consequence of the increase in blood flow to the lagoon spaces, such as this hemorrhage takes place around day 28 of the menstrual cycle can be mistaken as u na normal menstrual bleeding and in some cases it can make us fail in the calculation of the probable date of delivery the cytotrophoblast cells show local proliferation and penetrate the syncytiotrophoblast to organize cell columns surrounded by syncytium these cell columns with their syncytial covering are known as villi primary cells in the hypoblast, additional cells are produced that migrate following the interior of the exocoelomic membrane, these cells proliferate and give rise to a new cavity within the exocoelomic cavity and this new space is known as the secondary yolk sac or definitive yolk sac this yolk sac is much smaller than the primitive yolk sac and during its formation large portions are detached in the chorionic cavity called exocoelomic cysts the extra-embryonic mesoderm that lines the interior of the cytotrophoblast changes its name and the chorionic plate plus the syncytiotro is known as chronic plaque The phoblast plus the cytotrophoblast is known as the chorion and the chorion forms the wall of the chorionic sac where the definitive yolk sac and the amniotic cavity are suspended by the fixation pedicle this fixation pedicle with the development of the blood vessels becomes the umbilical cord Finally, if we approach this part of the bilaminar germinative disc we observe the parts that make up the epiblast and the hypoblast in the epiblast we will see the amnioblasts and in the hypoblast some cells have become cylindrical and this area becomes thickened. known as the prechordal plate and the prechordal plate on this site informs us of the location of the mouth and is also an organizing element of the head of the embryo.
Well, these are the phenomena that summarize all the events of the second week of development.
