Assisted reproductive techniques

Introduction

“Assisted reproduction” is the support provided to address infertility issues. It refers to all methods that assist infertile couples to achieve pregnancy and give birth to their own babies. Infertility rate in Pakistan is documented to be 21.9%, and the success rate is only 25% even after introduction of procedures like intracytoplasmic sperm injection (ICSI). “Assisted reproductive techniques (ARTs)” are procedures that represent an amalgamation of development in physiology, endocrinology, pharmacology, diagnostic technology, and clinical care of infertile couples. These techniques are intended to overcome natural barriers in fertilization and include in vitro fertilization (IVF) of human gametes and embryo replacement into the uterine cavity with the goal of achieving pregnancy.

Physiology of reproduction

During the process of reproduction, the male yields millions of gametes or spermatozoa, while the female yields a single female gamete or oocyte. In gametogenesis, haploid gametes are formed by meiosis of diploid progenitor cells that can fuse at the time of fertilization to produce a new, totally exclusive diploid organism. The homologous genetic recombination in prophase of meiosis I generates genetic diversity. This random combination of genetic material produces a variety of traits in the members of the species.

The ejaculation of sperms into the female reproductive tract enables the process of fertilization. While passing through the female reproductive tract, the process of capacitation occurs in sperms with the removal of cholesterol from the membranes along with calcium influx, thus making the plasma membrane less stable. In addition to this, the hyperactivated sperm motility and hyaluronidase secretion helps the sperm to penetrate the cumulus extracellular matrix for reaching the zona pellucida, where the capacitated sperm initiates acrosome reaction by binding of zona protein ZP3 with a sperm protein 1,4-galactosidase. This reaction releases hydrolytic enzymes, including proacrosin, which are activated to acrosin. Acrosin along with the flagellar beating of sperm is important in digesting zona pellucida and penetration of sperm into the oocyte plasma membrane. Then, the plasma membranes of sperm and oocyte fuse, by the interaction of sperm protein; Izumo1, and its cognate receptor, Juno, on the oolemma. Ultimately, sperm nucleus moves into the egg cytoplasm and chromatids of sperm and oocyte condensate to form pronuclei, thus completing meiosis. Polyspermy is blocked by certain changes that include shedding of Juno receptors from the oolemma, oocyte membrane depolarization, and cortical granule secretion.

Diploid zygote is formed by the combination and intermingling of the maternal and paternal chromosomes at syngamy, following the breakdown of pronuclear membranes. After the first cleavage division, two-cell embryo is formed 28 h after insemination, and the embryo undergoes several cleavage divisions forming a four-cell embryo at 44 h and an eight-cell embryo at 68 h. Beyond this eight-cell stage, embryo undergoes cell differentiation and enters the uterine cavity late on day 3 after fertilization. The embryo reaches the blastocyst stage in 5–6 days of fertilization, and after hatching from the zona pellucida, syncytial trophoblastic cells interact directly with the uterine epithelium through the stages of apposition, stable adhesion, and invasion.