CHAPTER 1
Structure and Function of the External Eye and
Cornea
BCSC Section 2, Fundamentals and Principles of Ophthalmology, also discusses some of the topics covered here.
Anatomy
Eyelids
The eyelid skin blends into the surrounding periorbital skin, varying from 0.5 mm thick at the eyelid margin to 1 mm thick at the orbital rim. Except for fine vellus hairs, the only hairs of the eyelids are the eyelashes, or cilia, which are twice as numerous along the upper eyelid margin as along the lower. Cilia are replaced every 3–5 months; they usually regrow in 2 weeks when cut and within 2 months if pulled out. The cilia catch small particles and also work as sensors to stimulate reflex eyelid closure. Blinking augments the lacrimal pump to rinse tears over the eye and flush off foreign material.
The epidermis of the eyelids abruptly changes to nonkeratinized stratified squamous epithelium at the mucocutaneous junction of the eyelid margin, along the row of meibomian gland orifices. Holocrine sebaceous glands and eccrine sweat glands are present in the eyelid skin. Near the eyelid margin are the apocrine sweat glands (the glands of Moll) and numerous sebaceous glands (the glands of Zeis) (Fig 1-1).
Lin MP, Park DJ, Harrison AR. The eyelids. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:41–46.
Figure 1-1 Cross section of the upper eyelid. (Illustration by Christine Gralapp.)
Conjunctiva
The conjunctival sac includes the bulbar conjunctiva, a fornix on 3 sides and a medial semilunar fold, and the palpebral conjunctiva. Smooth-muscle fibers from the levator muscle maintain the superior fornix, and fibrous slips extend from the horizontal rectus tendons into the temporal conjunctiva and plica to form cul-de-sacs during horizontal gaze. The caruncle is a fleshy tissue mass containing
hairs and sebaceous glands. The tarsal conjunctiva is tightly adherent to the underlying tarsus, and the bulbar conjunctiva is loosely adherent to the Tenon capsule. These tissues blend at the limbus, where a series of radiating ridges, the palisades of Vogt, appear. This area contains corneal epithelial stem cells.
The cell morphology of the conjunctival epithelium varies from stratified cuboidal over the tarsus to columnar in the fornices to squamous on the globe. Multiple surface folds are present. Goblet cells account for up to 10% of basal cells of the conjunctival epithelium; they are most numerous in the tarsal conjunctiva and the inferonasal bulbar conjunctiva.
The substantia propria of the conjunctiva consists of loose connective tissue. Conjunctivaassociated lymphoid tissue (CALT), which consists of lymphocytes and other leukocytes, is present, especially in the fornices. Lymphocytes interact with mucosal epithelial cells through reciprocal regulatory signals mediated by growth factors, cytokines, and neuropeptides.
The palpebral conjunctiva shares its blood supply with the eyelids. The bulbar conjunctiva is supplied by the anterior ciliary arteries branching off the ophthalmic artery. These capillaries are fenestrated and leak fluorescein just as the choriocapillaris does. Sensory innervation is controlled by the lacrimal, supraorbital, supratrochlear, and infraorbital branches of the ophthalmic division of cranial nerve V.
Nelson JD, Cameron JD. The conjunctiva: anatomy and physiology. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:25–31.
Lacrimal Functional Unit
The lacrimal functional unit (LFU; Fig 1-2) is a highly complex apparatus, comprising the lacrimal glands, ocular surface (cornea, conjunctiva, and meibomian glands), and eyelids, as well as the sensory and motor nerves that connect them. It is responsible for the regulation, production, and health of the precorneal tear film. The afferent component of the LFU is mediated through nociceptors found in the cornea and ocular surface and passing through the trigeminal nerve (cranial nerve V). These nociceptors synapse in the brainstem with autonomic and motor (efferent) nerves. The autonomic nerve fibers innervate the meibomian glands, conjunctival goblet cells, and lacrimal glands. The motor nerve fibers innervate the orbicularis muscle to initiate blinking, with the blink rate in adults being about 15 times per minute. During blinking, the meibomian glands express lipid, and the tears are replenished from the inferior tear meniscus and spread across the cornea while excess tears are directed into the lacrimal puncta. See also Chapter 6.
Figure 1-2 The lacrimal functional unit. (Modified with permission from Pflugfelder SC, Beuerman RW, Stern ME, eds. Dry Eye and Ocular Surface Disorders. New York: Marcel Dekker; 2004.)
The Tear Film
The tear film contains proteins that control the normal ocular flora; it serves as a protective layer to wash away irritants and pathogens; and it dilutes toxins and allergens. The tear film is the most important refractive interface of the eye and is primarily responsible for maintaining a smooth optical surface between blinks. Our understanding of the structure of the tear film has gradually evolved. Formerly described as a trilayered structure composed of lipid, aqueous, and mucin layers, the tear film is now thought of as a uniform gel consisting of soluble mucus, which is secreted by conjunctival goblet cells, mixed with fluids and proteins secreted by the lacrimal glands (Fig 1-3).
Pflugfelder SC. Tear dysfunction and the cornea: LXVIII Edward Jackson Memorial Lecture. Am J Ophthalmol. 2011;152(6):900–909.