Structure and Function of the Blastocyst with Diagram and Formation Stages
The hollow cellular mass formed during the first development consisting of cells forming the trophectoderm, the inner cell mass, and therefore the fluid-filled cavity or the blastocoel is understood as the blastocyst.
The process in which blastocyst forms is called blastulation and it takes place during 5-14 days after fertilization.
Placenta is the outermost layer furthermore other supportive tissues necessary for the development of the foetus are found. The body tissues are formed from the inner cell mass.
The embryonic stem cells isolated from the inner cell mass are utilized in tissue replacement after injury and in regenerative medicine.
The blastocyst is only found in mammals.
Blastocyst Formation
Zygote a single-celled structure is formed after fertilization. The zygote then undergoes rapid cell division leading to the formation of blastocyst which is a hollow cell mass.
The stages which lead to the formation of blastocyst are mentioned below:
The zygote encased in the zona pellucida segregates itself in such a way that the entire process progresses through 2-celled, 4-celled, 8-celled and 16-celled stages. The cells during the cleavage stage are referred to as blastomeres. The blastomeres within the 2-celled stage undergo mitosis and cytokinesis simultaneously.
The blastomeres form tight junctions with one another once the embryo reaches the 8-celled or 16-celled stage. This ultimately leads to the deformation of the round shape and in turn leads to the formation of a morula; a mulberry shaped cluster of cells.
The blastomeres form the outer and inner cell mass of the embryo in between the junctions.
In the zygote fluid starts to get accumulated which leads to blastocyst formation. The accumulation of fluid expands the blastocoel and forms blastocyst.
The blastocyst which is formed is composed of two very distinct tissues, namely the outer trophoblast and the inner cell mass.
Finally, the stretched zone breaks and releases the blastocyst. This is known as hatching. Implantation of the blastocyst takes place in the uterus.
The blastocyst cling or get attached onto the endometrial wall of the uterus. This process is known as implantation.
Blastocyst Vs Embryo
Similarities Between Blastocyst and Embryo
Embryo and blastocyst are diploid structures.
Both Blastocyst and embryo are formed after the fertilization of gametes in animals.
Both the structures possess potent cells and hence are often utilized in diagnostics.
Both are formed inside the mother.
What is an Embryo?
Embryo development refers to the various stages within the development of an embryo. Embryonic development of plants and animals vary. In animals, Different stages are observed during embryonic development where it differs from species to species.
After fertilization, the zygote is formed. The zygote divides mitotically to make 2, 4, 8, 16 celled stages. These cells are known as blastomeres.
Morula- Embryo having 8 to 16 blastomeres.
The morula divides mitotically and transforms into blastocyst. The outer layer of blastocyst known as the trophoblast gets attached to the wall of uterine known as the endometrium. The implantation starts within the first week but gets completed by 2nd week.
The inner cell mass of the blastocyst forms an embryo. The embryonic and extraembryonic tissues are formed due to differentiation of blastocyst. The implantation completes in the 2nd week.
The interdigitated chorionic villi of trophoblast and uterine cells form the placenta, which is the connection between the mother and the growing foetus.
The embryo gets nourishment and oxygen from the placenta moreover the placenta helps in removing waste produced and carbon dioxide by the embryo. It also acts as an endocrine and secretes various hormones like hCG (Human Chorionic Gonadotropin), estrogen, progestogens, etc. for maintenance of pregnancy.
Gastrulation starts within the 3rd week, the inner cell or embryo starts differentiating into three germinal layers, i.e. ectoderm, endoderm and mesoderm. These cells transform and obtain differentiation to all or any tissues and organs, like nerve, blood, muscle, bone, alimentary canal, etc.
Ectoderm- it forms brain, hair, spinal cord, nails, nervous system, epidermis etc.
Mesoderm- animal tissue, muscles, cardiovascular system, notochord, bone, kidney, gonads.
Endoderm- it forms the stomach, gut lining, liver, bladder, internal organs, lung, pancreas etc.
FAQs on Blastocyst in Human Embryonic Development
1. What is a blastocyst?
A blastocyst is an early stage of mammalian embryonic development formed about 5–6 days after fertilization. It develops from the morula and is characterized by:
- An outer layer called the trophoblast
- An inner group of cells called the inner cell mass (ICM)
- A fluid-filled cavity known as the blastocoel
2. How is a blastocyst formed?
A blastocyst is formed when the solid morula develops a fluid-filled cavity through cell differentiation and fluid accumulation. The process occurs in these steps:
- After fertilization, the zygote undergoes repeated mitotic divisions (cleavage).
- A solid ball of cells called the morula is formed.
- Fluid enters the morula, creating a cavity called the blastocoel.
- Cells differentiate into the trophoblast (outer layer) and inner cell mass (inner cluster).
3. What are the main parts of a blastocyst?
The main parts of a blastocyst are the trophoblast, inner cell mass, and blastocoel. These components include:
- Trophoblast – Outer cell layer that later forms part of the placenta.
- Inner cell mass (ICM) – Cluster of cells that develops into the embryo proper.
- Blastocoel – Fluid-filled cavity that supports structural organization.
4. What is the function of the blastocyst?
The primary function of a blastocyst is to implant into the uterine lining and initiate pregnancy. Its key roles include:
- Attaching to and invading the endometrium
- Forming the placenta through trophoblast differentiation
- Giving rise to the embryo from the inner cell mass
5. When does the blastocyst implant in the uterus?
The blastocyst typically implants in the uterus about 6–7 days after fertilization in humans. During implantation:
- The trophoblast attaches to the endometrial epithelium.
- It differentiates into cytotrophoblast and syncytiotrophoblast.
- The embryo becomes embedded within the uterine wall.
6. What is the difference between a morula and a blastocyst?
The main difference between a morula and a blastocyst is that the morula is a solid ball of cells, while the blastocyst has a fluid-filled cavity and differentiated cell layers. Key differences include:
- Morula – Solid mass of 16–32 cells, no cavity.
- Blastocyst – Contains a blastocoel and distinct trophoblast and inner cell mass.
- The blastocyst is specialized for implantation, while the morula is an earlier cleavage stage.
7. What does the inner cell mass of the blastocyst develop into?
The inner cell mass (ICM) of the blastocyst develops into the embryo proper. Specifically:
- It forms the three primary germ layers: ectoderm, mesoderm, and endoderm.
- These germ layers give rise to all tissues and organs of the body.
- ICM cells are the source of embryonic stem cells.
8. What does the trophoblast form in the blastocyst?
The trophoblast forms the major part of the placenta and supports implantation. Its functions include:
- Differentiating into cytotrophoblast and syncytiotrophoblast.
- Invading the uterine lining during implantation.
- Contributing to nutrient and gas exchange between mother and embryo.
9. What is the blastocoel in a blastocyst?
The blastocoel is the fluid-filled cavity inside the blastocyst. It forms when fluid accumulates between cells of the morula and serves to:
- Separate the inner cell mass from the trophoblast
- Provide space for cell migration and differentiation
- Help maintain the spherical structure of the blastocyst
10. Why is the blastocyst stage important in IVF?
The blastocyst stage is important in in vitro fertilization (IVF) because embryos transferred at this stage have a higher chance of successful implantation. Key reasons include:
- The embryo has reached a more advanced and stable developmental stage.
- Cell differentiation into trophoblast and inner cell mass has occurred.
- It better synchronizes with the uterine environment.
