Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 32

The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids, the anterior and posterior surfaces of the NM, and the exposed sclera. The conjunctiva consists of a thin layer of loose connective tissue beneath a simple to stratified epithelium that becomes consistently stratified squamous toward the eyelid margin, and provides the primary surgical source of tissues to cover deep and progressing corneal ulcerations (Figure 1.13). The palpebral conjunctiva lines the inner aspect of the eyelids and the anterior portion of the NM. As the conjunctiva reflects onto the globe, it is called the bulbar conjunctiva and becomes continuous with the limbal and corneal epithelium. The bulbar conjunctiva also lines the posterior portion of the NM. The junction between the palpebral and bulbar conjunctiva is the conjunctival fornix, and the epithelial lining in this region varies according to species, ranging from pseudostratified columnar to stratified cuboidal.

Ventrally, an additional fold is formed by reflection of the conjunctiva over the NM. The reflections at the conjunctival fornix and NM form the conjunctival sac. All parts of the conjunctiva are continuous, but for descriptive purposes, it is divided into the palpebral, bulbar, and fornix conjunctiva and further referenced to specific eyelids. The distribution of goblet cells in the conjunctiva is heterogeneous in the dog. The highest densities occur along the lower nasal and middle fornix, and the lower tarsal portion of the palpebral conjunctiva; this information is important when performing conjunctival biopsies. In cats, the conjunctival goblet cell density varies widely by region but is highest in the anterior surface of the NM and the conjunctival fornices. Additionally, in most domestic species, the bulbar conjunctiva has been reported to either essentially lack goblet cells or have a much lower population of these mucus‐forming cells. The substantia propria of the conjunctiva is composed of two layers: a superficial adenoid layer, which in the dog and cat contains a variable presence of lymphatic follicles and glands; and a deep, fibrous layer that contains the conjunctival nerves and vessels. The arteries of the conjunctiva arise from the anterior ciliary arteries, which are branches of the external ophthalmic artery, and from branches of the superior and inferior palpebral and malar arteries.

Figure 1.13 Bulbar conjunctiva of a porcine eyelid is externally lined by a stratified to pseudostratified columnar epithelium possessing numerous goblet cells (GC) near the fornix.

The lymphatics of the conjunctiva, called the conjunctiva‐associated lymphatic tissue (CALT), are arranged in two plexuses: a superficial and a deep system. CALT is generally diffuse with intermittent nodules or follicles. Often, the diffuse component of CALT infiltrates and is adjacent to tear‐secreting glands, especially those associated with the NM. Variations in the size and distribution of nodules occur between the upper and lower eyelids and are influenced by exposure to various foreign substances, including potentially infectious microorganisms. The conjunctiva at the fornix is very thin and translucent, and it lies loosely on the underlying connective tissue. In the domestic carnivore, approximately 3 mm from the limbus, the bulbar conjunctiva, Tenon's capsule, and sclera become closely united. The connective tissue is much more abundant in this location in the dog than in humans and other species. The primary functions of the conjunctiva are to prevent desiccation of the cornea, to allow mobility of the eyelids and the globe, and to provide a physical and physiological barrier against microorganisms and foreign bodies.