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Lecture 4

lecture 4 - lens.docx

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Department
Biological Sciences
Course
BIOC19H3
Professor
Ian Brown
Semester
Fall

Description
Lecture 4 – Lens Development * = added notes Lens development as a model system: (slide 2)  Characteristics of the differentiated lens as an important component of the eye  Developmental biology of the differentiation of the lens in the eye  An example of the differentiation of a highly specialized cell type exhibiting well defined morphological features and packed fill of a unique tissue – specific protein necessary for the functioning of the lens of the eye *Crystalline is a tissue specific protein that gives lens its functional property (focus light), it is a terminally differentiated cell Part of the Eye* Function/description* Cornea Transparent front layer of the eye that helps protect the eye from external matter. It also controls and focuses the entry of light into the eye Pupil Size changes depending on the amount of light that passes through it. Regulates the amount of light that enters. Iris Gives colour to the eye, it also controls the size of the pupil Lens It is responsible for changing the focal distance of the eye. The lens helps focus the light to the back of the eye onto the retina Zonular ligament Holds the lens in place Ciliary body Contains involuntary muscles that connect to the zonular ligament (helps change the shape of the lens) It also produces aqueous humor Lecture 4 – Lens Development * = added notes Vitreous humor Thick gelatinous material that takes up the center of the eye Retina Collects light in order to create an image. It has photoreceptors that helps send signal Choroid Contains a network of blood vessels which nourishes the eye Sclera Tough outer coating (which voluntary muscles are attached – ocular muscles, which control eye movement) Optic nerve Passes the signals from the retina to the brain (electrical impulses) Characteristics of the lens of the eye (slide 4)  Transparent (in order to transfer light), biconvex structure (curved on both sides)  Composed of individual lens cells  Polar structure: anterior (front) surface is less curved than posterior  Function: focuses incoming light onto the retina  In most animals, the lens is fully developed at birth but continues to grow throughout life  Suspended in place by the zonule fibers/ligaments to the muscles of the ciliary body Accomodation – ability to focus the lens (shifting the eye from distant objects to close objects), younger people are able to do this faster than older people (ability declines with age). *Distance decreases refraction (parallel) and the closeness increases refraction. Zonule fibers and Ciliary body – dark lines (slide 6)  Smooth muscles in ciliary body contract and relax by tugging on the zonule fibers  This causes a change in the shape of the lens  Flat front on lens for “far” focusing  Rounded front on lens for “near” focusing Lecture 4 – Lens Development * = added notes Lens nutrition (slide 7)  Avascular structure (no blood vessels) composed of individual cells, relies on diffusion of nutrients and water to maintain growth and transparency  Relative to other tissues, the lens has lower energy requirements  The lens obtains energy by glycolysis  *Lens can last longer in culture because they can survive for a longer period of time (avascular) The top corners consist of the iris (slide 7) *Cellular layers of the lens (slide 8) The 3 parts that make up the lens are Lens capsule, lens epithelium, lens fibre cells The outer layer is made up of the lens capsule ,which is the protective layer of the cell Lens epithelium is found under the capsule The dark figures along the lens epithelium is the nuclei (single cell layer) Lens fiber cells are packed with crystalline Lens Capsule (slide 9)  A smooth transparent layer that completely surrounds the lens  An extracellular matrix composed of collagen and glycosaminoglycans  The lens capsule is synthesized by the underlying lens epithelial layer Lens epithelium (slide 10)  Single layer of cuboidal cells that posses strong intercellular adhesion properties  Firmly attached to the overlying lens capsule layer  Maintains lens osmolarity and volume * Cuboidal cells are box shaped cells that maintain lens osmolarity and volume of differentiated lens cells (have nuclei) Lecture 4 – Lens Development * = added notes Lens Fiber cells (slide 11) Form the bulk of the lens Long, thin, transparent cells Lose their nuclei and organelles with maturation Packed with gamma crystalline *There are three types of genes in adults they consist of gamma, alpha and beta genes Gives lens optical properties *If the cells do have a nucleus they are highly condensed Why is lens Development a good model system? (slide 12) *it was the first tissue that was examined in embryology 1. Avascular tissue 2. Classical object studied in experimental embryology 3. Lens-specific protein: crystalline *(unique marker protein) 4. Simple system 5. Easy separation of developmental stages * Lens is composed of only one cell type (lens cells) * It is similar to an onion – size increases by layers being added to the outside * Without magnesium and calcium the adhesion molecules don’t stick together which causes the outer layer the strip off Morphological Phases of Lens Differentiation (slide 13) 1. Formation of lens vesicle in embryo 2. Differentiation of immature lens cells into mature lens cells Development of the Eye (slide 14) The eye develops from three different components: 1. Neural ectoderm = retina, epithelium of iris and ciliary body 2. Surface ectoderm = lens and epithelium of cornea 3. Mesoderm = sclera, choroid, connective tissue of the cornea, iris and ciliary body, eye muscles Neurulation (slide 15) * Neurulation is a stage of organogenesis in which the neural tube becomes the CNS * The ectoderm will roll in to form the neural groove * The neuroectoderm will then pinch off to form the neural tube * Finally the neural tube will become the brain  Formation of the neural tube (neural ectoderm)  Neural tube eventually becomes the central nervous system (brain and the spinal cord)  Occurs after gastrulation Lecture 4 – Lens Development * = added notes Formation of the lens placode (slide 16) The left and right optic vesicles invaginate from the developing forebrain regions * The neural tube elaborates to form the brain * On the left and right optic vesicle  invagination occurs * The ectoderm close to optic vesicle becomes elongated to become lens placode * They invaginate to form a double wall cup * The inner portion becomes the retina and the outer potion is the pigment epithelial layer Formation of lens vesicle (slide 19) – chick embryo Lecture 4 – Lens Development * = added notes Formation of Lens Vesicle (slide 17) * Starting from the upper left and upper middle it takes about 33 hours, upper right * = 42 h, lower right = 48 h, lower left = 72 h * Presumptive lens ectoderm elongates to form the lens placode * The optic stalk eventually becomes optic nerve * The lens pit invaginates and then pinches off to form the lens vesicle * At about 72 hours the lens is immature (not packed with crystalline) * The lens vesicle gives inductive signals to the ectoderm directly in front to differentiate to form the cornea Formation of Lens Vesicle (slide 18) 1. Optic vesicle approaches surface ectoderm (presumptive lens ectoderm) causing it to thicken (cells elongate due to rearrangement of cytoskeleton) to form the lens placode. 2. The optic vesicle begins to invaginate forming a double layered optic cup (future retina) 3. Lens placode invaginates to pinch off and form the lens vesicle positioned in the mouth of the optic cup 4. Surface ectoderm overlying the lens vesicle will next differentiate into the cornea Lecture 4 – Lens Development
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