“Much of your body is constantly renewing itself, but some parts of you are built to last a lifetime,” writes Whimsy columnist CLIVE WILLIAMS.

I said to my dentist “My teeth are going yellow”. He said, “Wear a brown necktie.” –Rodney Dangerfield.

The notion that your body completely replaces itself every seven years (or any set number of years) is a popular myth – but it’s partially true. 

Here’s what actually happens.

Much of your body is constantly renewing itself, but some parts of you are built to last a lifetime. Neurons in the cerebral cortex – the part of your brain responsible for thought, memory, and consciousness –  generally don’t regenerate. 

Similarly, the cells of the eyes’ lenses remain with you from birth to death. In other words, some parts of “you” are permanent, while others are in a constant state of renewal.

Here’s a rough guide to how quickly some parts of your body renew:

• Skin cells, 14-28 days

• Red blood cells, 120 days

• Stomach lining cells, 2-9 days

• Liver cells, 300-500 days

• Bone cells, 10 years

• Fat cells, 8 years

• Brain neurons, largely permanent

• Heart muscle cells, renew slowly (1 per cent a year)

• Eye lens cells, don’t renew

• Teeth, minor renewal

Teeth are a special case. Unlike skin, blood, or even bone, teeth do not regenerate once fully formed but they are extremely hard and long wearing.

When we were doing missing-in-action recovery work in Vietnam, we found that teeth were sometimes all that was left of a dead body after burial in lateritic soil, or exposure to fire – such as in an air crash. Full-size coffins were used for official burial purposes, but often there wasn’t much in them.

The regeneration potential of the teeth is as follows:

• Enamel (the outer layer of your teeth): Once damaged, the body cannot repair it.
• Dentin (the layer of tissue in your teeth that lies just beneath the enamel): Can form reparative dentin when stimulated, but only minimally.
• Pulp (the nerve and blood): Cannot regenerate if severely damaged; requires intervention.
• Cementum (the root cover): Slight regeneration possible if surrounding tissue is healthy.

Although your body can’t regrow a tooth on its own, modern dentistry does offer impressive ways to repair or replace damaged teeth through dental implants, biocompatible fillings and crowns, and regenerative endodontics. 

The bottom line is that your body does not completely renew itself – some parts are largely permanent.

On a lighter note – A young guy from Texas moves to California and goes to a big department store looking for a job.

The manager says: “Do you have any sales experience?”

The kid says: “I was a trainee salesman back home in Texas.” 

Well, the boss likes the kid, so he gives him a sales job. “You start tomorrow. I’ll come down after we close and see how you did.” 

The first day on the job is always rough but the kid gets through it. After the store is locked up, the boss comes down. “How many sales did you make today?”

The kid says, “One”. 

The boss fears he’s backed a lemon and says: “Just one?! Our salespeople normally average 20 to 30 sales a day. How much was the sale for?” 

The kid says: “$101,237.64.” 

The boss, astounded, says: “$101,237.64? What the heck did you sell him?” 

The kid says: “First I sold him a small fishhook. Then I sold him a medium fishhook. Then I sold him a larger fishhook. Then I sold him a Loomis fishing rod. Then I asked him where he was going fishing, and he said down at the coast, so I told him he was going to need a boat, so we went down to the boat department, and I sold him that twin-engine Chris Craft and trailer. Then he said he didn’t think his Honda Civic would pull it, so I took him down to the automotive department and sold him that 4 X 4 Blazer.” 

The boss says incredulously: “A guy came in here to buy a fishhook and you sold him a rod, boat and truck?!”

The kid says: “No, sir, he came in to buy a box of tampons for his wife, and I said, ‘Well, sir, since your weekend’s shot, you might as well go fishing’.”

Clive Williams is a Canberra columnist