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Histology of the Eye: Understanding the Eye

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Understanding the eye’s histology, or the structure of the eye’s tissue at a microscopic level, is crucial to understanding what is really happening when an eye becomes injured or damaged. If you know the eye’s structure, you can better assess which treatment options are best.

Accessory Structures of the Eye

The accessory structures are the parts of the eye not located within the eyeball itself.

Eyebrows

Eyebrows protect the eye from sweat and other types of moisture as well as other substances from the surrounding environment. In addition, the eyebrows help communicate to the nervous system through neuroreceptors. They can stimulate the immune system if any pathogens are identified.

Extraocular Muscles

These seven muscles are found around the eye, and they help control important functions of the eye itself.

The first six muscles, when used in combination with each other, allow for the movement of the eye in different directions. The levator palpebrae superioris, on the other hand, allows for the eyelid to be elevated.

  • Medial rectus: This is located on the side of the eye closest to the nose.
  • Lateral rectus: This is located on the side of the eye farthest from the nose.
  • Superior rectus: This is located on the top of the eye.
  • Inferior rectus: This is located on the bottom of the eye

Superior oblique: This is located on the top of the eye.

  • Inferior oblique: This is located on the bottom of the eye.
  • Levator palpebrae superioris: This is located above the eye, connecting to the eyelid.

Eyelid

Each eyelid is made up of different layers. The following parts of the eyelid are listed from outer to inner:

  • Epidermis: This is the outer layer of skin.
  • Dermis: This contains various glands, including sweat glands and the glands that lead into eyelash follicles.
  • Orbicularis oculi muscle: This muscle allows you to close the eyelid.
  • Orbital septum: This separates the structures of the eyelid from the structures of the rest of the eye.
  • Upper and lower tarsal plates: These two thick plates are made of connective tissue and allow the eyelid to keep its form. They are considered the main components that make up the structure of the eyelid.
  • Meibomian glands: These secrete a substance that helps the tear film not to evaporate.

    Lacrimal Apparatus

    • Lacrimal gland: This gland secretes tears, which are made of water and lysozyme, and help to kill bacteria. These tears keep the surface of the eye healthy and remove particles that may have landed on the eye.
    • Lacrimal canaliculi: This helps drain tears away from the eye.
    • Nasolacrimal duct: This brings tears to the nasal cavity.
    • Lacrimal sac: This connects the lacrimal canaliculi to the nasolacrimal duct.

    Conjunctiva

    The conjunctiva lines the inside of the eyelids as well as the surface of the sclera, which is the connective tissue that gives the eye most of its structure.

    • Epithelial layer: This protective layer is three to five cell layers deep. It contains goblet cells, which secrete mucus, as well as a variety of other cells, glands, and blood vessels.
    • Substantia propria: This is a connective tissue layer that contains blood cells and immune cells.
    • Glands of Krause and Wolfring: These secrete antibacterial substances and contribute to the tear film.

    Tear Film

    The tear film lines the outer layer of the eye and serves to protect the eye from harmful microbes. It is made up of three layers, listed from outermost to innermost:

    • Lipid layer: This stops the tear film from evaporating.
    • Aqueous layer: This maintains hydration of the eye and removes bacteria.
    • Mucin layer: This holds the tear film in place.

    Chambers of the Eye

    The eye itself is made up of three separate chambers. They are listed below, from outermost to innermost:

    Anterior

    The anterior chamber of the eye is found between the cornea (the clear, protective layer on the outside of the eye) and the iris (the colored part of your eye).

    This chamber is full of aqueous humor, a clear fluid that provides nutrients and oxygen to the cornea and the lens.

    Posterior

    The posterior chamber of the eye is found between the iris and the lens (the transparent disk located behind your pupil) of the eye. It is filled with the same fluid as the anterior chamber.

    Vitreous

    The vitreous chamber of the eye is found between the lens of the eye and the retina (the tissue lining the back of the eye). Unlike the anterior and posterior chambers, the vitreous chamber is full of a thicker fluid called the vitreous body.

    Internal Structures

    These are the internal structures of the eye:

    Sclera

    The sclera, commonly referred to as the white of the eye, is made of dense connective tissue, and has three layers. These layers are listed from outermost to innermost:

    • Episclera: This is a thin layer of connective tissue.
    • Stroma: This is a thick layer of connective tissue.
    • Lamina fusca: This is a thin layer of connective tissue, which is pigmented.

    Cornea

    The cornea is the transparent layer that covers the front of the eye. It is made up of parts that are organized to keep the cornea transparent.

    Its parts are listed below:

    • Epithelium: This acts as a barrier to stop chemicals, water, and microbes from entering the eye. This layer also works to help the tear film spread over the eye when you blink.
    • Bowman’s membrane: This helps to maintain the shape of the cornea.
    • Corneal stroma: This maintains the structure of the cornea and makes up the majority of its thickness. It is also organized to keep the cornea transparent.
    • Descemet’s membrane: This provides support between the stroma and endothelium.
    • Endothelium: This base layer helps move water in and out of the stroma.

    Iris

    The iris is the colored part of your eye. These are its parts:

    • Anterior surface: This is loose connective tissue that contains melanocytes cells that determine the color of the eye.
    • Stroma: This connective tissue contains vascular tissue as well as melanocytes.
    • Pupillary dilator muscle: This controls when the pupil dilates (widens).
    • Epithelium: This layer is heavily pigmented and found beneath the stroma. This continues into the ciliary body.
    • Pupillary sphincter muscle: This controls when the pupil constricts.
    • Angle formed by the iris and cornea: This drains aqueous humor out of the anterior chamber.

    Ciliary Body

    This tissue divides the posterior chamber of the eye from the vitreous chamber. It also allows the lens to be held in place. Its parts are listed below:

    • Ciliary muscle: This controls the structure of the lens through the zonules. These ligaments fasten the lens to the ciliary body and allow the ciliary muscles to determine how the lens curves, depending on if vision must focus on a near or far object.
    • Ciliary epithelium: This produces the aqueous humor that goes on to fill the anterior chamber. This aqueous humor passes through the pupillary aperture, the opening between the lens and the iris.

    Choroid

    This thin layer of loose connective tissue is found between the sclera and the retina. It contains blood vessels, which bring nutrients to the retina through a layer called the choriocapillary layer. It is separated from the retina by Bruch’s membrane.

    Lens

    The lens is the transparent disc found behind the pupil, which separates the anterior and posterior chambers from the vitreous chamber.

    These are its parts:

    • Subcapsular epithelium: Located just past the lens capsule, this layer allows molecules to move in and out of the lens.
    • Lens capsule: This membrane surrounds the lens and protects it from bacteria or viruses.
    • Lens fibers: The lens is made up of lens fibers, which impact how fast light moves through the lens

    Retina

    The retina is the layer of the eye that turns light into images. The retina is an extension of the optic nerve of the brain, which connects it to our eyes. Its parts are listed below, from outermost to innermost:

    • Retinal pigment epithelium: This is adjacent to the Bruch’s membrane in the choroid. It can absorb light, metabolize vitamin A, transmit nutrients the choroid to the retina, and maintain the blood-retina barrier.
    • Layer of rods and cones: The two types of photoreceptor cells are rods and cones. Rods make up the majority of the photoreceptors in our eye, and they are responsible for black and white vision, making them effective at night. Cones are responsible for bright light and colors, making them effective during everyday situations.
    • External limiting membrane: This connects the base of the rods and cones to Muller cells, which support them and give them structure.
    • Outer nuclear layer: This contains the bodies of the rods and cones.
    • Outer plexiform layer: This contains the synapses between the rods and cones and the horizontal and bipolar cells.
    • Inner nuclear layer: This contains the cell bodies of the horizontal cells, bipolar cells, amacrine cells, and Muller cells.
    • Inner plexiform layer: This contains the synapses between the bipolar cells and the ganglion and amacrine cells.
    • Ganglion cell layer: This contains the ganglion cells, which project images to the brain through the optic nerve.
    • Optic nerve fiber layer: This contains the axons of the ganglion cells.
    • Inner limiting membrane: This separates the retina from the vitreous humor found in the vitreous chamber. It contains Muller cells.

    The central area of the retina is known as the macula, which is responsible for central vision, and the fovea, which is the central area of the macula. It has the highest visual acuity in the eye.

    Optic Disc

    The optic disc is where the axons of the ganglion cells enter the optic nerve.

    Optic Nerve

    The optic nerve is what connects the eye to the brain and allows you to see. It is located at the back of the eye and connects to the retina and optic disc.

    Optic Nerve

    The optic nerve is what connects the eye to the brain and allows you to see. It is located at the back of the eye and connects to the retina and optic disc.

    References

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    16. Anterior Lens Capsule and Epithelium Thickness Measurements Using Spectral-Domain Optical Coherence Tomography. (June 2017). BMC Ophthalmology.
    17. Neuroanatomy, Retina. (August 2021). StatPearls.
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