Contributor: Gordon K. Klintworth
A typical coloboma is a developmental anomaly situated the inferonasal portion of the iris in the line of closure of the embryonic fissure and often extends through the entire uveal tract to involve the retina, choroid and/or the optic disc. It results from incomplete closure of the embryonic fissure (5-8 weeks of gestation). Histologic examination of the base of a typical iris coloboma reveals changes similar to those of aniridia. The edge of the coloboma is formed by rounded stroma as well as by folded iris pigment epithelium. The remains of the pupillary membrane and a few anomalous connective tissue strands may be present.
In the typical lens coloboma the inferonasal portion of the lens is notched in the region of the embryonic fissure. Lens tissue is not missing, but the notched portion of the lens equator is is thicker and rounder than normal at the notched site. Rarely an inferiorly displaced teardrop-shaped lens forms when the inferior part of the lens becomes adherent to the inferior midline raphe at the site of fetal fissure closure.