The formation of the 3 embryonic layers will give rise to all the tissues and organs of the body. This may be the most important stage of embryonic development and it happens during the third week of gestation. Hello!
Welcome or welcome to my YouTube channel, in this video we are going to address some of the main phenomena and changes that occur during the third week of pregnancy. If you have missed the videos of the first two weeks, I leave you here at the top right as a suggestion to that you access in a faster and easier way Let us recall some of the components that we had seen at the end of the second week, the epiblast and the hypoblast. We said that both formed the bilaminar germinative disc the cavities formed were the amniotic cavity and the definitive yolk sac and then we found to the syncytiotrophoblast, to the cytotrophoblast and to the chorionic plate with the fixation pedicle The most relevant event in the third week is Gastrulation and it is the process by which the three germ layers are established in the embryo From the outside to the inside we have the ectoderm, the mesoderm and endoderm And in this way, they constitute a trilaminar embryonic disc To study this phenomenon we must make 2 cuts to separate the disk Bilaminar One level of the amniotic cavity and the other at the level of the definitive yolk sac and we will observe from a dorsal view I make a stop, to let you know that if you need a refresher cuts and views that are used in embryology in my introduction video to human embryology I show you in the last 2 minutes of the video We continue .
. Once we make the cuts we find this structure On the one hand we observe the buccal or oropharyngeal membrane which later constitutes the oral cavity And on the other, the formation of the primitive line begins on the surface of the epiblast and this formation begins Gastrulation. Finally , if we section this portion of the part of the amniotic cavity and the yolk sac we can Observe the bilaminar disk formed by the epiblast and the hypoblast.
With the appearance of the primitive streak, the polarity of the embryo is established and we can It allows to distinguish a cranial axis caudal to the previous formation of its cranial and caudal ends It also allows to identify the dorsal and ventral surfaces The dorsal surface located towards the amniotic cavity and the ventral towards the place where the hypoblast cells are located And also allows us to identify the right side and the left side of this embryonic disc that begins to take on characteristics of a trilaminar embryonic disc During the later development in the cephalic end the primitive line is the node or primitive nodule This node comprises a slightly elevated area and surrounds the Primitive fossa To understand the following phenomena we must make a cross section at this level and observe it from the bottom. Here the epiblast cells migrate towards the primitive line when reaching the region of the line they detach and change their configuration sliding below it This sinking movement is known as invagination and at invaginate, some of these cells displace the hypoblast and give rise to the most intimate germ layer, which is the endoderm Another group of cells invaginate and will be located between this endoderm in formation and the epiblastic cells to constitute the intraembryonic mesoderm The rest of the cells that remain in the epiblast constitute the most superficial layer called ectoderm And in this way the trilaminar germ disc is formed Now we are going to talk about the formation of the notochord and for this we are going to return to this image In it we had said that we observed the primitive node and to the oropharyngeal membrane We will make a cut at this height and take a medial view and we will see that there are cells that invaginate at the level of the primitive node and move in a cranial direction along the midline until reaching the precordal plate these cells are called cells prenotochord and the prechord plate is an area located caudally to the oropharyngeal membrane These cells Prenotochordal cells intercalate with the cells of the hypoblast and form the notochordal plaque As the hypoblast is replaced by endoderm cells, the cells of the notochordal plaque proliferate and detach from the endoderm, resulting in a solid cord of cells that extends from the prechord plate to the primitive node and this cord is called the definitive notochord. The formation of the notochord is very important, especially because it plays an inductive role for the formation of the neural tube, a process also called Neurulation Y, in turn, around It will form the spinal column in other words, it will serve as the axis for the formation of the column.
To add, at the moment in which the primitive fossa forms a notch, it will form a duct that temporarily connects the amniotic cavities and the yolk sac and that duct is known as the Neuroenteric Duct At 16 days on the dorsal side of the germinal disc we can distinguish r that In the cranial region is the oropharyngeal membrane and in the caudal region the cloacal membrane is formed in a similar way to the oropharyngeal membrane. If we make a sagittal section and look medially, we will see that both the oropharyngeal membrane and the cloacal membrane are made up only of endoderm and ectoderm This is super important and much wondered! And finally, when the cloacal membrane is established in the posterior region of the yolk sac , a diverticulum is formed that extends into the fixation pedicle, this is called allantois or allantoenteric diverticulum.
Regarding the growth of the embryonic disc, its growth is in a cephalocaudal direction due to invagination and migration of cells from the primitive line in a cephalad and lateral direction and later in a caudal direction This continues until the end of the fourth week of gestation About the development of Chorionic Villi In the video of the second week We had seen how the cytotrophoblast invaded the syncytiotrophoblast to form the primary villi. If we make a cross section we can see how the primary villi is made up of a cytotrophoblast nucleus and a cover of syncytiotrophoblast. Later, mesodermic cells invade the cytotrophoblastic nucleus and grow towards the endometrial or deciduous tissue If we make a cross section we observe that a secondary villus will have an outer layer of syncytiotrophoblast followed by cytotrophoblast and in the central part a mesoderm nucleus At the end of the third week the mesodermal cells that are in the center of the villi they begin to differentiate into blood cells and small blood vessels that give rise to the villous capillary system.
From here, the villi is called tertiary villi or definitive placental villi and the capillaries within the tertiary villi merge and form arteriocapillary networks that soon after connect with the embryonic heart and supply nutrients and oxygen when the heart begins to beat in the fourth week Now let's look at the image that the langman offers us at the end of the second week and notice how the cytotrophoblast is found in the internal part and how in this image from the end of week 3 the cytotrophoblast proliferates and spreads in the syncytiotrophoblast establishing contact with the maternal endometrium and thus forms the external cytotrophoblastic capsule For its part, the chorionic cavity grows and by day 19 or 20 the embryo is attached to its trophoblastic capsule by the fixation pedicle This pedicle later becomes the umbilical cord and establishes the connection between the placenta and the embryo. To finish, I show you this excellent image that shows the evolution of the chorionic villi The primary villi, the secondary villi and the tertiary villi The villi that are extend from the chorionic plate to the decidua basalis are called stem or anchor villi and the villi that grow from the lateral areas of the anchor villi are called free or branched villi and in these the main exchange of material between the mother's blood and embryo.
