EYE: THE PHOTORECEPTOR SYSTEM. Prof. Dr. Huda Al Khateeb

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2 EYE: THE PHOTORECEPTOR SYSTEM Prof. Dr. Huda Al Khateeb

3 Lecture 1 The eye ball

4 Objectives By the end of this lecture the student should: 1. List the layers and chambers of the eye ball 2. Describe the sclera 3. Name and describe the layers of the cornea 4. Define parts of uvea 5. Discuss the choroid, ciliary body and iris 6. Outline the lens and its parts

5 EYE eye The is a complex and highly developed photosensitive organ that analyses light reflected from objects, providing the sense of sight..

6 EYE The eye is composed of 1. eye ball-(about 22 mm in adults) 2. its protective structures What are they?

7 EYE The protective structures of the eye ball are: 1. The bony orbit 2. The adipose tissue between eye ball and orbit 3. the eyelids 4. the conjunctiva 5. the lacrimal apparatus HOW THEY PROTECT THE EYE BALL???????

8 EYE Eye ball consist of three layers 1. Outer layer (Sclera and cornea) 2. Middle layer (Uvea) 3. Inner layer (Retina)

9 Chambers Inside the eye ball there is three chambers 1. Anterior chamber- contains aqueous humor 2. Posterior chamber- contains aqueous humor 3. Vitreous cavity- contains vitreous humor

10 Lens The lens is placed in front of vitreous cavity and posterior to posterior chamber

11 Sclera It is a white, fibrous, external layer of the eyeball that protects the more delicate internal structures and provides sites for muscle insertion It makes the white posterior five-sixths of the external layer of the outer layer of the eye ball

12 Sclera The sclera is relatively avascular, and consists of dense connective tissue containing flat type I collagen bundles Tendons of the extra-ocular muscles that move the eyes insert into anterior areas of the sclera Posteriorly the sclera joins with the epineurium covering the optic nerve

13 Cornea The cornea is avascular, transparent and the most anterior part of the eye ball. It lacks blood and lymphatic vessels It is very sensitive because it is rich in nerve endings.

14 Cornea The cornea consists of five layers Corneal epithelium Bowman s membrane Corneal stroma (90 %)- (substantia propria) Descemet s membrane Corneal endothelium single layer

15 Cornea Corneal epithelium is stratified squamous non keratinized. The surface cells (apical layer) have microvilli that retain a film of tears that keeps the anterior surface of cornea wet

16 Cornea The basement membrane of this epithelium is very thick (8 12 µm) and contributes to the stability and strength of the cornea, helping to protect against infection of the underlying stroma

17 Cornea The corneal stroma is transparent It represents 90% of the corneal thickness Composed of type I and V collagen fibers that are regularly arranged. Fibers and layers are separated by extracellular matrix rich in prteoglycans containing chondrotin and keratin sulfate

18 MEDICAL APPLICATION The curvature of the cornea can be changed surgically to improve visual abnormalities involving the ability to focus. In the common ophthalmological procedure (LASIK surgery), where the stroma reshaped by laser. The relatively rapid regenerative response reestablishes normal corneal physiology.

19 Cornea Descement membrane is a very thik basement membrane. It is produced by the corneal endothelium and contains type VII collagen

20 Cornea The corneal endothelium lines the posterior surface of the cornea and faces the anterior chamber of the eye. It consists of single layer of squamous epithelial cells.

21 Cornea The cornea can be transplanted without the risk of rejection..why?? The cornea is very sensitive.why?? The corneal anterior surface is always wet WHY?? Corneal epithelium has a remarkable wound healing capacity...why???

22 Limbus Encircling the cornea is the Corneo-scleral junction, or Limbus, a transitional area where the transparent stroma merges with the opaque sclera. This region has micro-vasculature which, along with aqueous humor in the anterior chamber, provides metabolites for the corneal cells by diffusion Discuss the modes of nutrition of cornea?????

23 Canal of Schlemm It is a circular canal that lie in the corneo-scleral junction (limbus). It drains the aquoes humor from the anterior chamber through the meshwork trabeculae into the veins that drain the eye ball

25 Middle layer: UVEA Vascular Layer It consists of three parts, from posterior to anterior: 1. the choroid 2. the ciliary body 3. the iris

26 The choroid It is a highly vascular layer in the posterior two-thirds of the eye, with loose connective tissue rich in collagen and elastic fibers, fibroblasts, melanocytes, macrophages, lymphocytes, mast cells, and plasma cells. The abundant melanocytes give the layer its characteristic black color. they block light from entering the eye except through the pupil

27 The ciliary body It is an anterior expansion of the choroid. It is a thickened ring of tissue lying just inside the anterior portion of the sclera. In transverse section the ciliary body is roughly a triangle, with its long base contacting the sclera, another side in contact with the vitreous body, and the third facing the posterior chamber.

28 The ciliary body It is composed of Ciliary muscle (smooth muscle) that is surrounded by a stroma of loose connective tissue, rich in microvasculature, elastic fibers, and melanocytes,

The ciliary body The ciliary muscle contraction (in response to parasympathetic nerves) decreases the internal diameter of the ciliary body ring, reducing tension on the fibers that run from

29 The ciliary body The ciliary muscle contraction (in response to parasympathetic nerves) decreases the internal diameter of the ciliary body ring, reducing tension on the fibers that run from this body to the lens. This allows the lens to become more rounded and better focus light from nearby objects onto the retina. The ciliary muscles are therefore important in visual accommodation

30 The ciliary body The surfaces of the ciliary body that face the vitreous body, posterior chamber, and lens has the ciliary processes (a series of about 75 radial ridges)

31 The ciliary body The surface epithelial cells is connected to the lens capsule by zonular fibers (suspensory ligament of the lens.)

32 The ciliary body Both ciliary body and processes are covered by a double layer of low columnar epithelium, the Ciliary epithelium. The epithelial cells directly covering the ciliary stroma are rich in melanin (Pigmented Layer). The surface layer of cells lacks melanin (Secretory Layer) (Non-pigmented layer)- secrets the aquous humor to the posterior chamber.

The ciliary body Secreted into the posterior chamber, aqueous humor flows between the lens and the iris to reach the anterior chamber through the pupil.

33 The ciliary body Secreted into the posterior chamber, aqueous humor flows between the lens and the iris to reach the anterior chamber through the pupil. The aqueous then flows into the channels of the trabecular meshwork at the corneo-scleral junction then to the Canal of Schlemm then drained to scleral venous sinus.

scleral venous sinus, intraocular pressure can increase, causing the condition called

34 MEDICAL APPLICATION Aqueous humor is produced continuously. If its drainage is impeded, typically by obstruction of the trabecular meshwork or scleral venous sinus, intraocular pressure can increase, causing the condition called glaucoma. Untreated glaucoma can cause pressing of the vitreous body against the retina, affecting visual function.

35 Iris It is the most anterior extension of the uvea (middle layer) that partially covers the lens, leaving a round opening in the center called the pupil.

36 Pupil Pupil

37 Iris: Flowers

38 Iris: Goddess of Rainbow

39 The anterior surface of the iris is covered by irregular, discontinuous layer of fibroblasts and melanocytes, densely packed and with interdigitating processes.

40 Iris The stroma is composed of loose connective tissue with microvasculature, dilator and constrictor pupillae muscles. The dilator and sphincter muscles of the iris have sympathetic and parasympathetic innervation, respectively, for enlarging and constricting the pupil. The posterior surface of the iris is composed of twolayered epithelium (both are pigmented epithelium).

42 Iris color Melanocytes of the iris stroma provide the color of the eyes. In individuals with very few lightly pigmented cells in the stroma = blue color is iris surface. As the number of melanocytes and amount of collagen increases in the stroma, the iris color changes through various shades of green, gray, and brown.

43 Iris Individuals with albinism have almost no pigment and the pink color of their irises is due to the reflection of incident light from the blood vessels of the stroma.

Lens The lens is a transparent biconvex structure immediately behind the iris, used to focus light on the retina. The lens is a unique avascular tissue.

44 Lens The lens is a transparent biconvex structure immediately behind the iris, used to focus light on the retina. The lens is a unique avascular tissue. It is highly elastic, a feature that is lost with age as lens tissue hardens. The lens has three principal components (capsule, anterior epithelium and lens fibers).

45 Lens Lens Capsule The lens is covered by a thick, homogeneous capsule rich in proteoglycans and type IV collagen. The lens capsule protects the underlying cells and provides the place of attachment for zonular fibers

Lens Lens Epithelium Subcapsular lens epithelium consists of a single layer of cuboidal epithelial cells and is present only on the anterior surface of the lens.

46 Lens Lens Epithelium Subcapsular lens epithelium consists of a single layer of cuboidal epithelial cells and is present only on the anterior surface of the lens. The basal ends of the epithelial cells attach to the lens capsule and their apical surfaces have interdigitations that bind the epithelium to the internal lens fibers..

47 Lens Lens fibers At the posterior edge of this epithelium, near the equator of the lens, the cells divide to provide new cells that differentiate as lens fibers. This process allows for growth of the lens and continues at a slow, decreasing rate near the equator of the lens throughout adult life

48 Lens Lens Fibers Lens fibers are highly elongated and appear as thin, flattened structures. Differentiating lens fibers eventually lose their nuclei and other organelles, fill the cytoplasm with a group of proteins called crystallins, and become very long.

Lens The lens is held in place by a radially oriented group of fibers, the elastic ciliary zonule, which inserts on both the lens capsule and on the ciliary

49 Lens The lens is held in place by a radially oriented group of fibers, the elastic ciliary zonule, which inserts on both the lens capsule and on the ciliary body. This system is important in the process known as accommodation, which permits focusing on near and far objects by changing the curvature of the lens..

50 Lens When the eye is at rest or gazing at distant objects, the lens is kept stretched by the zonule in a plane perpendicular to the optical axis. To focus on a near object, the ciliary muscles contract. This relieves some of the tension exerted by the zonule on the lens, allowing the latter to round up and become thicker, keeping the object in focus

This is a normal process (presbyopia, Gr.

51 MEDICAL APPLICATION Advancing age reduces the elasticity of the lens, making accommodation for near objects difficult. This is a normal process (presbyopia, Gr. eyes of elders), which can be corrected by wearing glasses with convex lenses (reading glasses).

When areas of the lens become opaque or cloudy and vision is impaired, the condition is termed cataract.

52 MEDICAL APPLICATIOn In old individuals, denaturation of crystallins commonly begins to occur in lens fibers, making them less transparent. When areas of the lens become opaque or cloudy and vision is impaired, the condition is termed cataract. Other causes of cataract include exposure to ultraviolet light or other radiation, trauma, and as secondary effects in diseases such as diabetes mellitus and hypertension.

53 Vitreous Body The vitreous body occupies the vitreous chamber behind the lens. It is composed of transparent connective tissue containing mostly (99%) water (vitreous humor)

54 summary Eye ball consist of three layers and three chambers Cornea has five layers Uvea composed of choroid, ciliary body and iris Lens

55 Quiz A 56-year-old woman complains of decreased vision in her left eye of 4 years duration. White discoloration of her left pupil is noted on physical examination (shown in the image). The white appearance of the pupil in this patient represents a pathological change affecting: 1. Conjunctiva 2. Cornea 3. Iris 4. Lens 5. Vitreous body

1. Introduction to Anatomy of the Eye and its Adnexa

1. Introduction to Anatomy of the Eye and its Adnexa Fig 1: A Cross section of the human eye. Let us imagine we are traveling with a ray of light into the eye. The first structure we will encounter is