Middle coat of the eye - lecture notes.pdf

THE MIDDLE COAT OF THE EYE THE UVEA: This vascular layer of they eye lies between the sclera and the retina, and is mainly composed of blood vessels which are in a loose network of connective tissue and rich in pigment cells. It is comparable to the pia mater of the brai...

THE MIDDLE COAT OF THE EYE THE UVEA: This vascular layer of they eye lies between the sclera and the retina, and is mainly composed of blood vessels which are in a loose network of connective tissue and rich in pigment cells. It is comparable to the pia mater of the brain. The uvea resembles the walls of a camera and prevents unwanted light from entering into the optical system, as well as, reducing internal stray light and reflections. It also supplies blood to the eye and facilitates its drainage. There are three parts to the uvea: 1. choroid (vascular membrane) 2. ciliary body 3. iris source: anatomy.iupui.edu/.../lecture.f04/Eyef04/fov.jpg 1 Choroid The choroid lies between the sclera and the retina stretching from the optic disc to the ora serrata. It thus, lines the entire posterior half of the globe, except at the optic nerve and extends into the anterior half as far as the beginning of the ciliary body. It is loosely attached to the sclera. It contains blood vessels and pigment. It is thickest at the posterior pole, about 0.2mm, and thins anteriorly to about 0.1mm. Its blood vessels provide nourishment to the anterior parts of the uvea and outer layers of retina. Its blood is derived from the short and long posterior ciliary arteries, but there are also connections with the anterior ciliary arteries. Drainage takes place via four vortex veins and the ciliary veins. The pressure with these vessels is higher than the IOP and pressure in most other parts of the body. The choroid is also very heavily pigmented in most animals. The pigment is melanin, and melanocytes are found in it. Together with the lipofuscin and melanin in the pigment cell layer of the retina, it acts as a "light trap" in the same way the black matte finish of the inside of a camera does. Absorbing stray light minimizes internal reflections and increases contrast, thus increasing the acuity of vision. If this layer lacks pigmentation, light passing through the retina can be reflected from the inner surface of the sclera, into the light-sensitive rods and cones. This seriously impairs the precision with which the image can be formed. Albinos, individuals who lack the ability to synthesize melanin, usually have poor vision for this reason. The choroid has five parts: i. suprachoroidea: This is a region of loose attachment between the choroid and sclera. It is composed of collagenous and elastic fibres. The long ciliary nerves and arteries move towards the anterior part of the eye through this layer. ii. Haller’s layer: A vascular layer. Contains large size vessels, primarily veins. iii. Sattler’s layer: A vascular layer. Contains small vessels (veins, venules, arteries and arterioles) with many anastomoses with those in Haller’s layer, and between which are many pigment cells (melanin). iv. Choriocapillary layer: A fine meshwork of capillaries primarily involved in nourishment of the outer retinal layer. Particularly dense behind the macula and fovea. Its capillary size is one of the largest in the body, supplied and drained by the choroidal vessel regions. v. Bruch’s membrane: A thin boundary between the choroid and retina, which marks the inner boundary of the choroid. It has an outer elastic lamina and an inner cuticular part which forms a basement membrane for the retinal pigment epithelium. 2  Sattler’s layer of small blood vessels source: education.vetmed.vt.edu/.../EYE/CHOROID.JPG source: medinfo.ufl.edu/year1/histo/images/g24a.jpg 3 Ciliary Body This is an annular portion of the uvea, extending from the ora serrata to the root of the iris. It is roughly triangular in section with the base of the triangle towards the iris. Is contains two main parts. The posterior region, the pars plana and is about 4mm wide. It is also called the orbiculus ciliaris. The anterior portion, called the ciliary corona or pars plicata, is about 2mm wide and is made up of between 70 to 80 radiating ridges, known as the ciliary processes. The ciliary processes are highly vascular and contain small lymphatic spaces and is here that the aqueous humour is produced. It is structurally similar to the choroid but lacks the choriocapillaris and the suprachoroidea is less developed. Source: education.vetmed.vt.edu/.../EYE/CHOROID.JPG The ciliary body is composed of: i. ciliary epithelium ii. stroma iii. ciliary muscle i. ciliary epithelium: The inner surface of the ciliary body is lined on its inner surface with a double layer of cells, the ciliary epithelium, also known as the pars ciliaris retinae. The outer of these two layers is heavily pigmented, the inner one is not. Strictly speaking, the inner layer of the ciliary epithelium (lying next to the aqueous or vitreous) is retinal in origin, in that it is a continuation of the retinal nerve layer, but it is so closely adherent to and functionally integrated with the outer layer. This layer contains 4 organelles and resemble secretory cells elsewhere in the body. The pigmented outer layer is a forward projection of the retinal pigment epithelium and its cells have many organelles. Both the inner and outer layers are involved with aqueous production which fills the anterior and posterior chambers of the eye. Source: education.vetmed.vt.edu/.../EYE/CHOROID.JPG ii. stroma The stroma of the ciliary body, is a loose net rich in pigment (melanin) and blood vessels. The blood vessels are derived from the ciliary arteries which produce an arterial ring around the region at the root of the iris (major iridic arterial circle). iii. ciliary muscle The ciliary muscle arises from the front of the choroid at the ora serrata and runs through the entire ciliary body. It is composed of smooth muscle fibres in two arrangements. The muscle of Bowman and Brucke (longitudinal) lies nearest to the sclera, originating at the ora serrata and orbicularis ciliaris, being inserted into the scleral spur. As the muscle is associated with elastic tissue near the ora serrata, on contraction it causes a 0.5mm forward movement of the ciliary body. Muller’s muscle (circular) lies internally to Brucke’s muscle. They take the form of a ring or sphincter. There is a continuous annulus of muscle surrounding and opposite the equator of the lens. On contraction, the ciliary body draws nearer to the lens. Radial muscle fibres are situated between the about two mentioned parts of the ciliary muscle. The function of the circular fibres is to contract and relax the zonular fibres thereby altering the tension on the capsule of the lens allowing for accommodation. The longitudinal fibres insert into the trabecular meshwork to influence it spore size. 5 Source: eyelearn.med.utoronto.ca/.../images/05Cili2.jpg Source: www.missionforvisionusa.org/anatomy/uploaded_... The Zonules The zonule of Zinn or suspensory ligament attaches the lens to the ciliary body. 6 Source: www.uveitis.org/images/Aqueous_flow.jpg Iris The name Iris, signifies rainbow in Latin or Greek. The iris is a thin disc in the frontal plane, and is the most forward part of the uvea. It is an annular, truncated cone about 12mm in diameter, and is thickest about 2mm from the edge of the pupil at the ‘collarette’, with the thickness decreasing at the root. The iris separates the space behind the cornea into the anterior and posterior chambers, and makes an angle with the cornea known as the filtration angle / angle of the anterior chamber. The periphery of the iris, its ciliary margin, is joined onto the ciliary body by loose connections into the cornea through a pectinate ligament. Source : education.vetmed.vt.edu/.../EYE/CHOROID.JPG The anterior surface of the iris has two zones i.e. the pupillary zone (near the pupil) and the ciliary zone (outer, broader part stretching into the root of the iris). 7 Source: www.rdecom.army.mil/.../cbn_biometric02.jpg The two zones meet as a delicate ‘zig-zag’ line called the collarette or iris frill. This is where the papillary membrane starts in the embryo and there the iris is thickest. The edge of the pupil has a highly pigmented border or ‘ruff’. Around the collarette are many deep spaces called crypts of Fuchs which atrophy in adult life. Iris tissues The bulk of the iris is stroma, made of loose connective tissue. Numerous blood vessels and nerves appear, in a radial arrangement, and become wavy when the pupil dilates. Pigment cells are found in the stroma. They will contain brown pigment (melanophores) or yellow (xanthophores) Anterior to the stroma, lies the anterior layer of finer connective tissue and an incomplete surface endothelium extends over the front surface. This layer is absent in the crypts where the aqueous comes into direct contact with the stromal blood vessels. Neonatal eyes have small amounts of pigmentation thus babies eyes tend to be light blue. As greater amounts of pigment appear, the children’s eyes become darker blue, grey or even brown depending on their future adult pigment density. COLOUR STROMA PIGMENT Blue Fine None in stroma Grey Coarse Little, if any White Very coarse None Green Medium Few xanthophores Yellow Medium Xanthophores Brown / black Dense, velvety Melanophores Pink Variable No pigment in stroma or epithelium Source: Spooner, JD. (1957) Ocular Anatomy. The Hatton Press Ltd: London The rear surface of the iris has a covering of two layers of pigmented epithelium (pars iridica retinae) which is a continuation of the pars ciliaris retinae. Within the stroma, are the sphincter and dilator muscles. The contractile properties of the iris come from the presence of smooth muscle. The sphincter muscle is about 1mm in 8 width and the fibres are arranged in a circle round the pupil. Thus contraction of this muscle produces miosis. This sphincter is under parasympathetic control from the III cranial nerve. The dilator muscle occupies the entire surface of the par iridica retinae. The dilator muscles run radially. It is under sympathetic nerve control and contraction results in pupil dilation. The iris has many blood vessels. Arteries are derived from the major arterial circle in the front of the ciliary body. The minor iridic circle is an incomplete ring of arteries and corresponding veins near the collarette. Nerves accompany the vessels through the various iris structures. source: anatomy.iupui.edu/.../lecture.f04/Eyef04/fov.jpg Source: www.missionforvisionusa.org The pupil The primary role of the iris is to control the size of the pupil. This is an aperture that may be centrally or slightly nasally place. Its diameter is under muscular control, and usually between 2.5 and 4mm. This aperture can be dilated to up to 8mm with mydriatic drugs. There is no difference in size between different coloured irises, or between sexes. Variation does occur with age, with the pupil being relatively small in a newborn and largest in childhood. It diminishes in size with age which is known as ‘senile miosis’. Pupil reactions to light also become slower with age. 9 Nerve supply of the iris The iris is richly supplied by sensory nerve fibres derived from the trigeminal. The sphincter pupillae is supplied by motor nerve fibres derived from the oculomotor nerve while the motor fibres of the dilatator muscle are derived from the cervical sympathetic chain. Blood supply of the Uvea The ophthalmic artery is a branch of the internal carotid artery. After entering the orbit, it divides into numerous branches to supply the globe and orbital structures. One such branch is the ciliary arteries. The short posterior ciliary arteries move through small holes in the sclera and make a ring around the optic nerve, and serve the choroid with blood. This ring is known as the circle of Zinn from which the optic nerve exits and nearby retina tissues are supplied. The long posterior ciliary arteries are two vessels which penetrate the sclera on the temporal and medial sides of the short arteries. They move forwards and produce branches ending in the ciliary body. Here, together with the anterior ciliary arteries, they form the major arterial circle and supply the iris and ciliary body. The anterior ciliary arteries arise from the arteries to the extraocular rectus muscles and from the lacrimal artery. They move through the sclera to the major arterial circle and send branches to the limbus and anastomose with the conjunctival vessels. The uveal veins follow the arteries, and drain the choroids by means of the four vortex veins which leave the eye just behind the equator, each draining one quadrant. Ciliary veins drain the front part of the eye, including the aqueous humour. The vortex veins drain all the blood from the choroid, the iris and the ciliary processes, while the anterior ciliary veins drain the blood from the ciliary muscle and the superficial plexuses. REFERENCES: 1. Spooner, JD. (1957) Ocular Anatomy. The Hatton Press Ltd: London. 2. Miller SJH. (1990) Parson’s diseases of the eye (18th edition). Churchill Livingstone: Edinburgh. 3. Saude T. (1993) Ocular Anatomy and Physiology. Blackwell Scientific Publications: London. 4. Riordan-Eva P and Whitcher JP. (2004) Vaughan & Asbury’s General Ophthalmology (16th edition) McGraw Hill: Boston. 5. Internet: education.vetmed.vt.edu/.../EYE/CHOROID.JPG 10