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Histology of the Eye II David L. McWhorter, Ph.D. Tunica Interna (Retina) • Inner layer of eye that consists of two portions separated by ora serrata (serrated extremity of optic part of retina) 1. Posterior portion is photosensitive 2. Anterior portion is nonphotosensitive • forms inner lining of ciliary body and posterior part of iris • Posterior part of retina is further divisible into two layers, based on structure, function, and embryonic origin: 1. Pigmented layer 2. Neural layer 1 2 Pigmented Epithelium • Melanin-rich simple cuboidal-tocolumnar epithelium • Basal surface adheres firmly to Bruch's membrane of choroid • Apical microvilli envelop outer segments of rods and cones in overlying neural retina: – Not anatomically joined to photoreceptors, which can lead to detachment of the retina: • Head trauma or other conditions can cause two layers to separate • Photoreceptor cells no longer have access to metabolic support from pigmented layer and choroid and will eventually die • Reattachment with laser surgery is an effective treatment • Diverse functions include: – Serve as part of blood-retina barrier – Absorb light passing through to prevent its reflection – Phagocytose shed components from photoreceptors – Remove free radicals – Isomerize and regenerate retinoids used as chromophores by rods and cones The posterior photosensitive part of retina (neural retina) has three major layers of neurons that receive, integrate, and transmit visual signals to the brain as nerve impulses 1. Outer layer of photoreceptors: 3 – rods – cones 2 2. Intermediate layer of: – bipolar cells that connects to rod and cone cells 1 3. Internal layer of: – ganglion cells that synapse with bipolar cells and send out axons that converge to form the optic nerve 1 Rods and cones are polarized photoreceptors (receptor sensitive to light) located next to pigmented epithelium • Named for the shape of their outer segments: – one pole is a single photosensitive dendrite other pole are synapses with bipolar cells – • • Light must penetrate more superficial layers of retina to reach and stimulate them Each cell has two structurally different segments: 1. 2. Outer segment or specialized dendritic (receptive) part Inner segment 2 1 Two Segments of Rods and Cones 1. Outer segment or specialized dendritic (receptive) part contain stacks of membrane-limited vesicles/saccules with visual pigment Inner segment is rich in polyribosomes and glycogen and is separated from the outer segment by a constriction 2. – – Basal bodies near constriction anchor one or two intracytoplasmic cilia that extend through constriction into outer segment Mitochondria near constriction provide energy for the visual process 2 1 Rod Cells are thin, elongated cells (50 x 3 µm) composed of two portions 1. Outer segment is photosensitive and composed mainly of numerous (600–1,000) flattened membranous disks stacked up like coins: – contain proteins, including the pigment visual purple, or rhodopsin • bleached by light and initiates the visual stimulus 2. Inner segment contains metabolic machinery necessary for cell’s biosynthetic and energy-producing processes (e.g., glycogen, mitochondria, and polyribosomes) • Human retina has approximately 120 million rods • Extremely sensitive to light and are considered to be receptors used when low levels of light are encountered (e.g., dusk or nighttime) Cone Cells are also elongated (60 x 1.5 µm) neurons • Each human retina has about 6-7 million cone cells • Structure similar to rods (elongated with outer and inner segments), but differ: – outer segments shorter and conical – stacked membranous disks remain continuous invaginations of the plasma membrane along one side • Less sensitive to low light • Specialized for color vision in bright light • Three functional types of cones (indistinguishable morphologically) contain variations of visual pigment, iodopsin: – maximum sensitivity is in the red, green, or blue region of the visible spectrum • Are believed to permit better visual acuity than do rods Color blindness is associated with functional deficits in what structures? • Color blindness is associated with the absence of the red-, green-, or bluesensitive photopigments in the cones Phototransduction is a process by which light is converted into electrical signals in the rod and cone cells of the retina • Main parts of process are similar in both rods and cones • Phototransduction begins when light hits the stacked membranous discs (next slide): – Membranes of the stacked membranous discs are densely packed with proteins: • Rhodopsin in rods (~ 1 billion molecules in each rod) • One of the Iodopsin proteins in cones • Each visual pigment (rhodopsin and iodopsin) contains: – a transmembrane protein, opsin – with a bound molecule of retinal, the lightsensitive chromophore • Rhodopsin and each of the three iodopsins: – absorb light most effectively at different specific wavelengths in the visible spectrum In darkness, rhodopsin is inactive • Intracellular concentration of the second messenger, cyclic GMP (cGMP) is high • One effect of cGMP is: – to keep open the abundant cation channels in the cell membrane, and therefore the cell is depolarized: • continuously releasing its neurotransmitter (glutamate) at the synapse with the bipolar neurons When photons of light are absorbed by the retinal of rhodopsin, the molecule isomerizes from 11— cis—retinal to all—trans—retinal • This change activates opsin, which in turn activates: – An adjacent peripheral membrane protein, transducin, a trimeric G protein: • allowing it to release its α subunit, which moves laterally and stimulates another membrane protein, phosphodiesterase: – hydrolyzes cGMP • With less cGMP: – many sodium channels close, producing hyperpolarization of the cell which: • decreases the release of neurotransmitter at the synapses • This change at the synapse: – depolarizes sets of bipolar neurons which then: • send action potentials to the various ganglion cells of the optic nerve: – will allow the brain to produce an image Conformation change induced by light in retinal, which initiates cascade of events producing neural activity, also causes… • Retinal, the chromophore, to dissociate from rhodopsin, leaving a more pale— colored (bleached) opsin • The free retinal all—trans-retinal diffuses into the surrounding pigmented epithelium, where it is converted back to 11-cis-retinal • 11-cis-retinal is then transported back into a rod or cone cell to combine with opsin for another round of phototransduction • Cycle of retinal regeneration and rhodopsin recovery from bleaching may take a minute or more: – Is part of adaptation of eyes that occurs Bipolar cells lie in middle of neural • Comprise two retina populations of interneurons that relay visual signals from photoreceptors to ganglion cells: 1. Each diffuse bipolar cell synapses with: • ganglion cells • two or more photoreceptors (mostly rods) 2. Each monosynaptic bipolar cell synapses with: • a single cone • a single ganglion cell • accounting for cones' greater visual acuity 1 2 Ganglion Cells lie near globe's inner surface and have large cell bodies and nuclei • Their dendrites make synaptic contact with bipolar cells • Axons of ganglion layer project to a specific region of the retina, where they come together to form optic nerve: – This region is devoid of receptors and is called blind spot of the retina (optic disc, optic papilla or optic nerve head) Other Cell Types in Neural Retina • Two minor populations of neurons whose function is thought to integrate visual signals before they reach the brain: 1. Horizontal cells processes terminate near synapses between photoreceptor and bipolar cells Amacrine cells processes terminate near synapses between bipolar and ganglion cells 2. • Muller cells are large, highly branched cells that span neural retina's entire width and embrace processes of retinal neurons: – – Heinrich, German anatomist functionally analogous to neuroglia: • support, nourish, and insulate retinal neurons and fibers 2 1 Fovea (L., pit) or fovea centralis is a specialized area of the retina • Shallow depression on the temporal side of the optic disc at the posterior pole of the optical axis • Contains retina's thinnest region, lies directly opposite the lens center: – Having only cone cells at its center with ganglion and bipolar cells at periphery of depression: • Cone cells in the fovea are long and narrow, resembling rod cells – permits closer packing of cones, increasing visual acuity • Light falls directly on cones in central part of fovea – helps account for extremely precise visual acuity of this region Macula lutea (L., spot + yellow) or Macula surrounds the fovea centralis • All layers of retina are present • Two plexiform layers are rich in various carotenoids: – Give this area its yellowish color – have antioxidant properties and filter potentially damaging short-wavelength light: • helping to protect the cone cells of the fovea Age-related macular degeneration is a leading cause of blindness in elderly individuals • • Causes blindness in the center of the visual field Degenerative changes in the retina around the macula include: – Depigmentation of the posterior epithelium – Focal thickening of the adjacent Bruch’s membrane – Major changes and blood loss in the capillaries in the choroid and retina – Eventual loss of photoreceptor cells producing blind spots • • Appears to be a genetic predisposition to the disorder, along with environmental triggers (e.g., excessive exposure to ultraviolet radiation) Progression of the disease can be slowed by laser surgery to destroy abnormal and excessive retinal capillaries Visual field of Age-related macular degeneration Fundoscopic or Ophthalmoscopic View 1. Fovea ___ 2. Macula ___ 3. Optic disc ___ A C B Vitreous Body is transparent, gel-like connective tissue (mostly water and hyaluronan) that fills the space between lens and retina (vitreous chamber) • Provide structural support to eyeball while offering a clear unobstructed path for light to reach the retina • Contained within a vitreous membrane: – Composed of type IV collagen and other proteins of external laminae • Contains a few macrophages and hyalocytes: – stellate cells with oval nuclei that produce the collagen and hyaluronate Eye Floaters (vitreous opacities) • Age-related changes with the gel-like vitreous body becoming more liquid cause debris within it to clump: – as light passes through the vitreous body, the debris forms shadows on the retina: • appear as black spots, streaks, strings, spider webs, etc. • move about in the visual fields – These are called eye floaters that can become an annoyance: • however, most people learn to disregard them – Usually, they resolve in a few weeks or so • When eye floaters are accompanied by bright flashes of light or fuzzy vision: – an ophthalmologist should be consulted: • may indicate a tear in the retina Lens is a transparent, elastic, biconvex structure immediately behind the iris • Used to focus light on the retina • Nourished by aqueous humor: – it has neither blood nor nerve supply • Suspended by the zonule of the ciliary body behind the pupil • Highly elastic, a feature lost with age • Ciliary muscle contraction changes curvature of lens to enable focus on objects near or far: – a process called accommodation Lens is a transparent, elastic tissue that focuses light on the retina • • • • • • Surrounding the entire lens, the lens capsule (LC) is a thick, homogenous external lamina formed by the epithelial cells and fibers Anterior surface of the lens, beneath the capsule, is covered by a simple columnar lens epithelium (LE) At the equator of the lens, near the ciliary zonule, the epithelial cells proliferate and give rise to cells that align parallel to the epithelium and become the lens fibers Differentiating lens fibers (DLF) still have their nuclei, but are greatly elongating and filling their cytoplasm with proteins called crystallins Mature lens fibers (MLF) have lost their nuclei and become densely packed to produce a unique transparent structure Lens is difficult to process histologically and sections usually have cracks or blebs among the lens fibers Lens Components 1. Lens capsule is an elastic and transparent basal lamina that covers entire lens and prevents wandering cells from penetrating it – – Insertion of zonule fibers from ciliary processes consists mainly of type-IV collagen fibrils and proteoglycans 2. Subcapsular epithelium is low cuboidal epithelium on the anterior lens surface, but increases to columnar near the lens equator, where cell division occurs – Its cells contain few organelles and form the lens fibers 3. Lens fibers are long, narrow, hexagonal, specialized epithelial cells derived from subcapsular epithelial cells that make up most of lens – During differentiation, they lose their nuclei, fill with proteins called crystallins, and develop a variety of specialized plasma membrane components, including junctional complexes and ridgelike processes • Crystallins belong to the family of heat-shock proteins 1 2 3 Accessory Structures of the Eye 1. Conjunctiva 2. Eyelids (palpebrae) 3. Lacrimal apparatus Conjunctiva is a thin transparent membrane comprised of two parts 1. Bulbar conjunctiva: – – nonkeratinized stratified squamous epithelium covers eye's anterior surface (sclera) to the cornea 2. Palpebral conjunctiva: – – stratified columnar epithelium with goblet cells covers inner eyelid surface Conjunctivitis (Pinkeye) is an inflammation of the conjunctiva producing red sclera and discharge • It may be caused by bacteria, viruses, parasites, and allergens • Some forms are contagious and may cause blindness if left untreated Infections near an opening of the tarsal gland are called styes • “eyelid bumps” • Generally caused by Staphylococcus aureus • Most common in infants, but can occur at any age • Can be painful • Can occur in periods of immunosuppression caused by: – Poor nutrition – stress Eyelid • Pliable tissue with skin (S) covering its external surface and smooth conjunctiva (C) lining its inner surface • At the outer rim of the eyelid are a series of large hair follicles (F) for the eyelashes • Associated with these hair follicles are small sebaceous glands and modified apocrine sweat glands • Internally eyelids contain fascicles of striated muscle (M) comprising the orbicularis oculi muscle and closer to the conjunctiva a thick plate of fibroelastic connective tissue called the tarsus (T) • Tarsal plate provides structural support for the eyelid and surrounds a series of large sebaceous glands, the tarsal glands (TG) (aka Meibomian glands), with acini secreting into long central ducts (D) that empty at the free edge of the eyelids Eyelid at higher magnification • Only the inner aspect of the eyelid is seen: – shows that the conjunctiva is a mucous membrane consisting of: • a stratified columnar epithelium with small cells resembling goblet cells and resting on a thin lamina propria (LP) • Large cells undergoing typical holocrine secretion are shown in the tarsal gland acini (TG), and the fibrous connective tissue in the tarsus (T) surrounding the acini: – Sebum from these glands is added to the tear film and helps lubricate the ocular surface Lacrimal Apparatus is a system of glands and ducts, providing tears to lubricate and protect the eyes • • • • • • Lacrimal glands are tear-secreting compound tubuloalveolar glands located superolaterally in the bony orbits Tears are released behind the upper eyelid and flow over the eye's anterior surface Excess tears drain through the lacrimal puncta (one small hole in the free margin of each lid near the medial palpebral angle) Tears enter by capillary attraction and, with the aid of a pumping action provided by the orbicularis oculi muscles, follow the lacrimal canaliculi into a short duct that empties into the lacrimal sac Lacrimal sac delivers tears to the nasolacrimal duct with the aid of gravity Nasolacrimal duct empties through a bony canal into the nasal cavity through the inferior meatus