Blastocyst: Overview, Definition, Stage, Implantation, Facts, Blastocyst

The Blastocyst is a key stage in the early embryo development in Human. The World Heath Organisation (WHO) states that it is formed 5-6 days after fertilisation. It has a fluid-filled cavity called a blastocoel. This fluid-filled cavity is lined by the inner cell mass, which eventually becomes the embryo. The blastocyst is an early stage in Human Reproduction when the embryo gets ready for implantation in the uterus.

This Story also Contains

1. What is a Blastocyst?
2. Basics of Embryonic Development
3. Formation and Structure of the Blastocyst
4. Functions of the Blastocyst
5. Difference Between Blastocyst and Embryo
6. Application of Blastocyst in Reproductive Technologies
7. MCQs on Blastocyst
8. Recommended Video on Blastocyst

According to the Indian Council of Medical Research (ICMR), the blastocyst stage is ideal for embryo transfer in IVF. Government-approved IVF clinics in India commonly follow blastocyst culture protocols for selecting embryo development. The blastocyst marks the beginning of the pregnancy-parturition-lactation cycle. A healthy blastocyst increases pregnancy success. This article includes the meaning, structure, and Role of Blastocyst in Biology.

What is a Blastocyst?

The blastocyst is a structure in early mammalian (vertebrate) development, entailing a hollow sphere of cells with an outer layer called the trophoblast and an inner cell mass (ICM) that is going to give rise to the embryo.

The blastocyst stage is critical for implantation in the uterine wall. This marks the beginning of a successful pregnancy and the development of the placenta, which will nourish the embryo throughout gastrulation.

The implantation of the blastocyst in the lining of the uterus during pregnancy triggers placenta formation. This allows the growing embryo to receive oxygen and nutrients from the mother needed for growth and development.

Basics of Embryonic Development

The fusion of sperm and egg initiates embryonic development and then passes through several subsequent steps, which ultimately lead to the blastocyst.

Fertilisation

• The sperm fuses with the egg, causing the initiation of stages of meiosis completion in the egg and the formation of the zygote.

• The fusion of the sperm and egg nucleus gives a diploid zygote nucleus that contains both parental genetic materials.

Zygote formation

• The zygote has a series of rapid mitotic divisions called cleavage, without increase in size.

• These produce small blastomeres, which go on to divide to give subsequent stages of development.

Early Cell Divisions

The stages of early cell divisions are:

Cleavage stages

• The mitotic cell division of the zygote develops a multi-cellular structure.

• Each division produces smaller cells and maintains the overall size of the zygote but increases the number of cells.

Morula stage

• After multiple rounds of cleavage division, a solid ball of embryonic cells is produced, known as a morula.

• The morula typically consists of 16 to 32 cells formed after multiple rounds of cleavage division.

• With additional divisions and compactions, the morula begins to prepare itself for the formation of the blastocyst.

Formation and Structure of the Blastocyst

The transition from the morula to the blastocyst stage shows dramatic changes in structure and function that will prepare the embryo to implant and continue development. The blastocyst helps form extraembryonic layers like the amnion and allantois after implantation.

Blastulation Process

• A fluid-filled cavity develops in the morula which then becomes the blastocoel and the blastocyst.

• The outer cells become the trophoblast, which will form part of the placenta.

• The inner cells become grouped to form the inner cell mass (ICM), which will eventually give rise to the embryo itself.

Structure of the Blastocyst

The structure is described in detail:

Trophoblast

• The trophoblast forms the outer layer of the blastocyst and has a crucial role in implantation.

• It secretes enzymes that allow the blastocyst to invade the uterine lining and establish a nutrient exchange.

Inner Cell Mass (ICM)

• The ICM contains cells that are pluripotent and hence have the capacity to give rise to all tissues of the embryo.

• It differentiates to form two layers: epiblast (which forms the embryo) and hypoblast which forms extra-embryonic tissues.

Blastocoel

• The blastocoel is a fluid-filled cavity that provides room for cell differentiation and growth.

• It contributes to the holding of the shape of the blastocyst. Additionally, it allows for the free flow of nutrients towards the ICM.

Functions of the Blastocyst

The blastocyst conducts several important functions during the early phases of pregnancy. It participates in implantation, placenta development, and embryonic differentiation.

Implantation

• It then implants into the endometrium, the lining of the uterus, some 6-10 days after fertilization.

• The trophoblast cells invade the uterine lining; this anchors the blastocyst and initiates the formation of the placenta.

• Organogenesis starts after the blastocyst implants and the germ layers begin forming organs.

Formation of the Placenta

The role of the trophoblast in the formation of the placenta:

• The trophoblast differentiates into two layers, the cytotrophoblast and syncytiotrophoblast.

• These layers are significant in the construction of the placenta which allows for the exchange of essential nutrients, gases, and waste between the maternal and fetal blood.

Differentiation and Development

• Cells of the ICM differentiate into three main fate of germ layers: ectoderm, mesoderm, and endoderm.

• These three germ layers further differentiate to give rise to the various tissues and organs of the embryo.

Stages Of Embryonic Development

• Gastrulation: Construction of germ layers

• Neurulation: Development of the nervous system.

• Organogenesis: Information about organs and tissues.

Difference Between Blastocyst and Embryo

Understanding the difference between blastocyst and embryo is important in the study of human reproduction and early development. While both are stages of the developing baby, they occur at different times and have different structures and functions. The table below shows the difference between Blastocyst and Embryo:

Application of Blastocyst in Reproductive Technologies

Problems in blastocyst formation or implantation can lead to disorders of human reproductive system, like infertility. So, the blastocyst in reproductive technologies has far-reaching applications, first in improving the success rates of procedures like IVF and in stem cell research

In Vitro Fertilization

• Transferring the blastocyst stage, rather than a cleavage-stage embryo has higher possibilities of implantation and subsequent pregnancy.

• Blastocyst culture selects more viable embryos that can result in successful implantation and pregnancy.

Stem Cell Research

• Embryonic stem cells (ESCs) are derived from the ICM of the blastocyst.

• Since ESCs are pluripotent, they can engender each possible cell type of the organism. That is the reason why ESCs are so useful for regenerative medicine.

Ethical Considerations

• Using a blastocyst for research means killing a potential human being.

• Therefore, proper weighting between scientific benefits and ethical issues is important in stem cell research.

MCQs on Blastocyst

Question: Which one of the following parts develops into a pseudocoelom?

1. Embryonic mesoderm

2. Blastopore lip

3. Blastocoel

4. Archenteron

Answer: Option C) is correct because blastocoel develops into pseudocoelom. Pseudocoelom means false coelom that is located between the gut and the body wall. The blastocoel is lined or filled partially with the cells of the mesoderm in pseudocoelomate animals. The cavity of the body is lined by mesoderm only towards the body wall. Mesoderm is absent towards the gut. Aschelminthes are examples of pseudocoelomate animals.

Option A) is incorrect because pseudo coelom is not developed by embryonic mesoderm. Pseudocoelom is developed by embryonic blastocoel which is also called the persistent blastocoel.

Option B) is also incorrect because the blastopore lip does not develop into pseudo coelom The blastopore develops into the anus.

Option D) is wrong because archenteron develops into the cavity of the digestive tract. It does not develop into pseudo-coelom.

Hence, the correct answer is option 3)Blastocoel

Question: Morula is:

1. 2-celled stage

2. 16-celled stage

3. 32- celled stage

4. None of them

Answer: Embryonic Development: Cleavage & Formation of Blastocyst -

• The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus and forms 2, 4, 8, and 16 daughter cells called blastomeres.

• When the blastomeres divide completely the cleavage is called holoblastic cleavage.

• The embryo with 8 to 16 blastomeres is called a morula.

• The embryo gradually becomes a 64-celled structure.

• A cavity is now formed within the cell mass. This ,cavity is called the blastocyst cavity or blastocoel.

• The embryo is now termed the blastocyst.

• The blastocyst is made up of an outer layer of cells called the trophoblast or trophectoderm and an inner cell mass or embryoblast.

• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.

Hence, the correct answer is option (2) 16-celled stage.

Question: Morula is a developmental stage:

1. Between the zygote and blastocyst

2. Between the blastocyst and gastrula

3. After the implantation

4. Between implantation and parturition

Answer: The morula is an early stage of embryonic development that occurs between the zygote (the single-celled fertilized egg) and the blastocyst (a hollow structure that implants into the uterine wall). After the zygote undergoes several rounds of mitotic divisions, it forms a solid ball of cells called the morula. The morula consists of 16-32 cells and is compact, resembling a mulberry. It eventually develops into the blastocyst as the cells continue to divide and organize, forming a fluid-filled cavity called the blastocoel. The transition from morula to blastocyst is a key step in early embryogenesis.

Hence, the answer is option (1) between the zygote and blastocyst.

Recommended Video on Blastocyst