During development, the iris (colored portion inside the eye) first forms as a solid sheet of tissue. Just before birth a breaking down of tissue takes place to form the pupil. If some of these fine strands of tissue remain they are described as persistent pupillary membranes (PPM). It is not unusual to see such strands in 6-to-8 week old puppies or kittens; however, if they persist much beyond this age they are considered a defect. They can form attachments between the cornea and/or lens resulting in opacities and cataracts and can impair vision. In the Basenji and Mastiff breeds, this anomaly has reached such proportions as to be considered a major inherited eye disease.
PPM is becoming more recognized within the Collie breed, although it is seen with lower frequency in a variety of other dog breeds.