PHILADELPHIA — For the millions who've torn a tendon — from the bagel-slicing homeowner who cuts his hand open to the pitcher who wrecks his rotator cuff by throwing one too many fastballs — the healing process generally results in scar tissue.

That means a joint that's less flexible and not nearly as strong. Many patients, once recovered from a tendon injury, will tear it again.

But when tendons are damaged in a fetal mouse, they grow back almost like new. In a recent breakthrough, researchers found that this occurs even when the fetal tissue is transplanted into an adult. Now they are starting to figure out why, in hopes of someday helping people heal better.

At the University of Pennsylvania, two main avenues are being explored:

Experiments with lab animals suggest that part of the answer lies with certain "growth factors" secreted by fetal cells. And scientists are trying to give the healing process an artificial boost by implanting "scaffolds" — pieces of stretchy fabric that guide the orderly growth of new, healthy cells.

"It's unlikely there's going to be one magic key that's going to solve the whole problem," says Pedro Beredjiklian, an assistant professor of orthopedic surgery at the University of Pennsylvania's medical school.

The work is part of a broader field, less than two decades old, called tissue engineering — coaxing the repair or regrowth of bodily tissues through a combination of artificial and natural means.

Scientists in the booming field have made headway with heart valves, bladders, liver and skin cells. And engineers at Lehigh and Princeton Universities are trying to repair bones with scaffolds made from a porous, glasslike substance.

"Fifteen years ago, tissue engineering was considered science fiction. Now it's reality," says Rice University's Tony Mikos, editor of the journal Tissue Engineering.

At the University of Pennsylvania's McKay Orthopaedic Research Laboratory, a key focus is tendons, the rubber-bandlike tissues that join muscle to bone. Lab director Louis Soslowsky subjects tendons from animals young and old to a battery of tests — comparing their strength, biochemistry and how connective fibers are organized.

Earlier this month, graduate student Heather Ansorge and research engineer David Beason tested a tendon from a 7-day-old mouse, stretching it very slowly so they could measure the resulting stress and strain.

First, Ansorge deftly used tweezers to remove the whitish strip of tissue from the calf of the euthanized rodent.

She then marked lines on the tendon exactly 2 millimeters apart, so she could keep track of how much it stretched. Beason used a laser to measure the tendon's cross-sectional area.

Then they clamped each end of the tiny tendon, dunked it in a warm saline solution, and attached it to a machine that pulled the ends apart ever so slightly — a few thousandths of a millimeter per second. A nearby computer screen displayed the amount of force on the tendon as a jagged red-line graph.

Eventually, the tissue began to tear — a key indicator of its strength.

They also test tendons from adult mice — both healthy tissue and injured tendons that have healed with scars.

Scar tissue is about one-tenth as strong as healthy tendon, says Soslowsky, who has collaborated with researchers in Cincinnati and at Children's Hospital of Philadelphia. He suspects he may have a partial tear in his own rotator cuff from playing volleyball, but is reluctant to seek a diagnosis.

"I know too much about it to want to know," he jokes.

While young athletes typically are aware they've been injured, older people often unknowingly suffer microinjuries because their joints have simply worn out with age.

Soslowsky says that more than half of those over 60 have suffered partial tears of their rotator cuff, a web of muscles and tendons that stabilizes the shoulder.

"Where they have trouble sometimes is lifting a gallon milk jug off their shelf," he says.

Fetal tendons seem to heal better because they have different levels of certain structural proteins surrounding the tissue cells. The cells stimulate production of the right mix of proteins by secreting certain "growth factors."

A key one is called interleukin-10, which the University of Pennsylvania group administers to adult lab animals, aiding the repair process to avoid formation of scar tissue.

But that alone might not be enough. So elsewhere in the lab, Robert Mauck is spinning webs.

He weaves stretchy plastic fabrics to implant in injured lab animals — a sort of scaffold for growing new tendon cells that dissolves after a few months, when it is no longer needed.

Mauck begins by dripping clear liquid polymer from a hole in the bottom of a tube. He applies voltage to the liquid, driving the molecules apart and turning the drips into a fine spray.

The spray lands on a metal cylinder that is spinning at 7,000 revolutions per minute, wrapping around it in layer after gossamer-thin layer until the result is a light, elastic material.

Implanted as a scaffold, it will take the place, temporarily, of the collagen and other extracellular proteins that will be produced during healing.

The scaffolding material can be implanted by itself; the cells grow into it from the surrounding tissue. Or they can be "seeded" with fetal cells or adult stem cells before implantation.

Scar tissue is a weak, disorganized mess. A scaffold, by contrast, mimics the structure of a healthy tendon.

"It's like a scaffold for a building," says Mauck, who first worked on such materials during a stint at the National Institutes of Health. "A scar is basically a pile of bricks," he adds. Healthy tissue "is a house."

The work is partly supported by NFL Charities, which is funded by the league and the players union. Both have an obvious interest in healthy joints.