At issue in this book are studies that combine genes, gametes, embryos, or embryonic stem (ES) cells from human and nonhuman species at the earliest stages of development. What is here called early interspecies research (ISR) involves the shared presence of human and/ or animal embryos and es cells in a potentially inheritable way. the prospect of such studies has been flagged, whether justifiably or not, by a number of observers and policy makers as problematic.
One example of early isr is the injection of human es cells into a mouse blastocyst (four- to six-day embryo of approximately three hundred cells) in order to understand how human es cells function over time in a living system. As the fetal mouse develops, the human cells differentiate and integrate. the ultimate goal is to understand the properties of es cells better in order to develop cell-based therapies for humans. the recipient mice, studied before or after birth, would be chimeras, with some human cells existing side by side with mouse cells.
A second example is the substitution of animal eggs for human eggs in investigations geared to developing cell therapies for humans. in theory, if the nucleus from a patient’s somatic cell is introduced to an enucleated egg, the egg can be stimulated to cleave and will yield an inner cell mass from which es cells can be derived after about five days. These es cells, which are capable of differentiating to virtually any type of body cell, can be coaxed to differentiate and used for cell replacement therapies. the goal is to derive es cells that have the same genome as the patient who provided the nucleus. Many eggs would be needed in these preliminary studies, so investigators have proposed using animal eggs, which . . .