Ccc

1. ANATOMY OF EYE
and
DRUG ABSORPTION
Presented by
Sneha A. Chavan
Department of pharmaceutics
M pharmacy 1st year II semester 1

2. Introduction
• The adult eyeball measures about 2.5 cm (1 in.) in diameter.
• Of its total surface area, only the anterior one-sixth is
exposed; the remainder is recessed and protected by the
orbit, into which it fits.
• Anatomically, the wall of the eyeball consists of three layers:
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3. Eyeball
Fibrous
Tunic
Cornea Sclera
Vascular
Tunic
Choroid
Retina
Ciliary
Body
Iris
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4. The External Structure of the Eye
• This includes the
• Eyelashes
• Eyelids
• Cornea
• Sclera
• Conjunctiva
• Tear Gland
• Iris
• Pupil
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6. Eyelashes
• Short, stiff hairs
• Protect the eyeballs from foreign objects, perspiration,
and the direct rays of the sun.
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7. Eyelids (Palpebrae)
• Function :-
i. shade the eyes during sleep,
ii. protect the eyes from excessive light and foreign objects, and
iii. spread lubricating secretions over the eyeballs
• The space between the upper and lower eyelids that exposes the
eyeball is the palpebral fissure .
• Its angles are known as the lateral commissure , which is narrower
and closer to the temporal bone, and
the medial commissure, which is broader and nearer the nasal bone.
• In the medial commissure is a small, reddish elevation, the lacrimal
caruncle , which contains sebaceous (oil) glands and sudoriferous
(sweat) glands. The whitish material that sometimes collects in the
medial commissure comes from these glands. 7

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9. lacrimal glands (lacrim - tears)
• This secretes tears (tears contain salts and a bacterial enzyme
lysozyme) to Wash out foreign particles or chemical irritants.
• Lubricate the conjunctiva which reduces friction between the eyelids
and eyeball.
• Prevent the eye from drying out. The tear film allows atmospheric
oxygen to dissolve and diffuse into the cornea.
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11. Fibrous Tunic
• The fibrous tunic is the superficial layer of the eyeball and consists
of the anterior cornea and posterior sclera.
CORNEA
• corneal diameter is about 11.7 mm with radius of curvature of
anterior surface is about 7.8 mm
• Thickness is 0.5 – 0.7 mm and thicker in center than limbus.
• Clear like glass transparent coat that covers the colored iris. Has no
blood vessels in it. This five-layered membrane is continuous with the
sclera.
• Because it is curved, the cornea helps focus light onto the retina.
• The cornea consists of collagen fibers and fibroblasts.
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12. 12

13. SCLERA ( SCLER- HARD),
• the “white” of the eye, is a layer of dense connective tissue made up
mostly of collagen fibers and fibroblasts.
• The sclera covers the entire eyeball except the cornea; it gives shape
to the eyeball, makes it more rigid, protects its inner parts, and
serves as a site of attachment for the extrinsic eye muscles.
• At the junction of the sclera and cornea is an opening known as the
scleral venous sinus (canal of Schlemm). A fluid called aqueous
humor.
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14. Conjunctiva
• known as mucus membrane.
• This is a transparent membrane that covers the sclera and lines the
inner surface of the eyelids.
• It also secretes mucous and a small volume of tears for the eye to be
moist.
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15. Iris
• Pigmented layer of muscular tissue which gives the eye its
color.
• It consists of melanocytes and circular and radial smooth
muscle fibers.
• The amount of melanin in the iris determines the eye color.
The eyes appear brown to black when the iris contains a large
amount of melanin.
• Blue when its melanin concentration is very low, and green
when its melanin concentration is moderate.
• The iris has two sets of involuntary muscles: the circular and
the radial muscles which control the diameter of the pupil and
hence, the amount of light that enters the eye.
• A principal function of the iris is to regulate the amount of
light entering the eyeball through the pupil.
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16. Pupil
• The Pupil is an opening at the center of the Iris.
• It permits light to enter the eye.
• Responses of the pupil to light of varying brightness.
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17. The Internal Structure of the
Eye
• Includes
1. Choroid
2. Ciliary body
3. Lens
4. Suspensory ligament
5. Retina
6. Optic Nerve
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19. Choroid
• The highly vascularized choroid which is the posterior portion of the
vascular tunic and
• Black-pigmented layer under the sclera that prevents the internal
reflection of light rays.
• Filled with blood capillaries, it is rich in blood vessels that bring
oxygen and nutrients to nourish the eyeball.
• The choroid is modified to form the iris and the ciliary body at the
front of the eye.
• Also contains melanocytes that produce the pigment melanin, which
causes this layer to appear dark brown in color.
• Melanin in the choroid absorbs stray light rays, which prevents
reflection and scattering of light within the eyeball.
• As a result, the image cast on the retina by the cornea and lens
remains sharp and clear.
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21. Ciliary Body
• In the anterior portion of the vascular tunic
• Like the choroid, the ciliary body appears dark brown in color
because it contains melanin-producing melanocytes.
• In addition, the ciliary body consists of ciliary processes and ciliary
muscle.
• The ciliary processes are protrusions or folds on the internal surface
of the ciliary body.
• They contain blood capillaries that secrete aqueous humor.
• Extending from the ciliary process are zonular fibers (suspensory
ligaments) that attach to the lens.
• The fibers consist of thin, hollow fibrils that resemble elastic
connective tissue fibers.
• The ciliary muscle is a circular band of smooth muscle.
• Contraction or relaxation of the ciliary muscle changes the tightness
of the zonular fibers, which alters the shape of the lens, adapting it
for near or far vision. 21

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25. Lens
• The Lens is a transparent, round, biconvex structure.
• It is elastic so that its curvative can be changed to adjust its
refractive or focusing power.
• It's responsible for focusing light onto the Retina.
• Behind the pupil and iris, within the cavity of the eyeball, is the lens.
• Perfectly transparent and lacks blood vessels.
• Held in position by encircling zonular fibers, which attach to the
ciliary processes.
• The lens helps focus images on the retina to facilitate clear vision.
• lens divides the interior of the eyeball into two cavities: the anterior
cavity and vitreous chamber.
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27. Anterior cavity
• The space anterior to the lens — consists of two chambers.
• The anterior chamber lies between the cornea and the iris.
• The posterior chamber lies behind the iris and in front of the
zonular fibers and lens .
• Both chambers of the anterior cavity are filled with aqueous humor
(aqua = water), a transparent watery fluid that nourishes the lens
and cornea.
• Aqueous humor continually filters out of blood capillaries in the
ciliary processes of the ciliary body and enters the posterior
chamber.
• It then flows forward between the iris and the lens, through the
pupil, and into the anterior chamber.
• From the anterior chamber, aqueous humor drains into the scleral
venous sinus (canal of Schlemm) and then into the blood.
• Normally, aqueous humor is completely replaced about every 90
minutes.
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30. Vitreous Chamber
• The larger posterior cavity of the eyeball is the vitreous chamber,
which lies between the lens and the retina.
• Within the vitreous chamber is the vitreous body, a transparent
jellylike substance that holds the retina flush against the choroid,
giving the retina an even surface for the reception of clear images.
• It occupies about four-fifths of the eyeball.
• The vitreous body does not undergo constant replacement.
• It is formed during embryonic life and consists of mostly water plus
collagen fibers and hyaluronic acid.
• The vitreous body also contains phagocytic cells that remove debris,
keeping this part of the eye clear for unobstructed vision.
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32. Cont…
• The pressure in the eye, called intraocular pressure, is produced
mainly by the aqueous humor and partly by the vitreous body;
normally it is about 16 mmHg (millimeters of mercury).
• The intraocular pressure maintains the shape of the eyeball and
prevents it from collapsing. Puncture wounds to the eyeball may
cause the loss of aqueous humor and the vitreous body.
• This in turn causes a decrease in intraocular pressure, a detached
retina, and in some cases blindness.
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33. Suspensory ligament
• A tissue that joins the lens to the ciliary body. This hold the
Lens in place.
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34. Retina
• The third and inner layer of the eyeball, the retina, lines the
posterior three-quarters of the eyeball.
• light sensitive layer of the eyeball, on which images are formed.
• They contain Photoreceptors which are light-sensitive cells.
• The surface of the retina is the only place in the body where blood
vessels can be viewed directly and examined for pathological
changes, such as those that occur with hypertension, diabetes
mellitus, cataracts, and age-related macular disease And are visible
through an ophthalmoscope. 34

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36. Photoreceptors
• Photoreceptors are specialized cells that begin the process by which
light rays are ultimately converted to nerve impulses.
• Each retina has about 6 million cones and 120 million rods.
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37. Photoreceptors
• Rods are responsible to detect light in dim light (black and white)
enabling night vision. They contain a pigment in rods
called Rhodospin which is light sensitive. If exposed to bright light
then it will be bleached. Rhodospin enables a person to see in the
dark.
• Cones are responsible to detect colored light. There are green, blue
and red cones. Red is used to respond to long wavelengths. Blue is
used to respond to short wavelengths and Green is used to respond
to medium lengths. Cones are closely packed to allow the
perception of details but cannot work well in dim light.
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38. Blind Spot
• The point in the Retina where the optic nerve exits the eye.
There are no photoreceptors which makes this Blind Spot
insensitive to light.
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39. Fovea
• A pit in the Retina where images are focused. It has a high
density of cones but it has no rods. The Fovea permits
detailed vision. It is concentrated with photoreceptors
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40. Optic Nerve
• The Optic Nerve is located at the back of the eye. It transmits nerve
impulses from photoreceptors to the brain.
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41. Blood ocular barrier
Blood aqueous barrier
• Ciliary epithelium and
capillaries of iris
Blood retinal barrier
• Non fenestrated
capillaries of the
retinal circulation and
tight junction
between retinal
epithelial cells
preventing passage of
large molecule from
choriocapillaries into
retina 41

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44. Absorption of drug in eye
• Absorption across sclera and conjunctiva into
the intraocular tissue
• Route is not productive
• It restrain entry of drug into aqueous humor
• e.g., inulin
Non
corneal
route
• Absorption of drug into cornea through
aqueous humor
• e.g., pilocarpine
Corneal
route 44

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47. Precorneal constraints
Spillage of drug by
overflow
Dilution of drug by
tear turnover
Nasolacrimal
drainage / systemic
drug absorption
Conjunctival
absorption
Enzymatic
metabolism 47

48. Cont…
1. Spillage of drug by overflow
 Normal volume of tear 7µl. (under normal condition)
 If blinking doesn’t occur the human eye can accommodate 30 µl without
spillage from palpebral fissure.
 With an estimated drop volume of 50 µl
 Due to overflow 70% of administered dose is expelled.
 If blinking occur, then the residual volume left is 10 µl.
( 90% OF DOSE IS EXPELLED )
2. Dilution of drug by tear turnover
• Normal human Tear turnover is approx. 16% per minute.
• Which is stimulated by factor drug entity, pH, tonicity of dosage form.
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49. Cont…
3. Nasolacrimal drainage / systemic drug absorption
• Absorbed across nasal mucosa and git
4. Conjunctival absorption
5. Enzymatic metabolism
• Occur in Precorneal space or in the cornea
• Loss of drug entity that posses labile bond.
• Reduced bioavaibility of instilled dose
• To overcome this, frequent dose of drug at high conc. are
recommended.
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50. Trans corneal penetration
• Trans corneal penetration is affected by
1. corneal barrier,
2. physicochemical property of drug
3. Active ion transport system present at cornea
 Corneal barrier
 Corneal epithelial is barrier to drug absorption into eye.
 Corneal epithelial is more permeable compared to stratum
corneum of skin.
 But relatively impermeable when compared to epithelial
tissue of nasal, intestinal, bronchial, tracheal.
 Protective barrier to ion transport and foreign molecule.
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51. Cont..
 Corneal stroma is highly HYDROPHILLIC tissue containing
mostly water.
 Drug with molecular size upto 500000 can diffuse in normal
stroma.
 This corneal stroma is rate limiting barrier for ocular
absorption of lipophilic drug.
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52.  Physicochemical properties of drug
• Main route for drug to penetrate across corneal epithelial
1. Transcellular pathway (lipophilic drug penetrate through)
2. Paracellular pathway (hydrophilic drug penetrate through)
• Lipophilicity, solubility, molecular size and shape, charge and degree
of ionization affect route and rate penetration in cornea.
• Esterase, peptidase, proteases enzymes present in ocular tissue
including cornea. So, drug metabolized during or after absorption.
• Ocular prodrug is converted into active parent drug by enzymatic
transformation of corneal epithelial. 52

53.  Ion transport
• In addition to, Passive corneal diffusion of ion via paracellular
pathway, ions can be actively transported across corneal epithelium
and endothelium.
• Corneal epithelial contains ionic channel that selective for cation
over anion.
• Also contain anion channel in apical membrane and conductive for
potassium channel in basal cell.
• Sodium penetrate from tears into epithelium via passive diffusion,
but it is actively transported from epithelium to the stroma. 53

54. Non corneal absorption
• Route involve drug penetration across bulbar conjunctiva and
underlying sclera into uveal tract and vitrous humor.
• Important for hydrophilic and large molecule e.g., insulin and p-
aminoclonidine (poor corneal permeability)
• Tight junction of superficial conjunctival epithelium main barrier for
drug penetration.
• Limiting molecular size for conjunctival penetration is between
20000 and 40000.
• Ocularly applied drugs penetrate across the sclera through
perivascular spaces through the aqueous media of gel like
mucopolysaccharide or through empty space within collagen
network.
• Sclera is more permeable in comparison to cornea e.g., sucrose,
inulin, PEG, β blocker.
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55. Reference
• Principles of ANATOMY & PHYSIOLOGY Gerard J. Tortora.
Bryan Derrickson, 13th Edition, John Wiley & Sons, Inc.
• Remington Essential of Pharmaceutics, edited by Linda Felton,
page no. 908 - 911
• http://thevisibleworld.weebly.com/the-internal-structure.html
• http://thevisibleworld.weebly.com/the-external-
structure.html
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