Lacrimal gland

The lacrimal gland (Latin: glandula lacrimalis) is a paired lobular gland located at the upper lateral corner of the orbit that is responsible for the production of tears. The tears protect and help to maintain the eyeball and refractive structures viable and functional.

Structure of the lacrimal gland

The lacrimal gland consists of orbital (larger) and palpebral (smaller) parts that are continuous posterolaterally around the concave lateral edge of the aponeurosis of the levator palpebrae superioris muscle.

The orbital part of the lacrimal gland is located in a shallow fossa on the medial aspect of the zygomatic process of the frontal bone within the orbital margin. The superior surface of the orbital part is convex and related to the bone, while the inferior surface is above the aponeurosis of the levator palpebrae superioris muscle and laterally above the lateral rectus muscle's upper margin. The orbital part's anterior border is the orbital septa, but the posterior border - the orbital fat.

The palpebral part of the lacrimal gland is subdivided into two or three lobules and extends below the aponeurosis of the superior palpebral levator muscle into the lateral part of the upper eyelid, where it is attached to the superior conjunctival fornix. The palpebral part of the lacrimal gland is visible through the conjunctiva when the lid is everted.

Approximately 12 lacrimal gland ducts travel from the orbital part to the palpebral part and end by opening into the superior conjunctival fornix.

Additionally to the main lacrimal gland, many accessory lacrimal glands are presented within the conjunctival sac. The accessory glands work as an additional way to keep the cornea moist in case the main gland is not functioning.

Microstructure of the lacrimal gland

The lacrimal gland has lobulated tubulo-acinar structure. The lobules are separated by loose connective tissue. The acini look like tube-shaped or round masses of columnar cells. The smallest intralobular ducts are lined with one layer of low columnar or cuboidal cells, while the periphery has myoepithelial cells. The larger ducts have a two-layer epithelial cover.

A discontinuous myoepithelial cells' layer together with the basal lamina surrounds the acini's secretory epithelial cells. The secretory cells have a shortened conical shape and microvilli on the apical surface. The cells have a basally placed nucleus. The secretory granules are round or oval and attached to the membrane. The largest amount of the secretory cells produce serous secrete.

Lacrimal secretion and tears

The acinar cells within the lacrimal gland produce the secrete. The secrete passes into the duct system. The cells lining the duct system possibly modify the secretion's composition. The ready tears are composed of lysozyme (antibacterial enzyme), IgA, and beta-lysin, all of which work to defend against microorganisms. Apart from tears, the lacrimal gland produces substances that affect the eye's surface by regulating epithelial cell turnover.

Hormones, especially androgens, support lacrimal secretion and suppresses any immunological activity within the lacrimal glands. The tears keep the corneal epithelium moist, maintaining a viable environment for the epithelial cells for them to do their refractive function. Tears also lubricate the anterior surface of the eyeball so it can move beneath the eyelids.

There are three main types of tears produced by the lacrimal gland. These are basal tears that are responsible for keeping the eye moist and nourished, reflex tears that are produced as an answer to irritation for lubrication and cleansing of the eyes, and psychic tears that are secreted during a process that is called crying most often in response to strong emotions, stress or pain. The composition of tears can vary based on their type and functions, but the main components are water, salts, antibodies, as well as antibacterial enzymes.

Vasculature and innervation of the lacrimal gland

Blood supply and venous drainage

The lacrimal gland receives its blood from the lacrimal artery originating from the ophthalmic artery. The infraorbital artery from the maxillary artery can participate in supplying the lacrimal gland.

Venous drainage happens through the superior ophthalmic vein.

Lymphatic drainage

The lymph from the lacrimal gland drains into the same route as that of the conjunctiva and ends in the superficial parotid lymph node.

Innervation

The lacrimal gland receives both autonomic and sensory innervation. The lacrimal gland receives sensory nerve supply from the lacrimal nerve, a branch of the ophthalmic nerve (CN V1), via the communicating branch and the zygomatic nerve, a branch of the maxillary nerve (CN V2). The greater petrosal nerve, a branch of the facial nerve (CN VII), conveys the parasympathetic and sympathetic fibers to the lacrimal gland.

The preganglionic parasympathetic fibers originate from a nucleus in the brainstem, synapse in the pterygopalatine ganglion, and the postganglionic parasympathetic fibers that join branches of the maxillary nerve to reach the lacrimal gland within the lacrimal nerve.

The postganglionic sympathetic arise from the superior cervical ganglion of the sympathetic trunk and travel along with the parasympathetic fibers (without synapsing in the pterygopalatine ganglion) and end up joining the deep petrosal nerve, the nerve of the pterygoid canal, the maxillary nerve, the zygomatic nerve, the zygomaticotemporal nerve, and lacrimal nerve to innervate the lacrimal gland.