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.
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.
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.
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.
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.
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.
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.
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.