Tear Film: Structure and Functions

Tear Film

• The tear film is the first thing light rays encounter when they impinge on the surface of the eye.
• It is a thin layer (~7 µl) of fluid with multiple functions, including: • Lubricating the eye • Acting as a barrier to foreign bodies and pathogens • Maintaining the health of underlying epithelial cells • Ensuring a smooth refractive surface for vision
• The tear film is composed of a sophisticated mix of mucous, serous, and sebaceous substances, arranged in layers.

Outer Lipid Layer (Sebaceous)

• The outer lipid layer has various roles, including: • Minimizing evaporation • Limiting contamination from particles and microorganisms • Increasing surface tension (stops tears from overflowing onto the skin)
• It is secreted primarily by meibomian (“tarsal”) glands, located within tarsal plates of upper and lower eyelids.
• These glands are simple, multicellular exocrine glands with one duct and multiple secretory units.

Central Aqueous Layer (Serous)

• The central aqueous layer has various roles, including: • Washing away foreign bodies and contaminations • Nourishing the avascular cornea (oxygen, proteins, salts)
• It is secreted primarily by the lacrimal gland, located above the upper eyelid.
• The lacrimal gland is a compound multicellular exocrine gland with multiple ducts.

Inner Mucin Layer (Mucous)

• The inner mucin layer has various roles, including: • Forming a glycocalyx over the ocular epithelium that prevents pathogen adhesion • Reducing friction during blinking
• It is secreted by goblet cells, located in the conjunctiva (a mucous membrane covering the posterior surface of the eyelid and the exposed surface of the eyeball up to the limbus).
• Goblet cells are unicellular exocrine glands with a distinctive triangular nucleus.

Tear Film Summary

• The tear film covers the anterior (front) surface of the eye.
• It has many important roles and is composed of three components: • Sebaceous secretions (tarsal glands) • Serous secretions (lacrimal gland) • Mucous secretions (conjunctival goblet cells)
• Blinking is required to distribute the tear film over the ocular surface.

Blinking

• Two skeletal muscles are involved in opening and closing the eye: • The orbicularis oculi, which runs in concentric bands around the eye • The levator palpebrae superioris, which raises the upper eyelid
• Other muscles are also involved, including the superior tarsal muscle, which is a smooth muscle that helps to raise the upper eyelid.