Embryology, IVF, PGD/PGS.................

Embryology  studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses

Embryology encompasses the study of congenital disorders that occur before birth, known as teratology.

Through mitosis, the zygote subdivides into blastomeres which contain the full complement of paternal and maternal chromosomes.

After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.
In bilateral animals, the blastula has 2 fates.
In the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome;  if the first pore becomes the anus then it is a deuterostome.
The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates.
In due course, the blastula changes into a more differentiated structure called the gastrula.

(https://www.britannica.com/science/prenatal-development)

The blastocyst has an inner cell mass which  forms the embryo. The outer layer called trophoblast gives rise to the placenta. 

A healthy blastocyst hatches from its outer shell, the zona pellucida between day 5 to day 7 after fertilization. Within 24 hours after hatching, embryo implantation after IVF (or a "natural" pregnancy) begins as the embryo invades into the uterine lining.

Blastocyst is implanted in the uterus, embryogenesis continues with the next stage of gastrulation.The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop (histogenesis)

The innermost layer, or endoderm, gives a rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites. The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system. The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.

(https://en.wikipedia.org/wiki/Human_embryogenesis)

Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.

(http://www.biozoomer.com/2011/02/evolution-embryology-evidences.html)

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

In first trimester screen, the maternal blood is tested (by NIPT or  Non Invasive Prenatal Test) for two normal first-trimester proteins.
Cell free maternal DNA and fetal DNA (placental DNA). Fetal DNA is 10% of all circulating cell-re DNA. Length of DNA is 150-200bp.

Then, an ultrasound is used to look at the nuchal translucency region under the skin behind the baby's neck. This test is done between the 11th and 14th week of pregnancy. Nuchal translucency test uses ultrasound to measure the thickness of the fluid buildup at the back of the developing baby's neck. If this area is thicker than normal, it can be an early sign of Down syndrome, trisomy 18, or cardiac problems.

AFP (alpha-fetoprotein), hCG, and Estriol level are tested.

AFP is a major plasma protein produced by the yolk sac of the fetus. The gene coding for this protein is in the q arm of chromosome 4. It is thought to be the fetal form of serum albumin. AFP binds to copper, nickel, fatty acids and bilirubin  and is found in monomeric, dimeric and trimeric forms.
It binds estradiol to prevent the transport of this hormone across the placenta to the fetus. It prevents the virilization of female fetuses. AFP may protect the fetus from maternal estradiol that would otherwise have a masculinizing effect on the fetus. AFP can tell about neural tube defects.

Nowadays, maternal serum is tested for fetal DNA.

Preimplantation genetic diagnosis/screening (PGD or PGS) is used prior to implantation to help identify genetic defects within embryos. PGD benefits couple at risk for passing on a genetic disease or condition. It prevents certain genetic diseases or disorders from being passed on to the child.

The embryos used in PGD are usually created during the process of in vitro fertilization (IVF).
Egg retrieval and fertilization in a laboratory. Over the next three to five days, the embryos will divide into multiple cells.
PGD steps:
Few cells (which would have become placenta) are micro-surgically removed from the embryos, which are about 5 days developed.
After this cell collection, the embryos are safely frozen. The cells derived by embryo biopsy (for genetic material or DNA) are  placed in a tube.
The embryo is screened for genetic abnormalities. The genetic material is evaluated by PCR, FISH, CGH  (comparative genomic hybridisation) arrays or Next Generation Sequencing (NGS) to determine if the inheritance of a problematic gene is present in each embryo.

 

(https://www.invitra.com/preimplantation-genetic-diagnosis-pgd/)

This process takes at least one full week (time taken is constantly being reduced).
Embryos that are free of genetic problems are kept frozen (frozen embryo transfer). Embryos with problematic genes are destroyed.
If PGD finds that the embryos is free of genetic problems, the embryo(s) will be placed in the uterus (by an IVF procedure), and the wait for implantation and a positive pregnancy test begins.
Embryos with the correct number of chromosomes (euploid embryos)
All women are at risk of producing chromosomally abnormal embryos. As a woman ages, the potential for chromosomally abnormal embryos increases, regardless of the number of embryos produced.
For each embryo tested, PGS results will fall into one of three categories: euploid, aneuploid, or mosaic.
Fertlity clinic: In vitro fertilization with standard insemination, Intracytoplasmic Sperm Injection (ICSI), Assisted Hatching (AHA), embryo cryopreservation, blastocyst culture, TESE () and MESA () for male factor, and embryo biopsy for pre-implantation genetic diagnosis.
GnRH-Agonist is used to suppress the secretion of gonadotropin hormones
Then multiple follicles are recruited by daily injections of gonadotropins. Ultrasound imaging and hormone assessments are used to monitor follicular development
Final maturation of eggs is done by HCG administration
Egg retrieval is scheduled 34-36 hours after HCG injection, in a surgical suite under intravenous sedation
Ovarian follicles are aspirated using a needle guided by trans-vaginal ultrasonography. Follicular fluids are scanned by the embryologist to locate all available eggs. The eggs are placed in a special media and cultured in an incubator until insemination
If sperm parameters are normal, approximately 50,000 to 100,000 motile sperm are transferred to the dish containing the eggs. This is called standard insemination.
ICSI (Intracytoplasmic sperm injection) technique is utilized to fertilize mature eggs if sperm parameters are abnormal
Embryologist picks up a single spermatozoa using a fine glass micro needle and injects it directly into the egg cytoplasm
If there are no sperm in the ejaculate, sperm may be obtained via a surgical procedure
Fertilization is assessed 16-18 hours after insemination or ICSI
The fertilized eggs are called zygotes and are cultured in a specially formulated culture medium that supports their growth
They will be assessed on the second and third day after retrieval
Blastocyst culture has several advantages. Embryos at this stage have a higher potential for implantation, therefore fewer embryos can be transferred on day 5 to reduce the chance of multiple pregnancies. Blastocyst culture makes it possible to select the best one or two blastocysts vs. two or three (or rarely four) early embryos to transfer back to the mother. This reduces the occurrence of potentially risky multiple births Low numbers of embryos and poor embryo quality reduce the chances for good blastocyst development.
Early in the morning on the day of your transfer the embryos are evaluated and photographed by the embryologist
The embryologist will decide based on the rate of development and appearance of the embryos, which and how many embryos are recommended to be transferred.
Typically embryos are transferred at the cleavage stage (4 – 8 cells) (Day 3 after oocyte retrieval) or at the blastocyst stage (a ball of cells with fluid inside)(Day 5)
Embryo transfer is a simple procedure that does not require any anesthesia. Embryos are loaded in a soft catheter and are placed in the uterine cavity through the cervix.
An embryo must hatch out of its outer membrane (zona pellucida) before implanting in the uterine wall (endometrium)
Sometimes the zona is abnormally thick. Laser assisted hatching is a technique that allows a small gap in the zona pellucida to be made. This will aid the embryo in breaking out of this membrane and facilitates implantation
This perforation/ assisted hatching is performed prior to embryo transfer and when doing trophectoderm biopsies
Assisted hatching improves IVF success rates in both fresh embryo transfers and frozen embryo transfers.

(https://emedicine.medscape.com/article/273415-overview)

#Risks of PGS....
Incorrect result
Biopsy stress may cause embryo to arrest growth
Freeze-thaw cycle may be harmful

#Benefits of PGS.......
Beneficial for older woman
Female having repeated miscarriages may be benfittrrd
IVF failure rate may decrease