Eyelid and Lacrimal Anatomy Quiz

Eyelid Anatomy

• The eyelid protects the eye, distributes tears, cleanses the ocular surface, regulates light exposure, and contributes to the tear film.
• The orbicularis oculi muscle closes the eyelids and is innervated by cranial nerve VII.
• The levator palpebrae muscle elevates the upper eyelid and is innervated by cranial nerve III.
• The Müller muscle, innervated by sympathetic nerves, also helps elevate the upper eyelid.
• The inferior tarsal muscle helps retract the lower eyelid.
• Eyelashes are the only hairs of the eyelid and serve as sensors to stimulate reflex eyelid closure.

Lacrimal Functional Unit (LFU)

• The LFU is responsible for tear film regulation, production, health, and integrity, maintaining corneal transparency and stem cell population, and contributing to retinal image quality.
• The LFU responds to environmental, endocrinologic, and cortical influences.
• Ocular surface and trigeminal nociceptors mediate afferent signals to the brainstem, where they synapse with efferent nerves (autonomic and motor).
• Autonomic nerve fibers innervate the meibomian glands, conjunctival goblet cells, and lacrimal glands.
• Motor nerve fibers innervate the orbicularis muscle to initiate blinking.

Corneal Structure

• The cornea is transparent and avascular.
• Its transparency is due to the organization of keratocytes, fibers, and the extracellular matrix within the corneal stroma, as well as the balance of stromal water content.
• Intraocular pressure (IOP) tends to compress the cornea, and the overall imbibition pressure of the corneal stroma is calculated as IOP—SP.

Dua layer

• The most posterior aspect of the stroma forms a thin, acellular layer called the pre-Descemet layer or Dua layer.
• This layer is essential for deep anterior lamellar keratoplasty (DALK).
• It is strong and resists air dissection from Descemet membrane.

Descemet Membrane

• This is the basement membrane of the corneal endothelium.
• Its thickness increases with age, from 3 µm at birth to 10–12 µm in adulthood.
• It consists of an anterior banded zone that develops in utero and a posterior amorphous, non-banded zone laid down throughout life.
• The Schwalbe line marks the end of Descemet membrane and the beginning of the trabecular meshwork.

Corneal Endothelium

• This single layer of cells lies on the posterior surface of the cornea.
• It is arranged in a mosaic pattern of mostly hexagonal shapes.
• If cell loss occurs (e.g., due to trauma or surgery), the defective area is initially covered by cell enlargement and spread of surrounding cells or perhaps peripheral stem cells.

Limbus

• The limbus is the transition zone between the cornea and sclera.
• It harbors corneal epithelial stem cells, which are crucial for corneal epithelial homeostasis and wound repair.
• The palisades of Vogt, concentrated in the superior and inferior limbus, are believed to be the site of the limbal stem cell niche.

Corneal Defense Mechanisms

• The external eye and cornea, along with the tear film, provide defense against infection.

Test your knowledge of eyelid anatomy and the lacrimal functional unit (LFU). This quiz covers the structures, functions, and innervations that protect the eye and regulate tear film. Dive into the details of muscular components and their significance in ocular health.