Within the world of Reproductive Biology, we can find many terms and concepts that people without specific health knowledge may confuse. This is the case of the words zygote, embryo and fetus, which definition and main differences will be explained hereunder.
It should be noted that these terms are used during the early stages of human development, known as Carnegie stages, which are based on the external and internal features of the embryo. The human embryonic period comprises 23 Carnegie stages that cover the first 8 weeks after fertilization.
Medical definition of ‘zygote’
By zygote or zygocyte we refer to the very first stage of life, which starts after the union of egg and sperm (male and female gametes). When gametes fuse together, a new cell with a core and 46 chromosomes is formed: 23 of these chromosomes are of paternal origin and 23 of maternal.
About 24 hours after the fusion, the first division of the new cell happens, and it is no longer called zygote. The embryonic period begins.
At what stage is a zygote called ‘morula’?
At the beginning, the embryo has two cells that keep on dividing themselves. When so many cells that cannot be distinguished exist, the embryo is called morula, because it acquires the appearance of a mulberry.
It is different from a blastocyst in the fact that a the term morula is used when the embryo is 3 to 4 days post fertilization, while it can be called blastocyst from day 4-5 onwards. Also, a morula is a 16-cell mass with a spherical shape.
It turns into a blastocyst through a process knows as compaction, which involves the following steps:
- Desmosomes and gap junctions begin to be formed within a few days after fertilization. It means that the cells on the outer part of the morula become bound tightly together.
- After that, a cavity forms inside the morula, which results in a “ball” of cells (inner cell mass or ICM). This group of cells and the outer part of the morula lead to the blastocyst, a stage which will be explained below.
Human embryo: definition & stages
From the moment two cells exist up until the 8th week of gestation, the development of the embryo takes place. At this stage, intense cellular changes occur.
Once the zygote is formed, the first cleavage of the human embryo occurs. Two cells are obtained from this process. Then, successive cleavage divisions will occur. The amount of cells and how rapid these divisions take place is known as morphokinetics.
Also, while there are certain embryonic cleavage patterns, each embryo is unique and can divide in a more or less quick way depending on its quality. Thus, it will be qualified as a high-quality embryo provided that it follows the common patterns of embryonic cleavage.
A day-2 embryo comprises usually 4 cells, which should be of similar size. Likewise, a day-3 embryo of optimal quality is usually composed of around 7-9 cells.
When the embryo is on day 5, it is called blastocyst. It is from this moment on when it acquires a particular shape with different cell types.
A cavity forms inside the embryo and a series of cells called trophoblast will be responsible for placental development. The inner cell mass, from which the baby’s body will form, is located within a section of this cavity. When it is at blastocyst stage, the embryo breaks down the membrane that is protecting it to enter the endometrial lining.
Embryo implantation occurs between day 7 and 8. This is a crucial moment, since some embryos won’t be able to implant. Implantation stands for the fixation of the embryo—at blastocyst stage—in the woman’s uterus, in particular, in an inner layer called endometrium.
Implantation will finish (more or less) on day 14 after fertilization. On this stage, we can say the endometrium is invaded by the embryo. If this process goes as expected, one can confirm that the woman is indeed pregnant. From this moment on, production of the beta-hCG hormone begins.
From this point on, the embryo grows at a rapid pace and goes from having a rounded and unrecognizable form to acquiring a slightly elongated shape, similar to the silhouette of a baby but without any recognizable details.
What is a human ‘fetus’?
From week eight of your pregnancy onwards, the embryonic period ends and makes space for the so-called fetal period of pregnancy.
Throughout the first month, overall body proportion is unequal and head volume is bigger than other parts of the body. As the months go by, the rest of the body will gradually acquire a larger proportion until the silhouette of the fetus is fully formed.
The fetus has very specific levels of cell specialization. This gives rise to tissue formation, from which the body’s major organs, such as the brain, the liver, and kidneys develop.
This is the longest stage of pregnancy. The baby will be called fetus until childbirth, that is, between weeks 38 and 40 of pregnancy. After childbirth, the fetus is not called like that anymore and is renamed baby.
Comparison with other animals
Vertebrates, that is, animals with backbones share a common ancestor, and the genetic information that guides their development is almost the same. However, every living organism has a molecule known as deoxyribonucleic acid (DNA), which is the set of codes that act as the “blueprint” of each being.
That is why, when they begin to grow, all vertebrates look so much alike: fish, chickens, dogs, lizards… If we see a picture of any of them at the same embryo stage, we might find almost no difference between their external characteristics:
At the early stages of development, a human fetus may look like a fish or a chicken embryo, but the different parts of the DNA molecule of the human will play their role and build the particular molecules that a human growing body needs.
Although it may seem that the folds of a fish embryo resemble those of a human embryo to a large extent, this resemblance stops as they grow. These are some of the most common resemblances between human embryos and other vertebrates:
- Gills: Humans are not related to fish. What seems to be the “gills” of a fish end up developing into the glands and structures of the neck and ears, not into the lungs, mouth and trachea.
- Tail: When the coccyx is developing, it resembles the tail of a reptile or other mammal. That is the reason why it is also known as tailbone, although it is not literally a tail. In fact, it is a unique part of the human body: the final segment of the vertebral column.
- Yolk sac: What seems to be the source of nourishment as in a bird egg is actually the source of the first blood cells of the human embryo.
Even though all human and animal embryos go through similar stages of development, the main difference is how long it takes for each specie to reach the same stages.
FAQs from users
What is the pronunciation of ‘zygote’?
The right pronunciation of this medical term is /ˈzaɪgəʊt/ or /zīˈgōtˌ/. The term comes from the root Greek word zygōtos, which means to yoke or to join. Some synonyms are zygospore or zygocyte.
Zygote or embryo, which comes first?
As explained above, the zygote is the first stage of human development right after fertilization occurs. It is the result of the fusion between gametes: the haploid ovum cell from the female and the haploid sperm cell from the male. The resulting diploid cell is what we know as zygote.
What does a human fetus look like?
The development of a human fetus changes by weeks or even by days. It is not possible to provide a detailed description of the appearance of a human fetus, because it changes as the different parts of the organism develop.
At 4 weeks, it begins to develop the structures that will form its face and neck, but it does not look like a baby yet. At 8 weeks, it is more or less half an inch in size and, by week 12, it measures about 2 inches and the sex organs start to become clear.
For more details, see also: The stages of pregnancy.
What is the difference between ‘zygote’ and ‘gamete’?
Gametes are reproductive cells that unite at fertilization and create a new being: the zygote. The female gamete is what we know as egg cell, ovum or oocyte; the male gamete is the sperm or spermatozoon.
Gametes are haploid cells, while the zygote is a diploid cell: while the former has just one complete set of chromosomes, the latter contains two complete sets. Another difference is that diploid cells reproduce by mitosis, while haploids do so by meiosis.
Do all human fetuses begin as female?
No, that is not true. The sex of humans is determined from fertilization, that is, as soon as it becomes a zygote: if it is XX, it will be a female, while, if it is XY, a male. However, at the beginning, human embryos have the precursors of both female and male gonads.
Taking this into account, if there is no Y chromosome, the embryo will proceed with the development of Müllerian ducts (which give rise to the ovaries). Also, when there is no Y chromosome, the embryo lacks the SRY region: the sex determining region.
To sum up: it is true that one could say that, in mammals, the initial plan for all embryos is female, which is only altered if the SRY region is present.
Suggested for you
All these changes and developments take place during the beginning of pregnancy. Would you like to know what are the most common symptoms and changes a woman experiences throughout this period? Do not miss this post: First moth of pregnancy.
Also, once the fetus is renamed baby, the moment of childbirth is closer, and new phases of your journey to motherhood begin. If you would like to find out what to expect during the next months, see also: Pregnancy month by month.
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On the other hand, a zygote is the first stage of human development right after fertilization, but how does implantation occur? This sought-after step toward pregnancy is an amazing process that we explain in detail here: How does embryo implantation occur?