Embryology: Some people break out in a cold sweat just from hearing the word. For many, it's a dreaded subject in their studies, and often, due to its complexity, students simply skim through it. But it doesn't have to be that way, because in this video series we want to approach the topic step by step. In this video, we start right at the beginning with the development of the embryo, namely in the first weeks of pregnancy. We will look at how the fertilization of the egg cell takes place and how the subsequent implantation in the uterine lining occurs. Enjoy! After the Kohl era, approximately 200 to 300 sperm cells reached the female genital tract. With the help of their motile tails, the sperm cells are able to move towards the egg cell. Once they are on their way, attracted by certain pheromones from the egg cell, they undergo an additional important maturation step, the so-called reproductive capacity. A number of changes now take place in the sperm cells. For example, certain... Glycoprotein removed from the sperm head. There, an important organ is located, namely the accumulator, which contains a multitude of columns and is shown here in red for you. Only through this important process is fertilization of the egg cell even possible. The accumulator's capacity increases the motility of the sperm and also enables the reaction, which we will look at again shortly. Now let's look at the further path of the sperm. As we can see, they have already passed through the vaginal canal and the uterus and are now swimming towards the fallopian tube. After ovulation, the egg cell is located there and is the fertilization sphere for approximately 24 hours. The first sperm have now also arrived in the ampulla. Therefore, we will now examine the processes between the sperm and the egg cell in more detail. Let's now turn to the egg cell. Here, it is important to have a basic understanding of its histological structure in order to understand the processes involved in fertilization. To better understand the structure surrounding the egg cell, we need to look at the so-called coronal radiator. This consists of several layers of glycoproteins located on the outside of the vasculature. The vasculature, which is marked here, is composed of various glycoproteins and plays a crucial role in fertilization. Furthermore, we can see that we have one vasculature and not three. This means that meiosis II is not yet complete. Now we come to the exciting part: how the sperm fuses with the egg cell. First, the sperm cell must penetrate the coronal radiator. A specific enzyme is located on the sperm cell membrane for this purpose. After overcoming this first barrier, the sperm cell reaches the vasculature. Here, it binds to a specific glycoprotein, for example, 3, which triggers the fertilization reaction. In this process, the proteolytic enzymes that were stored in the sperm head are released. In this way, the sperm can now... Overcoming the barrier and entering the gap (shown here in gray), the membranes of the egg and sperm cells fuse. This causes the entire contents of the bulky head and tail to be absorbed into the cytoplasm of the egg cell. Only one sperm cell can win this crucial race to prevent further sperm from penetrating the egg. Shortly after, several mechanisms prevent this. This creates the so-called politbloc. Among other things, there is a wave-like increase in the internal calcium concentration. This leads to the sperm releasing their contents into the gap (the area between the pilot zone, shown here in green, and the egg cell, shown in red). The result is a change in the composition of the proteins in the pelvic region, preventing sperm from binding and carrying out the necessary reaction. What are the further processes within the egg cell? The contents of the neck and tail area... The sperm cell deteriorates during the process, leaving only the head region with the genetic information. Precursors are now formed, initially containing the Hatloid chromosome sets from both father and mother. Furthermore, meiosis two of the egg cell is now complete, as we finally have three Pohl bodies. In the next step, everything is prepared for the upcoming cell division. The DNA in both precursors doubles, and the two structures now approach each other. In the final step, the nuclei of the precursors dissolve, and the paternal and maternal chromosomes align in a common spindle apparatus. In this way, the target cell, the first cell of the new organism, is formed. So, we have now seen how the chorionic chorion ( Cd) is formed through a series of steps. But what happens next? First, of course, more cells are needed, and that is exactly what happens now. During the passage through the fallopian tube towards the uterus, further division takes place, causing the Cd to reach the 22 stage. A little later, the embryo splits into the 14th stage. What's noticeable here is that the resulting embryonic cells, the so- called "burden," become progressively smaller because the outermost zone acts like a straitjacket. This prevents further changes in the zona pallidum, leading to early implantation in the fallopian tube. If we now jump ahead a bit, we see that we will eventually have 8, 16, and 32 cells. In the 16- to 32-cell stage, the structure is also called a Müllerian cell. Once in the uterus, our cluster of cells has already changed considerably. We now speak of a blastocyst, which consists of two different cell types, which we will examine in more detail shortly. Furthermore, the outer zone has now dissolved, making implantation actually possible. Overall, this journey through the fallopian tube took slightly less than a week. The number 10 points in the number of days indicates post-conceptions. In embryology, this is how the duration of pregnancy is measured from the date of conception. The time of fertilization is calculated as approximately five days after fertilization when the embryo arrives in the uterus. But how exactly is the blastocyst structured? Firstly, we have the embryo cells in turquoise and a dark blue one. Furthermore, a cavity has formed due to the influx of fluid, which is now called the blastocyst cavity. Additionally, the embryonic parts of the placenta develop from the uppermost cells during the course of development. The embryo cells are significantly more complex. All structures and organs of the future child develop from this cell mass. The actual implantation, or embedding of the blastocyst in the uterine lining, takes place approximately one week after fertilization. It is important to know exactly where this normally occurs in the uterus, namely usually in the upper part of the posterior wall. I have included a section through the uterus here, where the ideal implantation site is marked in green. Finally, let's look at exactly how the blastocyst... The first step in the endometriosis is when the blastocyst attaches itself to the mucosa with its embryonic structure, creating a firm bond. At this point, the embryo can no longer be easily washed out. In the next step, the blastocyst differentiates into two new cell types: the sesquiblast and the sesquiblast. As the name suggests, the sesquiblast is a more nucleated cell structure without cell boundaries. The cells of the previous blastocyst primarily serve as a supply for the sesquiblast, which burrows into the lower mucosa with the help of enzymes. In this way, the blastocyst enters the endometrium and can continue to grow and differentiate. That concludes the first week of embryonic development. From fertilization to the actual formation, a great deal happens. I will explain the subsequent developmental processes in upcoming videos. If you do n't have a video on embryology or If you don't want to miss other topics in anatomy or physiology, then please subscribe to the main channel.