L26 Chronology of Eye Development IB PDF

IBSSD 1525 2024-25 Maria Traka, Ph.D. [email protected] L26: Chronology of Eye Development Reading: Junqueira's Bas...

IBSSD 1525 2024-25 Maria Traka, Ph.D. [email protected] L26: Chronology of Eye Development Reading: Junqueira's Basic Histology (17th ed.), Chapter 23 and Purves Neuroscience (6th ed.), Chapter 11 OBJECTIVES: Be able to describe the formation of the optic vesicle, optic cup and lens vesicle. Be able to describe the formation of the optic stalk and name its adult structure. Be able to describe the derivatives of the optic cup. Be able to describe list the optic structures derived from mesenchyme. Be able to list and describe the adult structures of the eye. Be able to compare and contrast the sphincter pupillae and dilator pupillae mm. Be able to list and describe the function of each of the seven cell types in the retina. Be able to define the terms macula lutea and optic disc. Be able to state the cause of papilledema LECTURE OUTLINE I. DEVELOPMENT OF THE EYE – The eye develops from three embryonic layers: 1) neuroectoderm forms the retina, iris, and optic nerve, 2) surface ectoderm forms the lens, and 3) mesoderm located between these two layers forms the vascular and fibrous coats of the eye. A. Optic cup and lens vesicle 1. On day 22, the developing eye appears as shallow grooves called optic sulci on each side of the forebrain. 2. When the neural tube closes, these optic sulci form outpouchings of the forebrain called optic vesicles. a. Each optic vesicle makes contact with the surface ectoderm, inducing an elongation of ectodermal cells to form the lens placode. b. The lens placode invaginates, and in week 5, separates from the surface ectoderm. The lens vesicle is located at the mouth of the optic cup, and continued differentiation forms the lens. 3. The optic vesicle projects away from the prosencephalon into the surrounding mesenchyme, and the connection of optic vesicle to prosencephalon narrows to become the optic stalk. 4. The optic vesicle invaginates to form a double-layered optic cup. a. The invagination is more extensive inferiorly, and a cleft called the choroid fissure forms. This fissure allows the hyaloid artery to reach the inner chamber of the eye. b. During week 7, the choroid fissure closes, and the mouth of the optic cup forms a round opening that will be the future pupil. © Dr. Maria Traka & MWU 2024 1 B. Derivatives of the optic cup: the retina and portions of the ciliary body and iris. 1. Retina: formed from the two layers of the optic cup a. The outer layer forms a simple, cuboidal, pigmented epithelium. In the posterior retina, the retinal pigment epithelium is immediately adjacent to the choroid of the uvea. b. The inner layer of the optic cup forms the photoreceptive portion of the retina in the posterior 4/5 of the eye. The anterior 1/5 of this layer is non-photoreceptive. c. The intraretinal space between the two layers is obliterated and contains a small amount of fluid (~ 100 μl). NOTE: the approximation of inner and outer layers is a weak connection, and separation of these two layers results in retinal detachment, a potentially blinding condition. 2. Ciliary body: its posterior surface is covered by epithelial layers of the optic cup (neuroectodermal origin). Connective tissue of the ciliary body and the ciliaris muscle are derived from mesenchyme adjacent to the optic cup. 3. Iris: its posterior surface is covered by epithelial layers of the optic cup. Connective tissue of the iris is derived from adjacent mesenchyme. Sphincter pupillae and dilator pupillae muscles are derived from neuroectoderm of the optic cup. 4. Optic nerve: derived from the optic stalk a. When the choroid fissure closes, nerve fibers within the stalk increase in number as a result of the proliferation of the retinal epithelium. b. The hyaloid vessels become trapped within the optic stalk and are now called the central retinal artery and vein. c. The anterior portions of the hyaloid vessels obliterate and become a remnant in the vitreous humor called the hyaloid canal of the vitreous body. d. Myelination of the optic nerve is not complete until the 10th week postnatally. C. Choroid and sclera: formed from the mesenchyme surrounding the optic cup. Sclera is continuous with the dura mater surrounding the optic nerve. D. Cornea and chambers of the eye 1. The optic cup and lens vesicle are embedded in mesenchyme. a. Mesenchyme anterior to the lens vesicle vacuolizes, and these spaces eventually become one large space, the anterior chamber of the eye. b. The anterior chamber splits the mesenchyme into an inner layer in front of the lens and iris, the iridopupillary membrane, and an outer layer continuous with the sclera, the substantia propria of the cornea. c. The iridopupillary membrane obliterates and allows a communication between the anterior and posterior chambers of the eye. 2. The posterior chamber forms in mesenchyme between the future iris and the lens. 3. The cornea develops in conjunction with the formation of the anterior chamber and the eyelids. © Dr. Maria Traka & MWU 2024 2  a. The eyelids form as superior and inferior folds of the surface ectoderm that meet, and for a time, fuse, anterior to the developing eye. The eyelids open at about week 26 prenatally. b. The cornea forms from two sources: 1) Surface ectoderm forms the outer layer of the cornea. 2) Mesenchyme forms the substantia propria of the cornea and an epithelial layer adjacent to the anterior chamber. E. Congenital defects: 1. Coloboma iridis: the choroid fissure fails to close and a cleft persists. The cleft gives the pupil a ‘keyhole’ shape. This defect can affect more than the iris. It can affect the ciliary body, retina, choroid and optic nerve. 2. Persistent iridopupillary membrane: if the iridopupillary membrane fails to resorb completely, fibers may remain. 3. Congenital cataracts: the lens becomes opaque during intrauterine life. This may be genetically determined or the result of rubella infection during weeks 4-7. II. HISTOLOGY OF THE EYE A. Cornea and Sclera: the outermost connective tissue layer. 1. The cornea is transparent, and one of the body’s most densely innervated tissues for pain fibers. The cornea is avascular, and so must receive its nutrition via diffusion. The avascular nature of the cornea serves to antigenically isolate it, thereby allowing successful corneal transplantation. The cornea maintains its transparency by the constant removal of water from corneal surfaces. 2. The sclera covers the posterior 5/6ths of the eye. It is largely collagen and elastic fibers arranged in irregular bundles. The sclera is opaque and has a higher water content than the cornea. B. Lens: an avascular, biconvex structure lying posterior to the iris. It is suspended from the ciliary body by zonular fibers (suspensory ligaments). These zonular fibers help to maintain the ellipsoidal shape of the lens. The thickness of the lens is controlled by the action of muscles in the ciliary body. The lens is elastic is a young person, and beginning at ~age 40, becomes progressively less elastic with the effects of the ciliary muscle having less effect on refraction. The ability of the lens to accommodate is completely lost by ~age 50-55. The condition is corrected optically with bifocal glasses. A clouded or opaque lens is termed a cataract, and the lens is removed surgically. C. Uvea: considered the middle layer of the eye, it is vascular. 1. Choroid: thin layer of connective tissue with a specialized network of large capillaries termed choriocapillaris. 2.Ciliary body: circumferential structure attached to lens by suspensory ligaments and containing ciliary muscle for lens accommodation. © Dr. Maria Traka & MWU 2024 3  3. Iris: pigmented disc with a central aperture (the pupil). Contains smooth muscle fibers that regulate the size of the pupil, and therefore, the amount of light which reaches the retina. The sphincter pupillae muscle is ring-shaped and surrounds the pupillary margin. It is innervated by parasympathetic fibers from the ciliary ganglion, and contraction of the muscle constricts the pupil; miosis. The dilator pupillae muscle is a collection of radially arranged smooth muscle fibers. It is innervated by sympathetic fibers from the superior cervical ganglion, and contraction of this muscle enlarges the pupil; mydriasis. D. Retina: a delicate, transparent tissue measuring 0.4 mm in thickness which covers the inner surface of the posterior wall of the globe. It is an outgrowth of the diencephalon. The smaller anterior portion of the retina lies on the posterior surface of the ciliary body and iris and is not photoreceptive. The larger posterior portion is photoreceptive and will be the focus of the notes. 1. Seven distinct cell types reside in the neural retina: a. Retinal pigment epithelium (RPE): a single layer of cells whose basal surface is in contact with choriocapillaris, and apical surface is in contact with rod and cone outer segments. It has several important functions; absorption of excess light, storage of vitamin A for use by photoreceptor cells in visual transduction, maintenance of nutritional requirements for photoreceptors, and phagocytosis of discs shed by photoreceptors during visual transduction, to name a few. b. Photoreceptors: rods and cones, they constitute the light sensitive neurons of the retina with shapes characteristic of their names. Rods number 130 million/retina while cones number 6.5 million/retina. Therefore, rods outnumber cones 20:1. Rods are sensitive to low levels of light and are responsible for night vision. Thus, at lowest levels of illumination, we see only black and white. During relatively bright light (normal indoor light or sunlight), the visual pigment in rods is saturated and rods do not function. The visual pigment is regenerated under low illumination but takes 5-30 minutes. This is why we are unable to see well initially when going from a brightly lit to dimly lit area. Cones are specialized for acuity at the expense of sensitivity. So, we do not see color at low illumination. Individual cones contain one of three different visual pigments (blue, green, red), and differential stimulation of these three sets of cones results in our ability to see color. Equal stimulation of all cone types produces white light. c. Bipolar cells: dendrites receive impulse transmissions from rods and cones, and the axon of the bipolar cell synapses with a ganglion cell. 10-100 rods may synapse with a single bipolar cell, but only 1 cone will synapse with a single bipolar cell. This means that rods have high convergence and cones have low convergence, and this is the reason cones contribute most to visual acuity. d. Horizontal and Amacrine cells: these retinal association neurons modify visual data from photoreceptors to enhance borders and contours and increase contrast. © Dr. Maria Traka & MWU 2024 4  e. Muller cells: while cell bodies are located adjacent to bipolar cells, cytoplasmic processes extend through the retina to form both the outer and inner limiting membranes. Muller cells are believed to function like oligodendrocytes, and thus provide metabolic support for retinal cells. f. Ganglion cells: last link in the retinal component of the visual pathway. Ganglion cell dendrites receive impulse transmissions from bipolar cells, and each ganglion axon joins the other 1 million ganglion axons to form the optic nerve. 2. Regional distribution of rods and cones: Rods are entirely absent from the central fovea and increase in number toward the peripheral retina. Because rods are important for night vision, faint points of light (stars in the night sky) are best detected by looking slightly away from them. By doing so, the starlight stimulates the peripheral retina where rods are abundant. 3. Cones are densely packed in the central retina and decrease in number rapidly in all peripheral directions. 4. Macula lutea: small, pale yellow, circular area at posterior pole of globe, in direct line with visual axis. It represents the area for “central vision”. If we gaze at a fixation point in the center of our visual field, the image projects to the maculae of both retinas. In the center of the macula lutea is a shallow depression termed fovea centralis. It is the area for sharpest vision and most acute color discrimination. All retinal layers from ganglion cell layer to outer nuclear layer are displaced laterally so that light passes directly to the layer of photoreceptors. The fovea contains only cones. Most foveal cones establish a 1:1:1 synapse with bipolar cells and ganglion cells, and this lack of divergence creates the greatest visual acuity. 5. Optic disc, or papilla: located slightly medial to posterior pole of retina. It is the site of exit of optic nerve fibers. It lacks photoreceptors, and so is also termed “blind spot”. The ophthalmic blood vessels also enter and exit the eye at this site, so it has a pale pink color. 6. As the optic nerve exits the globe, it is invested by all three meningeal layers. Therefore, conditions of increased intracranial pressure, like cerebral edema, will cause CSF to compress the ophthalmic vessels and impede venous drainage. This results in a bulge of fluid at the optic disc, and a condition termed papilledema. E. Chambers: lens, suspensory ligaments and ciliary bodies partition the eye into a large posterior compartment and a smaller anterior compartment. The iris further subdivides the anterior compartment into anterior and posterior chambers. The posterior compartment contains gelatinous vitreous humor that is not replenished. The anterior compartment is filled with watery aqueous humor that is continually replaced. Aqueous humor drains from the eye at the corneoscleral junction, and blockage of this site results in glaucoma. © Dr. Maria Traka & MWU 2024 5