The 5 stages of embryonic development

Embryology is the science that studies the development of a new human being. This covers from fertilization to birth, although some books also include the formation of gametes called gametogenesis.

It is a complex science that includes the investigation and explanation of all the changes and processes that occur in the formation of a new being. In this article we detail the different stages from the beginning of pregnancy to its end, that is to say the stages of embryonic development.

    Stages of embryonic development

    In its development, the embryo goes through a series of decisive stages and processes over the course of 40 weeks. Embryology divides these weeks into pre-embryonic period, embryonic period and fetal period.

    The embryonic period covers from fertilization (which occurs on the day established as zero) until the acquisition of a three-dimensional configuration in week 3. In the embryonic period, the outlines of all the future organs of the baby are formed, this goes from week 4 to 8. From week 9, we enter the fetal period when the organs and systems finish growing and acquire all their functions so that birth is possible.

    1. Pre-embryonic period

    As we said in the introduction, embryonic development begins with fertilization, which is established on day 0 of pre-embryo development. Fertilization refers to the encounter of a male gamete (sperm) with a female gamete (oocyte type two) in the fallopian tube (tube-like structure that connects the ovaries to the uterus).

    The pre-embryonic period lasts until the true embryo is formed, that is, when it ceases to have a layered or laminar configuration. The meeting of gametes produces a single cell called an egg or zygote. The single-celled structure that is initially in the ampulla (the upper third of the fallopian tube) begins its journey to the uterus.

    1.1. First week of preembryonic development

    The focus of this week is to reach the endometrium (the lining of the uterus), as this is the most ideal point for the successful implantation of the cellular structure and its growth.

    On its journey through the fallopian tubes, the zygote goes through a process of cell division called segmentation.. This is divided into 2 daughter cells, then into 8… And so on. These cells are called blastomeres.

    Thus, although it grows in number, the mass of cells does not grow, since it is initially surrounded by two thin membranes: the internal pellucida and the external radiated crown. This gives rise to a phenomenon known as compaction. The cells acquire a polarity: they are concave on the outside and convex on the inside.

    This particular arrangement gives this mass a blackberry appearance which is called morula. The morula specifically appears on the third or fourth day of preembryonic development and contains between 16 and 32 cells. It should be noted that the process of segmentation – or cell divisions – is exponential. The first division takes place 24 hours after fertilization; however, the others significantly reduce this time. An average newborn has 15 billion cells.

    The morula and the phenomenon of compaction give rise to a cavity which is located in the center of the structure. good, the cell structure is now hollow and begins to enter a fluid called blastocoel. This is called a blastocyst (immature cavity) which already contains two types of differentiated cells (day 5). The trophoblast, from which the embryonic appendages are formed (amnios, yolk sac, allantois, chorion and placenta). The embryo, strictly speaking, derives from the outermost layer. The embryoblast produces all human tissues.

    When it reaches the endometrium (between 5 and 6 days), to implant itself in the mucosa, the blastocyst must rupture the membranes which surround it. This process is known as hatching. In summary, at the end of the first week of development, we have a spherical structure differentiated into two cell layers (trophoblast and embryoblast) which has reached the endometrium.

      1.2. Second week of preembryonic development

      During the second week, implantation in the uterine mucosa continues and several changes occur at the intraembryonic level.

      First of all, the innermost layer – the embryoblast – is divided into two distinct layers: the epiblast and the hypoblast. At this moment we can describe the embryo (remember that it comes from the embryoblast) as a cluster of flat cells. This takes the number of bidermal or bilaminar embryonic disc. This first differentiation already makes it possible to establish a dorsal (epiblast) ventral (hypoblast) axis of the embryo.

      It is from the epiblast that all the structures and tissues of the body arise. In addition, from there, the first embryonic cavity is formed: the amniotic cavitywhich at some point in development will contain the embryo.

      The amniotic cavity comes from an “excavation” of epiblast cells in contact with the trophoblast. This is quickly covered with flat cells which derive from the epiblast called amnioblasts. Amnioblast is responsible for the production of amniotic fluid. A layer of flat cells separates from the epiblast. These cells are called amnioblasts and produce amniotic fluid. Finally, it should be noted that this cavity grows progressively.

      Cells migrate from the hypoblast into the blastocoel cavity to form the primary yolk sac. This is called Heusser’s membrane or exocoelomic membrane. It is a combination of hypoblastic cells and a short-lived extracellular matrix.

      Meanwhile, the layer of cells that surrounds the sphere, the trophoblast, is also divided into two sheets or layers. The syncytiotrophoblast, an undifferentiated tissue whose mission is to invade the uterine lining; and the cytotrophoblast an inner cell tissue that will serve as the anchorage of the embryonic chorion to the maternal endometrium. These two tissues will form the utero-maternal circulation system.

      At the end of the second week, the pre-embryo is fully implanted in the endometrium of the mother’s uterus. Implantation may produce a small amount of bleeding which is sometimes mistaken for menstruation.

        1.3. Third week of preembryonic development

        A trilaminar embryonic mass emerges during the third week of development; this process is known as gastrulation. This trilaminar germinal disc houses three different embryonic layers: an ectoderm, a mesoderm and an endoderm.

        Epiblast cells proliferate very quickly, so they begin to migrate and occupy new places. Thus, the epiblast displaces and indirectly displaces the cells of the hypoblast, which in turn gives way to two new embryonic layers: the endoderm and the mesoderm. These three layers establish the beginning of all organs and tissues derived from our body.

          2. Embryonic period (4 to 8 weeks)

          The embryonic period is between the fourth and eighth week. At this moment, the conceptus or preembryo changes from a flat to a cylindrical shape. This process is known as folding.

          The main biological process that occurs during this stage is organogenesis. During this time, the organs of the embryo begin to develop, which ultimately leads to the creation of future systems and structures. The embryonic cells proliferate and begin to behave in specific ways. The heart, the muscle, the gland and the future nails draw the first contours in the embryo.

          Of all the systems, the nervous system is the first to appear. East it develops from a structure known as the neural tube or spine (referring to its appearance outside the embryo). The process of formation of the nervous system is known as neurulation. It should be noted that the lungs will only be functional at the time of birth; this means that all organs do not evolve in the same way. The heart, for example, already has its structure with the four chambers and the great vessels at week 8.

          During this period, the embryo goes through what is considered the most dangerous stage. It is more susceptible to teratogens, or harmful agents, which can cause mutations. Therefore, the possibilities of developing abnormalities, whether minor or serious, are greater.

          3. Fetal period (8 weeks to end)

          Such and as we have seen, the changes which occur in the embryo are progressive. However, moving from the number to the fetus means that there are already sketches of all the important systems. The growth of the fetus accelerates during this period, and the tissues and organs of the fetus differentiate and specialize in their different functions. Lately, the fetus remains in the uterus during this period, known as the fetal period.

          During the fetal period, the head stops developing faster than the rest of the structures. In addition, over time, the fetus matures and develops defenses that reduce the risk of spontaneous abortion.


          Learning the basic concepts of embryology can help doctors determine the status of a pregnant patient and the developing newborn. As this article shows, the life cycle begins with the formation of a single-celled embryo and ends with its appearance in the world. Thanks to its discoveries, this specialty helps families understand possible abnormalities before birth and also offers treatments that ensure that the embryo continues to develop normally, without complications.

          Bibliographic references

          • Larsen, WJ (2003). Human embryology. Editorial Elselvier, Madrid.
          • Saladin, K. (2011). Human anatomy. New York: McGraw Hill.

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