NARRATOR: YOUR JOURNEY THROUGH LIFE INVOLVES THE MOST REMARKABLE PHYSICAL TRANSFORMATION ON THE PLANET.

YOU GROW FROM A SINGLE CELL TO AN INCREDIBLE WALKING, TALKING, SENTIENT BEING OF 37 TRILLION CELLS, A METAMORPHOSIS CRAFTED BY A CASCADE OF HIDDEN PROCESSES, ENABLING YOU TO GROW AS MUCH AS HALF AN INCH IN A SINGLE DAY AND FROM BABY TO ADULTHOOD QUADRUPLE IN HEIGHT, CREATING YOUR OWN UNIQUE BRAIN, FORGED FROM YOUR CHILDHOOD EXPERIENCES... AND REGENERATING YOUR BODY EVEN INTO OLD AGE.

MAN: IF YOU DON'T USE IT, YOU LOSE IT.

NARRATOR: WITH A PROCESS SCIENTISTS ARE NOW USING TO EXTEND LIFE.

WE ARE LIVING IN AN EXTRAORDINARY ERA OF SCIENCE, WHERE NEW INSIGHTS ARE REVEALING THE HIDDEN WORKINGS OF OUR BODIES IN WAYS NEVER SEEN BEFORE-- HOW YOU LEARN, GROW, AND SURVIVE.

YOU MAY THINK YOU KNOW WHO YOU ARE, BUT IT'S TIME TO THINK AGAIN.

THIS IS "THE AMAZING HUMAN BODY."

WOMAN: ♪ OOH, OOH, OOH, OOH, OOH, OOH, OOH, OOH ♪ [BABY CRYING] NARRATOR: YOUR JOURNEY THROUGH LIFE FROM BABY TO FULLY FORMED HUMAN BEING IS ONE OF CONTINUAL TRANSFORMATION, AND WHEN YOU REACH ADULTHOOD, YOU PROBABLY THINK THAT YOUR YEARS OF GROWTH ARE WELL BEHIND YOU, BUT YOU COULDN'T BE FURTHER FROM THE TRUTH BECAUSE THE REMARKABLE PROCESS OF GROWTH THAT HAS GOT YOU THIS FAR ACTUALLY NEVER STOPS.

SO YOUR FINGERNAILS NEED TRIMMING, YOUR HAIR NEEDS CUTTING, AND THIS IS JUST THE GROWTH YOU CAN SEE.

INSIDE, YOU ARE CONSTANTLY BEING RENEWED AND REVITALIZED AS YOU AGE.

I FEEL LIKE IN MYSELF PROBABLY, LIKE, I'M ABOUT 25 OR 30, AND THEN I LOOK IN THE MIRROR.

I DON'T FEEL LIKE MY AGE HAS LIMITED ME IN WHAT I CAN DO OR WHAT I CHOOSE TO DO.

INSIDE I'M AT MY PEAK, I'M ABOUT 30.

YOUNG PEOPLE MIGHT LOOK AT ME AND SAY, "OH, SHE'S, SHE'S OLDER.

LOOK AT THAT OLDER WOMAN THERE."

UM, I DON'T FEEL IT.

I STILL FEEL QUITE YOUNG.

I DON'T FEEL OLD.

WOMAN ON P.A.

: WETSUITS ON.

MAKE YOURSELVES WARM AND COMFORTABLE, AND WE'LL BE GIVING YOU UPDATES AS SOON AS WE HAVE THEM.

NARRATOR: YOU ARE, IN FACT, FAR YOUNGER THAN YOU APPEAR BECAUSE THE TRILLIONS OF CELLS THAT MAKE UP YOUR BODY ARE CONSTANTLY REGENERATING.

TRIATHLETE LEW HOLLANDER IS 87 YEARS OLD AND THE PERFECT EXAMPLE OF THIS REMARKABLE PROCESS.

LEW: ESCAPE FROM ALCATRAZ IS A SPECIAL RACE.

WE GET ON THE FERRY BOAT WITH 2,000 OTHER LUNATICS, AND I'M SCARED.

YEAH.

ANY NORMAL PERSON WOULD BE REALLY SCARED, AND THEN IN 6 MINUTES, THEY HAVE TO UNLOAD ALL 2,000 PEOPLE.

THEY SAY, "IF YOU DON'T GO, WE PUSH YOU."

MAN: LET'S MOVE DOWN HERE.

KEEP ON MOVING.

DON'T GO TO THE BACK.

THIS IS THE ESCAPE FROM ALCATRAZ TRIATHLON.

TO SURVIVE THE GRUELING 1 1/2-MILE SWIM ACROSS THE SAN FRANCISCO BAY, FOLLOWED BY THE 18-MILE BIKE RIDE AND THE 8-MILE RUN, YOU HAVE TO BE AT THE PEAK OF YOUR PHYSICAL FITNESS.

LEW: ALCATRAZ IS A REAL CHALLENGE, AND I THINK THERE'S A TIME IN EVERYONE'S LIFE WHEN THEY RESIGN THEMSELVES, AND I'M 87, AND I HAVEN'T RESIGNED MYSELF YET.

NARRATOR: WHEN MANY PEOPLE OF HIS AGE ARE SITTING IN AN ARMCHAIR, LEW IS PUSHING HIS BODY TO THE LIMIT.

[CHEERING AND APPLAUSE] LEW: WHEN ARE YOU OLD IS REALLY THE QUESTION, AND WHEN YOU STOP DREAMING OF WHAT YOU CAN DO, THEN YOU'RE OLD.

I HAVEN'T STOPPED DREAMING YET.

NARRATOR: THE REASON WHY LEW IS ABLE TO COMPETE IS THAT MOST OF HIS BODY ISN'T ANYWHERE NEAR 87 YEARS OLD.

HIS BODY, JUST LIKE YOURS, IS UNDERGOING CONTINUAL REPAIR AND MAINTENANCE INSIDE AND OUT.

IT'S STILL GROWING.

YOUR SKIN IS TOTALLY REPLACED EVERY MONTH.

THAT'S OVER A TRILLION CELLS MAKING UP TO 16% OF YOUR BODY WEIGHT.

YOUR STOMACH LINING IS REPLACED EVERY 2-9 DAYS WHILE YOUR LIVER CAN TAKE UP TO A YEAR TO REGENERATE... AND EVEN THOUGH WE MAY THINK THAT ANY CELLS IN OUR BODIES CAN DIVIDE, MOST OF OUR CELLS FROM MUSCLES, BLOOD, TO NERVES CAN'T REPLICATE THEMSELVES.

THAT'S THE JOB OF A VERY SPECIAL TYPE OF CELL.

THESE ARE YOUR STEM CELLS, AND IT'S THESE THAT MULTIPLY, HIDDEN WITHIN TISSUES THROUGHOUT YOUR BODY, CREATING NEW SKIN, LIVER, OR BLOOD... AND EVEN THOUGH THERE'S LESS CELL REGENERATION AS YOU GET OLDER, LEW'S BODY, LIKE YOURS, IS STILL GROWING NEW CELLS EVERY DAY, SO OVER 200 MILLION ARE BRAND-NEW FOR THIS DEMANDING RACE.

LEW: SAN FRANCISCO'S GOT SOME HILLS.

I GOT PLENTY OF BREATH AND I TRAIN IN HILLS, BUT IT'S A TOUGH COURSE.

I MEAN, THIS IS A TOUGH RACE.

NARRATOR: THE COURSE MAY BE TOUGH FOR LEW, BUT WE ALL DAMAGE OUR BODIES WITH THE DAILY RITUALS OF LIFE.

THE SURFACE OF YOUR LUNGS UNDERGOES CONSTANT WEAR AND TEAR.

HEAT AND DUST FROM THE ATMOSPHERE DAMAGE THE DELICATE LINING, SO IT NEEDS TO BE REPLACED EVERY TWO WEEKS OR SO.

AS LEW PUSHES HIMSELF, HIS BODY DEMANDS MORE AND MORE OXYGEN TO KEEP IT GOING, AND IT'S THE WAY STEM CELLS REPLENISH RED BLOOD CELLS WHICH MEANS LEW GETS THE OXYGEN HIS BODY DEMANDS AS HE PUSHES THROUGH THE TRIATHLON.

IT'S RED BLOOD CELLS FLOWING ACROSS THE THIN SURFACE OF YOUR LUNGS THAT PICK UP OXYGEN AND CARRY IT TO YOUR ORGANS AND MUSCLES, BUT LIFE AS A RED BLOOD CELL IS HARD.

AS THEY ARE SQUEEZED THROUGH CAPILLARIES, THE MECHANICAL STRESS ON THEM MEANS AFTER JUST THREE MONTHS THEY DIE...

BUT DEEP IN THE MARROW OF YOUR LONG BONES, NEW RED BLOOD CELLS ARE CONTINUALLY GROWN.

STEM CELLS CALLED HEMOCYTOBLASTS DIVIDE AND, AFTER TAKING TWO DAYS TO MATURE, ARE RELEASED INTO THE BLOOD STREAM AS NEW BLOOD CELLS... ANOTHER REMARKABLE WAY THAT YOUR BODY CONSTANTLY REGENERATES... AND WITH A CONSTANT SUPPLY OF NEW RED BLOOD CELLS, LEW GETS ALL THE OXYGEN HE NEEDS.

LEW: IF YOU DON'T USE IT, YOU LOSE IT, AND EVERYBODY SAYS, "HOW CAN YOU POSSIBLY HAVE KNEES?"

YOU KNOW, THEY ALL GOT ARTIFICIAL KNEES THESE DAYS, AND MINE ARE JUST FINE, AND THESE ARE THE ORIGINAL ONES THAT THEY GAVE ME TO START THE GAME.

NARRATOR: EVEN THE PARTS OF YOU THAT SEEM PERMANENT ARE NOT.

YOUR BONES ARE CONSTANTLY BEING BROKEN DOWN AND REPLACED BY THOSE DIVIDING STEM CELLS.

IT TAKES NEARLY A DECADE TO GROW A NEW SKELETON... AND SINCE HE'S 87, LEW'S ON THE WAY TO HIS NINTH ENTIRELY NEW SET OF BONES.

MAN: COME ON, LEW, COME ON, BABY.

[CHEERING AND APPLAUSE] MAN, ON P.A.

: WELCOME TO THE FINISH, AND CONGRATULATIONS TO OUR ATHLETES.

YOU ALL ESCAPED FROM ALCATRAZ.

ANOTHER SHOUT-OUT TO OUR FRIENDS.

LEW, VOICE-OVER: I HOPE MY QUALITY OF LIFE WILL BE GOOD OUT TO 107, AND, UM, THAT'S LIKE A TARGET THAT I HAVE...

BUT I WOULD OF COURSE HOPE TO LIVE ANOTHER 6 OR 7 YEARS AFTER THAT.

IF I'M WANDERING AROUND AT 107, I'M STILL DOING RACES, IT WILL BE A MIRACLE.

MAN, ON P.A.

: LEW HOLLANDER CROSSING THE FINISH LINE.

87 YEARS YOUNG.

[CHEERING AND APPLAUSE] GREAT JOB.

NARRATOR: LEW IS A TESTAMENT TO THE INCREDIBLE POWERS OF HUMAN GROWTH THAT KEEP US ALIVE, BUT TO UNDERSTAND THE EXTRAORDINARY PROCESS OF GROWTH THAT CREATES YOU, YOU NEED TO GO BACK TO THE VERY BEGINNING, TO A TIME BEFORE YOUR FIRST BIRTHDAY WHEN ALMOST ALL OF YOUR ENERGY IS BEING USED TO CREATE NEW CELLS AND GROW YOUR BODY.

WOMAN: RIGHT, REALLY BIG PUSH, AND AGAIN, BIG PUSH.

KEEP GOING, KEEP GOING.

[MOANING] NARRATOR: FROM THE MOMENT YOU ARE BORN, YOU BEGIN AN EXTRAORDINARY JOURNEY OF GROWTH AND TRANSFORMATION.

A BABY GIRL!

NARRATOR: FROM A HELPLESS NEWBORN, OVER A LIFETIME, YOU WILL TAKE 700 MILLION BREATHS, AND YOUR HEART WILL BEAT 3 BILLION TIMES.

YOU'LL WALK 183 MILLION STEPS... MEET 80,000 NEW PEOPLE AND SPEND 6 MONTHS LAUGHING.

YOUR BODY WILL CHANGE BEYOND RECOGNITION, GROWING BONE AND MUSCLE AND ACQUIRING BRAIN POWER TO ENABLE YOU TO TAKE ON THE WORLD IN AN EXTRAORDINARY PROCESS OF CELL DIVISION AND GROWTH THAT WILL REQUIRE MUCH OF YOUR DAILY ENERGY INTAKE.

[BABY CRYING] 7.42.

NARRATOR: AND YOUR BIGGEST GROWTH SPURT STARTS WHEN YOU'RE BORN.

OVER THE FIRST FEW WEEKS OF LIFE, YOU GROW FASTER THAN AT ANY OTHER TIME, ALMOST HALF AN INCH A WEEK, AND OVER THE NEXT 3 MONTHS, YOUR BRAIN SIZE WILL INCREASE BY 60%... [GRUNTS] [CRYING] AND ALL THIS GROWTH CAN BE FUELED BY JUST ONE SINGLE FOOD, HUMAN MILK.

FOR THE BABIES, IT CONTAINS THE VERY STUFF OF LIFE, EVERYTHING A NEWBORN NEEDS TO POWER ITS GROWTH SPURT, BUT BIZARRELY, IT ALSO CONTAINS HUGE AMOUNTS OF SOMETHING THAT BABIES CAN'T DIGEST THAT PROTECTS YOU FOR LIFE.

SO AT THE UNIVERSITY OF CALIFORNIA, PROFESSOR BRUCE GERMAN AND HIS TEAM HAVE BEEN TRYING TO UNLOCK THE SECRETS OF HUMAN BREAST MILK.

GERMAN: SO THIS IS HUMAN MILK.

HUMAN MILK IS MOSTLY WATER.

BABIES NEED LOTS OF WATER.

THERE'S ALSO LIPIDS, FATS, COMPLEX LIPIDS FOR THE BABY.

THERE'S LOTS OF LACTOSE, SUGAR, FOR ENERGY FOR THE BABY, AND BABIES MAKE MORE BABY OUT OF PROTEIN.

THIS IS HOW YOU MAKE MORE BABIES, AND THERE'S LOTS OF PROTEIN IN HUMAN MILK.

NARRATOR: THE UNUSUAL INGREDIENT THEY'RE INVESTIGATING IS A COMPLEX SUGAR.

THE THING THAT MOST SURPRISED US, LITERALLY GOBSMACKED US, WAS THIS.

THIS MATERIAL, HUMAN MILK OLIGOSACCHARIDES-- THERE'S AS MUCH IN HUMAN MILK AS PROTEIN, AND THIS IS COMPLETELY UNDIGESTIBLE BY BABIES.

NARRATOR: SO IF BABIES CAN'T FEED ON HUMAN MILK OLIGOSACCHARIDES, OR HMOS, WHAT CAN?

SO OUR FIRST THOUGHT WAS, WELL, MAYBE IT'S NOT FOOD FOR THE BABY.

MAYBE IT FEEDS BACTERIA.

NARRATOR: PROFESSOR GERMAN GAVE A SAMPLE OF HMO TO A MICROBIOLOGIST TO SEE IF HE COULD FIND BACTERIA THAT FED ON IT.

GERMAN: HE TRIED BACTERIUM AFTER BACTERIUM AFTER BACTERIUM.

THEN HE FOUND ONE-- WHAT A SURPRISE-- FROM THE INTESTINE OF A BREASTFED BABY.

NARRATOR: AND THIS IS WHAT IT IS, BIFIDOBACTERIUM INFANTIS, AND SINCE IT FEEDS ON THE HMO IN BREAST MILK, IT THRIVES IN THE BABY'S GUT, PROVIDING MUCH NEEDED PROTECTION.

GERMAN: PUT YOURSELF IN THE PLACE OF A NEWBORN BABY.

THEY'RE COMING FROM A STERILE, PROTECTIVE ENVIRONMENT, AND THEY'RE LITERALLY DROPPED IN THE MUD, AND THEY BEGIN TO ACQUIRE BACTERIA FROM THE ENVIRONMENT THAT'S GOING TO COAT THEM INSIDE AND OUT FOR THE REST OF THEIR LIVES, AND THEIR LOWER INTESTINE IS PARTICULARLY SUSCEPTIBLE TO INFECTIOUS PATHOGENIC BACTERIA.

SO THIS PARTICULAR BACTERIA, FED BY THESE OLIGOSACCHARIDES, FILLS THE BABY'S INTESTINE, COATS ALL OF THE SURFACES, BLOCKS ANY PATHOGENS FROM GROWING, AND LITERALLY SQUEEZES ALL THESE INAPPROPRIATE BACTERIA OUT.

MOTHERS ARE LITERALLY RECRUITING ANOTHER LIFEFORM TO BABYSIT THEIR BABY.

NARRATOR: BY PROTECTING THEIR INTESTINES, THE BACTERIA ENABLES BABIES TO GROW HEALTHILY, AND AT THE NEONATAL UNIT IN SACRAMENTO, DOCTORS ARE NOW TESTING A NEW TREATMENT TO HELP PREMATURE BABIES.

MAN: WHEN A VERY PREMATURE BABY IS BORN, THEY'RE RISK FOR A SEVERE DISEASE CALLED NECROTIZING ENTEROCOLITIS.

IT HITS SUDDENLY, TYPICALLY BETWEEN 2 AND 8 WEEKS OF LIFE, AND THEY GO FROM BEING STABLE, HEALTHY BABIES, AND IN A MATTER OF HOURS, MUCH OF THE INTESTINE HAS BEEN DAMAGED OR DESTROYED, AND SO THE NUMBER OF KIDS WHO DIE FROM THIS DISEASE IS QUITE DRAMATIC.

NARRATOR: SO THE DOCTORS HERE ARE TREATING THE PRE-TERM BABIES WITH A MIXTURE OF MOTHER'S MILK MIXED WITH BIFIDOBACTERIUM INFANTIS.

THE LEVELS OF THEIR GOOD BACTERIA HAVE SOARED, AND THERE'S GOOD EVIDENCE THAT THESE TYPE OF BACTERIA HELP PREVENT NECROTIZING ENTEROCOLITIS.

THESE DELICATE NEWBORNS NOW HAVE A MUCH BETTER CHANCE OF SURVIVAL, AND THEIR GROWTH RATES HAVE INCREASED SIGNIFICANTLY.

UNDERWOOD: THE BABIES WHO HAVE MORE OF THAT BIFIDOBACTERIUM INFANTIS IN THEIR INTESTINES, THOSE BABIES HAD BETTER GROWTH, THEY HAD A BETTER RESPONSE TO THE VACCINES THEY RECEIVED, AND SO IT'S BECOMING CLEAR THAT THIS BIFIDOBACTERIUM INFANTIS IS HELPING WITH GROWTH AND WITH DEVELOPMENT OF THE IMMUNE SYSTEM.

NARRATOR: SO EVEN IF BABIES HAVE A TOUGH START IN LIFE, THE TREATMENT CAN HELP THEM CONTINUE TO THRIVE AND GROW LIKE OTHER NEWBORNS.

FUELED BY MILK AND ASSISTED BY FRIENDLY BACTERIA, THE GROWTH IN YOUR FIRST YEARS OF LIFE WILL SEE YOU CHANGE ALMOST BEYOND ALL RECOGNITION.

LET'S SEE HOW TALL YOU ARE.

BY THE TIME YOU ARE 2 OR 3, YOU'LL HAVE MORE THAN TRIPLED YOUR WEIGHT AND DOUBLED YOUR LENGTH.

MMM.

WOMAN: THAT FIRST TWO YEARS, IT, IT'S CONSTANT CHANGE AND CONSTANT MOVEMENT.

IT'S AN AMAZING FEELING TO KNOW THAT THIS PERSON WAS SOMEBODY YOU ONCE HELD IN YOUR ARMS.

SHE, LIKE, CONTINUOUSLY AMAZED US AT HOW QUICKLY SHE WOULD PICK THINGS UP AND RESPOND.

AND SOMETIMES YOU LOOK AT A PHOTO, AND YOU THINK, "IS THAT REALLY THE SAME CHILD?"

YOU THINK, "OH, YOU'RE HUGE, YOU'RE A GIANT, "YOU'RE HEAVY.

HOW DID YOU GET SO BIG SO FAST?"

NARRATOR: IT'S A GRADUAL PROCESS THAT DAY BY DAY YOU HARDLY NOTICE, BUT THE GROWTH OF YOUR BONES IS ONE OF THE TRUE WONDERS OF THE HUMAN BODY... THAT'S HOW TALL YOU ARE.

A PROCESS THAT WE NOW UNDERSTAND IN INTRICATE DETAIL.

NO FAMILY DEMONSTRATES THIS EXTRAORDINARY PROCESS OF BONE GROWTH BETTER THAN THE VAN KLEEF-BOLTON FAMILY FROM ENGLAND.

WITH A COMBINED HEIGHT OF 13 1/2 FEET, KEISHA AND WILCO ARE THE UK's TALLEST COUPLE.

KEISHA, VOICE-OVER: I'M 6'6".

I REALLY LOVE BEING TALL.

IT JUST MAKES ME A BIGGER PERSONALITY.

WILCO, VOICE-OVER: I'M 7 FOOT.

I DON'T NOTICE I'M 7 FOOT.

OTHER PEOPLE DO, BUT IF THEY HAVE AN ISSUE WITH IT, THAT'S THEIR PROBLEM TO BE HONEST.

KEISHA, VOICE-OVER: I MET WILCO THROUGH THE TALL PERSONS CLUB, AND HE JUST KEPT RUNNING UP TO ME, THIS REALLY TALL GUY.

WILCO: SHE WAS LOOKING FOR A DANCE PARTNER.

I CAN'T DANCE TO SAVE MY LIFE, BUT I THOUGHT, "WHAT THE HECK, YOU KNOW," AND WE ENDED UP CLUBBING FOR THE WHOLE NIGHT UNTIL 6:00 IN THE MORNING.

KEISHA: AND WE HAVE BEEN INSEPARABLE SINCE.

NARRATOR: WITH 5 CHILDREN RANGING FROM 18 MONTHS TO 11 YEARS, KEISHA AND WILCO'S GROWING FAMILY PROVIDES A PERFECT SNAPSHOT OF HOW THE HUMAN BODY TRANSFORMS FROM A 20-INCH BABY INTO A 6-FOOT-PLUS ADULT.

AND WITH PARENTS LIKE THIS, IT'S ALMOST CERTAIN ALL OF THE CHILDREN WILL END UP WAY OVER 6 FEET TALL.

EVA IS 8 YEARS OLD, AND THROUGHOUT HER CHILDHOOD, EVERY BONE, MUSCLE, AND SINEW IS CONSTANTLY LENGTHENING IN A COORDINATED FASHION.

AAH!

OHH!

EVA: WHEN I GROW UP, I'D LIKE TO GROW AS TALL AS MY MUM AND DAD BECAUSE I LIKE READING A LOT.

IF I GO TO THE LIBRARY, THEN I CAN GET THE BOOKS ON THE TOP SHELF, SO I WOULDN'T HAVE TO GET A LADDER.

NARRATOR: BY THE TIME SHE REACHES ADULTHOOD, EVA WILL BE AROUND 4 TIMES TALLER THAN WHEN SHE WAS BORN... AND WHAT DETERMINES HOW TALL SHE AND ALL OF US GROW STARTS NOT IN YOUR BONES BUT IN THE BRAIN.

FROM A PEA-SIZED GLAND KNOWN AS THE PITUITARY, GROWTH HORMONE IS RELEASED THROUGHOUT YOUR LIFE.

GROWTH HORMONE PLAYS A NUMBER OF IMPORTANT ROLES IN THE ADULT BODY, LIKE THE REGULATION OF OUR METABOLISM, BUT DURING CHILDHOOD AS HIGH LEVELS OF GROWTH HORMONE SURGE THROUGH YOUR ENTIRE BODY, IT HAS A FAR GREATER EFFECT.

TRAVELING THROUGH THE BLOOD STREAM, IT STARTS A REACTION IN THE LIVER, PRODUCING A HORMONE CALLED GROWTH FACTOR 1.

THIS CAUSES MUSCLE AND CARTILAGE CELLS TO MULTIPLY.

IT'S THE SECRET BEHIND HOW TALL YOU GROW.

AT THE ENDS OF THE LONG BONES OF YOUR ARMS AND LEGS ARE THE EPIPHYSEAL PLATES, SOFT DISCS MADE OF CARTILAGE.

IT'S HERE THAT GROWTH FACTOR 1 WORKS ITS MAGIC, STIMULATING STEM CELLS IN THE CARTILAGE TO RAPIDLY DIVIDE IN A PROCESS CALLED MITOSIS.

THE NEW CELLS PUSH THE OLDER CELLS TOWARDS THE CENTER WHERE THEY HARDEN, GRADUALLY ELONGATING THE BONE AND MAKING YOU TALLER.

DADDY, I FOUND SKIPPING STONE!

NARRATOR: WHAT DETERMINES YOUR HEIGHT IS STILL SOMETHING OF A MYSTERY.

KEISHA: CHOCOLATE.

HMM.

NICE.

NARRATOR: GENETICS PLAY A PART, BUT A RECENT STUDY FOUND NO SINGLE GENE WAS RESPONSIBLE.

IN FACT, THEY IDENTIFIED 4 DIFFERENT GENES THAT AFFECT HEIGHT.

KEISHA: IT IS INTRIGUING TO SEE THEM GROW.

IT'S IMPRESSIVE JUST SEEING THE DIVERSITY OF WHO THEY ARE JUST IN ONE FAMILY.

♪ HAPPY BIRTHDAY TO YOU ♪ NARRATOR: GROWING TO YOUR FULL ADULT HEIGHT TAKES A SURPRISINGLY LONG TIME.

PARTYGOERS: ♪ DEAR SUE ♪ NARRATOR: YOUR JOURNEY TO MATURITY IS FAR LONGER THAN ALMOST ANY OTHER ANIMAL ON THE PLANET, MAKING HUMAN CHILDHOOD HIGHLY UNUSUAL AND ENABLING A SEEMINGLY ENDLESS PERIOD OF CURIOSITY AND EXPLORATION.

HA!

I THINK I WAS CURIOUS TO THE POINT OF BEING SOMEWHAT, UH, IRRESPONSIBLE.

OUT IN THE GARDEN, I WOULD JUST CHARGE AROUND, BOUNCING ON THE TRAMPOLINE.

BEING YOUNG FEELS ENERGETIC.

ONE OF MY FOND MEMORIES OF CHILDHOOD IS DISSECTING A WHOLE FISH AFTER FISHING.

AS A CHILD I WAS INTO EVERYTHING, AND I ALWAYS ASKED "WHY?"

ERR!

YOU'RE HAPPY TO BE ALIVE, AND YOU WANT TO DO AS MANY THINGS AS POSSIBLE.

INTERVIEWER: ARE YOU LOOKING FORWARD TO BEING AN ADULT THEN?

NO.

NARRATOR: THE QUESTION IS WHY DOES IT TAKE US SO LONG TO GROW UP?

6-YEAR-OLD RECKER IS ABOUT HALFWAY THROUGH HIS CHILDHOOD... AND LIKE MANY CHILDREN, IT'S A TIME WHEN NEW TALENTS START TO EMERGE.

RIGHT NOW, THERE'S ONE TALENT HE'S ESPECIALLY KEEN ON DEVELOPING.

RECKER: I JUST LIKE BANGING ON STUFF BECAUSE IF YOU BANG ON STUFF IT MAKES A LOUD NOISE, BUT WHEN YOU HIT SOMETHING, IT MAKES A LOUD NOISE, TOO, BUT IT ACTUALLY MAKES A REALLY COOL SOUND.

[DRUMMING ON COUNTER] MAN: I JUST NOTICED HE'D ALWAYS BOB HIS HEAD TO GOOD BEATS.

HEY, EASY.

WOMAN: EVERYWHERE HE GOES, HE FINDS THE RHYTHM IN EVERYTHING.

COMING DOWN THE STAIRS, HE FINDS IT DOWN THE RAILING.

WHILE WE'RE IN THE KITCHEN, HE FINDS IT ON THE COUNTER.

ALL RIGHT.

TEACH ME IT.

SIMPLE.

THIS ONE HAS TO... NARRATOR: AS WELL AS GROWING NEW TALENTS, WE ALSO THINK OF THIS STAGE OF CHILDHOOD AS A TIME OF GROWING UPWARDS.

SINGLE.

HEY.

NO.

WE DID THAT.

NARRATOR: BUT THE STRANGE THING IS, AT CERTAIN TIMES IN YOUR CHILDHOOD, YOUR GROWTH ACTUALLY SLOWS DOWN.

WHEN YOU GET--WHEN YOU GET GOOD AT IT, YOU CAN GO LIKE THIS.

NARRATOR: IN THE PAST FEW YEARS, AN INTRIGUING NEW IDEA HAS EMERGED TO EXPLAIN THIS BIOLOGICAL ODDITY AND UNCOVER THE SECRET OF WHY WE HUMANS HAVE SUCH PROTRACTED CHILDHOODS.

RECKER'S MOM: IS IT HARD TO CHEW AND DRUM AT THE SAME TIME?

HEAD BACK.

NARRATOR: DURING CHILDHOOD AS THE BODY'S GROWTH RATE, SHOWN HERE IN BLUE, DECREASES FROM THE AGE OF 4, GROWING TALLER IS PRETTY MUCH PUT ON HOLD, BUT WHEN RESEARCHERS AT CASE WESTERN UNIVERSITY LOOKED AT THE ENERGY CONSUMPTION OF CHILDREN'S BRAINS OVER EXACTLY THE SAME PERIOD, SEEN HERE IN RED, THEY NOTICED SOMETHING STRIKING; IT GRADUALLY INCREASES, AN EXACT MIRROR OF GROWTH.

THE ADULT BRAIN USES UP A HUGE AMOUNT OF YOUR DAILY ENERGY.

WEIGHING JUST 2% OF YOUR BODY WEIGHT, IT DEMANDS 20% OF YOUR DAILY CALORIE INTAKE, BUT DURING CHILDHOOD, OVER 50% OF YOUR DAILY INTAKE IS DIVERTED TO YOUR BRAIN.

[PLAYING ROCK MUSIC] IN STUDYING ENERGY CONSUMPTION, SCIENTISTS HAVE PROVIDED ONE OF THE REASONS WHY YOU TAKE SO LONG TO GROW UP.

IT'S A TRADE-OFF BETWEEN GROWING YOUR BODY VERSUS GROWING YOUR BRAIN.

YOU SIMPLY CAN'T GET ENOUGH CALORIES TO FULLY GROW BOTH AT THE SAME TIME.

MAN: YEAH, YEAH.

SO DO THE BEAT WITH ME AGAIN.

♪ DOO DOO TASCH DOO DOO DOO TASCH ♪ RECKER'S MOM: WHEN I WAS PREGNANT, I PUT HEADPHONES ON MY BELLY WITH RECKER, SO I FEEL LIKE I MIGHT TAKE A LITTLE CREDIT FOR HIS MUSIC ABILITY.

MAN: RECKER'S A LITTLE UNUSUAL IN A COUPLE OF DIFFERENT WAYS.

HE OBVIOUSLY IS AN AMAZING DRUMMER AT SUCH A YOUNG AGE, AND HE JUST KIND OF-- HE JUST KIND OF GETS IT.

RECKER: MY FAVORITE THING ABOUT DRUMS IS THE DOUBLE KICK.

YOU JUST PLAY YOUR FOOT LIKE THAT, AND THEN IT JUST MAKES A, LIKE... PBBB, PBBB, PBBB!

MAKES ME FEEL HAPPY.

NARRATOR: DURING THIS EXTRAORDINARY PERIOD OF DEVELOPMENT, IT'S WHAT'S GOING ON IN RECKER'S HEAD THAT EXPLAINS WHY HIS BRAIN IS USING SO MUCH ENERGY, AND IT'S NOT JUST ABOUT BRAIN CELLS.

IT'S ABOUT THE CONNECTIONS THAT GROW BETWEEN THEM.

AS YOU GROW AND YOUR BRAIN TAKES SHAPE IN YOUR EARLY YEARS, THOUSANDS OF SYNAPSES ARE FORMED EVERY SECOND.

THESE SYNAPSES TURN A COLLECTION OF 80 BILLION BRAIN CELLS INTO A NETWORK, ALLOWING YOU TO SOLVE THE MOST COMPLEX PUZZLES, STORE YOUR MEMORIES, AND PERFORM TASKS THAT NO OTHER ANIMAL ON EARTH CAN ACCOMPLISH.

THERE YOU GO.

WHETHER RIDING A BIKE, PLAYING CHESS, OR PLAYING THE DRUMS, WHAT YOU DO DURING THIS CRUCIAL PERIOD CHANGES THE VERY STRUCTURE OF YOUR BRAIN FOREVER.

DO THAT... ♪ DOO DOO DOO TA TAKKA DOO TAKKA DOO, DOO, DOO ♪ DO YOU REMEMBER THAT ONE?

NARRATOR: AS RECKER PLAYS, NEW SYNAPTIC PATHWAYS GROW AND CONNECT.

WHAT HAPPENS TO THE UNUSED ONES IS ONE OF THE MOST EXTRAORDINARY ACTIONS IN THE HUMAN BODY.

SYNAPSES ARE THE MOST ENERGY-HUNGRY COMPONENTS OF YOUR BRAIN, SO MAINTAINING UNUSED ONES IS A WASTE OF PRECIOUS RESOURCES.

SO WHILE WE ARE GROWING, WE ARE ALSO SNIPPING WAY UNUSED PATHWAYS IN A PROCESS CALLED SYNAPTIC PRUNING.

AND VERY RECENTLY, SCIENTISTS THINK THEY'VE DISCOVERED HOW THE PROCESS WORKS.

SHAPE-SHIFTING CELLS CALLED MICROGLIA ARE THE BRAIN'S CARETAKERS, CLEARING AWAY INVADING CELLS AND DAMAGED TISSUES, BUT IT'S NOW BELIEVED THEY ALSO TRIM AWAY THE UNUSED CONNECTIONS, DISMANTLING ANY CONNECTIONS THAT AREN'T BEING USED AND SO SAVING PRECIOUS ENERGY.

RECKER'S DAD: READY TO ROCK OUT, SHOW THE NEIGHBORS WHAT'S UP?

NARRATOR: YOU PRUNE SYNAPSES THROUGHOUT YOUR LIFE, BUT IT'S IN CHILDHOOD WHEN YOU'RE GROWING THAT THE PROCESS IS PARTICULARLY ACTIVE.

AS YOUR YOUNG BRAIN IS SCULPTED BY ITS EVERYDAY INTERACTIONS WITH THE WORLD, YOU SLOWLY BUILD A BRAIN THAT IS UNIQUE TO YOU.

YOUR BRAIN IS STRUCTURALLY DIFFERENT FROM EVERY OTHER BRAIN ON EARTH, AND THIS COMPLEX PATTERN OF GROWTH AND PRUNING BACK IS WHAT MAKES US SO DIFFERENT AND INDIVIDUAL.

YOU GOT TO GIVE ME A HIGH-FIVE THERE, BUDDY, ALL RIGHT.

WAY TO GO, RECKER.

NARRATOR: THE UNIQUENESS OF YOUR BRAIN CAN PRESENT A PROBLEM.

BECAUSE EVERY BRAIN GROWS DIFFERENTLY, WHEN IT COMES TO FIXING IT, THERE'S NO SET ROAD MAP TO FOLLOW, BUT AT UNIVERSITY HOSPITAL SOUTHAMPTON IN ENGLAND, DR. PAUL GRUNDY IS AT THE FOREFRONT OF BRAIN SURGERY THAT TACKLES THE PROBLEM OF EACH BRAIN BEING ONE OF A KIND.

NICKY IS JUST 32 YEARS OLD AND IS HAVING AN OPERATION TO REMOVE A BRAIN TUMOR...

BUT BECAUSE YOUR EXPERIENCES SCULPT YOUR BRAIN IN A VERY INDIVIDUAL WAY, DR. GRUNDY HAS TO BE INCREDIBLY CAREFUL WITH WHICH PARTS OF NICKY'S BRAIN HE INTERFERES WITH.

GRUNDY: THE TRIANGULARIS HERE, AND ORBITALIS THERE.

NARRATOR: IN ORDER TO FIND OUT WHAT PARTS ARE CONNECTED TO SPECIFIC FUNCTIONS, NICKY HAS TO BE CONSCIOUS THROUGHOUT THE OPERATION... WOMAN: HOW YOU DOING THERE?

I'M FEELING ALL RIGHT.

NARRATOR: SO SHE CAN ANSWER QUESTIONS.

CAN YOU TELL ME NOW SOME GIRLS' NAMES?

NICKY: OK. HOLLY, SUE, PEGGY.

NARRATOR: AS SHE ANSWERS, DR. GRUNDY STIMULATES NICKY'S BRAIN.

GRUNDY: WE'RE PASSING A TINY ELECTRICAL CURRENT THROUGH THE SURFACE OF THE BRAIN, AND WE KNOW IF YOU STIMULATE OVER PART OF THE BRAIN CONTROLLING SPEECH, THAT THAT STIMULATION WILL INHIBIT SPEECH.

IT WILL STOP THE PATIENT FROM TALKING NORMALLY FOR A BRIEF PERIOD OF TIME.

HAVING IDENTIFIED THAT AREA, YOU CAN'T TOUCH THIS AREA.

YOU MUST PROTECT THIS AREA.

CAN WE GET YOU COUNTING?

YEAH.

CARRY ON UP TO 100.

28, 29, 30.

EXCELLENT.

31, 30--I STOPPED, SORRY.

THAT'S FINE.

GRUNDY: SO WE KNOW THAT IS HER EXPRESSIVE SPEECH CENTER.

YOU PICK THAT UP WHEN SHE--SHE JUST PAUSED.

GRUNDY, VOICE-OVER: AND WE CAME BACK TO THAT SAME AREA ON 3 SEPARATE OCCASIONS, AND ON EVERY OCCASION, WE TESTED IT, IT STOPPED HER SPEECH.

NICKY: 1, 2--HOLD ON.

SORRY.

I'M STILL HERE.

GRUNDY, VOICE-OVER: SO WE COULD HAVE ABSOLUTE CONFIDENCE THAT THAT WAS A CRITICAL AREA OF HER BRAIN FOR HER SPEECH FUNCTION.

NARRATOR: CAREFULLY PROBING THE BRAIN LIKE THIS, DR. GRUNDY IS ABLE TO MAP OUT EXACTLY WHERE NICKY'S SPEECH AREA IS, SO HE WON'T ACCIDENTALLY DAMAGE IT DURING SURGERY, AND IT'S A GOOD JOB HE DOES AS HER SPEECH AREA ISN'T WHERE HE THOUGHT IT WOULD BE.

GRUNDY, VOICE-OVER: THE PRECISE AREA VARIES QUITE CONSIDERABLY FROM PERSON TO PERSON, AND IN FACT, I'D INDICATED AT THE TIME THAT I THOUGHT THE SPEECH AREA WOULD HAVE BEEN ABOUT TWO CENTIMETERS AWAY FROM WHERE WE FINALLY IDENTIFIED IT EXISTED, SO THERE'S A LOT OF VARIABILITY IN SPEECH AND LANGUAGE.

THE ONLY WAY YOU CAN TELL WHERE THOSE IMPORTANT STRUCTURES LIE WITH ANY DEGREE OF ACCURACY IS TO DO IT IN AN OPERATION.

NARRATOR: YOUR BRAIN WILL BE DIFFERENT FROM ANYONE ELSE'S, AND THAT'S LARGELY DUE TO THE SYNAPTIC PRUNING THAT HAPPENS AS YOU GROW.

RESEARCH SUGGESTS THAT WHILE PRUNING CONTINUES, BY THE TIME YOU APPROACH YOUR TEENS, YOUR BRAIN IS WELL ON ITS WAY TO BEING FULLY DEVELOPED.

ONLY THEN CAN THE ENERGY PREVIOUSLY DEDICATED TO YOUR BRAIN BE USED TO POWER THE NEXT STAGE OF YOUR GROWTH... PUBERTY.

THERE YOU GO.

JUST THINKING WHAT'S GOING TO HAPPEN TO ME IN THE NEXT FEW YEARS.

AM I GOING TO STAY LIKE THIS SKINNY LITTLE KID, OR AM I GOING TO BECOME AN ADULT?

I WASN'T TOO SURE.

BOOBS.

SO I REMEMBER THE FIRST TIME I HAD, LIKE, A HAIR UNDER MY ARM.

OHH!

NIGHTMARE.

I DON'T KNOW IF I CAN SAY THE WORD.

PERIODS.

I REALLY HATED THAT.

I WENT FROM BEING SORT OF AVERAGE HEIGHT TO BEING THE TALLEST PERSON IN MY CLASS.

MY BODY DEVELOPED FAIRLY EARLY, SO THAT WAS DEFINITELY SOMETHING I STRUGGLED WITH.

COMING HOME FROM SCHOOL AND BEING HUNGRY EVERY DAY, AND SO, YOU KNOW, YOU GO STRAIGHT TO THE KITCHEN AND SEE WHAT THERE IS TO EAT.

[INDISTINCT CHATTER] THIS GROUP OF TEENAGERS RANGING FROM 12 TO 18 YEARS OLD FORM THE CHEERLEADING SQUAD ICE ALL-STARS OF CHICAGO.

LIKE ALL TEENAGERS, THEY ARE EITHER GOING THROUGH OR ABOUT TO EXPERIENCE THE MOST COMPLEX PERIOD OF OUR GROWTH, PUBERTY.

GIRL: I WAS PROBABLY ABOUT, LIKE, 12 OR 13, MIDDLE SCHOOL AGE, AND IT WAS A GOOD CHANGE.

LIKE, I FELT MYSELF MATURING, MY BODY WAS MATURING, AND I MOVED ON FROM, LIKE, A LITTLE GIRL.

NARRATOR: ALL THESE GIRLS ARE TAKING THEIR OWN UNIQUE PATH TO ADULTHOOD AND ARE AT DIFFERENT STAGES IN THEIR JOURNEY.

WOMAN: ♪ OOH, OOH, YEAH, YEAH ♪ NARRATOR: BUT WHAT DETERMINES WHEN YOU START PUBERTY HAS UNTIL RECENTLY BEEN A MYSTERY.

WOMAN: ♪ YEAH ♪ NARRATOR: WE'VE KNOWN FOR SOME TIME THAT THE PROCESS STARTS IN THE HYPOTHALAMUS, A SMALL GLAND IN THE BRAIN THAT SECRETES THE HORMONE GONADOTROPIN.

THIS STIMULATES THE PITUITARY GLAND.

FROM THIS PEA-SIZE GLAND, PULSES OF CHEMICALS, LUTEINIZING HORMONE AND FOLLICLE STIMULATING HORMONE, FLOOD THE VASCULAR SYSTEM.

WAVES OF THESE REACH THE OVARIES, STIMULATING THEM TO PRODUCE TESTOSTERONE.

IN FEMALES, MOST OF THIS IS IMMEDIATELY CONVERTED INTO ESTROGEN, WHICH FLOODS THE SYSTEM... AND IT'S THIS THAT TRIGGERS PHYSICAL CHANGES THROUGHOUT THE BODY.

IT'S A PROCESS THAT CAN TAKE UP TO 4 YEARS AS THE FEMALE BODY MATURES AND CHANGES FROM CHILD TO ADULT... WITH A MASSIVE GROWTH SPURT PROPELLING TEENAGERS UPWARDS.

GIRL: THERE'S A GIRL--ELLA OUT OF NOWHERE IT SEEMED LIKE, SHE WAS, LIKE, WAY MUCH TALLER THAN ME, LIKE, I HAD TO LOOK UP AT HER.

LIKE, THAT'S SO CRAZY.

"LIKE, WHEN DID YOU GET SO TALL?

I DIDN'T EVEN NOTICE IT."

SHE USED TO BE UP TO MY SHOULDER... YEAH.

AND NOW.

AND NOW I'M... SHE'S AS TALL AS ME.

BASICALLY THE SAME HEIGHT.

NARRATOR: THIS FAST GROWTH SPURT IN THE LONG BONES OF TEENAGE GIRLS AND BOYS IS TRIGGERED BY ESTROGEN.

WHEN IT ARRIVES IN THE BRAIN, IT CAUSES THE PITUITARY GLAND TO DOUBLE THE PRODUCTION OF GROWTH HORMONE.

RUSHING THROUGH THE BODY, GROWTH HORMONE STARTS A CASCADE THAT EXCITES CELLS INTO RAPID DIVISION, CAUSING THE GROWTH PLATES IN THE LONG BONES TO ELONGATE JUST AS THEY DID DURING CHILDHOOD...

BUT NOT EVERYONE EXPERIENCES THESE CHANGES AT THE SAME TIME.

GIRL: WELL, SINCE I'M SMALL, I'M ALWAYS THE ONE THAT'S BEING LIFTED UP BECAUSE OF MY HEIGHT, BUT I DON'T MIND IT, AND IT'S A REALLY EXHILARATING FEELING.

NARRATOR: WHY SOME PEOPLE START PUBERTY EARLIER OR LATER THAN OTHERS HAS ONLY JUST BEEN DISCOVERED.

NEW RESEARCH IS STARTING TO SHED LIGHT ON A POTENTIAL TRIGGER, MAKING A LINK BETWEEN THE LEVELS OF BODY FAT AND THE ONSET OF PUBERTY.

THANK YOU.

THANK YOU.

NARRATOR: IT SEEMS YOUR BODY HAS TO HAVE ENOUGH RESOURCES BEFORE THIS MASSIVE CHANGE CAN HAPPEN.

IT MAY EVEN EXPLAIN WHY TEENAGERS HAVE SUCH AN INSATIABLE APPETITE FOR HIGH-ENERGY FOODS SUCH AS PASTA AND BREAD... LOOK AT ALL KETCHUP.

UGH!

I HAVE TO SPREAD IT SOMEHOW.

NARRATOR: A BIOLOGICAL CRAVING THAT WE NOW KNOW IS WAY BEYOND YOUR CONSCIOUS CONTROL.

SCIENTISTS AT HARVARD UNIVERSITY HAVE RECENTLY DISCOVERED NEURONS THAT SENSE NUTRIENTS AS THEY TRAVEL THROUGH THE INTESTINE.

LOCATED AT THE END OF THE INTESTINAL VILLI, WHERE FOOD ABSORPTION TAKES PLACE, THESE RECEPTORS MONITOR THE LEVEL OF SUGARS, FATS, AND PROTEINS.

THIS INFORMATION SENT ALONG THE VAGUS NERVE TO THE BRAIN, DRIVES US TO EAT MORE WHEN FOODS ARE NUTRIENT-RICH.

I JUST CAN'T EAT JALAPENOS.

I'M MEXICAN!

NARRATOR: IN THIS GROUND-BREAKING STUDY, SCIENTISTS FOUND THAT OUR GUTS AREN'T JUST PASSIVELY DIGESTING WHAT WE EAT BUT CONTROLLING OUR BEHAVIOR SO WE GET THE RIGHT AMOUNT OF ENERGY TO POWER, CHANGE, AND GROW OUR BODIES.

YOU MAY THINK THAT TEENAGERS EAT LIKE THIS BECAUSE THEY'RE GOING THROUGH PUBERTY, BUT IN FACT, THEY MAY BE GOING THROUGH PUBERTY BECAUSE THEY EAT LIKE THIS.

GUYS, THERE'S TANNER.

[INDISTINCT CHATTER] NARRATOR: AFTER ALMOST 18 YEARS OF CONSTANT GROWTH, CHANGE, AND DEVELOPMENT, YOUR BODY IS FINALLY FULLY EQUIPPED FOR ITS JOURNEY THROUGH ADULT LIFE.

EMERGING FROM THE THROES OF PUBERTY STRONGER, FITTER, AND SHARPER THAN EVER, THE 20-SOMETHING HUMAN IS IN ITS PRIME.

IT'S A TIME IN YOUR LIFE WHEN MUCH OF YOUR ENERGY AND RESOURCES ARE DIRECTED TO ONE THING-- FINDING A MATE.

YOUNG COUPLE SVEN AND MICHAELA HAVE REACHED THE STAGE WHERE THEIR THOUGHTS ARE TURNING TOWARDS STARTING A FAMILY.

SIT YOURSELF UP HERE.

HERE?

YEAH.

SVEN, VOICE-OVER: WE HAVE GONE FROM BEING A NEW COUPLE TO KIND OF THAT PLACE WHERE YOU START THINKING, "YEAH, WHAT WOULD IT BE LIKE TO HAVE CHILDREN?"

MICHAELA: TO BE HONEST, WHEN I WAS YOUNGER, I DIDN'T SEE MYSELF HAVING CHILDREN, BUT WATCHING MY LITTLE NIECE AND NEPHEW GROW AND DEVELOP AND BECOME LITTLE PERSONALITIES AND LITTLE HUMANS, THAT DOES MAKE YOU THINK WHAT WOULD YOUR LIFE BE LIKE IF YOU DIDN'T EXPERIENCE THAT.

[CAMERA SHUTTER CLICKS] NARRATOR: THEY'RE LIKE ANY OTHER COUPLE IN LOVE... [SVEN AND MICHAELA LAUGHING] AND BIOLOGICALLY SPEAKING, LOVE IS ALL ABOUT CHEMICAL REACTIONS.

MICHAELA: HE'S GOT A GREAT SMILE AND OBVIOUSLY THE BEARD.

WHO DOESN'T LOVE A BEARD?

SVEN: I THINK EVERYTHING ABOUT HER IS BEAUTIFUL, YEAH, ABSOLUTELY.

JUST SMALL THINGS LIKE THE WAY SHE SMELLS DEFINITELY IS A PART OF THE ATTRACTION.

NARRATOR: TO SEE WHAT HAPPENS DURING THOSE MOMENTS OF INTIMACY, SVEN AND MICHAELA ARE ABOUT TO GET UP CLOSE AND PERSONAL IN AN MRI SCANNER.

WOMAN: CAN YOU HEAR US, GUYS?

MICHAELA: YEAH.

NARRATOR: IT OFTEN ALL STARTS WITH A KISS.

YOUR LIPS, ALONG WITH YOUR FINGERTIPS, ARE PACKED WITH SENSITIVE RECEPTORS, MAKING KISSING AND TOUCHING PLEASURABLE.

JUST CATCHING SIGHT OF YOUR BELOVED CAN BE ENOUGH TO START A CASCADE OF CHEMICAL RESPONSES.

THE BRAIN SENDS SIGNALS TO THE ADRENAL GLAND, WHICH SECRETES THE HORMONE ADRENALIN, CAUSING THE HEART TO BEAT FASTER AND STRONGER.

[HEART BEATING] NERVE CELLS IN YOUR BRAIN THEN RELEASE DOPAMINE, KNOWN AS THE REWARD HORMONE.

NEUROTRANSMITTERS SPARK CHEMICAL MESSAGES TO KEEP YOU ALERT AND ON TASK, GIVING YOU AN UNSTOPPABLE CRAVING AND FOCUSING YOUR ATTENTION ON YOUR LOVED ONE.

ALL THIS TO ENSURE ONE THING HAPPENS.

THE SMALLEST CELL OF THE BODY, THE MALE SPERM, UNITES WITH THE LARGEST CELL, THE FEMALE EGG.

IN 2016, SCIENTISTS AT NORTHWESTERN UNIVERSITY, CHICAGO, CAPTURED THE EXACT MOMENT A SPERM ENZYME REACTS WITH AN EGG.

BEFORE FERTILIZATION, THERE IS AN INCREASE IN ZINC WITHIN THE EGG, ESSENTIAL TO GROWTH, BUT THESE LEVELS THEN NEED TO DROP IF AN EGG HAS ANY CHANCE OF MATURING, AND HERE, CUTTING-EDGE IMAGING SHOWS A FLASH OF LIGHT AS ZINC IS RELEASED AS THE EGG IS FERTILIZED, AND AMAZINGLY, THEY THINK THE MORE LIGHT, THE MORE CHANCE THE EGG WILL GROW INTO A FETUS.

FROM INFANT TO ADULT, YOU ARE ON A LIFELONG JOURNEY OF GROWTH, A LIFETIME OF CELL GROWTH AND RENEWAL THAT IS NOW STARTING TO BE HARNESSED BY MEDICAL SCIENCE TO EXTEND OUR LIFESPANS.

AS WE AGE, OUR NATURAL CELL REGENERATION STARTS TO SLOW.

AS THE WEAR AND TEAR IN OUR BODIES BUILDS UP, EVENTUALLY OUR ORGANS WILL FAIL, BUT WHAT IF WE COULD GROW ENTIRELY NEW ONES?

DR. HARALD OTT IS ONE OF THE PIONEERS ATTEMPTING TO USE OUR UNDERSTANDING OF CELL GROWTH TO GROW NEW ORGANS IN THE LAB.

A FEW YEARS AGO, HIS RESEARCH WOULD HAVE BEEN PURE SCIENCE FICTION.

EVEN TODAY, OTT IS OPERATING AT THE VERY LIMITS OF SCIENTIFIC KNOWLEDGE...

ATTEMPTING TO USE STEM CELLS TO GROW HUMAN ORGANS FOR TRANSPLANT-- IN THIS CASE, A HEART.

OTT: AS WE PROGRESS THROUGH OUR LIVES, MANY DISEASE PROCESS LEADS TO LOSS IN FUNCTION AND IN THE END LEAVE US WITH ORGAN FAILURE, SO ORGAN FAILURE IS AN EPIDEMIC THAT IS INEVITABLE.

IT'S BASICALLY THE RESULT OF US LIVING, RIGHT?

IT'S WEAR AND TEAR THAT IN THE END ENDS UP DAMAGING OR WEARING DOWN ONE OR MULTIPLE ORGAN SYSTEMS IN OUR BODY, AND SO OUR GOAL IS IN THE END TO FIND THE SOLUTION FOR JUST THAT.

NARRATOR: A HUMAN HEART IS SO COMPLEX, IT WOULD BE IMPOSSIBLE TO GROW FROM SCRATCH, SO DR. OTT IS USING THE INNER STRUCTURE OF A HEART DONATED FOR MEDICAL RESEARCH.

REMOVING ALL THE OLD HEART MUSCLE CELLS WITH DETERGENT BUT KEEPING THE INNER SCAFFOLD INTACT IS A DELICATE BUSINESS.

OTT: WE TRIED TO PERFUSE HEARTS FOR MANY, MANY MONTHS WITH DIFFERENT CHEMICALS, AND WHEN WE CAME UP WITH THE RIGHT MIX OF DETERGENTS, THAT WAS A VERY EXCITING DISCOVERY BECAUSE THE CELLS ARE ALL GONE, BUT THAT SCAFFOLD IS PRESERVED.

FINALLY, WE'RE AT A STARTING POINT THAT ALLOWS US TO NOW BUILD FUNCTIONAL TISSUE OFF OF THAT.

NARRATOR: THE PROTEIN SHELL RETAINS THE EXACT ARCHITECTURE OF A WORKING HEART, BUT WITHOUT FUNCTIONING HEART MUSCLE CELLS, THE HEART CAN'T DO ITS JOB, SO DOCTOR OTT HAS TO TRY TO CREATE HEART MUSCLE FROM STEM CELLS.

HE TAKES STEM CELLS FROM THE SKIN AND REPROGRAMS THEM TO ACT LIKE EMBRYONIC STEM CELLS.

OTT: IN ORDER TO REGENERATE ORGANS FOR TRANSPLANTATION OR IN ORDER TO REGENERATE A FUNCTIONAL HEART, WE TAKE SKIN CELLS, AND WE REPROGRAM THEM INTO STEM CELLS.

THEN WE TAKE THESE STEM CELLS AND TURN THEM INTO BLOOD VESSEL CELLS OR HEART MUSCLE CELLS.

ONCE THEY REACH A CERTAIN MATURITY, ONCE THESE CELLS KNOW THAT THEY WANT TO BECOME A VASCULAR CELL OR A HEART MUSCLE CELL, WE SEED THEM ONTO THESE ORGAN SCAFFOLDS, ONTO THESE HEARTS.

NARRATOR: DOCTOR OTT HAS BUILT A HEART THAT LOOKS LIKE YOURS OR MINE, BUT IT'S NOT READY FOR TRANSPLANT YET... BECAUSE THE NEW CELLS ARE VERY YOUNG... MORE LIKE FETAL HEART CELLS.

THE SCIENTISTS STILL NEED TO GET TO THE NEXT LEVEL SO THE ORGAN CAN OPERATE LIKE AN ADULT HEART.

OTT: THE ULTIMATE DREAM HERE OF COURSE IS TO HAVE A PATIENT WALK INTO YOUR OFFICE AS A PHYSICIAN WHO NEEDS A HEART OR A LUNG OR A KIDNEY, AND YOU TAKE A SCAFFOLD OFF THE SHELF, AND YOU TAKE CELLS FROM THAT PATIENT, AND USING STEM CELL TECHNOLOGY, YOU THEN DERIVE AN ORGAN OR BUILD AN ORGAN ON DEMAND.

NOW THERE'S MANY YEARS OF RESEARCH ON THE WAY TO THAT, BUT THE BENEFIT OF THIS WOULD BE THAT, ONE, YOU COULD MAKE AN ORGAN ON DEMAND WITHOUT HAVING THE ISSUE OF DONOR ORGAN SHORTAGE, SO YOU WOULDN'T HAVE TO WAIT FOR ONE TO COME AROUND, AND YOU WOULD HAVE AN ORGAN THAT IS IMMUNO TOLERATED.

THAT'S A TERM THAT SAYS IF IT'S MADE FROM YOUR CELLS YOUR BODY DOESN'T RECOGNIZE IT AS A FOREIGN TISSUE, SO IT DOESN'T REJECT IT.

NARRATOR: THE RESEARCH, WHILE NOT PROMISING AN EVERLASTING LIFE, COULD EXTEND THE BODY'S NATURAL ABILITY TO REJUVENATE, ENABLING US TO CONTINUE OUR JOURNEY OF GROWTH FURTHER INTO OLD AGE.

THE STORY OF HUMAN GROWTH IS AS LONG AS LIFE ITSELF.

IN YOUR FIRST YEARS AFTER BIRTH, THE REMARKABLE POWER OF YOUR MOTHER'S MILK PROTECTS YOU AND PUSHES YOU INTO A PERIOD OF GROWTH THAT IS UNEQUAL DURING YOUR LIFETIME, WHERE YOU WILL MORE THAN DOUBLE IN HEIGHT AND TRIPLE IN WEIGHT.

THEN DURING YOUR CHILDHOOD, YOUR BRAIN TAKES CENTER STAGE, REQUIRING ENORMOUS AMOUNTS OF YOUR ENERGY AS IT GROWS AN INCREDIBLE NETWORK OF CONNECTIONS TO DEAL WITH ALL THE NEW EXPERIENCES AND LEANING BEFORE PRUNING THESE BACK TO LEAVE YOU WITH A BRAIN THAT IS CONNECTED AND EFFICIENT.

CHEERLEADERS: 5, 6, 7, 8!

NARRATOR: WITH RESOURCES NOW FREE, THEY WILL BE USED TO TAKE YOUR BODY THROUGH A WHOLESALE METAMORPHOSIS DURING PUBERTY, TRANSFORMING YOU FROM CHILD INTO ADULT.

WHILE YOUR JOURNEY UPWARDS WILL CEASE, THE AMAZING HIDDEN PROCESSES OF GROWTH WILL CONTINUE TO ENSURE YOU REGENERATE AS YOU EXPERIENCE THE WEAR AND TEAR OF DAILY LIFE, AND THOUGH OUR ABILITY TO REGENERATE DIMINISHES, OUR NEW UNDERSTANDING OF HOW WE GROW MEANS WE CAN LOOK TO A FUTURE WHERE OUR POWER TO REJUVENATE IS EXTENDED, HELPING US TO LIVE LONGER, HEALTHIER LIVES AND PUSH THE BOUNDARIES OF WHAT A HUMAN LIFETIME CAN BE.

♪♪ "THE AMAZING HUMAN BODY" IS AVAILABLE ON DVD.

TO ORDER VISIT SHOP.PBS.ORG OR CALL 1-800-PLAY-PBS ALSO AVAILABLE ON AMAZON PRIME VIDEO ♪♪