Epiblast: what is it and what are its characteristics

Embryology is a sub-discipline of genetics and biology that deals with the study of morphogenesis, embryonic and nervous development from gametogenesis to the birth of living things. Life in humans begins with an egg and a sperm, two specialized haploid (n) cells that, after intercourse, join together and form a zygote (2n).

Humans have 23 pairs of chromosomes in the nucleus of almost all of our cells, for a total of 46. At the time of fertilization, the two haploid cells mentioned merge, so half of the genetic information that they give us. code comes from our father and the other half from the mother. This simple mechanism explains the keys to heredity in our species and many other living things, as there are also genetic recombination processes and spontaneous mutations that generate long-term variability in living things.

Beyond the genetic mechanism of reproduction and the formation of a viable embryo, it is really interesting to know how we went from a fusion of two cells to a fetus, with distinct and clear anatomical structures. Today we tell you all about epiblast, one of the cell lines present during gastrulation of embryonic development in mammals, reptiles and birds.

    What is the epiblast?

    In the field of embryology, 1 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 hinted at if we go to its etymological basis: ear means envelope, while the Greek term βλαστός refers to a germ, a bud or a plant. In the epiblast resides the germ of life, for without it human development could not be completed.

    Histologically, this layer of cells is described as a columnar epithelium rich in microvilli in its apical part. These appear on the 8th day after fertilization, and undergo epithelial-mesenchymal change throughout development to give rise to the precursor layers of the various organs and structures of living things.

    We suddenly introduced a lot of complex terms, but don’t worry. To start from 0 and be able to understand the definition provided, we dissect each of the complex words presented in the following lines.

    What is gastronomy?

    Gastrulation is one of the first stages of embryonic development produced after implantation of the blastocyst in the endometrium.. After the implantation of the product of the female egg and male sperm, between weeks 4 and 5 of pregnancy, the embryo begins to undergo very significant changes, among which are the processes that we describe in the future lines.

    It should be noted that the first cell body of interest that we encounter during pregnancy is the already named blastocyst. It is made up of around 200 cells and appears within the first 5-6 days after fertilization.

    This is the stage of development prior to implantation of the embryo in the maternal uterus, and is differentiated into 2 main structures: the internal cell mass (ICM) or embryoblast, which will then form in the embryo, and the trophoblast, the outermost cell layer that protects the blastocyst.

    Gastrulation is a process by which a trilaminar embryo is formed by the migration of cell populations located in the epiblast.. These plaques correspond to the ectoderm, the mesoderm and the endoderm, but we will see their peculiarities in the subsequent lines.

      Epiblast and embryogenesis in mammals

      The inner cell mass (ICM) described above forms a bilaminar embryonic disc. From her, epiblast and hypoblast occur. The hypoblast is located above the epiblast, consists of a series of cubic 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, as it gives rise, during development, to the ectoderm, mesoderm and endoderm. We dissect the meaning of each of these sheets below.

      1. Ectoderm

      The ectoderm is the outer layer of the embryo gastrula in metazoa, that is, the animals themselves. It is one of the lamellae that the embryo has during its development, so it is found in the fetus at the stage of pregnancy, until it differentiates and forms the structures for which it was designed.

      The most important structure that forms as a result of the ectoderm is the nervous system. It is the layer responsible for the formation of the brain, spinal cord and motor nerves, retina and neurohypophysis, among other structures. The outer ectoderm is also responsible for the formation of the outer epithelial tissues which characterize different living things, 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 forms between it and the endoderm: the mesoderm. The cells in this lamina begin to divide into different cell lines, which will give rise to different organs and systems. Among them are tissues such as cartilage, muscles, skeleton and dorsal dermis, circulatory system and excretory, among others.

      3. Endoderm

      It is the inner layer of the gastrula of the metazoan embryo. Like the mesoderm, the endoderm is formed by mitotic differentiation from the ectoderm, the first lamellae to form. As the epiblast gives rise to the ectoderm, this cell line is also said to be responsible for the formation of the two consecutive layers, as it is a direct consequence of this event.

      endoderm 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 cells lining the cells of the stuffing glands of important organs (such as the liver and pancreas), the epithelium of the ear canal and tympanic cavity, bladder and urethra, scams and many other structures.

      The differentiation of the epiblast

      We already know that the epiblast gives rise to the ectoderm and therefore to the 3 cell lines which will form all our organs during the development of the embryo. Therefore, we can define the functionality of the epiblast in the following essential points:

      • Germ cells are produced by the epiblast. They are induced in the embryo, forming in the posterior region of this cell line, caused by the factors BMP4 and BMP8b.
      • Invagination, cell migration and differentiation of epiblasts are essential for the formation of all of the structures described above.
      • The epiblast is known to give rise to all fetal cell lines.

      Due to its functionality, the epiblast is also referred to as “primitive ectoderm”. It gives birth to the fetus itself throughout pregnancy, while from the hypoblast derives the extraembryonic endoderm, or what is the same, the yolk sac. It should also be noted that the epiblast is not unique to humans (not 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 what we know, there are still many unknowns to be deciphered.

      summary

      The explanations provided here may have seemed very complex, but if we want you to stay with one central idea, it is this: the epiblast and hypoblast form a bilaminar embryo, a product of internal cell mass (ICM ) previously described. Through the release of various factors, germ cells, ectoderm and, consequently, mesoderm and endoderm are produced from the epiblast. Without the epiblast, we wouldn’t exist, because all fetal cell lines are derived from it.

      During this time, the hypoblast is responsible for these extraembryonic structures, that is, they 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 level of the individual and the species.

      Bibliographic references:

      • Brons, IGM, Smithers, LE, Trotter, MW, Rugg-Gunn, P., Sun, B., de Sousa Lopes, SMC, … and Vallier, L. (2007). Derivation of pluripotent stem cells from epiblasts of mammalian embryos. Nature, 448 (7150), 191-195.
      • Epiblasto, medical publications. Collected February 15 at http://publicacionesmedicina.uc.cl/Anatomia/adh/embriologia/html/parte2/bil_fra.html
      • Epiblasto, quimica.es. Collected February 15 at https://www.quimica.es/enciclopedia/Epiblasto.html
      • Lawson, KA, Meneses, JJ and Pedersen, RA (1991). Clonal analysis of the fate of epiblasts during the formation of the germinal layer in the embryo of mice. Development, 113 (3), 891-911.
      • Tesar, PJ, Chenoweth, JG, Brook, FA, Davies, TJ, Evans, EP, Mack, DL, … and McKay, RD (2007). New mouse epiblast cell lines share definitive traits with human embryonic stem cells. Nature, 448 (7150), 196-199.
      • Yamanaka, Y., Lanner, F. and Rossant, J. (2010). Segregation of FGF signal from primary endoderm and epiblast in mouse blastocyst. Development, 137 (5), 715-724.

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