Layers of the eye.

The sagittal view of an eye shows its three major layers or tunics, with the lens in the anterior opening of the vascular layer and retina.

The lens of the eye is a biconvex transparent structure held in place by a circular system of zonular fibers, which extend from the lens into a thickening of the middle layer, the ciliary body, and by close apposition to the vitreous body on its posterior side. Partly covering the anterior surface of the lens is an opaque pigmented expansion of the middle layer called the iris. The round hole in the middle of the iris is the pupil.

The eye contains two fluid-filled cavities: the anterior chamber, which occupies the space between the cornea and the iris and the posterior chamber, between the iris, ciliary processes, zonular attachments, and lens. Interconnected at the pupil, these contain a clear fluid called aqueous humor.

The vitreous chamber lies behind the lens and its zonular attachments and is surrounded by the retina. This chamber is filled with a transparent, gelatinous mass

of connective tissue called the vitreous body.

Eye formation begins in the early embryo with epithelial optic vesicles bulging bilaterally from the developing forebrain. These elongate and form optic stalks bearing optic cups. Interaction between the optic cups and the overlying surface ectoderm causes the latter to invaginate and detach bilaterally, forming the lens vesicles. In the ensuing weeks, head mesenchyme differentiates to form most of the tissue in the eye's three layers and the vitreous, with the

ectoderm of the optic cup and optic stalk giving rise to the retina and optic nerve, respectively, and with surface ectoderm contributing to the cornea.

EYEBALL
Sclera	It is composed of dense fibrous tissue (collagen). Some elastic fibres, and connective tissue cells (mainly fibroblasts) are also present. Some of the cells are pigmented.
Choroid
- Suprachoroid lamina
- Choroid proper	It consists of a network of blood vessels supported by connective tissue in which many pigmented cells are present, giving the choroid a dark colour.
- Basal lamina
Retina
- Pigment epithelium	A single layer of polygonal cells containing pigment. Processes from pigment cells extend into the next layer.
- Layer of rods and cones	The rods are processes of rod cells, and cones are processes of cone cells. The tips of the rods and cones are surrounded by processes of pigment cells.
- External limiting membrane	It is formed by processes of Müller’s cells.
- Outer nuclear layer	It contains the cell bodies and nuclei of rod cells and of cone cells.
- Outer plexiform layer	It consists only of nerve fibres that form a plexus.
- Inner nuclear layer	It contains the cell bodies and nuclei of three types of neurons: the bipolar neurons, the horizontal neurons, the amacrine cells. It also contains the cell bodies retinal gliocytes or cells of Müller.
- Inner plexiform layer	It consists of synapsing nerve fibres.
- Ganglion cell layer	It contains the cell bodies of ganglion cells.
- Optic nerve fibre layer	It is made up of axons of ganglion cells.
- Internal limiting membrane	It is a basal lamina separating the central expansions of Müller’s cells from the vitreous.

Дата добавления: 2015-10-30