Epiblast: What It Is And What Are Its Characteristics
Embryology is a subdiscipline of genetics and biology that is responsible for studying morphogenesis, embryonic and nervous development from gametogenesis to the moment of birth of living beings. Life in the human being begins with an egg and a sperm, two specialized haploid cells (n) that, after sexual intercourse, join and form a zygote (2n).
Human beings have 23 pairs of chromosomes in the nucleus of almost all of our cells, that is, a total of 46. At the moment of fertilization, the two haploid cells mentioned fuse, so half of the genetic information that we have code comes from our father and the other half from our mother. This simple mechanism explains the keys to inheritance in our species and in many other living beings, since processes of genetic recombination and spontaneous mutations also occur that generate long-term variability in living beings.
Beyond the genetic mechanism of reproduction and the formation of a viable embryo, it is truly interesting to know how we go from being a fusion of two cells to a fetus, with differentiated and clear anatomical structures. Today we tell you everything about the epiblast, one of the cell lines present during gastrulation of embryonic development in mammals, reptiles and birds.
In the field of embryology, an epiblast can be defined as a layer of embryonic cells that appears during gastrulation (along with the hypoblast) and gives rise to the mesoderm and ectoderm The functionality of this cell line can be intuited if we go to its etymological basis: epi- means about, while the Greek term βλαστός refers to a germ, bud or shoot. The germ of life resides in the epiblast, since without it human development could not be completed.
At the histological level, this layer of cells is described as a columnar epithelium rich in microvilli in its apical portion These appear on day 8 after fertilization, and undergo an epithelial-mesenchymal change throughout development to give rise to the precursor layers of the different organs and structures of living beings.
We’ve thrown in a lot of complex terms all at once, but don’t worry. To start from 0 and be able to understand the definition provided, we dissect each of the complex words exposed in the following lines.
What is gastrulation?
Gastrulation is one of the stages of early embryonic development produced after the implantation of the blastocyst in the endometrium After the implantation of the product of the female egg and the male sperm, between weeks 4 and 5 of pregnancy, the embryo begins to undergo very important changes, among which are the processes that we describe in future lines.
It is necessary to clarify that The first cell body of interest that we encounter during pregnancy is the already named blastocyst This is made up of about 200 cells and appears the first 5-6 days after fertilization.
It is the stage of development prior to implantation of the embryo in the maternal uterus, and it is differentiated into 2 main structures: the inner cell mass (ICM) or embryoblast, which will subsequently form the embryo, and the trophoblast, the outermost cell layer that protects the blastocyst.
Gastrulation is a process by which a trilaminar embryo is formed through the migration of cell populations located in the epiblast These sheets correspond to the ectoderm, mesoderm and endoderm, but we will see their particularities in later lines.
The epiblast and embryogenesis in mammals
The inner cell mass (ICM) described above forms a bilaminar embryonic disc. Her, both the epiblast and the hypoblast arise The hypoblast is located above the epiblast, consists of a series of cuboidal cells and from it derives the extraembryonic endoderm (including the yolk sac).
Defining the role of the epiblast in mammals requires patience and prior knowledge, since it gives rise, during development, to the ectoderm, mesoderm and endoderm. We dissect the meaning of each of these plates below.
The ectoderm is the outer layer of the gastrula of the embryo in metazoans, that is, the animals themselves. It is one of the sheets that the embryo has during its development, so it is found in the fetus during the pregnancy stage, until it differentiates and forms the structures for which it was designed.
The most important structure formed by the ectoderm is the nervous system It is the layer responsible for giving rise to the brain, the spinal cord and motor nerves, the retina and the neurohypophysis, among other structures. The external ectoderm is also responsible for forming the external epithelial tissues that characterize different living beings, such as hair, nails, feathers, hooves, horns, cornea and many others.
2. Mesoderm
Through the process of mitosis of the ectoderm, A third layer of cells is formed between it and the endoderm: the mesoderm The cells of this sheet begin to divide into different cell lines, which will give rise to different organs and systems. Among them we find tissues such as cartilage, muscle, skeleton and dorsal dermis, circulatory and excretory systems, among many others.
3. Endoderm
It is the inner layer of the gastrula of the metazoan embryo. Like the mesoderm, the endoderm is formed thanks to the mitotic differentiation of the ectoderm, the first of the sheets to form. As the epiblast gives rise to the ectoderm, it is also said that this cell line is responsible for the formation of the two subsequent layers, since it is a direct consequence of this event.
The endoderm It is responsible for the formation of structures (cells and tissues) that are part of the histology of the digestive and respiratory system It also gives rise to the cells that line the gland cells that line important organs (such as the liver and pancreas), the epithelium of the ear canal and tympanic cavity, the urinary bladder and urethra, the thymus, and many other structures.
We already know that the epiblast gives rise to the ectoderm and, therefore, to the 3 cell lines that will form all our organs during the development of the embryo. So that, We can define the functionality of the epiblast in the following essential points:
Due to its functionality, the epiblast is also known as “primitive ectoderm.” It gives rise to the fetus itself throughout gestation, while the extraembryonic endoderm, or in other words, the yolk sac, derives from the hypoblast. It is also worth noting that the epiblast is not unique to humans (or even mammals), as it is also present in birds and reptiles. Anyway, The gastrulation process is different depending on the taxa consulted and, no matter how much is known about it, there are still many unknowns to decipher
Summary
The explanations provided here may have seemed very complex, but if we want you to stay with a central idea, this is the following: the epiblast and the hypoblast form a bilaminar embryo, a product of the Inner cell mass (ICM) previously described. Thanks to the release of various factors, germ cells, ectoderm and, consequently, mesoderm and endoderm are produced from the epiblast. Without the epiblast, we would not exist, since all fetal cell lines derive from it.
Meanwhile, the hypoblast is in charge of those extraembryonic structures, that is, those that do not affect the physical development of the fetus. Thanks to the joint action of these cell lines, all the organs and tissues are formed that allow us to be who we are, both at the individual and species level.
