The iris is the final layer of the uveal layer, & is a thin, contractile, pigmented diaphragm with a central aperture – the pupil. It is suspended in the aqueous humour between the lens & the cornea. It has two layers, the stroma & the epithelial layer (of which there are two). The stromal layer is highly vascular connective tissue & contains fibroblasts, melanocyes, mast cells, lymphocytes, macrophages & matrix, as well as smooth muscle for the sphincter papillae muscle. The anterior epithelial layer is in contact with the stroma & closely associated with the dilator papillae muscle & is continuous with the outer pigmented layer of the ciliary body. The posterior epithelial layer bathes in aqueous humour & is continuous with the inner non-pigmented layer of the ciliary body, & faces the posterior chamber. The iris receives its blood supply from the long posterior ciliary arteries & anterior ciliary arteries, & its function is to dilate/constrict the pupil, to control the amount of light entering the eye. People who have blue eyes have less melanocytes/pigment cells.
The last layer of the eye is the retinal layer, which lies between bruch’s membrane of the choroid & the vitreous body. It’s thickness varies, & is continuous posteriorly with the optic nerve, & anteriorly with the iris & ciliary body. It’s is made up of two layers – the outer pigmented & the inner nervous layers. (The space between the layers is called the intraretinal space). The pigmented layer (RPE) consists of a single layer of cells that extends forward from the optic nerve to the ora serrata anteriorly, & here it continues forward with the continuation of the nervous layer as the pigmented ciliary epithelium. The cells near the optic nerve are tall (columnar) & hexagonal in cross section, but become flattened as they near the ora serrata (cuboidal). They are strongly attached to the choroid & the apical surface faces the inner nervous layer (neural retina). The apical ends of the cell show lots of microvilli (5-7µm long) that project between & surround the outer segments of the rods & cones. The cells are embedded in glycosaminoglycans & joined together at the basal end with zonula adherens which encircles the cell, & in the apical regions by zonula occludens. These tight junctions are important in maintaining the isolation of the retina from the systemic circulation (the blood retina barrier; prevents large toxic molecules from the choroid capillaries from entering the retina, to the photoreceptors). The functions of the pigmented layer include absorbing light, participation in turnover of the outer segments of photoreceptors (except phagocytosis of tips of cones) & the formation of photopigment (made up of opsin & chromophore) by storing & releasing vitamin A). Opsin is rhodopsin for rods, iodopsin is for cones & the chromophore is the light-sensitive part of it, each photo receptor contains around a billion molecules of photopigment. Retinal detachment is where the retina detaches from the choroid; causes a shadow over one eye, floaters but no pain). The neural retina consists of three main groups of neurons: photoreceptors, bipolar cells & ganglion cells. Horizontal & amacrine cells are also present, which modulate their activity. Altogether there are ten layers that make up the retina: RPE, photoreceptors, external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, optic nerve fibre layer & internal limiting membrane. Photoreceptors are of two types: rods & cones. Rods work better in the dark (responsible for dim vision). The total number of rods in the retina is approx 125 million, & are located mostly around the periphery (~30,000/mm² at extreme periphery), they are absent at the fovea. The rod cells are ~110µm long. Their outer segment are layers of flattened discs containing light detecting proteins (continuously shed & replaced at dawn), while the inner segment is made up of the ellipsoid & myoid, & contains organelles. (Ellipsoid contains mitochondria, myoid contains rough & smooth ER, ribosomes & golgi apparatus). The outer & inner segments are connected by the connecting stalk. The bases of the rods are spherules, which contains presynaptic vesicles that synapse with the dendrites of the bipolar cells.
Cones are less sensitive to light than rods & therefore are adapted to bright light & can resolve fine details & colour vision. The measure to be about 70µm in length, & their total number in the retina is ~6 million. Their highest concentration is at the fovea centralis (14700/mm²). There are three types of cone cell which detect different wavelengths of light – S-cone, M-cone & L-cone. Their outer segments are invaginations of the plasma membrane which contain 3 different light-detecting pigments. Their inner segment is similar to rods & have a pedicle base (more stable) which also contains presynaptic vesicles that synapse with the dendrites of the bipolar cells.
Bipolar cells have bipolar axons & have a radial orientation; one or more dendrites of the cells pass outwards to synapse with the photoreceptor cell terminals. The single axon is directed inwards to synapse with amacrine & ganglion cells. There are several different types of bipolar cells – rod bipolar cells connect several rods to 1 – 4 ganglion cells. Flat or diffuse bipolar cells connect many cone cells with many ganglion cells, & midget bipolar cells connect one cone to one midget ganglion cell (direct pathway from the cone to a single optic nerve fibre).
Ganglion cells are multipolar & so named because they resemble cells found in the nervous ganglia, they are the second neurons in the visual pathway, & vary in diameter from 10 – 30µm, most of them are small (midget ganglion cells) but some are large. In most of the retina ganglion cells form a single layer, & the number of layers increases from the periphery to the macula, however they are absent in the fovea. Their axons converge at the exit of the optic nerve & the fibres exit the sclera by piercing the lamina cribrosa, after which they become myelinated.
Horizontal cells are multipolar & close to terminals of rods & cones. They have one long (up to 1mm) & several short processes. The short processes associated with cones have synaptic junctions with seven cone pedicles, whereas the short processes associated with rods synapse with 10-12 rod spherules. The long processes make contact with rods & cones & bipolar cells some distance away.
Amacrine cells are so named because it was initially thought that they had no axons. They have large cell bodies with lots of cytoplasm & lobulated indented nuclei. They are close to the ganglion cells; they’re stimulated by bipolar cells which in turn excite the ganglion cells.
Supporting cells: Müller cells fill most of the space in the neural retina not taken up by neurons. They are an example of neuroglia, which are cells that provide structural & metabolic support for surrounding neurons.
Neuroglia are abundant & diverse, are found in the CNS (oligodedrocyte – makes myelin sheath) & PNS (Schwann cells). (& retina example is Müller cells & astrocytes).
The macula lutea is an oval, yellowish (caused by carotenoid pigment xanthophyll) area at the centre of the posterior part of the retina. It measures about 5mm in diameter & lies about 3mm lateral to the optic disc. Fovea centralis is a depressed area in the centre of the macula lutea & measures about 1.5mm in diameter (floor of the depression is called the foveola). It is depressed because only the photoreceptors (cones) are present here whereas the other inner nerve fibres are located peripherally. The foveola has the most distinct vision as this arrangement allows incoming light to have greater access to photoreceptors. There are no blood vessels at the fovea.
The optic disc lies about 3mm medial to the macula lutea & measures about 1.5mm in diameter. The centre of it is depressed while he edge is slightly raised. In the central depression of the optic disc the central retinal vessels enter & leave the eye & optic nerve fibres exit the eye by piercing the sclera at the lamina cribrosa. This is a weak area & can be made to bulge out by a rise in intraocular pressure (pressure on optic nerve) & intracranial pressure, & this can lead to papilloedema. There are no rods or cones at the optic disc & is therefore insensitive to light – the blind spot. The nerve fibres are myelinated posterior to the optic disc & unmyelinated anterior to it.