My Favourite Tissue: The Eye

Fig. 1: The Human Eye

Introduction

The human eye is a complex tissue, and has many fascinating properties. It is used everyday to see the world around us.

Fig 2: A Common Optical Illusion

Our eyes can play many tricks on us, there are "optical illusions" around us everywhere we go. This is one of the main reasons that I chose the eye as my favourite tissue since such a small group of cells and tissues can allow us to see so many neat things! With vision we can see lots of this but we all perceive things differently.


Why do we as humans, need eyes?

I guess in the grand scheme of things, we don't really NEED eyes. Many people do without the ability to see every day. They have adapted to their lack of vision, by compensating with other senses to "see". However for those of us who can see we often take for granted the gift of sight that we have been given.

**FUN FACT**

Around the world, about 39 million people are blind and roughly 6 times that many have some kind of vision impairment.

Fig 3.

BUT for those of us who do have the ability to use our eyes we use them for a variety of reasons. The sense of sight, allows humans to take in light and perceive everything around us. Spatial Vision is the ability to detect patterns and distinguish them from a background and is said to be the most important aspect of vision.

Sight can help us in many circumstances, such as in times of stress. A physiological response to short term stress is for the Pupils to dilate, allowing for as much light as possible to be taken in, so the body can be more alert in its decision to "fight" or "take flight." (Taylor et al., 2012)

Fig 4: Fight or Flight Response

**FUN FACT**

We lose 6 seconds of visual information every minute from blinking - So, during a 150 minute movie, our eyes are shut for about 15 minutes.

HERE is a simplified version of how the eye works.

Basic Anatomy

The Human Eye Ball, is actually spherical, like a ball. Looking at a human face,  you might not expect the eye to be round, but it is! The eye is made up of a variety of structures that aid in support, protection and help it to function!

Fig 5: Basic Anatomy of the Human Eye

The main function of the human eye is to provide us with the sense of sight. 

Light is essential for us to see, but this light enters the eye through the pupil, passing through the cornea, aqueous humour, pupil lens and vitreous humour. (Campbell et al., 2009). The light is refracted as soon as it enters the eye, the cornea and the lens are responsible for bending the light rays to focus on the retina. This creates an inverted image on the retina, which is processed by the bipolar and ganglion cells to the optic nerve and sent to the brain for further processing. ( And to revert the image again. ) The funny thing is is that sight is a process that is LEARNED. (Taylor et al., 2012)

Look HERE for an experiment conducted by BBC on learning to see "upside down."

Anatomy and Histological Components of the Eye

The Human eye is composed of three layers or "tunics":
1. The Fibrous tunic or the sclera
2. The Vascular tunic or the choroid layer
3. The Neural tunic or the retina

Fig 6: Three Layers of the Human Eye

1. The Fibrous Tunic

Fig 7: The Sclera 

The word sclera comes from the greek word "skleros" meaning "hard". This layer of the eye is in fact the "hard," thick white outer layer which gives the eye it's round shape. The cornea, bulges outward from the rest of the sclera and becomes clear. The cornea has a non keratinized squamous epithelium that gives rise to six superficial layers.The cornea is clear due to the thin transparent membrane called the conjunctivia, which is kept moist through the tear glands.

Fig 8: Histological View of the Sclera.

The hard part of the sclera is made primarily up of collagen and elastic fibre. It's primary functions are to protect the eye from environmental factors and to support, providing the eye with constant volume and tone. (Slominka, 2009)

2. The Vascular Tunic

Fig 9: The Choroid Layer, showing the Iris and Ciliary body.

The middle layer of the eye is called the vascular tunic or the choroid layer. It contains the choriod, the cilary body and the iris. 

This layer is coloured and absorbs light and prevents internal reflection. The choroid is made up of loose connective tissue and forms the iris in the front of the eye. ( Because it is highly vascularized the choroid layer creates "Red Eye" in flash photography!) The Cells in the connective tissue contains many melanocytes, giving the choroid it's dark colour.

Fig 10: The iris and its musculature.

The iris is a muscle that adjusts the pupil (aperture in the middle of the iris) to regulate the amount of light that enters the eye at any given point. There are actually two muscles which which regulate pupil size, the dilator pupillae muscle and the sphincter pupillae muscle. Myoepithelial cells lie on the outer layer of the retina which have muscle extensions that form a flat sheet underneath the retina forming the dilator pupillae muscle and The annular sphincter pupillae muscle is formed by smooth muscle cells that are found in the centre of the iridial stroma. The stoma and the anterior border of the iris is what determines eye colour, depending on their pigmentation. If the pigments are low then eyes appear BLUE, if there are intermediate levels they are GREEN or GREY. And heavy pigmentation results in BROWN eye colour. (Slomianka, 2009).

**FUN FACT**

Some people are born with two differently coloured eyes. This condition is called heterochromia.

Fig 11: The Iris, Pupil and The Sclera

The choroid thickens behind the iris to form the ciliary body which contains smooth muscle to control the shape of the lens. This smooth muscle makes up the majority of the ciliary body, the rest is a small amount of loose connective tissue that extends from the choroid.(Slomanka, 2009). The lens is attached to suspensory ligaments attached to the ciliary muscles.

 (Campbell et al, 2009).

Fig 12: Histological View of eye, showing the Choroid and Retina layer, including the blind spot and optic nerve.

Between the choroid and the retina, there are two layers of collagen fibre separated by a network of elastic fibres called Bruch's membrane, also know as the vitreous lamina.(Slominka, 2009.)

3. The Neural Tunic

Fig 13: The Retina, composed of Ganglion, Bipolar and Photoreceptor cells

The outer layer of the retina consists of cuboidal epithelial cells. In this layer of the retina there is a lot of melanin, a pigment which absorbs light not captured by the photoreceptors.

This the innermost layer of the eye, composed of two different types of photoreceptors:

      1. Rods: Very light sensitive; but unable to distinguish colour.

      2. Cones: Require light to function; but can detect colour ( red, green and blue )

Fig 14: The makeup of The light sensitive
cells in the eye, the Rod.

Fig 15: The makeup of the colour detecting
cells of the eye, the Cone.

The fovea centralis is the buildup of cones on the retina and is located directly behind the lens. (This structure is essential to see fine detail!)

Bipolar cells are specialized neurons also found in the retina. They have two processes, one which receives signals from the photoreceptors and the other which conveys synapses to ganglion cells. The ganglion cells contained in the retinal layer transmit action potentials to the brain via the optic nerve. Where the optic nerve penetrates the eye (the retina) there is an absence of photoreceptors, hence the "blind spot." ( Renata, 2013). This is shown above, in Figure 12.

Fig 16: The eye lens fibres tightly packed.

THE LENS is very important in focusing. It consists of a lens capsule, supcapsular epithelium and lens fibres with an absence of blood vessels or nerves. The supcapsular epithelium is linked to a thick basal lamina.

The eye also has two chambers, the (1)Anterior Chamber and the (2)Posterior Chamber.

(1) The anterior chamber lies between the cornea and the lens, filled with aqueous humour.

(2) The posterior chamber is found behind the lens and is filled with vitreous humour which has a gel like consistency helping to maintain the shape of the eyeball.

Fig 17: Overview of Eye Histology

Look HERE for a rundown of eye histology on youtube. 

How cool is it that all of these Structures work together so that we can see, a process which we don't even have to think about!

Disease of the Eye/ Common Pathologies

1. Myopia(nearsightedness) /Hyperopia (farsightedness):

Fig 18: Focusing of the Lens in Near and Far sighted individuals.

Near-sightedness (myopia ) and far-sightedness (hyperopia or hypermetropia) are very common vision problems. They are problems with images not being focused correctly on the back of the eye or on the retina. In Figure 18 the line labeled focal plane shoes where an image would be clear. This is why glasses / corrective lenses are used to change light refraction so that the focal plane can be on the retina, for a clear image to be seen. 

2. Cataract:

Fig 19: Normal eye compared to an eye clouded by a cataract. 

Cataracts are defined by a cloudy lens that impairs vision and progressively gets worse. The natural lens of the eye become cloudy due to a buildup of protein and water that clumps and clouds and area of the lens. Cataracts can be treated with eye glasses, or brighter lighting but the most effective treatment is surgically replacing the cloudy lens with an artificial one. 

3. Astigmatism:

Fig 20: A normal eye compared to Astigmatic Eye

When the cornea of the eye is not completely round with some areas steeper than others images can be blurry or stretched out. This is known as astigmatism since light is bent differently and thus focuses on the retina unevenly. This condition can be detected by a comprehensive dilated eye exam.

4. Glaucoma:

Fig 21:  An Eye with Glaucoma 

This is the leading cause of blindness worldwide. It is progressive damage to the eye, where optic nerve cells are damaged by excess fluid pressure on the eyeball. There are four different types of glaucoma but all result in blindness. Since most people don't know they have it having your eyes check regularly can result in early detection, treatment and saving the eye sight!! ("Common eye disease," 2010)

**FUN FACT**

80% of vision problems worldwide are avoidable or even curable.

Thank-you for visiting my blog!

References

Campbell. , & Reece, (2009). Biology. (8th ed., pp. 1101-1105). San Francisco, CA: Pearson Education Inc.

Common eye disease. (2010). Retrieved from http://www.ottawahospital.on.ca/wps/portal/Base/ 
TheHospital/ClinicalServices/DeptPgrmCS/Programs/EyeInstitute/EyeDiseases

Hill. , Wyse, , & Anderson, (2012). Animal physiology. (3rd ed., pp. 385-395). Sunderland, MA: Sinauer.

Slomianka, L. (2009, August 05). Blue histology - the eye. Retrieved from                       http://www.lab.anhb.uwa.edu.au/mb140/corepages/eye/eye.htm

Taylor, S. E. (2012). Health psychology. (2 ed., p. 223). Canada: McGraw-Hill Ryerson.

[Web log message]. (2013, October). Retrieved from http://mynotes4usmle.tumblr.com/post/55004016035/medicalschool-histology-of-the-human-eye

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