Lens fibre from an eye. Coloured scanning electron micrograph (SEM) of the fibre-like cells that form the crystalline lens of the eye. During their development, these cells lose their nuclei and become an inert, transparent protein structure. They are called fibres because of their long, thin dimensions. The lens is responsible for precisely focusing light onto the light sensitive cells at the back of the eye (retina) after it has been roughly focused by the cornea at the front of the eye.