Topics Discussed: iris; iris diseases; ophthalmology.
Sections: Pathogenesis, Clinical Presentation, Iris Disorders, Pupil Disorders, References.
Excerpt:"The iris is a structure composed of connective tissue and blood
vessels that lies just anterior to the lens. The central opening
in the iris forms the pupil. The color of the iris is determined
by pigmented cells within the stroma. Pigment may accumulate in
these cells during the first year of life, and the color of the
iris often changes during this time. The posterior layer of the
iris is deeply pigmented. It extends slightly onto the anterior
surface at the edge of the pupil.At 6 weeks of gestation the iris begins to form in association
with the tunica vasuculosa lentis.
This is a network of blood vessels that extends through mesenchymal
tissue on the anterior surface of the developing lens. The muscles
of the iris sphincter begin to develop at around 3 months' gestation,
followed at 6 months by formation of the dilator muscles. The iris
stroma forms from neural crest cells. The posterior pigmented epithelium
and muscles form from neuroectoderm. Much of the iris remains incompletely
formed at birth, including pigmentation of the anterior layer and
formation of the dilator muscles. This is why the iris often appears
somewhat hypoplastic, even in normal newborns. During normal embryogenesis
the tunica vasulosa lentis resorbs in the pupillary opening by birth.
Remnants of the membrane may be seen in premature infants (Figure
29–2)...."
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