Normal Vs Abnormal Embryo Development
Normal embryo development: From Day 1 to Day 5.
Abnormal embryo development:
*) Pronuclear abnormalities
*) Cytoplasmic anomalies
*) Elongated embryo
*) Chromosomal abnormalities.
Normal Embryo Development:
There are several stages of development of the embryo and these stages are marked by cell division.
It started from cell gametes and divide into many cells.
This development occurs around 5 days.
Day 1 : 2-cell stage
Day 2 : 4-cell stage
Day 3 : 8- cell stage it is known as “Cleavage Stage”.
Day 4 : 16-cell stage it is known as “Morula”
Day 5 : Blastocyst stage.
This stage is known as 2-Cell Stage.
At 17±1 h post insemination,Fertilization of cell gametes occurs which are the sperm and the oocyte with the presence of 2 pronuclei. Cells are in meridional division.
Each nucleus contain chromosome material from the genetic parents.
This stage is known as 4-cell stage.
In this dividing stage, mitotic cell division occurs one cell divides meridionally while the other cell divides equatorially which result in four cells.
At 44±1 h post insemination, a embryo displays 4 equal sized mononucleated blastomere in a three dimensional tetrahedral arrangement; and embryo surrounded by thick layer called Zona Pellucida.
This stage is known as Cleavage Stage.
At 68±1 h post insemination, embryo displays 8 equal sized mononucleated blastomeres.
There are grade system of embryo on day 3 based on fragmentation which is to be considered for embryo transfer.
Grade 1: Embryos with less than 10 percent of the fragmentation.
Grade 2 : Embryos with greater than 15 percent of the fragmentation
Grade 3: Embryos with greater than 50 percent of the fragmentation and are not selected for embryo transfer.
This stage is known as Morula.
At 92±2 h post insemination, embryo displays a 16-32 blastomeres and a structure known as Morula ( solid-ball like structure).
Stage of Compaction begins, in which the blastomeres continue to dividing and begin to adhere tightly to each other.
This stage is known as Blastocyst Stage.
At 106 to 108 h embryo will be in Blastocyst. Presence of small fluid filled cavity called blastocoel inside the embryo.
It possesses an Inner Cell Mass(ICM), it will become the fetus and outer layer known as Trophectoderm it will become the placenta.
Grading of Blastocyst:
Blastocyst are graded from 1 to 6 based on the degree of expansion and hatching status.
Grade 1: Early blastocyst with blastocoel less than half of the embryo volume.
Grade 2: Blastocyst with more or equal to half of the embryo volume.
Grade 3: Full blastocyst with blastocoel completely fills the embryo.
Grade 4: Expanded Blastocyst with blastocoel volume larger than early embryo with a thinning zona.
Grade 5: A hatching Blastocyst with trophectoderm starting to herniated through the zona.
Grade 6: Hatched Blastocyst with complete escape of Blastocyst from zona.
In Blastocyst, the assessment of Inner Cell Mass development:
Grade A: Tightly packed, many cells.
Grade B: Loosely packed, several cells.
Grade C: Very few cells.
In Blastocyst, the assessment of trophectoderm:
Grade A: Many cells forming cohesive epithelium.
Grade B: Few cells forming a loose epithelium
Grade C: Very few large cells.
Abnormal Embryo Development
Single Pronucleate is due to asynchronous appearance or fusion of both pronuclei.
In Three Pronucleate, there will be two pronuclei with a third additional small nucleus.
Blastomere presented with more than one single interphase nucleus.
Multinucleation is associated with genetic disorders of the embryo.
It impairs cleavage rates and implantation potential of embryo.
It is tiny structures in the embryo that is detached from the cells.
Fragmentation is caused due to chromosomal abnormalities, abnormal cell cycle, apoptosis and oxidative stress of embryo.
The degree of fragmentation is expressed as the percentage of total cytoplasmic volume.
Degree of fragmentation is expressed as:
Mild(less than 10%),
Severe( more than 25%).
The Cytoplasmic anomalies are Cytoplasmic pitting, Cytoplasmic granularity and presence of vacuoles.
It is characterized by the presence of numerous small pits in cytoplasm. It may result in risk of early loss of gestational sacs.
It has excessive darkened with centralized granularity associated with cortical halo.
These embryo reduce implantation potential and degenerate.
Cytoplasmic Vacuolization is the most common cytoplasmic dysmorphism in embryo.
Vacuoles vary in size. Small vacuoles and Large Vacuoles.
Embryo with uneven size and irregular blastomere.
Zona pellucida is also elongated.
In Chromosomal abnormalities, embryo will not contains 46 chromosomes.
And this abnormality is known as Aneuploidy.
Aneuploidy has a risk of implantation failures, miscarriage, birth defects.
Mosaic embryo is another defect of chromosome abnormality. It is mixed of Euploid( normal) and Aneuploid(abnormal).
The most common test to detect the abnormal chromosome is PGT-A ( Preimplantation Genetic Testing – Aneuploidy);which specifically looks at the number of chromosome of an embryo.
PGT involves taking a biopsy of an embryo, which involves a removing a tiny part of the embryo.
Embryonic arrest is also occurs due to chromosomal or other genetic abnormalities. These embryos are too “weak ”to continue normal development and to sustain implantation.
During cleavage stage division, programming of maternal and paternal chromosome takes place to create the Embryonic genome activation (EGA) to start the Preimplantation embryo development.
Embryonic genome activation (EGA) fails, the development does not continue because of the inability of the embryo to have cellular functions.