The eyelids are thin, mobile folds that cover the eyeball anteriorly. They offer protection from excessive light or injury, and maintain lubrication by distributing tears over the surface of the eyeball. Show The eyelids are split into upper and lower portions, which meet at the medial and lateral canthi of the eye. The opening between the two eyelids is called the palpebral aperture or opening. In this article, we shall look at the anatomy of the eyelids - their layers, vasculature and innervation. [caption id="attachment_22588" align="aligncenter" width="335"] Fig 1 - The upper and lower eyelid.[/caption]Layers of the EyelidThe eyelid consists of five main layers (superficial to deep):
Skin and Subcutaneous TissueThe skin and subcutaneous tissue form the most superficial layer of the eyelid. The layer of skin is among the thinnest in the human body. In the subcutaneous layer, there is loose connective tissue but no subcutaneous fat - and subsequently, the eyelids are readily distended by oedema or blood. The eyelashes are attached here with their accompanying modified sweat glands - the ciliary glands of Moll. There are also sebaceous glands located in this layer, known as the glands of Zeis. Orbicularis OculiThe orbicularis oculi muscle has three distinct parts - palpebral, lacrimal and orbital.
[caption id="attachment_22892" align="aligncenter" width="1024"] Fig 2 - Sagittal section of the orbit, demonstrating the layers of the eyelid.[/caption]Tarsal PlatesThe tarsal plates are located deep to the palpebral region of the orbicularis oculi muscle. There are two plates; the superior tarsus (upper eyelid) and inferior tarsus (lower eyelid). They act to form the scaffolding of the eyelid, and are composed of dense connective tissue. The superior tarsus also acts as the attachment site of the levator palpebrae superioris. In the tarsal plates lie the Meibomian glands (also known as tarsal glands). These are a specialised type of sebaceous gland that secretes an oily substance onto the eye to slow the evaporation of the eye's tear film. The oily substance also prevents the eyelids from sticking together when closed. [start-clinical] Clinical Relevance: Styes and ChalazionA stye (or hordeolum) is an infection of a hair follicle or Meibomian glands around the eyelash. If affecting the hair follicles, they are described as external, and if they affect the Meibomian glands they are classified as internal. Usually they are self-limiting, although they can sometimes be drained by removing the hair from the affected follicle. A chalazion is a painless granuloma of the Meibomian glands. It can be distinguished from a stye by the absence of pain in a chalazion, whereas styes are normally painful. [end-clinical] Levator ApparatusThe levator palpebrae superioris and superior tarsal muscles both act to open the eyelid. They are only present in the upper eyelid.
[caption id="attachment_8508" align="aligncenter" width="432"] Fig 3 - Attachment of the levator palpebrae superiors to the superior tarsal plate.[/caption]ConjunctivaThe palpebral conjunctiva forms the deepest layer of the eyelid. It is a thin mucous membrane, which is reflected onto the sclera of the eyeball (bulbar conjunctiva). VasculatureThe eyelid has a rich arterial supply from numerous vessels:
Venous drainage is provided by a rich network around the eyelid. Medially, blood drains via the medial palpebral vein into the angular and ophthalmic veins. Laterally, blood drains into the superficial temporal vein from the lateral palpebral vein. InnervationSensory innervation to the eyelids is supplied by branches of the trigeminal nerve:
As discussed above, innervation to the muscles of the eyelid is via the facial nerve (orbicularis oculi), oculomotor nerve (levator palpebrae superioris) and sympathetic fibres (superior tarsal muscle).
Knowledge of the histology of the eye is important for understanding disease pathophysiology and treatment, as many diseases of the eye are manifestations of pathology within specific histological layers. This article describes the histological classification of the tissues of the eye and its external structures. Accessory StructuresFrom superficial to deep, the eyelid is made of epidermis, dermis, the orbicularis oculi muscle, the orbital septum, the tarsal plate, and the palpebral conjunctiva.
Lacrimal ApparatusThe lacrimal apparatus of the eye produces lacrimal tears in order to prevent dehydration of the eye. Dry eye syndrome is the most common eye disease, and if untreated, it can cause corneal ulcerations, scarring, and even perforation. The lacrimal apparatus is composed of four parts:
ConjunctivaThe conjunctiva is a transparent mucous membrane that lines the inner surface of the eyelids (palpebral conjunctiva) and the anterior surface of the sclera (bulbar conjunctiva).
Chambers of the EyeThere are three recognized chambers of the eye:
Internal Structures of the EyeThe eyeball is composed of three principal layers: the fibrous tunic, the vascular tunic, and the neural tunic.
ScleraThe white, opaque sclera is a tough fibrous connective tissue layer that provides the architectural support for the shape of the eyeball. Structure/histology:
CorneaThe cornea is a modified mucous membrane that covers the anterior portion of the eye. Its tissue elements are highly specialized to order to maintain transparency. It is composed of six histologically distinct layers, from superficial to deep:
IrisThe iris is a colored, opaque ring that covers the front of the lens and functions to regulate the pupil diameter. Structure/Histology
Ciliary BodyThe ciliary body is a ring of smooth muscle that spans the inner wall of the eye at the level of the lens. It suspends the lens in place via suspensory ligaments and functions primarily to control the shape of the lens and produce aqueous humor. Aqueous humor production
Lens control
LensThe lens is a transparent biconvex disc located directly behind the pupil. It consists of three parts: the lens capsule, the subcapsular epithelium, and the lens. Structure/Histology
ChoroidThe choroid is a heavily pigmented layer containing a dense network of blood vessels that serves to provide nutrients to the retina and absorb light. It is loosely attached to the sclera and separated from the retina by Bruch’s membrane. Structure/Histology
RetinaThe retina is formed by the outer retinal pigment epithelium (RPE) and the inner neural retina, a multilayered structure containing photoreceptor cells and supportive cells. It can be organized into 10 distinct layers, beginning with the RPE, which lies adjacent to Bruch’s membrane and the choroid, to the internal limiting membrane, which meets the vitreous humor: 1. Retinal pigment epithelium (RPE): The RPE consists of cuboidal to columnar epithelial cells that contain an abundance of melanin granules in order to absorb light and reduce random reflections of unabsorbed light.
2. Layer of rods and cones (photoreceptor layer): This layer contains the outer segments of the photoreceptor cells—the rods and cones. Rods are the photoreceptor cells specialized for perceiving dim light, while cones are specialized for perceiving bright light and color.
3. External limiting membrane (ELM): The ELM is not a true membrane, but it is a region of zonulae adherents between the photoreceptor cells and Muller cells. Muller cells provide structural and nutritional support for the retinal neurons. 4. Outer nuclear layer: This layer contains the cell bodies of the photoreceptor cells. Rods and cones can be distinguished histologically by their nuclei—the nuclei of rods are smaller, more rounded, and more darkly stained than the nuclei of cones. 5. Outer plexiform layer: This layer contains the synapses between the axons of photoreceptor cells and the dendrites of intermediate neurons (bipolar and horizontal cells). 6. Inner nuclear layer: This layer contains the cell bodies of the intermediate neurons (bipolar, horizontal, and amacrine cells) and Muller cells. 7. Inner plexiform layer: This layer contains the synapses between the axons of bipolar cells and the dendrites of ganglion cells and amacrine cells. 8. Ganglion cell layer: This layer contains the cell bodies of ganglion cells, the neurons whose axons travel in the optic nerve and project to the brain. 9. Optic nerve fiber layer: This layer contains the axons of ganglion cells. The nerve fibers are formed of unmyelinated axons, which become myelinated as the nerve exits the sclera. 10. Internal limiting membrane (ILM): The ILM is composed of the basement membrane of Muller cells, which separates the retina from the vitreous humor. Optic discThe optic disc is the site at which the axons from the retinal ganglion cells converge and exit the eye via the optic nerve. The optic disc contains no photoreceptor cells and creates the blind spot of the retina. Macula and FoveaThe macula is a yellow-pigmented zone lateral to the optic disc, approximately 5.5mm in diameter. The fovea is an approximately 1.5mm area of specialized avascular retina that can be identified as a depression in the retina in cross-section. The foveola is the central floor of the fovea, approximately 0.35mm in diameter, and is the area of retina with the greatest visual acuity. It can be differentiated from the fovea histologically by an absence of ganglion cells and rods. The cones in this area are narrower than in other parts of the retina, allowing for more cones per unit area. Additional Resources
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