Objectives

1. Identify the three tunics of the eye
2. Identify the layers of the retina and cornea
3. Identify the ciliary body, iris, limbus and drainage system
4. Identify the optic nerve, eyelid and lacrimal gland

Overview of the Eye

The eye can be divided into three concentric tunics (layers) from superficial to deep include: the corneoscleral tunic, the vascular/uvea tunic and the neuroectodermal/nervous tunic (retina).

Corneoscleral tunic:

• Sclera– opaque, fibrous capsule composed of dense connective tissue, provides attachment for extraocular muscles. Appears as white in adults, can have a bluish tint in children due to thinness and can appear more yellow in elderly people due to the accumulation of lipofuscin. Can also appear yellow in the case of jaundice due to circulating bilirubin in liver disease.
• Cornea– transparent convex region in the center of the anterior eye continuous with the sclera. 6 layers of the cornea superficial to deep: corneal epithelium (nonkeratinized stratified squamous) > anterior epithelial basement membrane > Bowman’s membrane/layer (collagen fibers) > corneal stroma (avascular, collagen fibers with fibroblasts) > Descemet’s membrane (posterior BM) > corneal endothelium (simple squamous)
• The junction between the cornea and sclera is known as the limbus

Vascular tunic (Uvea):

• Choroid– makes up the majority of the uvea, vascular layer
• Ciliary body– ring-like thickening that contains the ciliary muscle for lens accommodation and is located between the choroid and the iris and is continuous with both structures. Consists of a layer of smooth muscle (ciliary muscle), an inner vascular region and outer ciliary processes lined with basal pigmented cells and apical columnar cells. This epithelium produces aqueous humor which flows into the posterior chamber, then into the anterior chamber. Excess fluid drains back into systemic circulation through the trabecular meshwork and canal of Schlemm. Zonula fibers extend from the ciliary processes to the lens capsule to change the shape of the lens in response to contraction and relaxation of the ciliary muscle.
• Iris– contracting and dilating diaphragm anterior to the lens that consists of vascularized connective tissue with a posterior layer of pigment epithelium, continuous with the ciliary body. As the 2 layers of epithelial cells extends to the iris, both layers become pigmented and do not produce aqueous humor in this region. The pigment makes it difficult to see cell structures like the nucleus and boundaries of adjacent cells.
  • The pupil is the central aperture (opening).

Neuroectodermal tunic/Retina:

• Neural retina– located posterior to the vitreous chamber consisting of several distinct layers of neurons including photoreceptors
• Retinal pigment epithelium (RPE)– outer layer consisting of simple cuboidal, melanin containing cells
• The optic disc (or optic papilla) is the site where the optic nerve exits the eye, there is a lack of photoreceptor cells here which is why it is also referred to as the “blind spot”, see Michigan Eye slide in “More Practice” section.
• Layers of the retina from outside inward (superficial to deep, choroid layer to vitreous chamber):
  1. RPE
  2. Rods and cones- outer and inner segments of photoreceptor cells
  3. Outer limiting membrane- apical portion Müller cells
  4. Outer nuclear layer- cell bodies (nuclei) of rods and cones
  5. Outer plexiform layer- processes of rods and cones and processes of cells that connect to them
  6. Inner nuclear layer- cell bodies of horizontal, amacrine, bipolar and Müller cells
  7. Inner plexiform layer- processes of horizontal, amacrine, bipolar, and ganglion cells
  8. Ganglion cell layer- cell bodies of ganglion cells
  9. Optic nerve fibers- processes of ganglion cells
  10. Inner limiting membrane- basal lamina of Müller cells
• Cells of the retina:
  • RPE cells- simple cuboidal connect with adjacent cells through gap junctions and zonulae occludentes and adherentes creating a blood-retina barrier; contain debris (lipofuscin) from phagocytosed material
  • Rods and Cones- photoreceptor cells that consist of an outer segment, connecting stalk (containing a basal body) and inner segment
  • Müller cells- form the scaffolding by extending through the retina with microvilli extending between the photoreceptor cells
  • Conducting Cells:
    • Bipolar cells- processes extend to both inner and outer plexiform layer
    • Horizontal cells- processes extend to outer plexiform layer
    • Amacrine cells- contribute to a complex interconnection of cells
    • Ganglion cells- multipolar neurons
    • Rods and Cones- also non-photosensitive conducting cells

Other Structures:

• Lens– transparent, biconvex structure, avascular, no nerves, contained within a thick capsule of basal lamina suspended between the edges of the ciliary body by the zonular fibers. Zonula fibers help flatten the lens through contraction of the ciliary muscle. Subscapular epithelial layer is composed of simple cuboidal cells.
• Vitreous body– transparent, jelly-like substance in the vitreous chamber
• Eyelids– anterior surface is covered by skin while the inner posterior surface is lined by stratified columnar epithelium with goblet cells known as conjunctiva. The conjunctiva on the inner surface of the eye lid extends to cover the anterior aspect of the sclera covering the exposed part of the eye (white) up to the limbus. The tarsal plate in the eyelid consists of dense fibrous and elastic connective tissue to provide structures. There are fibers from the orbicularis oculi muscle and levator palpebrae superioris muscles (both skeletal muscle). There is also a smooth muscle known as the superior tarsal muscle. There are also eyelashes, short curved hairs arising from hair follicles inserted into the eyelid dermis.
• Glands:
  • Tarsal glands (Meibomian glands)- long sebaceous glands, ducts open at lid margin, secrete oil layer to decrease evaporation from tear layer
  • Sebaceous glands of eyelashes (glands of Zeis)- modified sebaceous glands that empty into follicles of eyelashes
  • Apocrine glands of eyelashes (glands of Moll)- sweat glands
  • Lacrimal glands– compound serous tubuloalveolar glands located in the superior lateral orbit (main gland) and accessory glands located on inner surface of upper eyelid posterior to the tarsal plate (glands of Wolfring) and located in the fornix of lacrimal sac (glands of Krause).