Blastocyst Definition
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
1. What exactly is a blastocyst in human development?
A blastocyst is an early-stage embryo that develops in mammals about 5 to 6 days after fertilisation. It is a hollow, fluid-filled ball of cells, representing a critical transition from the solid morula. This is the specific stage that must implant into the uterine wall to establish pregnancy. It is characterised by having two distinct cell types: the inner cell mass and the trophoblast.
2. What are the main parts of a blastocyst and what are their functions?
A blastocyst is composed of three primary parts, each with a crucial function for future development:
- Inner Cell Mass (ICM): This is a cluster of cells on the interior of the blastocyst. These cells are pluripotent, meaning they will develop into the embryo itself, eventually forming all the tissues and organs of the foetus.
- Trophectoderm (or Trophoblast): This is the outer layer of cells that surrounds the inner cell mass and the blastocoel. Its main functions are to facilitate implantation into the uterine wall and to develop into the embryonic part of the placenta.
- Blastocoel: This is the fluid-filled cavity within the blastocyst. It provides a nutrient-rich environment and helps separate the cells into the inner cell mass and the trophectoderm.
3. How is a blastocyst formed from a morula?
The formation of a blastocyst from a morula is a process called blastulation. After the zygote undergoes several cleavage divisions to form a solid ball of cells called a morula, fluid begins to seep into the morula from the uterus. This fluid accumulates and creates a central cavity, the blastocoel. As this cavity expands, it pushes the cells apart, organising them into the outer trophectoderm and the distinct inner cell mass, thus transforming the solid morula into a hollow blastocyst.
4. What is the main difference between a morula and a blastocyst?
The primary difference lies in their structure and level of cell differentiation. A morula is a solid, compact mass of 16-32 undifferentiated cells (blastomeres). In contrast, a blastocyst is a hollow structure with a fluid-filled cavity (blastocoel) and is the first stage to show clear cell differentiation into two distinct types: the inner cell mass, which forms the embryo, and the trophectoderm, which forms the placenta.
5. At what stage of pregnancy does the blastocyst form and implant?
The blastocyst forms around day 5-6 after fertilization. The process of implantation, where the blastocyst attaches to and embeds within the uterine lining (endometrium), typically begins around day 6-7 and is completed by the end of the second week. From a clinical perspective, this corresponds to the third week of a pregnancy, counted from the last menstrual period.
6. What is the significance of the inner cell mass (ICM) in future development?
The significance of the inner cell mass (ICM) is immense because it contains pluripotent embryonic stem cells. This means the ICM is the source of all cells that will eventually differentiate to form the three primary germ layers (ectoderm, mesoderm, and endoderm) of the embryo. These layers, in turn, give rise to every single tissue and organ system in the body, such as the nervous system, muscles, bones, and internal organs.
7. Why is the process of 'hatching' important for a blastocyst?
Before implantation, the blastocyst is enclosed within a protective glycoprotein layer called the zona pellucida. 'Hatching' is the process where the blastocyst breaks free from this layer. This is a critical step because the zona pellucida prevents the embryo from attaching to the uterine wall. Only after hatching can the trophectoderm cells of the blastocyst make direct contact with the endometrium and begin the process of implantation.
8. How does the blastocyst ensure it receives nourishment before the placenta is fully formed?
Initially, the blastocyst gets nourishment from the uterine fluid that fills the blastocoel. Upon implantation, the trophectoderm cells rapidly multiply and invade the maternal endometrial tissue. This invasion erodes maternal blood vessels, allowing maternal blood to pool around the developing embryo, providing a rich source of oxygen and nutrients long before the mature placenta is established.
9. Is a blastocyst the same as an embryo?
This is a common point of confusion. A blastocyst is a specific, very early stage of an embryo. The term 'embryo' is broader and refers to the developing human from the first cell division after fertilization up to the end of the eighth week of gestation. Therefore, while every blastocyst is an embryo, not all embryonic stages are blastocysts (e.g., the morula and the later gastrula are also embryonic stages).
