Construction crews had been using the crane and scaffolding to work on a 14-story college dormitory on Boylston Street, which runs along the south side of Boston Common. No students were injured, said Emerson Vice President of Public Affairs David Rosen.

"It's terrible," said Mayor Thomas M. Menino, who drove to the accident site after the collapse, around 1:30 p.m. "People just driving down the street in their cars, construction workers just doing their jobs - they never knew what hit them."

John Hynes of Boston was driving up Boylston Street when he saw the scaffolding start to tumble just ahead of him.

"You could see it coming down and then I started rolling forward. It started to hit my car, and then I sped up," said Hynes, grandson of former Boston Mayor John Hynes. He said his car and several others on the street were crushed.

The building - a dormitory and campus center - has been under construction for nearly two years and was scheduled to open for the fall semester, Rosen said.

"It's very distressing," Rosen said. "We're waiting for the police and fire to investigate."

A spokeswoman at Macomber Builders, the contractor hired by Emerson, had no immediate comment.

The contractor is based in South Boston and has been in business for more than 100 years, according to the company Web site. The site also promotes its employee safety program.

Navy officials were searching for the missing boat after fishermen found the bodies of two men floating at sea. The open, mostly wooden fishing boats normally used by illegal migrants are often unable to withstand storms or rough seas.

Both vessels had left the Mauritanian port of Nouadhibou, close to the northern border with Western Sahara, on the perilous 500-mile voyage to the Canary Islands.

"We left Nouadhibou more than a month ago and we went 17 days without food," one 25-year-old survivor told Reuters. "We had nothing to drink but sea water."

The man, who asked not to be identified, said each passenger had paid the equivalent of $1,200 for the trip. Around 20 of them were being treated in Nouakchott for exhaustion, starvation and dehydration.

Their launch was rescued 20 miles from the capital Nouakchott after having drifted on ocean currents 300 miles south of Nouadhibou.

More than two-thirds of the population of West Africa are under 30 years of age, and unemployment in some countries tops 50 percent, leaving many with no hope of finding a job.

Mauritania signed a deal with Spain last month to try to stem the rising tide of migrants attempting the dangerous crossing. The two governments agreed to mount joint coastal patrols and to crack down on people-trafficking networks.

The governor of Nouadhibou estimated in March that around 1,000 sub-Saharan Africans were arriving in the port every month to attempt the crossing, one of the main routes into Europe since Moroccan officials clamped down last year on attempts to enter Spain's north African enclaves of Ceuta and Melilla.

"It's really science fiction at its best," said Kaitlyne's mother, Tracy McNamara.

The bladder transplants, performed on seven patients ages 4 to 19, were being reported online Tuesday in The Lancet medical journal - a breakthrough that could hold exciting promise for someday regenerating ailing hearts and other organs.

"This suggests that tissue engineering may one day be a solution to the shortage of donor organs in this country for those needing transplants," said Dr. Anthony Atala, the lead researcher.

Growing other organs will likely hold unforeseen challenges, however, since organs are so specialized in their functions, scientists stress.

Even for people with bladder disease - and there are an estimated 35 million in the United States alone - Atala's technique requires testing on more patients and for longer times, researchers say. Replacing an entire bladder would pose many more problems, including reconnecting urine tubes, blood supply and nerve signaling, according to Dr. Steve Y. Chung, an Illinois urologist who wrote a commentary for The Lancet.

Still, he called the work "a tremendous, tremendous advance."

For the children and teenagers in the study, the transplants reduced leaking from their bladders - a potentially big gain in quality of life. For Kaitlyne McNamara, the transplant has meant a new social life.

Since birth, the teen from Middletown, Conn., has coped with spina bifida, a defect that leaves the spine incompletely closed. She's trudged through a grinding series of medical tests, treatments and procedures.

There was the urinary infection that nearly killed her as a toddler. There were the surgeries to correct her spine and legs. By age 11, there was the kidney damage from her weak bladder that again put her life in jeopardy.

Yet maybe the worst part, for a teenager, was simply being different. "I didn't fit in with the kids," she said. "Sometimes the kids made fun of me."

It wasn't necessarily her crutch, braces, or 4-foot-2 height that set her apart, at least in her mind. It was more the accidents from her diseased bladder, which would frequently leak.

That was before her surgery five years ago. Now, her kidneys are working again, and she no longer wears a diaper. Instead, she was waiting for alternations on a low-cut champagne-colored dress for her junior prom.

"Now that I've had the transplant, my body actually does what I want it to do," she said last week. "Now I can go have fun and not worry about having an accident."

Scientists, marveling at how animals like salamanders regenerate lost limbs, have long toyed with the futuristic possibilities of regrowing worn-out or injured human parts.

Over the past decade, researchers began fashioning better scaffold-like platforms that hold growing cells and dissolve inside the body. The study of stem cells, which can mature into all the body's other tissues, has also supercharged progress in regenerative medicine.

The researchers at Children's Hospital in Boston used a more mature cell type known as a progenitor. They first operated on the patients to remove bad tissue that made up more than half their bladders. They fished out muscle and bladder wall cells, seeded them on cup-like bladder-shaped scaffolds of collagen, then let the cells reproduce in the lab for seven weeks. Starting with tens of thousands, they ended up with about 1.5 billion cells. The cell-bearing molds were then surgically sewn back to the remnants of the patients' original and partly working bladders, where the lab-nurtured cells kept maturing.

The team, which began its work in 1999, followed the last patient for almost two years. In undergoing the experimental procedure, the patients skirted the typical side effects of grafts that would otherwise have been made with their own intestinal tissue.

Atala, who has since moved to Wake Forest University, has already begun commercializing his transplant techniques through Tengion, a company he helped found in King of Prussia, Pa. It has licensing rights to patents on his work.

Some researchers were more cautious about the promise shown with the new procedure, saying the study lacks any direct comparison group of patients getting the traditional graft.

Dr. Joseph Zwischenberger, who edits the journal of the American Society of Artificial Internal Organs, questioned how well the new bladders worked in the first few patients and raised a "red flag" about two patients who left the study for personal reasons and were ultimately omitted from the results. He also said Atala's attempts to commercialize the technique should add some skepticism toward the findings, which he nonetheless called "very interesting preliminary data."

The patients in the study must still deal with the ravages of spina bifida, which can turn off nerve signals that keep the bladder healthy. It causes the stiff, leathery bladder to leak frequently.

The rebuilt bladders, though, were up to three times more elastic and better at holding urine, the researchers report. In all seven patients, kidney function was preserved, the study said. The patients must still empty their bladders regularly with a tube but can avoid leaking in between.

"You don't want your little girl to wear a diaper all her life," said Tracy McNamara.

She used to worry about her daughter dying from kidney damage or urinary infections. That's all faded into the past. Now, she worries about all the time her daughter fritters away on the telephone, talking to friends.

The research concludes that at least 283,000 U.S. children aged one to six are too heavy for standard seats, including booster seats commonly used for children aged four and up.

The study appears in the April issue of Pediatrics, the journal of the American Academy of Pediatrics. It does not deal with obesity rates in Canada or other countries.

It says the problem is most noticeable among youngsters who exceed the weight limit for seats with built-in harnesses rated at up to 40 pounds (18 kilograms).

An estimated 182,000 three-year-old children in the U.S. fall into that category.

The danger is that the momentum of a crash could overpower the harness, allowing the child to spring free.

"Year after year, injuries resulting from motor vehicle crashes continue to the leading cause of death for children in the United States," the study said.

The study was launched after researchers at a Baltimore safety centre began seeing children who weighed more than the limit for almost all available seats.

"Estimates of child obesity are as high as 10 per cent for children two to five years of age in the United States," the authors wrote. "In the past three decades, the rates of childhood obesity have doubled for children two to five years of age ... and have tripled for children six to 11 years of age."

Doctors are still trying to figure out what to do about the problem, they said. "We do know, however, that childhood obesity is increasing, and we can expect even more children to face the prospect of limited or no child-safety seats available to protect them."

Responding to the demand for stronger seats, manufacturers are now producing beefed-up versions costing $240 US and up, the study said.

The prices cause extra problems for low-income families, whose children are more likely than average to be obese and more likely to be hurt or killed in a car crash, the study said.