5 stages of embryonic development

These are the stages of embryonic development and their roles in the formation of the baby.

Embryology is the science that studies the development of a new human being. This ranges 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, the stages of embryonic development .

Stages of embryonic development

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

The embryonic period ranges from fertilization (which occurs on the day established as zero) to 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 the week 4 to 8. From week 9 we enter the fetal period where 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 as day 0 of the development of the pre-embryo. Fertilization refers to the meeting of a male gamete (sperm) with a female gamete (type two oocyte) 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 lamellar configuration. The meeting of the gametes produces a single cell called an egg or zygote. The single-celled structure originally found in the ampulla (the upper third of the fallopian tube) begins its journey toward the uterus.

1.1. First week of pre-embryonic development

The goal 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 known as cleavage . This divides into 2 daughter cells, then 8… And so on. These cells are known as blastomeres.

Thus, although it grows in number, the cell mass does not grow in size, since it is initially surrounded by two thin membranes: the internal pellucid membrane and the external corona radiata. 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-like appearance that is now called morula. The morula specifically appears on the third or fourth day of pre-embryonic 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 occurs 24 hours after fertilization; however, the others reduce this time considerably. An average newborn has 15 billion cells.

The morula and the compaction phenomenon give rise to a cavity that is located in the center of the structure. So, the cellular structure is now hollow and a liquid called blastocoel begins to penetrate . This happens to be called blastocyst (immature cavity) that already contains two types of differentiated cells (day 5). The trophoblast, from which the embryonic appendages (amnion, yolk sac, allantois, chorion, and placenta) are formed. The embryo, proper, derives from the outermost layer. The embryoblast produces all human tissues.

At the time of reaching the endometrium (between days 5 and 6), in order to implant itself in the mucosa, the blastocyst has to break the membranes that surround it. This process is known as hatching. In short, at the end of the first week of development we have a spherical structure differentiated into two cell layers (trophoblast and embryoblast) that has reached the endometrium.

1.2. second week of pre-embryonic development

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

First, the innermost layer – the embryoblast – divides into two distinct layers: the epiblast and the hypoblast . At this point we can describe the embryo (remember that it originates from the embryoblast) as a mass of flat cells. This takes the name of bilayer or bilaminar embryonic disc. This first differentiation already makes it possible to establish a dorsal axis (epiblast) ventral axis (hypoblast) of the embryo.

It is from the epiblast that all the structures and tissues of the body originate. Also, from this, the first embryonic cavity is formed: the amniotic cavity , which at a certain point in development will contain the embryo.

The amniotic cavity originates from a “digging” of the epiblast cells in contact with the trophoblast. This is rapidly covered by flat cells derived from the epiblast known as amnioblasts. The amnioblast is responsible for producing amniotic fluid. From the epiblast a layer of flat cells dissociates. These cells are called amnioblasts and they 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. This is a combination of hypoblastic cells and short-lived extracellular matrix.

Meanwhile, the layer of cells that surrounds the sphere, the trophoblast, also divides into two sheets or layers. The syncytiotrophoblast, an undifferentiated tissue that has the mission of invading the uterine mucosa; and the cytotrophoblast, an internal cellular tissue that will serve as an anchor of the embryonic chorion to the maternal endometrium. These two tissues will allow to 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 can cause a small amount of bleeding that is sometimes confused with menstruation.

1.3. Third week of pre-embryonic development

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

Epiblast cells proliferate very rapidly, so they begin to migrate and occupy new places. Thus, the epiblast moves 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 the derived organs and tissues of our body .

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2. Embryonic period (4 to 8 weeks)

The embryonic period occurs between the fourth and eighth week. At that time, the conceptus or pre-embryo changes from a flat shape 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 embryo’s organs begin to develop, eventually leading to the creation of future systems and structures. 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. This develops from a structure known as the neural tube or epinura (referring to its appearance on the outside of the embryo). The process of formation of the nervous system is known as neurulation. It should be noted that the lungs will not be functional until the moment of birth; this means that not all organs evolve in the same way. The heart, for example, already has its structure with the four chambers and the great vessels around 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, that can cause mutations. Consequently, there are greater chances of developing abnormalities, whether mild or severe.

3. Fetal period (8 weeks at the end)

As we have seen, the changes that occur in the embryo are progressive. However, the passage from name to fetus means that sketches of all the important systems already exist. The growth of the fetus accelerates during this time, and the tissues and organs of the fetus differentiate and specialize in their different functions. Lastly, the fetus remains in the uterus during this period, which is known as the fetal period .

In the fetal period, the head stops developing faster than the rest of the structures. Also, over time, the fetus matures and develops defenses that reduce the chances of miscarriage.


Learning the basics of embryology can help clinicians determine the status of a pregnant patient and the developing newborn. As shown in this article, the life cycle begins with the formation of a single-celled embryo and ends with its appearance in the world. Thanks to its findings, this specialty helps families understand possible anomalies before birth and also provides treatments that ensure that the embryo continues to develop normally, without complications.


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