Progress in Cloning Embryos from Domesticated Livestock

Abstract

The techniques necessary to successfully clone embryos by nuclear transfer have been developed in the last half of this century and applied to domestic animals in the last decade. The concept of transferring nuclei from early embryos to oocytes to coerce the nuclei to recapitulate the early developmental events is credited to Spemann (1). Spemann's interest was a question of nuclear equivalence or commitment. The techniques initially developed in amphibians (2) have also allowed the researcher to “clone” an embryo. Nowadays, individuals often clone genes or portions of a DNA molecule from a nucleus(i); in contrast, this nuclear transfer technique permits cloning of the entire genome and the creation of animals that have identical nuclear genomes.

Because many excellent reviews have been written on the subject of cloning by nuclear transfer (3, 4) and a similar review was presented in this journal earlier (5), the focus here will be an update of our understanding since that time and will highlight one of my interests: nuclear reprogramming. The review will begin with a general review of the current status of the techniques and a discussion of nuclear reprogramming and oocyte activation. This section will be followed by examples of some general problems of having embryos in vitro, and the review will finish with a discussion of the development of embryonic stem cells and the direction in which this leads.

General Procedures and Theory

The techniques for cloning by nuclear transfer were initially developed by Briggs and King in 1952 (2) for use in amphibians. In general, the procedures use unfertilized oocytes and donor nuclei. Chromosomes from unfertilized recipient oocytes are removed or destroyed with a micropipette or by exposure to ultraviolet irradiation, respectively. Next, the cells from a donor embryo or cell line are dispersed, and individual cells are aspirated into a micropipette.