Complex sentence: Another form is atrophic scarring (sunken scarring), which also has an over expression of collagen blocking of regeneration, this scar type is sunken, the collagen bundles do not over extend the tissue.
Split into simpler sentences: Another form is atrophic scarring (sunken scarring), which also has an overexpression of collagen blocking regeneration. This scar type is sunken, because the collagen bundles do not overextend the tissue.