Complex sentence: Root resorption occurs as a result of differentiation of macrophages into (odontoclasts) in surrounding tissue which, if in close proximity to the root surface will resorb the root surface cementum and underlying root dentin.
Split into simpler sentences: Root resorption occurs as a result of differentiation of macrophages into odontoclasts, in surrounding tissue. Which, if in close proximity to the root surface will resorb the root surface cementum and underlying root dentin.