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 beings alive. Life in humans begins with an egg and a sperm, two specialized haploid (n) cells that, after the sexual act, unite and form a zygote (2n).

Human beings present 23 pairs of chromosomes in the nucleus of almost all our cells, that is, a total of 46. At the moment of fertilization, the two aforementioned haploid cells fuse, so half of the genetic information that encodes us comes from our father and the other half from the mother. This simple mechanism explains the keys to heredity in our species and in many other living beings, since it is also produce processes of genetic recombination and spontaneous mutations that generate variability in living beings over the long term term.

Beyond the genetic mechanism of reproduction and the formation of a viable embryo, it is truly interesting to know how we went from being a fusion of two cells to a fetus, with anatomical structures distinct and clear. Today we tell you all about

instagram story viewer

the epiblast, one of the cell lines present during the gastrulation of embryonic development in mammals, reptiles and birds.

Related Article: "Epithelium: types and functions of this type of biological tissue"

What is the epiblast?

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 turn to its etymological basis: epi- means on, 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.

Histologically, 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 the development to give rise to the precursor layers of the different organs and structures of beings alive.

We've introduced a lot of complex terms out of the blue, 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 implantation of the blastocyst in the endometrium.. After 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 lines coming

It is necessary to clarify that The first cell body of interest that we encounter during gestation 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 the implantation of the embryo in the maternal uterus, and it differs in 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 embryo. blastocyst.

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

You may be interested in: "Neurulation: The Neural Tube Formation Process"

The epiblast and embryogenesis in mammals

The inner cell mass (ICM) described above forms a bilaminar embryonic disc. Her, both epiblast and hypoblast arise. The hypoblast lies 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 cards below.

1. ectoderm

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 stage of pregnancy, until it differentiates and forms the structures for which it was designed.

The most important structure that forms from the ectoderm is the nervous system.. It is the layer in charge of 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 other many more.

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, the skeleton and the dorsal dermis, the 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 be formed. As the epiblast gives rise to the ectoderm, it is also said that this cell line is responsible of the formation of the two consequent 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 systems.. It also gives rise to the cells that line the gland cells that line major 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 structures further.

The differentiation of the epiblast

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:

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

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 what is the same, the yolk sac, derives from the hypoblast. It should also be noted that the epiblast is not unique to humans (not even to mammals), as it is also present in birds and reptiles. Anyway, The gastrulation process is different depending on the taxa consulted and, despite the fact that it is known about it, 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 a central idea, this is the following: the epiblast and the hypoblast form a bilaminar embryo, 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, as all fetal cell lines derive from it.

Meanwhile, the hypoblast is in charge of those 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 individually and as a species.

Bibliographic references:

Brons, I. g. M., Smithers, L. E., Trotter, M. W., Rugg-Gunn, P., Sun, B., de Sousa Lopes, S. m. c.,... & Vallier, L. (2007). Derivation of pluripotent epiblast stem cells from mammalian embryos. Nature, 448(7150), 191-195.
Epiblast, Medicine Publications. Collected on February 15 in http://publicacionesmedicina.uc.cl/Anatomia/adh/embriologia/html/parte2/bil_fra.html
Epiblast, chemistry.es. Collected on February 15 in https://www.quimica.es/enciclopedia/Epiblasto.html
Lawson, K. A., Meneses, J. J., & Pedersen, R. TO. (1991). Clonal analysis of epiblast fate during germ layer formation in the mouse embryo. Development, 113(3), 891-911.
Tesar, P. J., Chenoweth, J. G., Brook, F. A., Davies, T. J., Evans, E. P., Mack, D. L.,... & McKay, R. d. (2007). New cell lines from mouse epiblasts share defining features with human embryonic stem cells. Nature, 448(7150), 196-199.
Yamanaka, Y., Lanner, F., & Rossant, J. (2010). FGF signal-dependent segregation of primitive endoderm and epiblast in the mouse blastocyst. Development, 137(5), 715-724.