Human stem cell research and its possible connection to human cloning (i.e., somatic cell nuclear transfer) has been the subject of much debate. Significant confusion exists, however, in the public discourse on this topic--about what an embryo is, as well as about the ethical, religious, and legal issues involved. The aim of this essay is to clarify these issues and lay out as impartially as possible some of the ethical, religious, and scientific questions surrounding these complex matters, in the hope of stimulating and facilitating informed public debate.
Gametes--that is, ova and spermatozoa--carry one half of the genetic material, i.e., the DNA, of the female and male putative parents. When a spermatozoon enters an ovum and its DNA is introduced into the nucleus of the ovum, this DNA pairs up with the corresponding DNA already in the nucleus of the ovum, reconstituting its total genetic material in forty-six chromosomes. A fertilized ovum or zygote is thus formed. This can occur in the female genital tract (usually in the fallopian tube) as part of normal sexual reproduction, or in vitro fertilization (IVF) in the case of assisted fertility. In both instances the zygote starts to divide if conditions are favorable; first to form a blastomere of eight, then sixteen cells, and ultimately to form around day five a blastocyst. The blastocyst consists of 120 to 150 cells, some of which are the stem cells that can develop into all forms of normal human tissue. These are called totipotent. It is important to remember that at this stage the blastocyst, which is an early embryo, can divide into two or more living entities that can then develop into normal embryos. Thus, blastocysts are not individuals or unique entities. They are twinnable.
