Does a phono cartridge have a break-in period??

The short answer is "yes". Regardless of what you think about break-in of electronics/wires, etc, there is no question that the elastic characteristics of suspension components will change over time. So will the "shape" of the stylus. Unfortunately, that also means that they will eventually require replacement. Luckily for you, with an MM, you can get a new stylus/cantilever/suspension to plug in for relatively modest $.

"My question is: do cartridges have a burn-in period after which the sounds is fuller or better?? If so, what exactly is breaking in?? stylus, cartridge coilings??"

Of all the components in an audio system, the phono cart changes the most in sound from break in. Its not even close. I've had cartridges that sounded defective when they were new. For the first 10 or 20 hours, you can usually hear a difference from record to record.

As far as what's breaking in, its a mechanical device like a speaker.

I find, for most cartridges, 40-50 hours is about the average for break-in. Some less, some more.

IME they certainly do, more than any other component (along with speakers).

IMO and as noted by Swampwalker, this is largely due to changes in the suspension. The process is inevitable, because subjecting a new, never stressed elastomer to repeated flexion/compression cycles causes changes at the molecular level. A perfectly elastic suspension wouldn't change, but unfortunately a perfectly elastic material doesn't exist.

Dynamic cone speakers have a break-in period for exactly the same reason.

The length of break-in varies widely from one cartridge to another. I've had cartridges stabilize after just 10-20 hours while others have continued to exhibit wild swings through 100 hours and beyond.

Thanks to all who have responded so far. I appreciate the info and education.

How do you like your new Ayre CD player? For the money, I don't think you could have done better.