During the development of the eye, the front or anterior chamber of the eye is filled with a primitive embryonic tissue called the pupillary membrane. As this tissue disappears the pupil forms. Normally the pupillary membrane totally disappears but occasionally remnants of the structure remain in the eye as strands bridging the iris, or strands which travel from the iris across to the lens or cornea.
In some breeds such as the Basenji or the English Mastiff they are known to be heritable. The heritability is not known in most other breeds but the condition rarely causes vision problems unless the strands cause extensive opacification of the lens or cornea. Usually the recommendation is that the breeder can decide if he or she will or will not breed dogs with PPMs (ie breeder option).