Ocular Cytopathology

An atlas that features the cytologic findings of the normal features and diseases of the eye.

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Friday, October 07, 2005


CHAPTER 2 Normal Anatomic and Cytologic Features
Accurate cytologic interpretation of ocular specimens requires a fundamental knowledge of normal ocular histology. A general overview is presented here with emphasis on those areas relevant to cytology specimens. There are numerous treatises available for more complete study of ocular anatomy (1-4). The average adult eye measures about 25 mm horizontally, 23 mm vertically, and 21 to 26 mm anterior-posteriorly (Figures 2-1 and 2-2) (5). The eye has an external approximate volume of 7.6 milliliters (ml), the aqueous has a volume of about 200 microliters, and the vitreous a volume of 4.0 ml. The eye is contained in the pear-shaped orbit that has dimensions of about 35 mm vertically, 45 mm horizontally, and 40-45 mm anteroposteriorly (6). The lacrimal gland is located superolaterally in the orbit and is divided by the orbital septum.

The conjunctiva covers the posterior surface of the eyelids (palpebral conjunctiva), curves anteriorly at the fornix to reflect onto the anterior surface of the eye as the bulbar conjunctiva (Figures 2-3 and 2-4). There are subtle histologic differences in the conjunctiva of the lid margins, tarsus, fornix, and bulbar conjunctivae (3). The conjunctiva covering the lid margin and bulbar conjunctiva is a modified nonkeratinized, stratified Squamous epithelium. The tarsal and fornix conjunctiva is covered by stratified cuboidal to columnar epithelium of varying thickness (Figures 2-5). This epithelium is unusual because it retains some squamoid features, such as numerous desmosomes, yet has a microvillus surface architecture (7,8). Goblet cells are abundant over the tarsus, fornix, and specialized areas such as the plica semilunaris. Goblet cells are scarce near the lid margin and adjacent to the cornea at the limbus. Most swabs of the conjunctiva are taken from the inferior fornix and show clusters and single epithelial cells with abundant cytoplasm, eccentric nuclei, and occasional single nucleoli . Goblet cells exhibit clear vacuoles filled with mucin. The presence of keratinized epithelium in the conjunctival smear is distinctly abnormal unless the sample is taken from the caruncle or accidentally from the eyelid.

The cornea is covered by five to six layers of a modified stratified Squamous epithelium (Figures 2-7) (9). The basal cells are smaller and have a higher nuclear-to-cytoplasmic ratio than the other epithelial cells in the cornea. There are two to three layers of wing cells with interdigitating cytoplasmic processes connected by desmosomes to other wing cells. These attachments may explain why corneal epithelium tends to be removed in sheets. The two top layers are flattened, superficial cells with small, round nuclei and inconspicuous nucleoli. The superficial epithelial cells are normally uniform in size and shape and have many microvilli that form a microplical complex on the external surface of the cornea (10). Epithelial cells are attached to a basement membrane, beneath which lies Bowman’s layer, a specialized layer of collagen that does not regenerate after injury. The stroma is composed of lamellar sheets of collagen arranged perpendicularly. The posterior surface of the cornea is covered by Descemet’s membrane, and endothelial cells line its posterior surface (Figure 2-7). Cytologic surface smears from the normal cornea will demonstrate cohesive sheets of non-keratinized Squamous epithelium. Individual cells exhibit intermediate-size, round nuclei with bland and uniform chromatin (Figure 2-8). The presence of keratinized cells in smears from the cornea is abnormal.

1. Jakobiec FA. Ocular anatomy, embryology, and teratology. Philadelphia: Harper & Row, 1982.
2. Hogan MJ, Alvarado JA, Weddell JE. Histology of the human eye. Philadelphia: W.B. Saunders, 1971.
3. Last RJ. Eugene Wolff's anatomy of the eye and orbit. Philadelphia: W.B. Saunders, 1961.
4. Fine BS, Yanoff M. Ocular histology, a text and atlas. New York: Harper & Row, 1972.
5. Stenstrom S. Untersuchungen uber die variation unk kovariation der optishen elemente des menshlickhen auges. Acta Ophthalmol 1946;26:1.
6. Duke-Elder WS. The anatomy of the visual system. In: System of ophthalmology. St Louis: CV Mosby, 1961;2:410-413.
7. Greiner JV, Covington HI, Allansmith MR. Surface morphology of the human upper tarsal conjunctiva. Am J Ophthalmol 1977;83:892-905.
8. Dark AJ, Durrant TE, McGinty F, Shortland JR, et al. Tarsal conjunctiva of the upper eyelid. Am J Ophthalmol 1974;77:555-564.
9. Hogan MJ, Alvarado JA, Weddell JE. Histology of the human eye. Philadelphia: W.B. Saunders, 1971.
10. Blumcke S, Morgenroth K Jr. The stereo ultrastructure of the external and internal surface of the cornea. J Ultrastruct Res 1967;18:502.


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