Histology Lecture 5: Special Senses - Eye (2020-2021) PDF

Nervous system module 2020-2021 Histology Lecture: 5 Special senses: Eye By: Dr Safaa Salman [email protected] The eye is a complex highly developed photosensitive organ responsible for light reception. Each eye is located in a protective bony chamber in the skull, the orbit. Hist...

Nervous system module 2020-2021 Histology Lecture: 5 Special senses: Eye By: Dr Safaa Salman [email protected] The eye is a complex highly developed photosensitive organ responsible for light reception. Each eye is located in a protective bony chamber in the skull, the orbit. Histologically, the eye consists of three concentric layers: 1. External (fibrous) layer: the tunica fibrosa. 2. Middle (vascular) layer: the tunica vascoulosa. 3. Internal (nervous) layer: the retina. An eye has three chambers: 1. The anterior chamber lies between the cornea and the iris. 2. The posterior chamber lies between the iris and the lens. 3. The vitreous chamber lies between the posterior surface of the lens and the neural retina. 3 1 2 The Tunica Fibrosa The external layer (tunica fibrosa) is subdivided into sclera and cornea.  Cornea is a transparent layer and covers the anterior one- sixth of the eye.  Sclera covers the posterior five-sixths of the eye. The junction between the cornea and the sclera is called limbus. The Cornea The most anterior part of the eye, a highly transparent convex structure made of 5 layers (anteroposteriorly): ABCDE 1. The Anterior Epithelium (Pavement Epithelium): stratified squamous epithelium (nonkeratinized) consisting of 5-6 cell layers. The basal layer regenerates other cells (corneal epithelial turnover occurs each 7 days) & mitotic figures are seen especially at the perephery. The surface cells have microvilli protruding into the tearfilm. The epithelium has a very rich sensory nerve supply. 2. Anterior limiting (Bowmann's) membrane: the very thick (8-12 µm) basement membrane of the epithelium, consisting of randomly runing collagen fibers, it is responsible for corneal strength. 3. The Corneal stroma (Substantia Propria): about 60 layers of parallel highly organized collagen bundles crossing at right angles to each other. The uniform orthogonal array of these collagen fibrils contributes to the corneal transperacy. Fibroblast-like cells (keratocytes) have flattened cytoplasmic extensions (like butterfly wings) between collagen fibrills, with proteoglycan-rich extracellular substance. Lymphoid cells are seen in the stroma. 4. Posterior limiting (Descemet’s) membrane: thick homogenous layer composed of fine interwoven collagen fibers organized in a 3D network. 5. The Posterior epithelum (Cornael Endothelium): a simple squamous epithelium, with the cells showing the features of active transport & protein synthesis. The cornea is said to have three cellular layers (epithelial layers and stroma). and two noncellular layers (Bowman membrane and Descemet membrane). The Sclera  It is the white part of the eye, an opaque layer of dense connective tissue that protects delicate internal structures and gives the eye its shape.  It is relatively avascular, consisting of: flattened bundles of type I collagen fibers running in different directions parallel to the eyeball surface, moderate amount of ground substance, & few fibroblasts.  The innermost layer of sclera (adjacent to the choroid) is less dense, with thinner collagen fibers, more fibroblasts, melanocytes, & elastic fibers.  The sclera thickens posteriorly, reaching 1 mm at the optic nerve attachment, where it becomes continuous with the epineurium of the nerve. The corneoscleral junction (The Limbus) It is the highly vascularized transitional zone between the cornea & sclera. In the stromal layer of the limbus, there is the scleral venous sinus or Schlemmm's canal, an irregular endothelium- lined space that communicate with the anterior chamber of the eye via many tiny openings at the iridocorneal angle. Schlemm's canal is connected to the venous system & act to drain the aqueous humor to it. The cornea is avascular, it is nourished by diffusion from: 1. Vessels in the limbus. 2. The aqueous humor The Tunica Vasculosa (Uveal Tract) Tunica vasculosa consists of three parts: choroid, ciliary body and iris 1- The Choroid The choroid is a highly vascularized thin layer, with a loose C.T between its blood vessels. The choroid is rich in C.T cells, collagen & elastic fibers & melanocytes (that give the choroid its dark colour). The outer layer of choroid beneath the sclera is the suprachoroidal lamina. The inner layer (the choriocapillary lamina) is richer in small blood vessels and has a major role in the nutrition of the retina, from which it is separated by the hyaline Bruch's membrane. This membrane consists of three layers: elastic fibers network in the middle, & collagen layers on each side. Bruch's membrane is covered externally by basement membrane of choriocapillary vessels, & internally by basement membrane of the pigmented epithelium of retina. Sclera, choroid, and retina. The lateral wall of an eye includes of the Sclera (S) dense connective -tissue choroid (C). loose, vascular connective -tissue With rich melanocytes especially at the suprachoroidal lamina (SCL). choroidocapillary lamina (CCL) Bruch’s layer (B). Between the choroid and the retina is a thin layer of extracellular material. pigmented layer (P) The outer layer of the retina cuboidal epithelium containing melanin. Rods and Cones (R&C), photoreceptor components whose cell bodies make up the outer nuclear layer (ONL). Junctional complexes between these cells and glia are aligned and can be seen as a thin line called the outer limiting layer (OLL). Axons of the rods and cones extend into the outer plexiform layer (OPL) forming synapses there with dendrites of the neurons in the inner nuclear layer (INL). These neurons send axons into the inner plexiform layer (IPL), where they synapse with dendrites of cells in the ganglionic layer (GL). Axons from these cells fill most of the nerve fiber layer (NFL) which is separated by the inner limiting layer (ILL) from the gelatin-like connective tissue of the vitreous body (VB). X200. H&E. 2- The Ciliary Body This middle part of the uveal tract extends from ora serrata to the root of iris. It is a thick ring with a triangular cross section, having one surface in contact with the sclera, one with the vitreous body & the third irregular surface facing the posterior chamber of the eye. Histologically, ciliary body consists of loose C.T rich in blood vessels, elastic fibers & melanocytes. The ciliary body is covered by 2 layers of simple columnar epithelium, both are derived from the retina. The first (inner) layer (directly adjacent to ciliary stroma) consists of melanin-rich (pigmented) cells & represents the anterior continuation of the pigment epithelium of the retina. The second layer covers the first & consists of non-pigmented cells that represent the anterior continuation of the sensory layer of the retina. The ciliary body is divided into: 1. Ciliary muscles: 2 bundles of smooth muscle fibers divided into thick inner circular & thin outer longitudinal layers, they are important in visual accomodation. 2. Ciliary processes: about 75 ridge-like or finger-like projections from the ciliary body, each consists of a loose connective tissue core rich in fenestrated capillaries & covered by 2 layers of epithelium. Ciliary processes serve two functions: 1) Give attachment to the fibers of suspensory ligament of the lens (that extend from the basement membrane of the pigmented epithelium to the capsule of the lens). 2) Secrete the aqueous humor (by the non-pigmented epithelium) into the posterior chamber. Aqueous humor secretion and circulation: Non-pigmented cells of the cilliary processes have tight junctions and extensive basal & Na+/K+-ATPase in their lateral membrane, they filtrate blood in the ciliary process vessels secreting aqueous humor (1). Aqueous humor is similar to plasma but with a minimal amount of proteins. It is secreted into the posterior chamber & flows via the pupil (2) to the anterior chamber to supply the cornea by nutrients & then enters the canal of Schlemm (through the trabecular meshwork in the iridocorneal angle)(3), then drained to the veins in the limbus. 3- The Iris It is the anterior part of the uveal tract (the coloured part of the eye), a disc-like structure attached to the ciliary body peripherally and having a rounded aperture (the pupil) centrally. It has the following layers: 1. Anterior Iridial border ( anterior surface of the iris): this is not covered by epithelium, but formed by a discontinuous layer of fibroblasts & melanocytes, with interdigitating processes giving an irregular, rough grooved appearance. 2. The Iris Stroma (stroma iridis):: a loose C.T with an anterior zone poorly vascularized & rich in fibroblasts & melanocytes, & a highly vascularized posterior zone. 3. Posterior surface of the iris: a smooth surface covered by the same two epithelial layers covering the ciliary body. i. The posterior layer (facing the posterior chamber) cells are heavily pigmented, preventing light from entering the eye except via the pupil. ii. The anterior layer (adjacent to the stroma) consists of less pigmented myoepithelial cells that have radially arranged processes forming the dilator pupillae muscle. Iris The colour of the eye Posterior pigmented epithelium and melanocytes in the stroma of the iris are responsible for the eye colour. People with few melanocytes have blue eyes, people with more melanocytes and collagen in the iris stroma have darker eyes. People with albinism lack pigment in their cells, they have pink eyes from the visible blood vessels of the iris. The Referacive Media of the Eye The eye has four refractile structures: Cornea, Aqueous humor, Lens, and Viterous body. The Lens The lens is a biconvex transparent structure with great elasticity (that decreases with age). The lens has three components: 1. Lens capsule: a thick homogenous refractile external layer that represents the basement membrane of the lens epithelium. 2. Subcapsular epithelium: simple cuboidal or columnar cells lining the anterior half of lens capsule. Cells at the lens periphery divide to give new lens fibers. 3. Lens fibers: extremely elongated highly differentiated epithelial cells that fill the lens. They originate from the subcapsular epithelium and loose their nuclei & organelles to become very long, thin, flattened structures filled with proteins (crystallins). The suspensary ligament of the lens (Zonule): a group of radially oriented fibers extending from the ciliary processes to the lens capsule. Zonular fibers are similar to the microfibrills of elastic fibers. The Vitreous Body It is a transparent gelatinous medium filling the vitreous space between the lens & retina. It consists of water (99%) with hyaluronate & small amounts of collagen. Vitreous body is surrounded by the vitreous membrane, made by type IV collagen. The only cells in the vitreous body are few macrophages & a small number of hyaluronate- producing cells (hyalocytes) near the membrane. The Retina (Nervous Layer) The retina consists of two basic layers: 1) The neural retina or retina proper is the inner layer that contains the photoreceptor cells. 2) The retinal pigementary epithelium (RPE) is the outer layer that rests on and is firmly attached through the Bruch’s membrane to the choriocapillary layer of the choroid. In the neural retina, two regions or portions that differ in function are recognized: 1) The nonphotosensitive region (nonvisual part), located anterior to the ora serrata, lines the inner aspect of the ciliary body and the posterior surface of the iris. 2) The photosensitive region (optic part) lines the inner surface of the eye posterior to the ora serrata except where it is pierced by the optic nerve Types of cells found in retina neurons and supporting cells can be classified into four groups of cells: 1) Photoreceptor cells—the retinal rods and cones 2) Conducting neurons—bipolar neurons and ganglion cells 3) Association neurons and others—horizontal and amacrine neurons (Amacrine cells are interneurons. They are named from the Greek roots a– ("non"), makr– ("long") and in– ("fiber"), because of their short neuritic processes.) 4) Supporting (neuroglial) cells—Müller’s cell The ten layers of the retina, from outside inward, are: 1. Retinal pigment epithelium (RPE). 2. Photoreceptor layer: contains the outer and inner segments of photoreceptor cells. 3. Outer limiting membrane: the apical boundary of Müller’s cells. 4. Outer nuclear layer: contains the cell bodies (nuclei) of retinal rods and cones. 5. Outer plexiform layer: contains the processes of retinal rods and cones and processes of the horizontal, amacrine, and bipolar cells that connect to them. 6. Inner nuclear layer: contains the nuclei of horizontal, amacrine, bipolar, and Müller’s cells. 7. Inner plexiform layer: contains the processes of horizontal, amacrine, bipolar, and ganglion cells that connect to each other. 8. Ganglion cell layer: contains the cell bodies (nuclei) of ganglion cells 9. Optic nerve fibers layer: contains ganglion cells processes that lead from the retina to the brain. 10. Inner limiting membrane: composed of the basal lamina of Müller’s cells The specific arrangement and associations of the nuclei and processes of these cells result in the retina being organized in ten layers that are seen with the light microscope. The Photoreceptors The rods and cones are the outer segments of photoreceptor cells whose nuclei form the outer nuclear layer of the retina. The retina contains approximately 120 million rods and 7 million cones. Functionally, the rods are more sensitive to light and are the receptors used during periods of low light intensity (e.g., at dusk or at night). The rod pigments have a maximum absorption at 496 nm of visual spectrum, and the image provided is one composed of gray tones (a ―black and white picture‖). In contrast, the cones contains a different visual pigment molecule that is activated by the absorption of light at the blue (420 nm), green (531 nm), and red (588 nm) ranges in the color spectrum. Cones provide a visual image composed of color by mixing the appropriate proportion of red, green, and blue light. Each rod and cone photoreceptor consists of three parts: 1. The outer segment of the hotoreceptor is roughly cylindrical or conical (hence, the descriptive name rod or cone). This portion of the photoreceptor is intimately related to microvilli projecting from the adjacent pigment epithelial cells. 2. The connecting stalk contains a cilium composed of nine peripheral microtubule doublets extending from a basal body. The connecting stalk appears as the constricted region of the cell that joins the inner to the outer segment. 3. The inner segment is divided into an outer ellipsoid and an inner myoid portion. This segment contains a typical complement of organelles associated with a cell that actively synthesize proteins.  The outer segment is the site of photosensitivity, and the inner segment contains the metabolic machinery that supports the activity of the photoreceptor cells. The optic nerve The optic nerve is a nerve fiber trunk formed by the convergence of retinal ganglion cell axons at the posterior pole of the eye. From there, they leave the globe on their way to the brain. Each optic nerve contains about one million myelinated axons and even more neuroglial cells. The surface of the optic nerve is covered by pia mater,which is continuous with that on the surface of the brain. The optic disk (optic nerve papilla) is the small circular site in the retina where the retinal nerve fiber layer (nonmyelinated nerve fibers) continues into the optic nerve. The nonmyelinated nerve fibers begin to acquire myelin at the level of the lamina cribrosa (the thin dotted line in the figure) a perforated, sievelike region of the sclera through which optic nerve fibers and blood vessels pass. The myelinated segments of ganglion cell axons, therefore, form the optic nerve. The neuroglial cells include oligodendrocytes, which produce myelin for axons in the CNS, and astrocytes, which perform several nutritive and supportive functions Accessory Structures of the Eye  Conjunctiva The conjunctiva is a thin, transparent mucosa that covers the exposed, anterior portion of the sclera and continues as the lining on the inner surface of the eyelids. It consists of a stratified columnar epithelium, with numerous small goblet cells, supported by a thin lamina propria of loose vascular connective tissue. Mucous secretions from conjunctiva cells are added to the tear film that coats this epithelium and the cornea.  The Eyelids Are mobile folds of tissue that protect the eye. Each eyelid consists of the following layers: 1. Skin: thin, elastic skin with eyelashes at its free margin. 2. Loose C.T containing two muscles: orbicularis oculi and levator palpebri superioris. 3. Tarsal plate: tough plate of dense connective tissue that contains the meibomian glands (long sebaceous glands) 4. Conjunctiva. The eyelid contain four types of glands: 1. Meibomian glands: long sebaceous glands in the tarsal plate. They don't communicate with the hair follicles of eyelashes and produce an oily layer on the surface of the tearfilm preventing its rapid evaporation. 2. Glands of Zeis: smaller modified sebaceous glands connected to the hair follicles. 3. Glands of Moll: spiral sweat glands that open in the hair follicles of the eyelashes. 4. Acessory lacrimal glands of Krause and of Wolfring The Lacrimal Apparatus Lacrimal gland: It is the tear secreting gland, located in the antero-supero-lateral part of the orbit. It consists of several lobes with several ducts that open in the superior conjunctival fornix. The lacrimal gland is tubuloalveolar gland composed of serous columnar cells rich in secretory granules. The secretory portion is surrounded by myoepithelial cells. Tear film moisturizes the front of the eye and is drained by: 1- Lacrimal canaliculi, that begin at two lacrimal puncta (tiny openings at the medial end of the free margin of each eyelid), run medially about 8mm then unite forming one canaliculus (all lined with stratified squamous epithelium) before opening into: 2- Lacrimal sac, which passes tears to the nasal cavity via the: 3- Nasolacrimal duct