Artificial Activation of Mammalian Oocytes for Cloning: Present Status and Future Perspectives

Since the birth of the first cloned sheep, "Dolly", many studies have reported the successful use of cloning by somatic cell nuclear transfer in other mammalian species. In spite of these successes, the efficiency of this technique is low, with less than 10% of generated embryos resulting in offspring. Factors of genetic, epigenetic, and technical origin seem to affect the efficiency of cloning. Further, because these reconstructed embryos do not have the benefit of being activated by the sperm, egg activation relies on artificial procedures such as those delivered by chemical or physical stimuli. In general, these stimuli aim to replicate either the signal delivered by the sperm, namely, elevations in the intracellular concentration of calcium ion ([Ca²⁺]_i), or the outcome of this signal, inactivation of the M-phase kinases that are responsible for the metaphase II arrest. In mammalian fertilization, the sperm initiates repetitive increases in [Ca²⁺]_i that are also known as oscillations, and experimentally replicating these oscillations has been difficult to accomplish, except in mouse eggs where oscillations can be attained by replacing extracellular CaCl₂ with SrCl₂. In other mammalian species, in the absence of oscillations, most activation protocols incorporate chemical methods to subdue the activity of the M-phase kinases. Here, we will briefly describe oocyte activation protocols presently in use in different species, highlight some of their advantages and disadvantages, and discuss possible improvements.