Dilator Muscle and Anterior Pigmented Epithelium
The dilator muscle develops from the anterior pigmented epithelium and is derived from the neuroectoderm. It lies parallel and anterior to the posterior pigmented epithelium (Fig 2-22; see Fig 2-20). The smooth muscle cells contain fine myofilaments and melanosomes. The myofibrils are confined mainly to the basal portion of the cells and extend anteriorly into the iris stroma. The melanosomes and the nucleus are found in the apical region of each myoepithelial cell. The remaining anterior pigmented epithelium is smaller and less pigmented than its posterior counterpart, making it difficult to visualize even on histologic sections.
Figure 2-22 Anterior and posterior pigmented epithelia of the iris. The posterior pigmented epithelium is larger than the anterior epithelium and contains more pigment granules than does the latter.
(Illustration by Thomas A. Weingeist, PhD, MD.)
There is dual sympathetic and parasympathetic innervation. The dilator muscle contracts in response to sympathetic α1-adrenergic stimulation; cholinergic parasympathetic stimulation may have an inhibitory role. See BCSC Section 5, Neuro-Ophthalmology, for additional discussion of physiology and pathology of the dilator muscle.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.