Glands are group of specialized secretory epithelial cells that may be unicellular or multicellular. Glands can be classified as exocrine and endocrine. Exocrine (externally secreting) glands are the group that release their secretions onto an epithelial surface. Endocrine (also called duct-less or internally secreting) glands pour their secretions directly into the bloodstream because they lose their contacts with the epithelial surface from which they originate during embryonic development.
In addition, glands may also be described on the bases of the nature of their secretions as serous glands and mucous glands. On the basis of their secretory cell arrangements, they are termed as acini if their secretory cells are arranged as rounded sacs, tubular if their secretory elements are straight, coiled, or branched, and alveoli if they form flask-shaped structures.
The lacrimal glands release their secretions directly onto the eyeball and are thus classified as serous type of exocrine glands with acini secretory elements. Lacrimal gland is an important part of the lacrimal apparatus (the lacrimal system) that plays an essential role in maintaining the normal anatomy of the eye. Other structures constituting the lacrimal apparatus include the lacrimal sac (approximately 12 mm long), nasolacrimal duct (which is about 18 mm in length) and the lacrimal canaliculi (singular: canaliculus and is approximately 10 mm in length). Thus the lacrimal system can be divided into a part which produces the lacrimal fluid - the lacrimal gland, and the lacrimal drainage pathway – the ducts, sac, cavity and canals.
The lacrimal glands secrete tears which act to lubricate and protect the cornea and eyeball from injury which may result from small particles such as dust. Tears are a watery fluid with an electrolyte content similar to that of plasma and contains:
- bactericidal enzymes known as lysozyme and lactoferrin
- immunoglobulin A (IgA)
The lacrimal ducts constitute the secretory elements of the lacrimal gland and serve as the direct channels through which lacrimal fluids are poured onto the conjunctival sac and eyeball. Once these fluids arrive at the lacrimal puncta, they are collected and channeled across by the lacrimal canaliculi, while the nasolacrimal ducts also serve to convey the fluid to another part of the lacrimal apparatus. Each of the two lacrimal puncta is 2 mm in length.
Gross and Microscopic Anatomy
The lacrimal glands are paired almond-shaped structures, one for each orbit, of approximately 2 cm in length, and lie in the lacrimal fossae (also referred to as ‘fossae for the lacrimal glands’). This fossa is situated on the superolateral aspects of the zygomatic process of the frontal bone forming part of the orbit (eye socket or orbital cavity). Laterally, the gland sits above the lateral rectus muscle while posteriorly, it attaches to the orbital fat.
The lateral expansion of the tendon from levator palpebrae superioris muscles spans across the lacrimal gland and divides it into a superior and an inferior part. The superior half is called the orbital part, while the inferior part (which is smaller) is the palpebral part. Small ducts from the palpebral aspect of the lacrimal gland join about 8-12 ducts from orbital part and open into the superior conjunctival fornix which is the superior line of reflection of the palpebral conjunctiva to the eyeball.
Accessory lacrimal glands
Also associated with the lacrimal gland are small accessory lacrimal glands (glands of Krause and Wolfring) which are more numerous in the superior eyelid than in the inferior eyelid. They mainly function to contribute about 40% of the total volume of lacrimal fluid. These accessory glands occur in or near the conjunctival fornix.
Under microscopic examination, the secretory elements of the lacrimal gland are seen to be composed of serous acini cells partitioned into many lobules by loose connective tissues, which contain many immune cells, mainly B-lymphocytes and plasma cells (most especially the IgA secreting cells).
Flow of Lacrimal Fluid (Tears)
Although some tears are lost by evaporation or absorption across the conjunctiva, most of the tears produced are eliminated via the nasolacrimal drainage system described as follows. Once the lacrimal fluids or tears are secreted by the lacrimal glands, they flow from the superolateral angle of the bony orbit and move through the lacrimal ducts to flow over the eyeball through the blinking action of the eyelids. The lacrimal fluid empty into the lacrimal sac by flowing through two small pits called the lacrimal puncta (singular: punctum) which lies in the superior and inferior eyelids. These puncta lead to the lacrimal canaliculi (or lacrimal canals) which are hollow passages in the upper and lower eyelids connecting the lacrimal sac to the eyeball. Lacrimal sac exists in the lacrimal fossa at the medial wall of the orbit between anterior and posterior lacrimal crests.
From the lacrimal sac, tears are drained into the nasolacrimal duct, from which, they are emptied into the posterior nasal meatus after flowing through the inferior opening (inferior meatus) of the nose. The inferior opening of the nose is a passage along the lateral wall of the nasal cavity which is formed by the projection of the inferior nasal concha. The lacrimal fluid is finally absorbed in the nasopharynx after draining into the posterior nasal meatus.
This is the normal flow of tears which occurs periodically to maintain the health of the eye (exposed parts of the ocular surface framed by margins of the eyelid), and more especially the cornea which is constantly kept moist with lacrimal fluid. This periodic process ensures a good optical quality, as well as provide the cornea with oxygen since the cornea lacks blood supply. The lacrimal fluid is also important in protecting the eye against microbial colonization, by acting as a cleanser which reduces the adhesion of microbes to the ocular surface and fighting such microorganisms with its antimicrobial proteins.
However, in some instances such as in lacrimation reflex, exposure to a gas which induces tears, or when an individual is in an emotional state, there is an increase in the secretion of tears. This results in an overflow over the lipid barrier on the edge of the eyelids and onto the cheeks. This occurs because the lacrimal apparatus can only collect and absorb about 7 - 10 µL at a time. Out of this, 1 - 3 µL is distributed within the precorneal tear film and 7 µL within the upper and lower marginal tear strips which runs along the posterior border of the eyelid margins.
The lacrimal branch of the ophthalmic artery runs into the lacrimal gland to provide its blood supply. Little amount of arterial supply is also received from the infraorbital artery. It drains out its venous blood into the superior ophthalmic vein.
The sensory supply to lacrimal gland is by lacrimal nerve which is a branch of ophthalmic nerve. Parasympathetic secretomotor fibers are received through zygomatic and lacrimal branches of the maxillary nerve coming from the pterygopalatine ganglion. Preganglionic fibres reach the ganglion in the greater petrosal nerve arising from the facial nerve. Essentially, the production of lacrimal fluid is stimulated by the parasympathetic impulses from the facial nerve.