Blastocyst vs. Embryo

What's the Difference?

Blastocyst and embryo are both stages in the early development of a fertilized egg, but they differ in terms of their structure and developmental potential. The blastocyst is formed around 5-6 days after fertilization and consists of a hollow ball of cells with an inner cell mass and an outer layer called the trophoblast. It is responsible for implantation into the uterine wall and the subsequent formation of the placenta. On the other hand, an embryo is formed after the blastocyst stage, around 2 weeks after fertilization. It is characterized by the differentiation of cells into three primary germ layers, which give rise to various tissues and organs in the developing organism. The embryo has a higher developmental potential compared to the blastocyst, as it is capable of forming all the structures necessary for the development of a fully functional organism.


Development StageEarly stage of developmentAdvanced stage of development
Cell CountApproximately 100 cellsMultiple cells, more differentiated
ImplantationNot yet implanted in the uterusImplanted in the uterus
FormationFormed from the morulaFormed from the blastocyst
Cell DifferentiationCells are undifferentiatedCells start to differentiate into specific tissues
Development PotentialCan develop into an embryoCan develop into a fetus

Further Detail


When it comes to the early stages of human development, two key terms often come up: blastocyst and embryo. Both blastocyst and embryo represent crucial stages in the formation of a new life, but they differ in various aspects. In this article, we will explore and compare the attributes of blastocyst and embryo, shedding light on their unique characteristics and roles in the development process.

Definition and Formation

A blastocyst is an early stage of development that occurs approximately five to six days after fertilization. It is a hollow ball of cells that forms when the fertilized egg, known as a zygote, undergoes multiple cell divisions. The blastocyst consists of two distinct cell types: the inner cell mass (ICM) and the trophoblast. The ICM eventually develops into the embryo, while the trophoblast contributes to the formation of the placenta.

On the other hand, an embryo is the stage of development that follows the blastocyst. It begins around day 14 after fertilization and continues until approximately eight weeks. During this period, the embryo undergoes significant growth and differentiation, with the formation of various organ systems and the establishment of the basic body plan.

Size and Structure

In terms of size, a blastocyst is typically smaller than an embryo. It measures around 0.1-0.2 mm in diameter, making it barely visible to the naked eye. The blastocyst consists of a fluid-filled cavity called the blastocoel, surrounded by a single layer of trophoblast cells. The ICM is located within the blastocoel, forming a cluster of cells.

On the other hand, an embryo is larger and more complex in structure compared to a blastocyst. By the end of the third week, the embryo develops three distinct layers: the ectoderm, mesoderm, and endoderm. These layers give rise to different tissues and organs in the body. The embryo also develops a neural tube, which eventually becomes the brain and spinal cord.

Implantation and Development

After fertilization, the blastocyst travels through the fallopian tube and reaches the uterus. It then undergoes implantation, which is the process of attaching to the uterine lining. Implantation typically occurs around six to seven days after fertilization. Once implanted, the blastocyst starts to receive nourishment from the mother's blood supply, allowing it to continue developing.

As for the embryo, it continues to develop after implantation. The cells within the embryo rapidly divide and differentiate, forming the various tissues and organs. By the end of the eighth week, the embryo has developed all major organ systems, and it is then referred to as a fetus.

Developmental Potential

One of the key differences between blastocyst and embryo lies in their developmental potential. The blastocyst has the potential to develop into an embryo and eventually a fetus, given the appropriate conditions and support. However, not all blastocysts have the same developmental potential. Some may have genetic abnormalities or other factors that prevent successful development.

On the other hand, an embryo has a higher developmental potential compared to a blastocyst. By the time it reaches the embryo stage, the cells have already undergone significant differentiation and organization. The embryo has the capacity to develop into a fully formed human being, with all the complexities of the human body.

Ethical Considerations

Both blastocysts and embryos have raised ethical considerations in various contexts. Blastocysts, particularly those created through in vitro fertilization (IVF), are often subject to debates regarding their moral status and the appropriate use of surplus embryos. Some argue that blastocysts should be afforded the same rights and protections as fully developed human beings, while others believe that their use for scientific research or fertility treatments is ethically justifiable.

Similarly, embryos have been at the center of ethical debates, especially concerning reproductive technologies and stem cell research. The destruction of embryos for research purposes or the selection of embryos based on certain genetic traits raises questions about the value and sanctity of human life at its earliest stages.


In summary, blastocyst and embryo represent distinct stages in the early development of a human being. While blastocysts are smaller and simpler in structure, embryos are larger and more complex, with the potential to develop into a fully formed human being. Both blastocysts and embryos have their unique attributes and roles in the development process, and their study and understanding contribute to advancements in reproductive medicine and our understanding of human life.

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