Culture and nuclear transfer make premeiotic female germ cells (left) fertilization competent.

Hatada/Macmillan

For anyone proposing to generate a clone army, or even to do therapeutic cloning for tissue repair, the culturing of oocytes is a major stumbling block. The poor success rate of cloning means that hundreds of oocytes are needed to receive transferred nuclei. Luckily, every woman has millions of oocytes. But almost all of those oocytes are trapped in an immature state that is not competent for either fertilization or productive receipt of a transferred nucleus.

Yayoi Obata, Izuho Hatada (Gunma University, Gunma, Japan), and colleagues have made some progress along these lines by successfully culturing female germ cells derived from mouse fetuses. Unfortunately, progression through meiosis and then efficient blastocyte development required successive nuclear transfers into the cytoplasm of mature oocytes. These nuclear transfer steps make this approach useless from the cloning point of view—you cannot get around the need for mature oocytes with a procedure that requires the use of mature oocytes. But Hatada points out that women about to undergo chemotherapy could store immature oocytes and then use a variation on his procedure to conceive later in life.

Hatada is not sure why the nuclear transfer steps are needed, although he points out that the in vitro cultured oocytes never reach the full size of a mature oocyte. For now, he is happy that his cultured oocytes establish imprinting, which will allow him to study this process in vitro. ▪

Reference:

Obata, Y., et al. 2002. Nature. 418:497–498.