Present Status Of Equine Cloning And Clinical Characterization Of Embryonic, Fetal And Neonatal Development Of Three Cloned Mules

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The ability to clone mammals via somatic cell nuclear transfer has been recognized as a major scientific milestone because it proved that a fully differentiated somatic cell can be genetically reprogrammed back to the undifferentiated state of a 1-cell zygote (embryo) and initiate and undergo complete embryonic-fetal development, resulting in the birth of an animal that is genetically identical to the original cell donor. Although the specific methods used for cloning via nuclear transfer can differ, it is generally performed by micromanipulating and fusing 2 cells (Figure 1). One cell, referred to as the nuclear donor or karyoplast, is derived from the animal to be cloned; typically, donor cells are maintained in tissue culture, from which 1 cell is selected for each nuclear transfer procedure. The other cell, referred to as a cytoplast, is a mature unfertilized oocyte from which the genetic material (polar body and metaphase plate) has been removed. The cytoplast contains numerous cellular factors (eg, mRNA and proteins) that play an important role in the reprogramming of the genetic material (ie, genes) of the donor cell, which enables the cloned embryo to initiate the complex sequence of events leading to embryonic and fetal development. The reconstituted embryo uses the donor cell DNA as the template for subsequent gene expression, which results in a genetic clone of the donor animal.