Uvea III - Iris

-The anterior stroma is of mesodermal origin, although Bron argues that a more appropriate designation would be mesenchymal due to the contribution of the neural crest cells to its development. -The two posterior rows of epithelium are of a neurodermal origin.

-The iris is a diaphragm controlling the pupil aperture and therefore controls the amount of light that enters the eye.

-The iris is the most anterior portion of the uveal tract.-The peripheral portion of the iris, which is attached to the ciliary body is not visible by direct viewing but may be observed using a gonio lens.-Clinically, the visible iris diameter is used as a measurement for the corneal diameter, which for instance is often helpful to know in guiding the correct choice of contact lens overall diameter.

-The diameter is approximately 12mm and its circumference is 38mm.-This iris is thickest at the collarette (0.6mm) and thinnest at the root (0.5mm)

-Anteriorly, the iris is formed by stromal tissue-Posteriorly, it is formed by two layers of densely pigmented epithelial cells.

-The pupil pierces the iris, slightly superior and nasal to its geometrical center by 0.2 to 0.3mm in each oerientation.

-Its function is to regulate the amount of light that enters the eye.-To accomplish this, it constricts in light to dilate in the dark.

-The pupil diameter ranges from 1.5 to 8mm in youth.-In the older population the average pupil size is often smaller due to the fibrotic changes in the sphincter and atrophy of the dilator muscle.

-The color of the iris is determined mainly by the melanocytes in the stroma and the anterior border layer.-Melanocytes contain the only pigment in the eye, melanin, which is brown.-In a brown iris the melanocytes are perfused and well pigmented.-In a blue iris, there is a decrease in the number of melanocytes and while the longer wavelenghts are absorbed, the shorter wavelengths (in the blue region of the spectrum) are reflected through back scatter.-Green eyes are a combo of blue and brown.

-In an albino iris, there is a buff color due to the absence of pigmented melanocytes.-As the light levels increase, the albino iris takes on a pink color because the light that has revealed

-The collarette lies about 1.6mm from the pupillary margin and divides the surface into an inner pupillary zone and an outer ciliary zone, which often differ in color.-The iris is slightly thicker at the collarette, which overlies an incomplete vascular circle (circulus vaculosus iridis minor)

-The anterior iris surface has a trabecular structure which is most prominent in the pupillary zone and collarette region where there are discontinuities in the superficial layer; as a result, large, pit-like depressions are formed and are referred to as Fusch's crypts (or crypts of fuchs).-The trabecular structure which form the crypts are remnants of obliterated vessels that passed to the pupillary membrane during embryonic life.

-The pupillary ruff is formed as the posterior epithelial layers of the iris extend forward at the pupillary margin.-The crenations of the pupillary ruff result from a forward extension of the radial folds of the posterior iris surface.-The crenations are most noticeably observed during miosis.

-The posterior surface of the iris is dark brown due to dense pigmentation and displays radial and circular furrows.-Schwalbe's contraction folds are numerous, small folds that extend radially for approximately 1mm from the edge of the pupil.-The posterior pigmented layers extend forward around the pupil and are responsible for the crenated surface of the pupillary margin.-Schwalbe's structural furrows begin 1.5mm from the pupillary margin where they are narrow and deep.-Toward the peripheral iris they broaden and become shallow.-These structural furrows pass into the valleys of the ciliary processes.-Also found within the posterior iris are fine circular furrows that cross the structural furrows at regular intervals and are due to the variation of the thickness of the pigmented epithelium.

1. Anterior Border Layer2. Stroma and sphincter muscle3. Anterior epithelium and dilator muscle4. Posterior pigmented epithelium