IRIS :- Definition , function, Microscopic structure.

*Iris is the Anterior most part of the UVEAL TRACT.
*The average diameter and thickness of Iris is 12 mm and 0.5 mm.
*In the center of Iris "slightly Nasal" , is a hole or aperture called PUPIL (3-4 mm).
*Pupil control the amount of light reach the Retina.
*Iris lies between the Cornea and lens into Anterior to posterior chamber.
*Iris is attached to the middle of anterior surface of the ciliary body at periphery.
*Iris is thinnest at its root( tears away easily from ciliary body).


MICROSCOPIC APPEARANCE :-

Anterior part of the Iris -

*IRIS in Anterior divided into a Ciliary Zone and a Pupillary zone by zigzag line called COLLARETTE (thickest region of the Iris).

1- CILIARY ZONE -

* Radial streaks are straight when pupil is small and wavy when it is dilated.
*Crypts are depression where the superficial layer of Iris is missing.
crypts are arranged in two rows -

A- Peripheral crypts
B- Central crypts.

*Contraction furrow are the tenuous line concentric to the collarette.

2- PUPILLARY ZONE- 

*This zone lies between the collarette and pigmented pupillary frill.
*It is about 1.6 mm wide.

*Pigment frill is a fringe of black pigment present at the pupillary margin.

Posterior part of the Iris -

*Posterior part of the Iris is dark brown or black in color.
*It looks smooth with naked eye.
*Following radial , circular furrows and folds are present in posterior surface of Iris.

MICROSCOPIC STRUCTURE :-

Anterior to posterior it consists four layers  are as follows -
*Anterior limiting layer
*Iris Stroma
*Anterior epithelial layer
*Posterior pigmented epithelial layer.

1- Anterior limiting layer -

*The definitive color of the Iris depends upon this layer.
*Blue Iris - layer is thin and contain few Pigment cells.
*Brown Iris - thick layer and doubly pigmented.

*It is the Anterior most condensed part of the Stroma.
*It consists of melanocyte, and fibroblasts.

2- Iris Stroma -

*It form the main bulk of the Iris tissue.
It consists of loosely arranged collagenous Network.


3- Anterior epithelial layer -

*It is the Anterior continuation of the pigment epithelium of retina and ciliary body.
*Lacking of melanocyte.

4- Posterior pigmented epithelial layer -

*This layer is derived from the internal layer of the optic cup.
*It is the continuation of the non- pigmented epithelium of the Ciliary body.

FUNCTION :-

1. It is responsible for controlling the diameter and size of the pupil.
2. Eye color is defined by that of the iris.
3. Maintains the gradient of Intraocular pressure between the front and back of the eye