Inferior tarsus

The inferior tarsus, also known as the inferior tarsal plate, is a dense connective tissue structure located within the lower eyelid. It provides structural support and maintains the shape of the lower eyelid, contributing to the overall function of the eyelid. The inferior tarsus is less rigid and smaller compared to the superior tarsus (located in the upper eyelid), but it plays a crucial role in the movement and stability of the lower eyelid.

Location

The inferior tarsus is located within the lower eyelid, just beneath the skin and muscle layers. It extends horizontally along the lower eyelid margin, from the inner corner (medial canthus) to the outer corner (lateral canthus). The inferior tarsus is positioned between the palpebral conjunctiva (lining the inside of the eyelid) and the orbicularis oculi muscle (the muscle responsible for closing the eyelids). It forms the supportive framework of the lower eyelid and anchors the tarsal glands within its tissue.

Structure and Anatomy

The inferior tarsus is a crucial structural component of the lower eyelid, providing support and shape. It is primarily composed of dense connective tissue and contains important glandular structures. Below is a detailed description of its anatomy.

Structure and Composition

The inferior tarsus is a semi-rigid, crescent-shaped structure made up of dense fibrous connective tissue. It provides strength and structural integrity to the lower eyelid, allowing it to maintain its shape and position during eye movement and blinking.

• Dense Connective Tissue: The tarsal plate consists of tightly packed collagen fibers that give it rigidity while allowing flexibility. This structure helps support the eyelid’s movements without collapsing.
• Thickness and Size: The inferior tarsus is thinner and shorter than the superior tarsus (found in the upper eyelid). On average, the inferior tarsus is about 4 to 5 mm in vertical height, whereas the superior tarsus is larger and thicker.

Location within the Lower Eyelid

The inferior tarsus is located centrally within the lower eyelid and runs horizontally along the eyelid margin.

• Position: It lies just beneath the skin and muscle layers of the lower eyelid, situated between the palpebral conjunctiva (which lines the inner surface of the eyelid) and the orbicularis oculi muscle (which encircles the eye and controls eyelid closure).
• Medial and Lateral Attachments: The inferior tarsus is anchored at its medial and lateral ends to the medial and lateral canthal tendons, which secure the eyelid to the bony orbit. These attachments provide stability to the lower eyelid and help maintain its shape.

Relationship with Surrounding Structures

The inferior tarsus is closely associated with several other structures of the eyelid and eye, which contribute to its function and stability.

• Palpebral Conjunctiva: The inner surface of the inferior tarsus is lined by the palpebral conjunctiva, a thin, transparent mucous membrane that is continuous with the conjunctiva covering the surface of the eyeball. This membrane protects the tarsus from direct exposure to the eye and provides a smooth surface for tear distribution.
• Orbicularis Oculi Muscle: The orbicularis oculi muscle lies just superficial to the tarsal plate. This muscle is responsible for eyelid closure, and its fibers surround the tarsus, providing the mechanical force needed for blinking and eye closure.
• Inferior Fornix: The inferior tarsus borders the inferior conjunctival fornix, a fold of conjunctiva that connects the lower eyelid to the eyeball. This flexible fold allows for free movement of the lower eyelid during blinking and eye movements.

Glands within the Inferior Tarsus

The inferior tarsus houses the tarsal glands (Meibomian glands), which are embedded within its connective tissue and run vertically along the length of the tarsal plate.

• Tarsal Glands (Meibomian Glands): These are large sebaceous glands that produce meibum, an oily substance that contributes to the tear film’s lipid layer. There are about 20 to 30 tarsal glands in the lower eyelid, fewer than in the upper eyelid. The ducts of these glands open at the eyelid margin, releasing their secretions onto the ocular surface to prevent tear evaporation and keep the eyes lubricated.
• Ductal Openings: The ducts of the tarsal glands pass through the anterior surface of the inferior tarsus and open at the eyelid margin, near the base of the eyelashes. These openings are aligned in a row along the lower eyelid margin, ensuring a consistent distribution of the gland’s secretions.

Blood Supply

The inferior tarsus and the lower eyelid receive blood supply from branches of the ophthalmic artery and the facial artery.

• Inferior Palpebral Artery: This branch of the ophthalmic artery supplies blood to the lower eyelid, including the inferior tarsus.
• Facial Artery Contribution: Branches of the angular artery (from the facial artery) also supply the lower eyelid, contributing to the vascular network that nourishes the tarsal plate and the surrounding tissues.

Venous drainage occurs via the ophthalmic veins and the facial vein, ensuring that blood circulates effectively through the lower eyelid.

Innervation

The inferior tarsus is innervated by both sensory and autonomic nerves.

• Sensory Innervation: Sensory innervation of the lower eyelid, including the inferior tarsus, is provided by branches of the trigeminal nerve (cranial nerve V). Specifically, the infraorbital nerve, a branch of the maxillary division of the trigeminal nerve, supplies sensation to the lower eyelid and tarsus.
• Autonomic Innervation: The autonomic nervous system provides innervation to the tarsal glands within the inferior tarsus, regulating their secretion of meibum. Sympathetic fibers stimulate the glands to secrete, while parasympathetic fibers may modulate their activity.

Histology

The histological structure of the inferior tarsus is characterized by dense connective tissue, glandular structures, and blood vessels.

• Collagenous Connective Tissue: The tarsal plate is composed of dense, collagen-rich connective tissue that provides the necessary rigidity to maintain the shape of the lower eyelid. This tissue is interspersed with elastic fibers that allow flexibility during eyelid movement.
• Glandular Tissue: The acini of the tarsal glands (Meibomian glands) are embedded within the connective tissue of the tarsus. These glands are made up of lipid-producing cells, which release their contents into the central ducts that open at the eyelid margin.
• Vascularity: The inferior tarsus is well-vascularized, with small blood vessels coursing through the connective tissue to supply nutrients to the glandular cells and surrounding structures.

Function

The inferior tarsus, or inferior tarsal plate, plays a critical role in supporting the structure and function of the lower eyelid. It ensures proper eyelid mechanics, provides a framework for essential glandular activity, and contributes to the protection of the ocular surface. Below is a detailed description of its functions.

Structural Support and Shape Maintenance

One of the primary functions of the inferior tarsus is to provide structural support to the lower eyelid.

• Maintaining Eyelid Shape: The inferior tarsus is made of dense connective tissue, which gives the lower eyelid its rigidity and form. This helps the eyelid retain its shape and prevents it from collapsing or drooping, ensuring that it remains properly aligned with the eyeball.
• Foundation for Eyelid Movement: The tarsal plate serves as a stable foundation that allows the lower eyelid to move smoothly during blinking and eye movements. It provides enough flexibility to allow the eyelid to move freely while maintaining the structural integrity needed for proper function.

Anchoring and Stability of the Lower Eyelid

The inferior tarsus is securely attached to the bones of the orbit via tendons at the medial and lateral canthi, which helps maintain eyelid stability.

• Medial and Lateral Canthal Tendons: The attachments of the inferior tarsus to the medial and lateral canthal tendons keep the lower eyelid tightly anchored to the orbital bones, preventing it from becoming lax or sagging. This attachment is crucial for maintaining the normal position of the eyelid and ensuring that the lower lid follows the contour of the eyeball.
• Eyelid Margin Alignment: The structural support provided by the inferior tarsus helps maintain the proper alignment of the eyelid margin along the eye, ensuring that it fits snugly against the surface of the eye without pulling away or turning inward (entropion) or outward (ectropion).

Housing and Support for the Tarsal Glands

The inferior tarsus serves as the structural base for the tarsal glands (Meibomian glands), which are embedded within its connective tissue.

• Support for Meibomian Glands: The tarsal plate houses the vertically arranged Meibomian glands, providing a stable and protective environment for these essential sebaceous glands. The structure of the tarsus ensures that the ducts of these glands are properly aligned to open at the eyelid margin.
• Facilitating Gland Function: The rigid structure of the inferior tarsus ensures that the Meibomian glands can function effectively, allowing them to secrete meibum, the lipid component of the tear film. The tarsus maintains the gland’s positioning and orientation, enabling the proper release of meibum onto the ocular surface.

Contribution to Tear Film Maintenance

By supporting the function of the tarsal glands, the inferior tarsus indirectly plays a role in maintaining the tear film, which is essential for ocular surface health.

• Meibum Secretion for Tear Film: The inferior tarsus supports the tarsal glands, which secrete meibum that forms the lipid layer of the tear film. This lipid layer is crucial for reducing tear evaporation and keeping the ocular surface moist and protected.
• Stabilizing the Tear Film: The proper functioning of the Meibomian glands within the tarsus ensures that the tear film remains stable, preventing the formation of dry spots on the cornea. A stable tear film is essential for maintaining ocular comfort and providing a smooth optical surface for clear vision.

Protection of the Ocular Surface

The inferior tarsus helps protect the ocular surface by ensuring that the lower eyelid functions effectively as a barrier and lubricator.

• Barrier Against Foreign Particles: The structural rigidity of the inferior tarsus helps the lower eyelid form a tight seal against the surface of the eye. This prevents foreign particles, dust, and other environmental debris from entering the eye and causing irritation or damage to the cornea and conjunctiva.
• Assisting in Tear Film Spread: During blinking, the lower eyelid, supported by the tarsus, plays a role in spreading the tear film across the surface of the eye. This ensures that the tear film is evenly distributed and that the entire ocular surface is coated with protective moisture.

Facilitating Smooth Eyelid Movements

The inferior tarsus enables smooth, controlled movements of the lower eyelid during blinking and eye closure, which is essential for ocular protection and comfort.

• Foundation for Blinking: The tarsal plate provides the necessary structure for the eyelid to close properly during blinking. Blinking spreads the tear film over the eye, lubricates the ocular surface, and removes debris. The inferior tarsus ensures that the lower eyelid moves smoothly and in sync with the upper eyelid during this process.
• Mechanical Stability: By maintaining the eyelid’s shape and position, the inferior tarsus allows for controlled, precise movements during eyelid opening and closing. This mechanical stability prevents excessive movement or misalignment of the eyelid, ensuring optimal function.

Preventing Entropion and Ectropion

The inferior tarsus helps prevent abnormal eyelid positioning, such as entropion (inward turning of the eyelid) or ectropion (outward turning of the eyelid).

• Preventing Eyelid Malposition: The structural integrity of the tarsus helps the lower eyelid maintain its proper alignment against the eye. A weakened or damaged tarsal plate can lead to eyelid malposition, resulting in conditions such as entropion, where the eyelid turns inward and the lashes rub against the eye, or ectropion, where the eyelid turns outward, exposing the inner surface of the eyelid to irritation.
• Supporting Eyelid Tension: The tarsal plate’s attachment to the canthal tendons provides the necessary tension to keep the lower eyelid taut against the surface of the eye, preventing sagging or laxity that could lead to malposition.

Clinical Significance

The inferior tarsus plays a crucial role in maintaining the structural integrity and function of the lower eyelid. Dysfunction or damage to this structure can lead to various clinical issues:

• Entropion: A weakened or damaged inferior tarsus can cause entropion, where the lower eyelid turns inward, causing the eyelashes to rub against the cornea and conjunctiva. This can lead to discomfort, irritation, and even corneal damage.
• Ectropion: If the inferior tarsus becomes lax or loses its structural support, ectropion can occur, where the lower eyelid turns outward. This condition can result in exposure of the inner eyelid surface, causing dryness, irritation, and an increased risk of infection.
• Chalazion: Blockage of the Meibomian glands within the inferior tarsus can lead to the formation of a chalazion, a painless lump on the lower eyelid caused by the accumulation of glandular secretions.
• Tarsal Plate Trauma or Scarring: Trauma or scarring of the inferior tarsus from injury, surgery, or chronic inflammation can compromise the shape and function of the lower eyelid, leading to malposition and difficulty in eyelid closure.