Tarsal glands

The tarsal glands, also known as the Meibomian glands, are specialized sebaceous glands located within the eyelids. These elongated glands are responsible for producing an oily secretion that plays a critical role in maintaining the stability of the tear film and preventing tear evaporation. They are a key component of the eyelid’s anatomy and contribute to the overall health of the ocular surface.

Location

The tarsal glands are embedded within the tarsal plates of both the upper and lower eyelids. The tarsal plates are dense connective tissue structures that provide rigidity to the eyelids. The glands run vertically along the length of the tarsal plates, with their openings located at the eyelid margin, just behind the eyelashes. There are more tarsal glands in the upper eyelid than in the lower eyelid, corresponding to the larger size of the upper tarsal plate.

Structure and Anatomy

The tarsal glands, also known as Meibomian glands, are a critical part of the eyelid’s structure, responsible for producing a lipid-rich secretion. These glands play an essential role in maintaining the tear film and ocular surface. Below is a detailed description of their anatomy.

Structure of the Tarsal Glands

The tarsal glands are long, branched sebaceous glands that extend vertically within the tarsal plates of both the upper and lower eyelids. Each gland consists of multiple acini, which are the small, sac-like clusters where secretion production takes place.

• Acini (Secretory Units): The acini of the tarsal glands are responsible for the production of meibum, the oily substance secreted by these glands. Each acinus contains cells that secrete lipids, and these secretions are collected and transported through a central duct.
• Central Duct: Each tarsal gland has a long, single central duct that runs through the middle of the gland and opens at the eyelid margin. The secretions from the acini are collected in this duct and then excreted onto the ocular surface. The duct is lined with stratified squamous epithelium.
• Gland Size: The tarsal glands are large, elongated structures that span the height of the tarsal plates. The glands in the upper eyelid are larger and more numerous than those in the lower eyelid.

Location in the Eyelids

The tarsal glands are located within the tarsal plates, which are dense connective tissue structures that provide shape and rigidity to the eyelids.

• Upper Eyelid: The upper eyelid contains more tarsal glands than the lower eyelid, with around 30 to 40 glands running vertically along the length of the tarsal plate. The larger size of the upper eyelid allows for more glands, which ensures adequate secretion of meibum.
• Lower Eyelid: The lower eyelid contains around 20 to 30 tarsal glands. These glands are smaller than those in the upper eyelid but function similarly by producing meibum to support the tear film.

Openings of the Tarsal Glands

The openings of the tarsal glands are located at the edge of the eyelid margin, behind the eyelashes.

• Eyelid Margin: The ducts of the tarsal glands open at the posterior margin of the eyelid, just anterior to the mucocutaneous junction, where the skin of the eyelid meets the conjunctiva. These openings are aligned in a row along the eyelid margin, ensuring the distribution of meibum across the ocular surface.
• Tear Film Interface: The positioning of the tarsal gland openings allows the secretions to mix directly with the tear film. This helps create the lipid layer of the tear film, which prevents tear evaporation and maintains ocular surface hydration.

Relationship with Tarsal Plates

The tarsal glands are embedded within the tarsal plates, which are fibrous, supportive structures that give the eyelids their shape and structural integrity.

Tarsal Plates: These dense connective tissue structures form the structural framework of both the upper and lower eyelids. The tarsal plates provide a rigid base in which the tarsal glands are housed, ensuring that the glands are securely positioned within the eyelids.

Blood Supply

The tarsal glands receive their blood supply from branches of the ophthalmic artery and the facial artery, which supply the surrounding eyelid tissues.

• Ophthalmic Artery: The ophthalmic artery gives rise to branches such as the superior palpebral artery and inferior palpebral artery, which supply blood to the upper and lower eyelids, respectively.
• Facial Artery: The facial artery also contributes to the blood supply of the tarsal glands and surrounding eyelid tissues.

Innervation

The tarsal glands are innervated by both sensory and autonomic nerve fibers that control their secretory activity.

• Autonomic Innervation: The tarsal glands are under the control of the autonomic nervous system. Sympathetic nerves stimulate the glands to secrete meibum, while parasympathetic innervation is thought to modulate gland function and secretion levels.
• Sensory Innervation: The sensory innervation of the eyelids, including the tarsal glands, is provided by the ophthalmic division and maxillary division of the trigeminal nerve (cranial nerve V). This innervation helps detect irritation or blockages at the gland openings.

Histological Structure

The histological structure of the tarsal glands is typical of sebaceous glands, with large acini surrounded by connective tissue.

• Sebaceous Acini: The acini of the tarsal glands are filled with lipid-producing cells. These cells synthesize meibum, which is essential for creating the lipid layer of the tear film. The cells undergo holocrine secretion, meaning the entire cell disintegrates to release its contents.
• Ductal System: The ducts that collect the secretions from the acini are lined with stratified squamous epithelium and are supported by surrounding connective tissue. The ducts channel the meibum to the gland openings at the eyelid margin.

Function

The tarsal glands, also known as the Meibomian glands, have several crucial functions that maintain the health of the ocular surface and ensure the proper lubrication and protection of the eye. Their primary role is the production of an oily secretion called meibum, which forms the lipid layer of the tear film. Below is a detailed description of the functions of the tarsal glands.

Production of Meibum

The main function of the tarsal glands is to produce meibum, a lipid-rich secretion that plays a critical role in the tear film. This oily substance is essential for maintaining a stable tear film and protecting the ocular surface.

• Lipid Secretion: The acinar cells within the tarsal glands produce meibum, a mixture of lipids, including cholesterol, free fatty acids, and wax esters. This lipid is secreted through the gland’s ducts onto the surface of the eye.
• Holocrine Secretion: Meibum is released through a process known as holocrine secretion, where entire cells disintegrate to release their lipid contents into the central duct. This secretion process ensures a continuous supply of meibum.

Formation of the Lipid Layer of the Tear Film

The tarsal glands contribute to the outermost layer of the tear film, known as the lipid layer. This layer is essential for tear film stability and maintaining ocular surface health.

• Lipid Layer Formation: The meibum secreted by the tarsal glands forms the lipid layer of the tear film. This layer floats on top of the aqueous layer, which is produced by the lacrimal glands, and the mucin layer, which is produced by goblet cells in the conjunctiva.
• Prevention of Tear Evaporation: The lipid layer reduces evaporation of the aqueous portion of the tear film. By creating a hydrophobic barrier, the oily layer prevents the tears from evaporating too quickly, helping maintain proper hydration of the ocular surface.

Maintenance of Tear Film Stability

One of the most important functions of the tarsal glands is to stabilize the tear film, which is crucial for maintaining a clear optical surface and providing comfort to the eye.

• Tear Film Stability: The lipid layer created by the meibum reduces surface tension and spreads evenly across the eye, preventing the formation of dry spots on the cornea. This stability ensures that the tear film remains intact between blinks, providing continuous protection to the ocular surface.
• Support for Optical Clarity: A stable tear film is essential for proper refraction of light, ensuring that the corneal surface remains smooth and clear for light transmission. Disruptions in the tear film can lead to blurred vision and discomfort.

Lubrication of the Eyelid Margins

The tarsal glands also play a role in lubricating the eyelid margins, ensuring smooth movement of the eyelids during blinking.

• Reduction of Friction: The meibum secreted by the tarsal glands coats the eyelid margins, reducing friction between the eyelids and the surface of the eye. This lubrication ensures that the eyelids move smoothly over the ocular surface during blinking and eye movements, preventing irritation and mechanical damage.
• Eyelash Protection: The oily secretions of the tarsal glands also help lubricate the base of the eyelashes, contributing to the health of the eyelid margin and preventing issues such as eyelash breakage or irritation.

Prevention of Tear Overflow

The oily layer produced by the tarsal glands helps prevent tears from spilling over the eyelid margin, reducing the risk of excessive tearing (epiphora).

Tear Retention: The lipid layer prevents tears from flowing over the eyelid margins by forming a barrier that helps keep tears confined to the ocular surface. This function is important for maintaining proper tear drainage and preventing tear overflow onto the face.

Protection Against Infections

The secretions from the tarsal glands play a protective role in preventing bacterial infections at the eyelid margin and ocular surface.

Antimicrobial Properties: Meibum contains certain components that may have antimicrobial properties, helping to protect the eyelid margin and ocular surface from bacterial colonization. The lipid-rich environment created by meibum discourages the growth of bacteria that can cause infections such as blepharitis (inflammation of the eyelid margins).

Regulation of Tear Film Composition

The tarsal glands play an important role in regulating the overall composition of the tear film, ensuring that the lipid, aqueous, and mucin layers are in balance.

• Lipid-Aqueous Balance: The proper secretion of meibum helps maintain the balance between the lipid and aqueous layers of the tear film. An imbalance between these layers can lead to conditions such as dry eye syndrome, where the tear film becomes unstable or evaporates too quickly.
• Control of Tear Film Evaporation Rate: By regulating the thickness and consistency of the lipid layer, the tarsal glands control the rate at which the tear film evaporates. This function is essential for maintaining hydration of the ocular surface, especially in dry or windy environments.

Clinical Significance

The tarsal glands (Meibomian glands) are crucial for maintaining a healthy tear film and preventing dry eye conditions. Dysfunction in these glands can lead to several clinical issues:

• Meibomian Gland Dysfunction (MGD): This is one of the most common disorders associated with the tarsal glands. It occurs when the glands become blocked or fail to secrete enough meibum, leading to evaporative dry eye syndrome. MGD can cause irritation, redness, and discomfort due to inadequate lubrication of the ocular surface.
• Blepharitis: Inflammation of the eyelids, often linked to dysfunction of the Meibomian glands, can lead to chronic irritation and scaling along the eyelid margins. Blepharitis can cause burning, itching, and a gritty sensation in the eyes.
• Chalazion: Blockage of the tarsal glands can result in a chalazion, a painless lump in the eyelid due to the accumulation of meibum. While generally benign, chalazia can become large and require medical or surgical intervention.