Ординатура / Офтальмология / Английские материалы / Dry Eye and Ocular Surface Disorders_Pflugfelder, Beuerman, Elliot Stern_2004

chief histological constituents of the ocular surface include the nonkeratinized epithelia and underlying stroma of the entire conjunctiva, the cornea and the corneoscleral limbus, and the tear film. In a broader sense, structures that produce (all lacrimal glands and ocular surface epithelia), distribute (eyelids), and eliminate (lacrimal drainage system) tears could be considered as part of the ocular surface. If the neural network that connects all these components is added, the lacrimal functional unit results [1], which is described in detail in Chapter 2.

The role of the ocular surface is to maintain optical clarity of the cornea by regulating its hydration and by protecting it from infectious, toxic, or mechanical trauma. All elements of the ocular surface participate as an integrated set of tissues toward a common goal of corneal transparency and health. The shared embryological origin of epithelial and subepithelial components from the ectodermal and subectodermal tissues overlying the developing optic vesicle [2,3] further support consideration of the ocular surface as a physiopathological unit. Because the cornea depends on the ocular surface for nutrients, protection, and defensive mechanisms, anything that compromises the tear film, the eyelids, the limbus, and/or the conjunctiva will eventually affect corneal transparency, and consequently, quality of vision. As an important component of the ocular surface, conjunctival function is crucial for maintenance of vision.

II.FUNCTIONAL ANATOMY OF THE CONJUNCTIVA

The conjunctiva, the thin translucent mucous membrane of the eye, which is part of the maintenance visual system of the ocular surface, serves as the junction between the eyeball and the eyelids. Its most superficial epithelial layer is totally covered by the tear film. It is continuous with the nasal mucosa through the lacrimal puncta, with the corneal epithelium at the corneo-scleral limbus, and with the eyelid skin at the mucocutaneous junctions of the lid margins. Multiple folds of the conjunctiva are formed, especially in the fornices, which allow free movement of the globe [4].

Although the conjunctiva is a continuous membrane, three topographic zones are considered: bulbar, palpebral, and forniceal (Fig. 1). The bulbar conjunctiva overlies Tenon’s capsule and permits visualization of the underlying white episclera and sclera. It covers the anterior surface of the globe, and the insertions of the extraocular muscles, except the obliques. The bulbar conjunctival stroma binds loosely to underlying tissues. Consequently, the bulbar conjunctiva is freely movable and can be easily pulled away from the globe with forceps. For this reason, a considerable amount of fluid can accumulate underneath the bulbar conjunctiva, including from subconjunctival injection of drugs. At the

Figure 1 The bulbar (b), palpebral or tarsal (p), and forniceal (f) zones of the conjunctiva. The limbal area (l) marks the transition between conjunctiva and cornea. The caruncle

(c) and semilunar fold (s) are located at the medial interpalpebral angle.

limbus, however, the bulbar conjunctiva (also called the limbal conjunctiva in this zone) becomes firmly attached to the underlying Tenon’s capsule, forming a 3-mm-wide ring. The palpebral or tarsal conjunctiva lines the inner surface of both eyelids. It is continuous with the forniceal conjunctiva and the eyelid skin at the mucocutaneous junction, where both epithelia merge abruptly. This demarcation between conjunctiva and skin is discernible clinically as the so-called gray line. This conjunctival area is smooth and contains numerous infoldings of epithelium, or crypts (Henle’s crypts), which increase the conjunctival surface area and its functional capacity [5]. The subepithelial stroma is thin in this region and firmly adheres the epithelium to the tarsal plates of the lids. This area of the conjunctiva is routinely examined by everting the upper eyelid and pulling the lower eyelid down. Its normal appearance must be distinguished from abnormal appearance, which clearly suggests pathology of the ocular surface. The superior and inferior forniceal conjunctiva connect the tarsal and bulbar areas of conjunctiva. The adherence of its stroma to the underlying orbital septum is the loosest of the three conjunctival areas, being redundant and forming folds. The ducts of the

main lacrimal gland open into the temporal portion of the upper fornix, and those of the accessory glands of Krause and Wolfring open into the upper and lower fornices. The superior fornix is approximately 13 mm from the lid margin, the inferior 9 mm, and the lateral 5 mm. The upper and lower fornices meet at the medial and lateral canthi, forming a continuous fornix or cul de sac. The volume of this conjunctival sac is about 7 L when eyelids are closed. This is important, because the inferior cul de sac is where topical medications are usually applied. This means that even one drop will overflow unless the lower eyelid is pulled away from the eyeball, and application of more than one drop at a time is ineffective [3].

The conjunctiva possesses two specialized tissues, the semilunar fold and the caruncle, located in the medial interpalpebral angle of the eye, both of which are highly vascularized (Fig. 1). The plica semilunaris, or semilunar fold, is a loose, narrow arc-shaped fold of the conjunctiva which facilitates movement of the eye. It is thought to be homologous to the nictitating membrane or third lid present in many vertebrate animals. Its epithelium is similar to that in other areas but contains many goblet cells. Its stroma contains fat, some nonstriated fiber muscles, and immune cells. The caruncula lacrimalis or caruncle, at the inner canthus in the lacrimal lake, is attached externally to the plica and blends medially with the canthal skin. It is an ovoid nodular mass of fleshy modified skin with stratified nonkeratinized squamous epithelium, and stroma containing sebaceous gland ducts, sweat glands, smooth muscle (portions of Horner’s muscle), adipose tissue, and fine lanugo hairs, in addition to accessory lacrimal glandular tissue [2].

The blood supply of the conjunctiva is mainly provided by the anterior ciliary arteries (ophthalmic artery), and also by branches from the eyelid vasculature. The conjunctival capillaries are a fenestrated type. The lymphatic channels can be seen at a deep level, where they are larger and directly under the stroma. The trunks on the medial side drain into the submaxillary node, whereas the lateral trunks drain into the pretargian and parotid nodes. Conjunctival sensory innervation comes from the ophthalmic division of the trigeminal or fifth cranial nerve; only the medial portion of the inferior forniceal and palpebral conjunctiva derives its nerve supply from the maxillary division of the trigeminal (infraorbital nerve) [3]. Sympathetic and parasympathetic innervation (autonomic nerves) supply the conjunctival vessels and secretory epithelia [6].

Embryologically, the conjunctiva develops from ectodermal and subectodermal tissue located directly underneath the posterior surface of the eyelid folds and along the margin of the developing cornea. By the 10th to the 12th week of gestation, goblet cells are discernible in the future palpebral, forniceal, and bulbar areas. By the 12th to the 14th week, the upper and lower forniceal and palpebral conjunctival epithelium form invaginations that will become the accessory glands of Krause and Wolfring [2,3].

Figure 3 Histological micrographs of the bulbar, palpebral and forniceal conjunctiva of normal adults. (Courtesy of the Miguel N. Burnier Jr Registry of Ocular Pathology, IOBA, University of Valladolid.) (A) This area of bulbar conjunctiva shows 6–8 epithelial layers of predominantly cuboidal cells; Goblet cells are absent because it is located near the limbus. Occasional intraepithelial lymphocytes (arrows) can be seen. Lymphatic channels (lc) and blood vessels (bv) are seen in the stroma. Masson trichromic stained, 252×.

(B) The upper palpebral conjunctiva is shown in the upper part of the photograph, near the tarsal meibomian glands (mg) and one accessory lacrimal gland (alg). Epithelial infoldings in this area form Henle’s crypts, with a large number of goblet cells and a diffuse layer of numerous subepithelial lymphocytes, belonging to the diffuse conjunctival associated lymphoid tissue (CALT). The stroma here is thinner and more compact than in other areas. The forniceal conjunctiva (f) can be seen in the left part of the photograph, showing a loose and abundant stroma. Note the conjunctival lymphoid follicle (lower arrow), part of the organized CALT (arrow). 31.5×. (C) Magnification of the follicular arrangement of lymphocytes seen in (B) (a view extending downward from the lower arrow), belonging to CALT system. The thin epithelium (M cells) covers a rounded area (upper left) heavily populated by lymphocytes. In the transition to the more diffusely arranged CALT, the epithelium becomes thicker, lymphocytes are less numerous and compact, and some plasma cells are evident. ×252.

conjunctival epithelium to more regular corneal epithelium. The limbal epithelium has fingerlike projections into the lamina propia, named Vogt’s palisades, that contain the stem cells of the cornea. Here, the limbal epithelium contains up to 15 cell layers in the valleys of the palisades and up to 3 layers in the apical parts of the palisades. The superficial epithelial cells are flat here, becoming more cuboidal toward the conjunctival zone. The epithelium in this zone is devoid of goblet cells, but, in the basal layers, Langerhans’ cells and melanocytes are abundant, with the number of melanocytes depending on the degree of pigmentation. At the mucocutaneous junction, there is a relatively abrupt transition from nonkeratinized stratified squamous mucosal epithelium to keratinized epidermal epithelium [7].

The different layers of the human conjunctiva are organized into three main zones. The basal cells are a cuboidal monolayer attached to the basement membrane by hemidesmosomes and adherent complexes. Their cytoplasm contains cytokeratin intermediate filaments. These basal cells are capable of mitosis, a characteristic of undifferentiated cells. The intermediate layers are more columnar and are present only in areas of thicker epithelium, especially at the fornices and limbus. These cells contain intermediate filaments grouped in fascicles thinner than those of the basal cells. Cells of the intermediate layer become progressively more polygonal and flattened as they reach the conjunctival surface, remaining in a nonkeratinizing state under normal healthy conditions. The superficial cells, however, have variable morphology depending on their location. They are cuboidal in the bulbar and tarsal areas, more polygonal in the forniceal regions, and flattened in the limbus. This more pronounced flattening is considered an adaptation to increased mechanical pressure [8]. The superficial layers can be easily removed from the deeper epithelial layers by impression cytology, which allows visualization of epithelial cells and quantitation of goblet cells (Fig. 4). This technique can also identify pathological alterations that develop in ocular surface diseases [9] and define the state of keratinization of the superficial epithelial layers. A nonkeratinized state of the ocular surface is crucial to its health. Clusterin (formerly named apolipoprotein J, apoJ, or SP-40), one of a variety of molecules responsible for this nonkeratinized state [10,11], is present in the superficial layers of normal conjunctival and corneal epithelia, and absent from keratinized epithelia such as epidermis. Its expression is markedly reduced in keratinizing ocular surface disorders such as ocular cicatricial pemphigoid and Stevens-Johnson syndrome [10,11]. These disorders have in common the pathological transition of a secretory nonkeratinized, stratified epithelium, such as that of the normal ocular surface, into a nonsecretory, keratinized epithelium, a pathological condition termed squamous metaplasia. It is accompanied by loss of goblet cells, increase in cellular stratification, enlargement of superficial cells, and keratinization [12]. In addition to cicatrizing conjunctivitis, squamous metaplasia is a characteristic change found in lacrimal keratoconjunctivitis (LKC) [13]. The

Figure 5 Transmission electron microphotograph of a group of conjunctival goblet cells, showing the secretory mucin granules (arrow) that push down the nuclei. The microvilli (mv) are also evident.

expression of clusterin in LKC and its potential role in the pathogenesis of this disease remains to be elucidated.

Although the exact location where stem cells of the conjunctival epithelium reside has not been exactly defined, some studies suggest that the fornices and some areas present in the bulbar and limbal conjunctiva contain these cells [14]. Although their localization needs further confirmation, the fornices are a logical spot because stem cells in the fornices are protected from environmental insults.

The apical surface of the superficial cells exhibit microplicae and microvilli similar to those of the cornea that can be easily seen in transmission (Fig. 5) and scanning electron micrographs. Superficial cells appear hexagonal and are completely covered by microvilli (0.5 µm diameter × 0.5–1 µm high; Fig. 6), some of which coalesce into microplicae [15]. These microscopic features have the twofold function of enlarging the resorbent surface of the conjunctival epithelium and stabilizing and anchoring the overlying tear film to the epithelial cell membranes and overlying glycocalyx [16,17]. The variable density and length of microvilli makes it possible to distinguish three populations of cells by scanning