Retina consists of two portions:
- The inner sensory retina, which has two clinically and anatomically important landmarks. These are the fovea centralis, a shallow depression of about 2.5mm in diameter and the macula lutea, a yellow rim surrounding the fovea centralis. The fovea is the area of the retina where vision is the sharpest and is crossed by the visual axis. In this area, the retina is specialized for discrimination of details and colour vision.The site where the axons converge to form the optic nerve is called the optic disc. Because the optic disc is devoid of photoreceptor cells, it is a blind spot in the visual field. In the central region of the retina is a yellowish spot called the macula lutea.
- The outer nonsensory retinal pigmented epithelium. It is a single layer of cuboidal cells extending from the edge of the optic disk to the ora serrata, where it continues as the pigmented layer of the ciliary epithelium. The apical domain of the cuboidal nonsensory pigmented epithelium is sealed by tight junctions to form the external retinal barrier. Granules of melanin are present in the apical cytoplasm and apical cell processes. Melanin granules absorb excess light reaching the photoreceptors. The apical surface contains microvilli that surround the outer segments of the photoreceptors (cones and rods). At this location, the sensory retina and the pigmented epithelium are attached to each other through an amorphous extracellular material, the interphotoreceptor matrix.
There are four cell groups found in the sensory retina:
- The photoreceptor cells (rods and cones). Rods and cones occupy specific regions in the sensory retina. Cones are predominant in the fovea centralis and perceive color and detail. Rods are concentrated at the periphery and function in peripheral and night vision. Both rods and cones are elongated cells with specific structural and functional polarity. They consist of two major segments, an outer segment and an inner segment. The outer segment contains stacks of flat membranous disks harboring a photopigment. The disks are infoldings of the plasma membrane that pinch off as they move away from the modified cilium, the outer-inner segment connecting region.
- The various components of the disks are synthesized in the inner segment and are transported by molecular motors (kinesins and cytoplasmic dyneins) along microtubules toward the outer segment across the narrow cytoplasmic bridge containing the modified cilium. The production and turnover of the disks is continuous. New disks are added near the cilium. Older disks move apically toward the pigmented epithelium of the retina and once they reach the tip of the outer segment, they are phagocytosed by the cells of the pigmented epithelium. The inner segment displays abundant mitochondria involved in the synthesis of adenosine triphosphate (ATP), the Golgi apparatus and rough and smooth endoplasmic reticulum. The modified cilium consists of nine peripheral microtubule doublets but lacks the central pair of microtubules. The terminal portion of the photoreceptors is equivalent to an axon forming synaptic contacts with cytoplasmic processes-neurites-of bipolar cells and horizontal cells.
- There are numerous differences between rods and cones. The outer segment is cylindrical in the rodes and conically shaped in the cones. The rods terminate in a small knob or rod spherule, which contacts dendrites of bipolar cells and neurites of horizontal cells. The cones end in a thicker cone pedicle, which also synapses with bipolar and horizontal cells. The synaptic ending of cones and rodes (spherules and pedicles) contains a synaptic ribbon surrounded by synaptic vesicles. Rods contain the photopigment rhodopsin, which operates during night vision. Cones contain a set of similar pigments referred to as iodopsin, which perceives detail and discriminates colour (blue, green, and red).
- The conducting cells (bipolar and ganglion cells). They conduct the impulse received by the photoreceptor cells. There are two major classes of bipolar cells that can be distinguished, the rod bipolar cells, linked to rod spherules, and cone bipolar cells, linked to cone pedicles. Ganglion cells extend their dendrites into the inner plexiform layer; the axons form part of the optic nerve.
- The association cells (horizontal and amacrine cells). They do not have axons or dendrites, only neuritic processes conducting in both directions. The nuclei of the horizontal and amacrine cells contribute to the inner nuclear layer. Horizontal cells give rise to neurites synapsing with both rod spherules and cone pedicles. Amacrine cells are found at the inner edge of the inner nuclear layer. They have a single neuritic process that branches to link the axonal terminals of the bipolar cells and the dendritic branches of the ganglion cells.
- The supporting cells (Müller cells). Their nuclei are located in the inner nuclear layer. The cytoplasmic processes extend to the outer and inner limiting membrane. The inner limiting membrane represents the basal lamina of the Müller cells and serves to separate the retina from the vitreous body. The cytoplasmic processes of Müller cells fill the spaces between photoreceptors, bipolar, and ganglion cells. At the outer segment photoreceptor contact sites, zonula adherens and microvilli extending from Müller cells stabilize the association between neuronal photoreceptors and glial Müller cells. This contact region is represented by the distinct boundary of the outer limiting membrane. Müller cells maintain the homeostatic functions of the retina and thereby assure correct tissue function.