The conjunctiva is a thin mucous membrane that lines the inside of the eyelids and covers the sclera (the white of the eye). It is composed of non-keratinized, stratified squamous epithelium with goblet cells, stratified columnar epithelium and stratified cuboidal epithelium (depending on the zone). The conjunctiva is highly vascularised, with many microvessels easily accessible for imaging studies. Structure The conjunctiva is typically divided into three parts: Blood supply Blood to the bulbar conjunctiva is primarily derived from the ophthalmic artery. The blood supply to the palpebral conjunctiva (the eyelid) is derived from the external carotid artery. However, the circulations of the bulbar conjunctiva and palpebral conjunctiva are linked, so both bulbar conjunctival and palpebral conjunctival vessels are supplied by both the ophthalmic artery and the external carotid artery, to varying extents. Nerve supply Sensory innervation of the conjunctiva is divided into four parts: Microanatomy The conjunctiva consists of unkeratinized, both stratified squamous and stratified columnar epithelium, with interspersed goblet cells. The epithelial layer contains blood vessels, fibrous tissue, and lymphatic channels. Accessory lacrimal glands in the conjunctiva constantly produce the aqueous portion of tears. Additional cells present in the conjunctival epithelium include melanocytes, T and B cell lymphocytes. Function The conjunctiva helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland. It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye. Clinical significance Disorders of the conjunctiva and cornea are common sources of eye complaints, in particular because the surface of the eye is exposed to various external influences and is especially susceptible to trauma, infections, chemical irritation, allergic reactions, and dryness. The conjunctival microvascular hemodynamics are affected by diabetic retinopathy (DR), hence can be useful for DR diagnosis and monitoring, and discriminating stages of DR. Type II diabetes is associated with conjunctival hypoxia, increased average blood vessel diameter, and capillary loss. Sickle-cell anemia is associated with blood vessel sludging, altered blood flow and blood vessel diameter, and capillary micro-haemorrhages. Hypertension is associated with an increase in the tortuosity of bulbar conjunctival blood vessels and capillary and arteriole loss. Carotid artery occlusion is associated with slower conjunctival blood flow and apparent capillary loss. With age, the conjunctiva can stretch and loosen from the underlying sclera, leading to the formation of conjunctival folds, a condition known as conjunctivochalasis. The conjunctiva can be affected by