Vessels and Nerves
Blood vessels form the bulk of the iris stroma. Most follow a radial course, arising from the major arterial circle and passing to the center of the pupil. In the region of the collarette (the thickest portion of the iris), anastomoses occur between the arterial and venous arcades to form the minor vascular circle of the iris, which is often incomplete. The major arterial circle is located at the apex of the ciliary body, not the iris (see Chapter 1, Fig 1-20).
The diameter of the capillaries is relatively large. Their endothelium is nonfenestrated and is surrounded by a basement membrane, associated pericytes, and a zone of collagenous filaments. The intima has no internal elastic lamina. Myelinated and unmyelinated nerve fibers serve sensory, vasomotor, and muscular functions throughout the stroma.
Figure 2-21 Composite drawing of the surfaces and layers of the iris, beginning at the upper left and proceeding clockwise. The iris cross section shows the pupillary (A) and ciliary (B) portions; the surface view shows a brown iris with its dense, matted anterior border layer. Circular contraction furrows are shown (arrows) in the ciliary portion of the iris. Fuchs crypts (C) are seen at either side of the collarette in the pupillary and ciliary portions and peripherally near the iris root. The pigment ruff is seen at the pupillary edge (D). The blue iris surface shows a less dense anterior border layer and more prominent trabeculae. The iris vessels are shown beginning at the major arterial circle in the ciliary body (E). Radial branches of the arteries and veins extend toward the pupillary region. The arteries form the incomplete minor arterial circle (F), from which branches extend toward the pupil, creating capillary arcades. The sector below demonstrates the circular arrangement of the sphincter muscle (G) and the radial processes of the dilator muscle (H). The posterior surface of the iris shows the radial contraction furrows (I) and the structural folds (J) of Schwalbe. Circular contraction folds are also present in the ciliary portion. The pars plicata of the ciliary body is shown at bottom (K).
(Reproduced with permission from Hogan MJ, Alvarado JA, and Weddell JE. Histology of the Human Eye. Philadelphia: WB Saunders; 1971.)
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.