What do mouse-like creatures reveal about human growth?

Meet the jerboa, a rodent with long springy legs. About the size of a large mouse, the jerboa has two mighty back legs that can help propel it as high as 3 feet in the air. Now, scientists are studying their appendages to better understand and to manipulate the growth of human bones.

TRANSCRIPT

UP NEXT, WE MEET A RODENT WITH A UNIQUE FEATURE -- LONG, VERY SPRINGY LEGS.

ABOUT THE SIZE OF A LARGE MOUSE, THE JERBOA HAS TWO MIGHTY BACK LEGS THAT CAN HELP IT JUMP AS HIGH AS THREE FEET IN THE AIR.

NOW, SCIENTISTS ARE LOOKING TO THESE INTERESTING APPENDAGES TO HELP US BETTER UNDERSTAND AND MANIPULATE THE GROWTH OF HUMAN BONES.

MEET THE JERBOA.

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THE JERBOA YOU CAN SORT OF THINK OF AS A FURRY, LITTLE RODENT VERSION OF A TYRANNOSAURUS REX.

THEY'RE ABOUT THE SIZE OF A LARGE MOUSE.

THEY HAVE THESE REALLY STRIKINGLY LARGE EYES.

THEY HAVE FOUR LIMBS, WHICH PEOPLE SOMETIMES OVERLOOK BECAUSE THEY'RE TUCKED UP UNDERNEATH THEIR JAW.

THE HIND LIMBS ARE REALLY, REALLY LONG AND DRAMATICALLY DIFFERENT.

THEY ARE CAPABLE OF JUMPING THREE FEET STRAIGHT UP IN THE AIR.

THEY'RE VERY SPRINGY.

AND BECAUSE OF THIS TAIL THAT THEY HAVE, WHICH IS ALSO REALLY, REALLY LONG, THE KAZAKH PEOPLE LIVING IN NORTHERN CHINA CALL THEM 'LO MEIN TUK-TUK,' WHICH IS TRANSLATED TO 'NODDLE HOP-HOP.'

OF THE MANY UNIQUE FEATURES OF THE JERBOA, IT'S THEIR HIND LIMBS THAT ARE PERHAPS THE MOST INTRIGUING.

WHAT LOOKS LIKE A BACKWARDS-TURNED KNEE IS ACTUALLY THEIR ANKLE, AND THAT LONG BONE STRETCHING DOWN FROM THE JOINT ISN'T THEIR SHIN.

IT'S THEIR FOOT.

AND WHAT YOU MIGHT THINK IS THEIR FEET IS ACTUALLY THEIR TOES.

THIS UNIQUE MORPHOLOGY IS QUITE A DEPARTURE FROM THEIR ANCESTOR, THE COMMON MOUSE.

THE MOUSE HAS WHAT WE CALL THE ANCESTRAL BODY FORM OF ARMS AND LEGS THAT ARE THE SAME LENGTH.

THE JERBOA HAS TAKEN THAT BASIC BODY PLAN AND HAS MODIFIED IT EXTREMELY TO HAVE LONG LEGS AND LONG FEET.

WHAT WE REALLY HOPE TO UNDERSTAND IS HOW DO YOU GET SKELETAL ELEMENTS -- BONES -- THAT REACH DIFFERENT LENGTHS IN A BODY?

DO THE BONES GROW FASTER?

DO THEY GET LONGER BECAUSE THEY GROW FOR A LONGER AMOUNT OF TIME IN DEVELOPMENT?

TO ANSWER THESE QUESTIONS, SIMPLY TAKE A SKELETAL ELEMENT FROM A RECENTLY DECEASED JERBOA AND EMBED IT IN A SMALL BLOCK OF ICE.

USING A CRYOSTAT, WHICH IS BASICALLY A VERY HIGH-TECH DELI SLICER, SHAVE OFF A TINY PORTION AND PLACE IT ONTO A SLIDE.

USE A SERIES OF SPECIAL DYES TO STAIN THE BONE ELEMENTS.

ALIZARIN STAINS THE BONE RED WHILE ALCIAN STAINS THE CARTILAGE BLUE, MAKING EACH EASIER TO IDENTIFY AND STUDY.

THEN, POP THE SLIDE UNDER A MICROSCOPE AND OBSERVE.

THE CELLS IN THE CARTILAGE ARE CALLED 'HYPERTROPHIC CHONDROCYTES.'

'HYPER' IS FAST, ELEVATED.

'TROPHIC' IS GROWTH.

AND THEY'RE CHONDROCYTES, WHICH IS THE CELL TYPE THAT MAKES UP CARTILAGE.

SO, THE HYPERTROPHIC CHONDROCYTE IS A CARTILAGE CELL THAT GETS VERY, VERY BIG VERY, VERY FAST.

THE FASTER-GROWING BONES IN THE JERBOA HAVE LARGER CARTILAGE CELLS.

AND THAT MUCH WE KNEW.

BUT WHAT WE REALIZED EARLY ON WAS THAT WE DIDN'T EVEN KNOW HOW THESE CELLS WERE GETTING BIGGER TO BEGIN WITH.

ANIMAL CELLS TYPICALLY ENLARGE BY MAKING STUFF -- PROTEINS, FATS, AND COMPLEX SUGARS -- AND ALSO BY TAKING UP WATER TO KEEP THE CONCENTRATION OF THAT STUFF CONSTANT.

IF THE WATER CONTENT OF MOST CELLS INCREASES BY 10%, THE CELL RUNS THE RISK OF RUPTURING.

CARTILAGE CELLS, HOWEVER, CAN INCREASE THEIR WATER VOLUME BY A STAGGERING 60%, BLOWING UP LIKE WATER BALLOONS.

THIS LETS THEM GROW BIGGER FASTER, SINCE IT TAKES LESS ENERGY AND TIME TO SUCK UP WATER THAN IT DOES TO MAKE MORE STUFF.

THE PURPOSE OF THOSE CELLS IS TO LAY THE FOUNDATION, THE SCAFFOLDING STRUCTURE, AROUND THEM.

THEN, THOSE CELLS DIE, AND THEN BONE CELLS COME IN, AND THEY REMODEL ALL OF THAT.

SO, THE MAJOR PURPOSE OF THAT CELL IS JUST TO GET REALLY BIG, REALLY FAST AND MAKE A SCAFFOLD.

IT'S LIKE BUILDING A BUILDING.

SO, NOW THAT WE'VE ANSWERED THIS QUESTION ABOUT HOW CELLS GET BIG, WE CAN CIRCLE BACK AROUND TO OUR ORIGINAL QUESTION, WHICH WAS HOW THAT GROWTH IS CONTROLLED SO THAT CELLS REACH DIFFERENT SIZES.

IF WE CAN UNDERSTAND THE MECHANICS OF HOW A BONE GROWS, THEN WE CAN BEGIN TO UNDERSTAND HOW TO MANIPULATE THAT GROWTH.

THAT KIND OF KNOWLEDGE CAN PROVE USEFUL WHEN TREATING BONE DEFICIENCIES AND ASYMMETRIES IN PEOPLE.

TREATING BONE DISEASE WAS NOT SOMETHING THAT KIM COOPER SET OUT TO TAKE ON BUT INSTEAD WAS THE RESULT OF ASKING THE RIGHT SERIES OF QUESTIONS.

IT'S NOT AN EITHER/OR THAT WE SHOULD STUDY HUMAN DISEASE OR WE SHOULD STUDY WEIRD ANIMALS.

TO ME, I THINK ONE OF THE MOST IMPORTANT THINGS ABOUT WHAT WE'RE DOING IS THAT IT HELPS US TO UNDERSTAND HOW ALL BODIES WORK, OURS INCLUDED.

WE'RE ALL BUILT BASICALLY THE SAME WAY.

IF WE CAN UNDERSTAND THE MECHANISMS THAT ALLOW THE JERBOA TO DEVELOP THEIR HIND LIMBS, THOSE ARE PROBABLY THE SAME FUNDAMENTAL PRINCIPLES THAT SHAPE OUR BODIES.

WITH MY OWN LITTLE ONE GROWING AND SHAPING INSIDE OF ME, IT GIVES ME GREAT COMFORT JUST UNDERSTANDING THESE PROCESSES, KNOWING HOW REMARKABLE IT IS WHEN EVERYTHING GOES WELL.

FOR 'SCIENCE FRIDAY,' I'M CHRISTIAN BAKER.