Deep secrets> Study of skate’s eggs may yield medical cure

While medical research often focuses on synthetic solutions to human health problems, scientists still find answers in the natural world.

Consider the lowly skate, cousin to the stingray and shark. Two Florida researchers think proteins in the skate’s egg case could be used to repair human bones.

“I think this material could potentially work as a bone substitute,” said Thomas Koob, a biologist and researcher at Shriners Hospital for Crippled Children in Tampa, Fla.

Koob and a colleague, David Cox, are studying skates and their egg cases at Mount Desert Island Biological Laboratory in Maine. Koob began the project 10 years ago, and he returns to the island each fall to collect pregnant skates and study the eggs they produce.

MDI Biological Laboratory is ideal for the research, according to Koob.

“It has one of the best seawater systems on the East Coast for keeping the animals alive,” he said, “and skates are readily available in Maine waters.”

The life cycle of the skate is not well understood, according to Koob. There are at least five species around Mount Desert Island, ranging in size from 12 inches to 4 feet when measured across the disk.

Koob said the skate appears to lay eggs year-round, with peak periods in midwinter and midsummer. Unlike most fish, it lays only one egg at a time. Much like a chicken’s eggshell, the capsule forms around the egg while it is still in the skate’s uterus.

In tanks at the laboratory, skates drop their eggs loose in the water. When freshly laid in the ocean, the cases have sticky fibers which could be used to attach the capsules to rocks or algae, but that has never been observed by humans.

“We don’t know where they are laying the eggs (in the open ocean),” said Koob.

As it hardens, the egg case undergoes a process known as polymerization. Proteins in the capsule gradually link up to form larger strands of molecules, giving the case the strength and resiliency of plastic, which is a synthetic polymer.

“We analyze the proteins that make up the capsule and examine the polymerization process,” said Koob. “… So far, we have characterized six major structural proteins and two enzymes crucial to the process.”

Koob is intrigued by the way a skate egg case can change from a gelatinous mass to a leathery capsule less than 12 hours after it is laid. The cases protect skate eggs for a year or more in the harsh waters of the North Atlantic before the juvenile skates hatch.

“The material has to be extremely stable,” said Koob. “It also has anti-microbial properties, because the ocean is filled with bacteria.”

Koob has seen the capsules survive in Frenchman Bay for five years or more without breaking down. Old egg cases, blackish and barnacled, are a familiar sight on Maine beaches. They are often called mermaid’s purses.

At Shriners Hospital, Koob’s research focuses on tendons, ligaments, cartilage and bone. He said scientists have yet to find a material that can adequately fill spaces in children’s bones after corrective surgery.

The ideal material would be easily molded, strengthen quickly, adhere well to surrounding tissue and be slowly absorbed by the body so that it didn’t have to be removed. Synthetic material that mimics the properties of the skate’s egg case would be perfect, he said.

“If you could assemble the proteins in a test tube, they could be used … something like caulking compound that you could mold to the defect, but it would polymerize and have properties similar to natural bone,” said Koob.

In addition to helping children, the material also could be used to repair breaks in the bones of senior citizens. Because existing materials aren’t ideal, the elderly often face months of bed rest while they recuperate from a bad break.

Koob thinks the skate proteins could get senior citizens back on their feet quickly, reducing the chance of pneumonia and other illnesses. The researcher is almost ready to begin trials of the material in rabbits.

“We have just received funding to study animal implantation and see how the proteins react,” he said.