——有望帮助精子产出极低的男性实现做父亲的梦想

据《自然》杂志7月3日在线报道，美国科学家利用大鼠的克隆精子繁育后代取得成功，这一成果可能为那些精子产出极低的男性带来做父亲的希望。

美国康奈尔大学威尔医学院（Weill Medical College）的科学家使用了与克隆整只动物如多利羊相类似的技术。他们将大鼠精子的精头注入去除了DNA的卵细胞，这些“假精子”就开始复制产生一批与原始精子基因完全相同的细胞，随后研究人员利用已经在人类试管受精中广泛使用的技术——单精子卵母细胞浆内显微注射（intracytoplasmic sperm injection），将这些复制细胞注入完整的大鼠卵细胞，并将得到的晶胚植入母鼠子宫，最后得到了13只幼鼠，其中4只成年并且繁育成功。

该项研究的领导人之一Gianpiero Palermo说：“即使只有三四个样本精子，我们也能得到克隆精子。”但他们同时也承认，这项技术的问题还很多，比如他们一共移植了80个晶胚，只有13个足月产下，其中几只一出生就很快死亡。

科学家目前尚不清楚这项技术可否用于人类，因为人类的细胞分化要比大鼠复杂得多。Palermo认为，从每个精子最多能够得到8个克隆精子，因为如果这些细胞复制次数超过了3次，精子中的男性特有基因就不再表达。此外，克隆精子呈球形，也就是说没有精尾，不能游动，只能用于体外受精。

研究人员希望在五年之内能够克隆人类精子。目前大约每300个男性中有一位精子数量趋近于零，如果这项研究取得成功，将为这些男性带来巨大的希望。

《自然》在线报道：

Mice born from cloned sperm

Technique raises hopes for infertile men.

Cloning might one day restore the fertility of men with severely low sperm counts, say researchers in the United States who have cloned mouse sperm and used it to create apparently healthy adult mice.

Embryologists at Cornell University's Weill Medical College in New York cloned the sperm using similar methods to those used to clone whole animals, most famously Dolly the sheep.

They are hoping to clone human sperm within five years. If the technique works in humans, it could offer a lifeline to the roughly 1 in 300 men who are rendered effectively infertile by sperm counts approaching zero.

"If we can find just three or four sperm in a sample, we could grow them up," says Gianpiero Palermo, who led the research with his colleague Takumi Takeuchi.

Far from perfect

Palermo and Takeuchi cloned mouse sperm by injecting the heads of sperm cells into eggs that had had their DNA removed. The resulting 'pseudosperm' began to replicate, producing a cluster of cells all containing the same genetic material as the original sperm.

The researchers then fertilized intact mouse eggs with these cells, using a method called intracytoplasmic sperm injection. Already used in human in vitro fertilization, this involves using a fine needle to inject the sperm into the egg.

The resulting embryos were transferred to the wombs of female mice for gestation. So far, 13 mice have been born using the method, of which four survived to adulthood and produced offspring of their own.

The technique is a long way from perfect, Palermo and Takeuchi admit. The researchers implanted more than 80 embryos, and of those that did survive to full term, several died soon after birth.

It is not clear whether the technique would work for human sperm, in which cell division is more complex. Palermo estimates that each sperm could yield no more than eight clones, because allowing the cells to replicate more than three times could mean that the sperm DNA stops expressing male-specific genes.

And the cloned sperm are spherical. Lacking the tails that normal sperm use to swim, they are suitable only for test-tube fertilization.

Legal hurdle
In many countries, the technique would also require changes to the law. In Britain, regarded as leading the way on embryological regulations, it is illegal to implant any embryo created using novel research techniques into a woman's womb. Embryos created using cloned sperm would presumably fall into this category.

Nevertheless, Palermo hopes that, as well as helping infertile men to become fathers, studying cloned sperm could help diagnose the cause of low sperm counts. "We can use the process to work out what exactly is wrong with the sperm," Palermo says.