As it develops, the skin may thicken and crack.

The skin thickens with age, making older cacti more fire resistant.

The skin tends to be more sensitive and may thicken, crack or scale.

But after that, his skin thickened, and he stuck around.

As men age, their skin thickens and shows wrinkles less.

They gained lean body mass, lost fat, and their skin thickened, taking on a more youthful look.

The skin thickens, embedded scales develop and the body divides into segments.

She would have imagined his skin had thickened a bit.

Because of my job, Grace's skin has thickened to violence through osmosis.

And, although the skin thickens in the sun to protect itself, it doesn't on a sunbed.