The advent of in vitro fertilization (IVF) marked a watershed in the scientific understanding of the human embryo. This, in turn, led to a renaissance of human embryology, accompanied by the ability to manipulate the human embryo in the laboratory. This ability has resulted in yet further developments: refinements of IVF itself, preimplantation genetic diagnosis, the derivation and extraction of embryonic stem cells, and even various forms of cloning. There are immense social and scientific pressures to utilize the artificial reproductive technologies in ways that have little or no connection with overcoming infertility. As the original clinical goals of IVF have undergone transformation ethical concerns have escalated, so much so that they are condemned by some as illustrations of ‘playing God’, while any babies born via some of these procedures are labelled as ‘designer babies’. Both terms reflect the fear and repugnance felt by some at the interference with the earliest stages of human life by the artificial reproductive technologies. It is at these points that bioethical analyses have an important contribution to make.
Embryonic development begins when an egg is successfully fertilized by a single sperm, a process that takes between 26 and 30 hours to complete. The resultant single cell, the zygote, is totipotent, that is, it has the potential to give rise eventually to a complete new individual. On the second day of development, this single cell undergoes cleavage, during which it divides with little intervening growth to produce two, then four, then eight smaller, identical cells. These are the blastomeres, which at the eight-cell stage are only loosely associated with one another, each retaining its totipotency. By the 32-cell stage, they have become increasingly adherent and closely packed, and have almost definitely lost this equal developmental potential.
By day five the embryo consists of well over 100 cells and is termed a blastocyst. The outer cells of the blastocyst differentiate to form a surface layer, the trophectoderm, which becomes the trophoblast, which in turn eventually gives rise to the placenta. By contrast, the inner cells of the blastocyst constitute the inner cell mass (ICM) and are still undifferentiated (unspecialized), retaining the potential to form every type of tissue involved in the construction of the fetus (the cells are pluripotent). Some of these cells will later form the embryo proper and subsequently the fetus. Around day seven the blastocyst embeds in the uterine wall, marking the beginning of implantation, which is usually completed by day 14. Hence, the term preimplantation embryo refers to the embryo up to 14 days’ gestation.
Key Terms in this Chapter
Surplus Embryos: Embryos created as a part of fertility treatment that are left over once the treatment has finished; they are capable of development but were not implanted because more embryos were created than were ultimately required.
Embryonic Stem Cells: Stem cells derived from the inner cell mass of early embryos (blastocysts).
Preimplantation Genetic Diagnosis (PGD): A procedure devised to test early human embryos for serious inherited genetic conditions, with the subsequent transfer of only unaffected embryos to a woman’s uterus
Primitive Streak: A thickening of the ectoderm which appears in the human embryo at fourteen to fifteen days’ gestation; often considered to represent the transition from a non-organized to an organized state during embryonic development; its appearance marks the end of the time during which research can be undertaken on embryos.
Cloning (Somatic cell nuclear transfer): Asexual reproduction, in which the nucleus (and chromosomes) of an ovum (egg) is replaced with the nucleus of a somatic (body) cell of an adult. This causes the ovum to develop as if it had been fertilized without the involvement of sperm.
Preimplantation Embryo: A name given to the entire product of the fertilized egg up to the end of the implantation stage (fourteen days).
In Vitro Fertilization (IVF): The process of fertilizing a (human) egg with a (human) sperm in vitro in the laboratory and therefore outside the body of the woman
embryo transfer may follow: and the term ‘IVF’ is used to cover both the fertilization and the embryo transfer