During the embryonal phase, the "pupil" of the primitive iris tissue is covered by a membrane, the pupillary membrane. This membrane begins to regress about two weeks before birth and has completely disappeared by 2 to 4 weeks after birth. Occasionally remnants of this membrane remain as congenital defects.
As the term indicates, persistent pupillary membrane is a condition in which the delicate membrane covering the anterior surface of the lens during the greater part of the intrauterine life, fails to undergo complete resolution and persists as fibers, either singly or in strands, passing across the pupil, or slightly opaque membrane floating or adherent to the capsule of the lens.
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In the eye, these strands, or "threads", sometimes resemble a spider's web. They can also attach to the cornea and cause scars there. The condition seems to hereditary in some cases. Where but a few fibers persist, vision is not impaired and their presence is usually unnoticed until some disorder prompts close inspection of the eye. In some cases, the membrane interferes with the dog's vision, particularly in bright light. This condition may be found alone or in combination with other eye defects. Strands and floating membranes can be removed surgically.
- Adam Rijnberk. Medical History and Physical Examination in Companion Animals.