The iris sphincter and dilator muscles originate from which tissue type?

• A. Neural ectoderm
• B. Mesoderm
• C. Neuroectoderm
• D. Ectoderm

Answer: (C)

The iris sphincter and dilator muscles come from neuroectoderm. During eye development, neural crest–derived mesenchyme migrates into the developing iris and differentiates into the smooth muscle fibers that form the sphincter pupillae and dilator pupillae. Neural crest cells originate from the ectoderm and are considered part of the neuroectoderm lineage, which is why these iris muscles are described as neuroectodermal in origin.

While the optic cup itself is neural ectoderm, the muscular components of the iris are not from mesoderm; they arise from neural crest–derived tissue. Broad ectoderm would be too general, and mesoderm would incorrectly imply a mesodermal origin for these iris muscles.