Iris Anatomy: Ultrastructure Notes

Iris Ultrastructure

Iris Layers

  • The iris has four main layers:
    • Anterior border layer.
    • Stroma and sphincter muscle.
    • Anterior epithelium and dilator muscle.
    • Posterior pigmented epithelium.

Anterior Border Layer (ABL)

  • Surface layer of the iris.
  • Thin condensation of stromal tissue.
  • Facilitates light absorption and allows aqueous to penetrate the stroma.
  • Composed of interwoven collagen fibers interspersed with Fuchs’ crypts.
  • Fibroblasts are concentrated at the surface, with melanocytes below.
  • Variable numbers of melanocytes, but no large blood vessels.
  • ABL is absent at the crypts and ends at the iris root.
  • Thicker in brown irises and thickest at the collarette.
  • Freckles are accumulations of pigmented melanocytes in the ABL.
  • Capillaries sometimes seen in ABL - rubeosis iritis, associated with chronic retinal ischemia.

Iris Stroma

  • Contains structural elements similar to ABL (melanocytes, fibroblasts) but in a looser form.
  • Contains:
    • Collagen bundles with spaces.
    • Scattered melanocytes and fibroblasts.
    • Sphincter muscle.
    • Blood vessels.
    • Nerves.
    • Immune cells.
  • Looser, sponge-like structure for pupil constriction and dilation.

Sphincter Pupillae Muscle

  • Located in the pupillary portion of the iris stroma.
  • Smooth muscle: 0.75 – 1 mm wide and 0.1 to 0.17 mm thick.
  • Encircles the pupil; contraction causes pupil miosis.
  • Innervated by the parasympathetic system.
  • Supported by collagen and connective tissue of the stroma.
  • Muscle cells are spindle-shaped and closely packed.

Dilator Pupillae Muscle

  • Thin myoepithelial layer from iris root to sphincter.
  • Single layer ~ 12.512.5 µm thick, continuous with anterior pigment epithelium.
  • Apical epithelial portion and basal muscular portion (4 µm thick) comprise spindle-shaped processes (7 µm wide, 60 µm long).
  • Non-myelinated post-ganglionic sympathetic neurones terminate near muscle processes.

Two Epithelial Layers

  • The anterior epithelial cells are flatter with lower melanin density and long extensions that form the strands of the dilator.
  • Posterior epithelium cells are columnar with a high density of melanosomes.

Posterior Pigmented Epithelium

  • Derived from the internal layer of the optic cup.

  • Height of 36 to 55 µm and width of 16 to 25 µm.

  • Densely packed with melanin granules.

  • Becomes unpigmented in the periphery (iris root).

  • Continuous with non-pigmented epithelium of the ciliary body.

  • Curls around the pupillary margin to produce the pupillary ruff.

  • Makes the iris opaque; missing areas transilluminate.

  • Ocular albinos have a melanin deficiency in the RPE and iris.