TOKYO (AP) — The Japanese laboratory that retracted a paper reporting a potentially major breakthrough in stem cell research said Wednesday its researchers have not managed to replicate the results.

Scientists at the government-affiliated Riken Center for Developmental Biology said they are still trying to match results reported in two papers published by the journal Nature in January and then retracted in July. But they refused to say whether or not they expected to succeed in doing so.

"It's like asking scientists to try to predict the odds of winning a lottery," said Shinichi Aizawa. Later in the press conference he said his analogy was not entirely apt in explaining his reluctance to speculate on the chances for success.

Riken scientist Haruko Obokata, originally lauded for leading the research that raised hopes for a discovery of a simple way to grow replacement tissue for various diseases, is participating in the experiments. They are to continue until next March.

Obokata and other researchers in Boston and Japan participating in the project said they used a simple procedure to turn ordinary cells from mice into stem cells. They exposed cells from spleens of newborn mice to a more acidic environment than they are used to.

Having failed to generate the stem cells using spleen cells from one type of mouse, the scientists plan to use cells from another type of mouse, and from other organs, and to alter the methods of stressing those cells, said lead researcher Hitoshi Niwa.

The discovery of problems with the original research caused an uproar and prompted an investigation at Riken. One senior Japanese scientist involved in the research died earlier this month in an apparent suicide.

Riken announced plans Wednesday for an organizational overhaul to prevent any further problems, changing its director and halving the number of researchers.