Eye and ear histology

Definitions & Key takeaways

The eye and ear are both organs of the body that are responsible for receiving sensory information. The eye is responsible for receiving visual information, while the ear is responsible for receiving auditory information.

The eye structures include the eyelashes, lids, muscles, accessory glands, and conjunctiva, and internal structures which include sclera and cornea, the uvea which is the vascular layer in the middle, subdivided into the iris, ciliary body, and choroid; and finally, the retina that consists of nervous tissue.

The ear has three main parts, which are the outer ear, middle ear, and inner ear. The outer ear consists of the auricle, and the external auditory canal and are both lined by keratinized stratified squamous epithelium. The middle ear extends from the tympanic membrane to the oval window of the inner ear, and it is lined with non-keratinized stratified squamous epithelium. Finally, the inner ear consists of the cochlea, semicircular canals and the vestibule.

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Introduction0:00–0:28

Special senses, such as vision, smell, hearing, and balance are senses that use specialized organs and sensory receptors in order to convey information about a person’s surroundings to the central nervous system.
This video will focus on the eyes, which are the sensory organs responsible for vision; and the inner ears, which are the sensory organs responsible for both hearing and balance.The eyes are complex and highly developed photosensitive organs.
Each eye has three concentric layers or tunics that make up the outer portion of the eye. The tough external fibrous layer is made up of both the sclera and transparent cornea.

Anatomy of the eye0:28–1:21

The vascular layer consists of the pigmented iris, choroid, & ciliary bodies. And the innermost sensory layer is formed by the retina.
The optic nerve transmits the signal from the retina to the cerebrum. Even at low magnification, we can see that the inner portion of each eye is split into two main chambers by the lens: the aqueous chamber and vitreous chamber.
The aqueous chamber is filled with a clear fluid called aqueous humor, and the vitreous chamber contains a gelatinous and transparent connective tissue called the vitreous body.
Let’s first take a closer look at the outer fibrous layer or tunic. The cornea covers the anterior 1/6th of the eye and can be further divided into five sub-layers.

Fibrous layer of eye - Cornea1:21–2:40

Starting from the surface of the cornea is the corneal epithelium, then the bowman membrane, stroma, descemet’s membrane, and corneal endothelium along the inner surface.
The corneal epithelium is made of non-keratinized stratified squamous epithelium that stains darker than the remaining layers.
Bowman membrane is composed of collagen fibers and has a thickness between 7-12 µm. When stained with hematoxylin and eosin it can be difficult to identify, but it has a more solid pink appearance when compared to the underlying stroma.
The stroma is the thickest layer of the cornea and is made of collagen fibers and fibroblasts that form a clear and avascular layer of connective tissue that bends or refracts incoming light to help the lens more easily focus light onto the retina.
The descemet’s membrane is the basement membrane that supports the innermost layer, the corneal endothelium. The corneal endothelium consists of a single layer of simple squamous epithelial cells that are in direct contact with the aqueous humor that fills the anterior aqueous chamber.
The posterior 5/6 of the outer fibrous layer is the sclera. The sclera is not transparent and is a very vascular layer of dense and irregular connective tissue.

Fibrous layer of eye - Sclera and conjunctiva2:40–3:10

The anterior portion of the sclera that’s exposed to the air is also covered by a thin and transparent mucous membrane called the conjunctiva.
This membrane also lines the inner surface of the eyelids and consists of stratified columnar epithelial cells as well as small goblet cells.The next main tunic of the eye is the vascular layer.

Vascular layer of eye - Iris and pupil3:10–4:01

The anterior third of the vascular layer is the pigmented iris, which separates the aqueous chamber into two parts, the anterior chamber and posterior chamber.
The opening at the center of the iris forms the pupil.If we take a closer look at the iris, the anterior surface of the iris is lined with simple squamous epithelial cells, but the posterior surface has a double layered epithelium that is highly pigmented so that the only light that enters the inner eye has to travel through the pupil.
The core of the iris is made up of mostly connective tissue and smooth muscle. The smooth muscle makes up the dilator and constrictor muscles that control the amount of light that enters the eye by increasing or increasing the diameter of the pupil.The posterior two-thirds of the vascular layer consists of the choroid, which is very vascular and is another pigmented layer of the eye.

Vascular layer of eye - Choroid4:01–4:48

The pigmentation in this layer helps absorb extraneous light that would hinder the retina’s ability to accurately detect incoming light rays.
The bruch membrane is the thick basement membrane that separates the choroid from the retina. In this image, the multi-layered retina is seen to the left of the choroid and on the right we can see the connective tissue of the sclera that surrounds the choroid.
The majority of the choroid is composed of loose connective tissue with many blood vessels that supply nutrients to the retina.
The blood vessels in this layer are also the reason why eyes sometimes look red in photographs of people taken with a flash.The ciliary body, a significantly thicker portion of the choroid that’s found at the junction between the iris and choroid.

Vascular layer of eye - Ciliary body4:48–5:50

The ciliary body is thick because it contains a ring of smooth muscle called the ciliary muscle. On the surface facing the lens, there are ciliary processes are the projections on the ciliary body that extend towards the lens.
These processes have an inner layer of pigmented cells with an outer layer of unpigmented simple columnar epithelial cells that secrete aqueous humor into the posterior chamber.
The unpigmented epithelial cells are contiguous with the retina. Zonular fibers are the ligaments that attach the choroid to the capsule of the lens.
These fibers keep the lens suspended in the eye and also allows the ciliary muscle to control the shape of the lens, which allows the eyes to focus on both near and far objects.
In histologic images, the zonular fibers can be difficult to see and oftentimes only fragments of the fibers are present after slide preparation.The sensory layer is the innermost layer or tunic of the eye that consists of the retina.

Sensory layer of eye - Retina5:50–8:53

The retina has two main parts, the inner neural layer or neural retina, which is the photosensitive portion of the retina that can detect light, and an outer retinal pigment epithelium or RPE, which consists of a single layer of pigmented simple cuboidal cells that help absorb extraneous light and also provide vitamin A for the neighboring photoreceptor cells.
The ora serrata is the junction between the photosensitive part of the retina and a noticeably thinner non-photosensitive part of the retina that is found in the most anterior part of the retina.
The anterior retina is thinner because it does not have a neural layer and only consists of the retinal pigment epithelium.
Moving even further anteriorly, the non-photosensitive layer of cells continues as part of the epithelium that lines the ciliary body and posterior surface of the iris.
Now, if we take a closer look at the neural retina, we can see that it’s actually composed of 10 sub-layers. The bottom or outermost layer is the retinal pigment epithelium, then a thicker and eosinophilic layer of rods and cones; a very thin outer limiting membrane; a thick and basophilic outer nuclear layer; the outer plexiform layer; another thick and basophilic inner nuclear layer; the inner plexiform layer; a third basophilic and thick ganglion cell layer; a nerve fiber layer; and finally a very thin inner limiting membrane.
Three of these layers stand out because of the prominent basophilic or purple nuclei that make up the majority of these layersThe outer nuclear layer contains the cell bodies and nuclei of photoreceptor cells, including both the rod and cone cells.
The inner nuclear layer contains the cell bodies and nuclei from various neurons, which include bipolar, amacrine, and horizontal cells.
And the ganglion cell layer contains the cell bodies and nuclei of ganglion neurons. The ganglion cell layer varies in thickness and will be the thickest near the central area of the retina called the macula.
The very long axons coming from the ganglion cells converge to form the optic nerve and also make up the nerve fiber layer that’s immediately above the ganglion cell layer.
The inner limiting membrane is the basement membrane that forms the inner surface of the retina. In between the three layers that contain nuclei, are the inner plexiform layer and outer plexiform layer.
These two layers are fibrous or “plexiform” layers that only contain neuron synapses between axons and dendrites. Moving deeper to the layer of rods and cones, we can see that the individual rod and cone cells are aligned perpendicular to the layer itself.
The axons from these cells actually extend up into the outer plexiform layer where they synapse with dendrites from the inner nuclear layer.
Alright, let’s now move to the inner or internal ear, which is located within the temporal bone and detects sound, movement, and position of the head.

Anatomy of Inner ear 8:53–9:36

The inner ear consists of fluid filled sacs that make up a membranous labyrinth that can be split into three main parts, the cochlea, vestibule, and semicircular canals.
The vestibule and semicircular canals together make up the vestibular apparatus. The cochlea is the spiral structure that detects sound.
The vestibule contains sensory receptors that can detect the static position of the head, as well as linear movement. And the semicircular canals consist of 3 semicircular ducts that detect rotational movements of the head.

Cochlea9:36–10:32

The cochlea consists of a single coiled tube that spirals approximately 2.7 times in a cone-like shape. The cochlea’s tube is divided into three compartments by two ribbon-like membranes that run the length of the tube.
The upper membrane is the vestibular membrane and the lower membrane is the basilar membrane. The upper compartment is called the scala vestibuli, the middle compartment is the scala media or cochlear duct, and the lower compartment is the scala tympani.
Both the scala vestibuli and tympani are filled with extracellular fluid called perilymph, whereas the scala media contains endolymph.
If we take a look at the very end or apex of the cochlear compartments, we can see that the scala vestibuli and tympani are no longer separated by a membrane, which allows the perilymph to flow between these two compartments.
This passageway is called the helicotrema.The organ of corti is the sensory portion of the cochlea that’s able to convert sound vibrations to nerve impulses.

Organ of corti10:32–11:30

At high magnification, we can see many of the cells and structures that make up the organ of corti, including the long tectorial membrane on the top, the outer hair cells, inner hair cell, and supporting cells.
In the bottom right of the image, we can also see part of the cochlear nerve. All these structures are within the scala media and rest on the basilar membrane.
The cochlear nerve carries nerve impulses received by hair cells to spiral ganglia. If we zoom out, we can see that both the cochlear nerve and spiral ganglia are found within a central bone that acts as the central axis for the cochlea.
This spongy bone is called the modiolus. The nerve fibers exit the modiolus and form the cochlear branch of cranial nerve VIII.
Alright, let’s now take a closer look at the vestibule, which is the irregularly-shaped central cavity that contains two interconnected membranous labyrinths called the utricle and saccule.

Vestibule11:30–13:01

Each labyrinth forms a sac that’s lined with a simple squamous epithelium. Both the utricle and saccule have a specialized sensory region called a macula that’s able to detect linear movement and static position of the head by sensing the movement of the endolymph within their sac.
If we take a closer look at the utricular macula, we can see the sensory epithelium that consists of hair cells with stereocilia and a basal layer of supporting cells beneath them.
Underneath the sensory epithelium is a thick layer of connective tissue that also contains the nerve fibers coming from hair cells.
Above the hair cells is a layer of gelatinous material that doesn’t stain well called the otolithic membrane. The outer surface of this membrane has crystal-like structures that stain darker than the rest of the membrane called otoliths or otoconia.
These otoconia use gravity to help determine the head’s position. The otoconia are heavier than the endolymph, so when the head changes position, for example, when the head tilts forward, the otoconia shifts anteriorly, pulling the otolithic membrane and stereocilia of the hair cells in the same direction.
The hair cells detect the change in position and send nerve impulses that travel through the vestibular branch of cranial nerve VIII to reach the brain.
Now, if we take a look at the semicircular canals, each canal has an expanded area at the end called the ampullae. Each ampulla has a raised portion of their wall that forms a ridge called the crista ampullaris.

Semicircular canals13:01–13:45

Similar to the maculae found in the vestibule, the crista ampullaris consists of hair cells with a base or basal layer of supporting cells.
The most notable difference is that the gelatinous material covering these hair cells form a dome-shaped structure called the cupula.
When the head rotates, the fluid endolymph that fills the semicircular canals doesn’t rotate or move as fast as the rest of the head.
As a result, the endolymph pushes the cupula in the opposite direction of the rotating head, allowing the hair cells to detect the