In lab rats, ‘Who’s your daddy?’ can now yield a surprising answer. Scientists have generated rats from mice that developed rat sperm. The breakthrough marks the first time researchers produced healthy offspring [Photo] from sperm cells fostered in a different species. The hope is this method could help generate sperm from endangered species or prize bulls. A decade ago, scientists successfully developed sperm in one animal that had come from cells in another. Researchers began by growing rat sperm in mice, and proceeded to foster sperm from hamsters, rabbits, pigs, bulls and humans in mice as well. However, until now it remained unknown whether any of these sperm were fertile. In several instances they developed abnormally in their foreign hosts. The breakthrough Reproductive biologist Takashi Shinohara at Kyoto University in Japan and his colleagues first began with rats genetically engineered to produce a green fluorescent protein. Their cells and progeny would thus prove easy to recognize. Shinohara and his colleagues then removed the stem cells that sperm arise from in the rats and implanted them into testicles of mice. The scientists collected fluorescent green rat sperm from the mice and injected them into rat […]