SENSORY ORGANS: EYE

Description
Figure 1. Eye, Transverse Section
Figure 2. Cornea and lens, transverse section
Figure 3. Iris and lens, transverse section
Figure 4. Retina, transverse section
Figure 5. Optic nerve, sagittal section
Figure 6. Choroid body, sagittal section

DESCRIPTION

Organization of the fathead eye follows the general structure found in higher vertebrates (Fig. 1). Eyes are large and round with a flattened corneal surface and no eyelids. They orient medial-laterally within bony cavities in the skull, attach by striated oculomotor muscles, and bulge from the plane of the body surface. Eyes are organized into two chambers; an outer aqueous chamber bounded by the cornea and iris, and an inner vitreous chamber bounded by the iris and sclera. The round lens is suspended by ligaments and muscles between the chambers and projects into both. Aside from the lens, the eye is composed of three layers; an outer sclero/corneal layer, a medial uveal layer, and an inner retinal layer.

Lens. The teleost lens is completely round and protrudes into the aqueous chamber nearly to the cornea providing a wide "fish eye" field of vision (Fig. 2). The lens is made up of three layers, an outer acellular sheath, an underlying cuboidal epithelial cell layer, and an innermost fibrous layer. Due to its rigidity, visual accommodation is accomplished not by deflecting the shape of the lens but by moving it forward and back via retractor muscles and suspensory ligaments.

Sclero-Corneal Layer. The outermost layer of the eye is divided into external cornea and internal sclera. The cornea consists of an unpigmented squamous epithelium, a membranous stroma, and a thin endothelium. Corneal epithelium is multilayered and contiguous with the integument of the head. The wall of the posterior portion of the eye is made up of the thick fibroelastic sclera which is contiguous with the corneal stroma. While the cornea is unpigmented and therefore transparent, the sclera is nonrefractile with an external sheath of hyaline cartilage.

Uveal Layer. The middle layer of the eye is composed of choroid body (Fig. 6) and iris (Fig. 3). In fathead minnows the choroid body, surrounding the optic nerve, is a well developed and intricate array of arterioles and capillaries forming a reté mirabile. This organ is designed to satisfy the high oxygen demands of the poorly vascularized retina. The iris, a continuation of the choroid body, projects in a thin layer over the anterior surface of the lens. The iris separates aqueous and vitreous chambers with its leading edge defining the pupil.

Retinal Layer. The retina, lying inside the choroidea within the vitreous body, is divided into ten distinct layers; pigment epithelium layer, photoreceptor cell layer; external limiting membrane, external nuclear layer, external plexiform layer, internal nuclear layer, internal plexiform layer, ganglion cell layer, nerve fiber layer, and internal limiting membrane (Fig. 4). In fatheads these layers remain distinct until terminating at the margins of the iris. Visual images are relayed from the rods and cones of the photoreceptor cell s to the nerve fiber layer which in turn synapses with the optic nerve (Fig. 5). Visual information is conveyed via the optic nerve to the mesencephalon of the brain.

EYE: Transverse section

Figure 1. Eye, transverse section (Formalin, H&E, Bar = 335 µm).
1. cornea; 2. lens; 3. iris; 4. retina; 5. optic nerve; 6. choroid body (reté
mirabile); 7. sclera (w/ cartilage).

EYE: Cornea and lens, transverse section

Figure 2. Cornea and lens, transverse section (Formalin, H&E,
Bar = 18.0 µm). 1. squamous corneal epithelium; 2. corneal stroma
(Bowman's membrane); 3. corneal endothelium; 4. aqueous chamber;
5. acellular lens capsule; 6. cuboidal epithelial lens cells; 7. lens fibers.

EYE: Iris and lens, transverse section

Figure 3. Iris and lens, transverse section (Bouins, H&E, Bar = 33.6 µm).
1. iris; 2. corneal stroma; 3. lens capsule; 4. cuboidal lens epithelium;
5. lens fibers; 6. aqueous chamber; 7. vitreous chamber.

EYE: Retina, transverse section

Figure 4. Retina, transverse section (Bouins, H&E, Bar = 20.5 µm).
1. pigment epithelium; 2. photoreceptor layer (cones and rods); 3. outer
limiting membrane; 4. outer nuclear layer; 5. outer plexiform layer;
6. inner nuclear layer; 7. inner plexiform layer; 8. ganglion cell layer;
9. nerve fiber layer; 10. inner limiting membrane.

EYE: Optic nerve, sagittal section

Figure 5. Optic nerve, sagittal section (Bouins, H&E, Bar = 32.4 µm).
1. optic nerve; 2. retina; 3. vein; 4. vitreous chamber.

EYE: Choroid body, sagittal section

Figure 6. Choroid body, sagittal section (Formalin, H&E, Bar = 17.8 µm).
1. arterioles and capillaries forming the rete mirabile; 2. endothelial cells
lining capillaries.