system
Given the following post, respond as if you're commenting on Reddit:
human
Are stretch marks permanent?

There are a lot of products and home remedies, but what does science have to say?
gpt
Stretch marks are similar to a scar from a wound. What happens is the skin is stretched by rapid growth (example: growing tall quickly or gaining weight quickly). 

The skin (all layers) are stretched beyond the physical limitations of the cellular composition of the tissue. The skin and is unable to grow to keep pace. This causes subtotal tearing. It's not a open wound, the internal/external barrier is still intact (although weakened).

The body responds to this in a damage control response. Simply; Fibroblasts migrate into the voids left by the stretching and crudely begin to proliferate to increase the thickness of the barrier to the external environment. This is different than normal growth because these cells are not derived from semi-differentiated stem cells which are capable of forming more specialized cells. The various stem cells are able to form multiple cells of different types and give an organ like the skin its complexity. This is different from scarring as collagenous fibroblasts replicate and form cell types of limited complexity; these are unable to emulate skin that formed in utero.

This 'patch' never fully returns to its original state before the damage. The stem cells and underlying tissues are damaged, missing or both. However, over time the scaring becomes less pronounced as long as additional damage does not occur. These patches also have limited tanning, sensing and protective capacities. 

Some topical treatments may increase the "elasticity" of the skin, causing it to pull in on itself more also reducing the size of the scars. If the marks are from rapid weight gain, weight loss may reduce their appearance. Exercise in general tends to make tissues more dense and stronger. Even the skin.