CHOROID :- Definition, Function, Microscopic structure

Choroid is the vascular layer of the Eyeball.
It is the Posterior most part of the vascular layer of the eyeball.
Its inner surface is smooth, brown and lies in contact with pigment epithelium of the Retina.
It extends from the optic disc to ora serrata.
The outer surface is rough and lies in contact with sclera .
It is thicker posteriorly - 0.22 mm
Thicker at Anterior - 0.1 mm.


MICROSCOPIC STRUCTURE :-

It consists the following four layer -

*Suprachoroidal lamina (lamina fusca)
*Stroma of the choroid.
*Choriocapillaris.
*Basal lamina (Bruch's membrane or lamina vitrae).

1- Suprachoroidal lamina -

It is thin membrane 10-34 mm.
It consists condensed collagen fibres, fibroblasts and melanocyte.
Transition zone of choroid and sclera.
Blends with choroid and lamina fusca of sclera.

2- Stroma of the choroid -

It contains plenty of pigment cells .
It contains other cells like microphage, masts cells, plasma cells and lymphocytes.
It consists of loose collagenous tissue .
Arteries are branch of short posterior Ciliary arteries.

3- Choriocapillaris -

It consists of a rich capillary Network.
It nourishes the pigment epithelium and the outer layers of sensory retina.
Capillaries are fenestrated.
Extends anteriorly to ora serrata.
Greatest density of capillaries at macula.

4- Basal lamina (Bruch's membrane or lamina vitrae) -

It is inner most layer of choroid and approx- 2-4 mm in thickness.
It is multilayered structure.
It lies between the Choriocapillaris and pigment epithelium of the Retina.
It prevents choroid vessels from penetrating the Retina but allow nutrients, protein etc.

It is described to consists of five layers :

*Basement membrane of the retinal pigment epithelium.
*An inner collagen layer.
*A middle elastic layer.
*An outer collagen layer.
*The basement membrane of the Choriocapillaris.

FUNCTION:-

1. it gives protection to the eyeball.
2. supply of nutrients to tissues of the eyeball.
3. converging the light.