Ophthalmic System Laboratory

Introduction

The eye is a high specialized, photosensitive organ which is housed in a bony orbit and glides in a bed of orbital fat. Associated skeletal muscles aid in controlling eye movements. The eye is able to focus on near objects (accommodation) and far images as well. Each of you have one slide of the eye. Some of these slides demonstrate a section of the retina only; some slides are hemisections of the eye showing all major structures. If you have a slide which shows retina only, please borrow the more complete slide from a neighbor. You will also have access to excellent slides from the Mayo Clinic during lab. These must be returned at the end of the session but can be checked out from the HEO office.

Exercise

Select a partner and take a moment to examine the external eye. The sclera is partially visible as the "white" of the eye. The transparent cornea can be visualized by viewing the eye from the side. Note that its curvature is greater than the remainder of the eyeball causing it to protrude from the eye surface. Note the iris which gives "color" to the eye and pupil. You will be examining the histologic configuration of these and other structures on the slide today.

Slide 133, Human eye:
Locate the following structures:

  1. Locate the cornea. Note that the external, convex surface is covered by about 5 layers of stratified epithelium and the inner, concave surface is covered by a simple squamous epithelium, the endothelium. Identify Bowman's membrane underlying the stratified epithelium and Descemet's membrane underlying the endothelium. Locate the relatively acellular stroma.
  2. Identify the iris. It looks like a leaf cut in cross section. Note that the external portion contains scattered melanocytes (pigmented cells) and smooth muscle cells which form the pupillary sphincter muscle. The inner surface contains two layers of epithelial cells; the innermost one is fully pigmented, the outermost is a layer of partly pigmented myoepithelial cells. Note that half of the myoepithelial cells stain bright pink, similar to smooth muscle and the remainder of the cell contains melanin granules. These myoepithelial cells form the pupillary dilator muscle.
  3. Locate the lens. This transparent, refractive structure is suspended by zonular fibers arising from the ciliary processes. Note that it stains very acidophilic because of its high protein content. The lens is encased in a thick basement membrane which serves as a capsule and is composed of an anterior layer of subcapsular epithelium and densely packed, anucleate lens fibers (cells). Lens changes shape as tension is inceased or decreased on zonule. Lens often appears disrupted in histologic section because low water content makes sectioning difficult.
  4. Locate the zonular fibers. Collectively these form the zonule. In histologic section, the fibers are wispy and pink; they extend from the ciliary processes and insert into the lens capsule.
  5. Ciliary body. The ciliary body encircles the inner aspect of the eyeball like a belt. In histologic section it is a triangular shaped area with small finger-like extensions radiating toward the lens. The zonular fibers arise from the ciliary processes. Identify the ciliary processes. Note that they are covered by two layers of simple, cuboidal epithelium. The innermost layer is non-pigmented, but the outermost layer is contains abundant melanin. Note the pink-staining smooth muscle cells in the ciliary body. These form the ciliary muscle that contracts to EASE tension on the zonule and allow the lens to thicken.
  6. Locate the three cavities of the eye. The anterior chamber is formed by posterior surface of cornea and the anterior surfaces of the iris and lens. The posterior chamber is small and is formed by posterior surface of iris, anterior surface of lens and ciliary processes. These two chambers contain aqueous humor. The vitreous chamber is the large space posterior to the lens that contains the vitreous body. It appears as a space in histologic section.
  7. Identify the sclera. This outer tunic of the eyeball is composed of compact, fibrous connective tissue and stains acidophilic due to high type I collagen content. It begins anteriorly at the limbus, its site of junction with the cornea. The canal of Schlemm is locate in this region. The sclera extends posteriorly to the region of the blind spot (optic disc or papilla) where the optic nerve exits the eyeball.
  8. Find the choroid coat. This tissue layer lies just internal to the sclera. It contains numerous vessels and melanocytes. Identify the pigment-containing cells and the blood vessels. The choroid vessels provide nutrients to the retina.
  9. Find the retina. The outer layer of the retina is the pigment epithelium, a simple columnar (cuboidal) epithelium which is highly pigmented and lies sandwiched between the choroid and the rod and cone layer of the retina. Find the pigment epithelium. Locate the neural retina, the photosensitive layer of the retina. The neural retina lies between the vitreous cavity and the pigment epithelium. Please identify 6 of the neural retina layers: 1) The rods and cones. The acidophilic rod and cone outer segments lie adjacent to the pigment epithelium. Their basophilic nuclei are just internal to the outer segment layer. 2) Outer (external) plexiform layer. This thick layer is site of synapses of rod and cones with bipolar cells. It stains slightly acidophilic. 3) Bipolar cell layer. This layer can be identified by locating the middle layer of nuclei. 4) Inner (internal) plexiform layer. Again, a lightly acidophilic layer just internal to the layer of bipolar ganglion cells. 5) Ganglion call layer. Identify this layer by locating the innermost layer of basophilic nuclei. 6) Inner limiting membrane. Pale acidophilic, innermost layer of neural retina.
  10. Find the optic nerve. Note that it exits through the lamina cribrosa at the blind spot near the posterior pole of the eyeball. Also notice that all layers of the retina disappear at the blind spot except the inner limiting membrane.