Secondary Corneal Disease: Developmental Abnormalities of the Anterior Segment

The development of ocular anterior segment structures is a precisely coordinated process that is determined by both genetic and environmental factors. In humans, this process begins from week six of gestation and is characterized by the formation of the lens placode from overlying surface ectoderm. The cornea is derived after lens detachment, while several waves of tissue invade the primary mesenchyme that lies behind the surface ectoderm, ultimately giving rise to an anterior epithelium and a posterior endothelium, with the corneal stroma laying between these layers. A number of genes that include transcription factors, nuclear proteins, structural proteins, and enzymes are known to be involved in this sophisticated process, and defects in these key genes may lead to severe congenital anterior segment dysgenesis (ASD).