lens (vertebrate anatomy) lens (vertebrate anatomy) the lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. along with the cornea, aqueous and vitreous humours it refracts light, focusing it onto the retina. in many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. this adjustment of the lens is known as accommodation (see also below). in many fully aquatic vertebrates such as fish other methods of accommodation are used such as changing the lens's position relative to the retina rather than changing lens shape. accommodation is analogous to the focusing of a photographic camera via changing its lenses. in land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical. lenslens of eye changing shape to focus near and far.schematic diagram of the human eye.detailspart ofeyeballsystemvisual systemfunctionrefract lightidentifierslatinlens crystallinmeshd007908ta98a15.2.05.001ta26798fma58241anatomical terminology accommodation in humans is well studied and allows artificial means of supplementing our focus such as glasses for correction of sight as we age. the refractive power of a younger human lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power of about 60 dioptres. by 25 years of age the ability of the lens to alter the light path has reduced to 10 dioptres and accommodation continues to decline with age. structure position in the eye the lens is located towards the front part of the vertebrate eye called the anterior segment which includes the cornea and iris positioned in front of the lens. the lens is held in place by the suspensory ligaments (zonule of zinn), attaching the lens at its equator to the rest of the eye through the ciliary body. behind the lens is the jelly-like vitreous body which helps helps hold the lens in place. at the front of the lens is the liquid aqueous humor which bathes the lens with nutrients and other things. land vertebrate lenses usually have an ellipsoid, biconvex shape. the front surface is less curved than the back. a human adult the lens is typically about 10mm in diameter and 4mm thick though changes shape with accommodation and size due to grow throughout a person's lifetime. anatomy sheep eye lens para-formaldehyde fixed front view. small lenses are about 1cm in diameter. small bumps at edge are remnants of suspensory ligamentssheep lens fixed side view. note the largest lens has damaged capsule and iris attachedmicroscope image of lens cell types and capsulelens diagram with photosthe lens has three main parts: the lens capsule, the lens epithelium, and the lens fibers. the lens capsule is a relatively thick basement membrane forming the outermost layer of the lens. inside the capsule much thinner lens fibers form the bulk of the lens. the cells of the lens epithelium form a thin layer between the lens capsule and the outermost layer of lens fibers at the front of the lens but not the back. the lens itself lacks nerves, blood vessels, or connective tissue.anatomists will often refer to positions of structures in the lens by describing it like a globe of the world. the front and back of the lens are referred to as the anterior and posterior "poles", like the north and south poles. the "equator" is the outer edge of the lens often hidden by the iris and is the area of most cell differentiation. as the equator is not generally in the light path of the eye the structures involved with metabolic activity avoid scattering light that would otherwise affect vision. lens capsule main article: capsule of lens sheep lens capsule removed. decapsulation leads to a nearly formless blob.the lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. the capsule is elastic and its main structural component is collagen. it is presumed to be synthesized by the lens epithelium and its main components in order of abundance are heparan sulfate proteoglycan (sulfated glycosaminoglycans (gags)), entactin, type iv collagen, laminin. the capsule is very elastic and so allows the lens to assume a more spherical shape when the tension of the suspensory ligaments is reduced. the human capsule varies from 2 to 28 micrometres in thickness, being thickest near the equator (peri-equatorial region) and generally thinner near the posterior pole. specialist area of mouse capsule forming part of the zonule of zinnthe photo from an electron microscope shows an area of the capsule near the equator where one of the thousands of suspensory ligaments attach. attachment must be strong enough to stop the ligament being detached from the lens capsule. forces are generated from holding the lens in place and added to when focusing. the anterior and posterior capsule is thinner. lens epithelium the lens epithelium is a single layer of cells at the front of the lens between the lens capsule and the lens fibers. by providing the lens fibers with nutrients and removing waste the cells of the epithelium regulate maintain lens homeostasis. as ions, nutrients, and liquid enter the lens from the aqueous humor, na+/k+-atpase pumps in the lens epithelial cells pump ions out of the lens to maintain appropriate lens osmotic concentration and volume, with equatorially positioned lens epithelium cells contributing most to this current. the activity of the na+/k+-atpases keeps water and current flowing through the lens from the poles and exiting through the equatorial regions. the cells of the lens epithelium also divide into new lens fibers at the lens equator. the lens lays down fibers from when it first forms in embryo until death. lens fibers lens fibers from mid to outer cortex. balls of the ball and sockets can be seen protruding for the corners of cells in the lower picturefibers from lens nucleus the lens fibers form the bulk of the lens. they are long, thin, transparent cells, firmly packed, with diameters typically 4–7 micrometres and lengths of up to 12mm long in humans. the lens fibers stretch lengthwise from the posterior to the anterior poles and, when cut horizontally, are arranged in concentric layers rather like the layers of an onion. if cut along the equator, it appears as a honeycomb. the approximate middle of each fiber lies around the equator. these tightly packed layers of lens fibers are referred to as laminae. the lens fiber cytoplasms are linked together via gap junctions, intercellular bridges and interdigitations of the cells that resemble "ball and socket" forms. the lens is split into regions depending on the age of the lens fibers of a particular layer. moving outwards from the central, oldest layer, the lens is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, the inner and outer cortex. new lens fibers, generated from the lens epithelium, are added to the outer cortex. mature lens fibers have no organelles or nuclei. cell fusion, voids and vacuoles cellular and supercellular structure in the mouse lens. photos at increasing depth: a-epithelium b-broadening fiber ends c-fiber ends lock together d-f- voids g-vacuoles i-suturesleft to right we have a smooth capsule, a small patch of epithelium next to fused lens fibers or perhaps a void straighter fibers and finally wrinkled fiberswith the advent of other ways of looking at cellular structures of lenses while still in the living animal it became apparent that regions of fiber cells, at least at the lens anterior, contain large voids and vacuoles. these are speculated to be involved in lens transport systems linking the surface of the lens to deeper regions. very similar looking structures also indicate cell fusion in the lens. the cell fusion is shown by micro-injection to form a stratified syncytium in whole lens cultures. development similar to a human, this is a lens forming in a chicken eyedevelopment of the vertebrate lens begins when the human embryo is about 4mm long. the accompanying picture shows the process in a more easily studied chicken embryo. unlike the rest of the eye which is derived mostly from the inner embryo layers, the lens is derived from the skin around the embryo. the first stage of lens formation takes place when a sphere of cells formed by budding of the inner embryo layers comes close to the embyro's outer skin. the sphere of cells induces nearby outer skin to start changing into the lens placode. the lens placode is the first stage of transformation of a patch of skin into the lens. at this early stage, the lens placode is a single layer of cells. as development progresses, the lens placode begins to deepen and bow inwards. as the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells bud off from the embryo's skin to form a sphere of cells known as the "lens vesicle". when the embryo is about 10mm long the lens vesicle has completely separated from the skin of the embryo. the embryo then sends signals from the developing retina, inducing the cells closest to the posterior end of the lens vesicle to elongate toward the anterior end of the vesicle. these signals also induce the synthesis of proteins called crystallins. as the name suggests the crystallins can form a clear highly refractive jelly. these elongating cells eventually fill in the center of the vesicle with cells, that are long and thin like a strand of hair, called fibers. these primary fibers become the nucleus in the mature lens. the epithelial cells that don't form into fibers nearest the lens front give rise to the lens epithelium. pattern of lens fibers (anterior and lateral aspect)additional fibers are derived from lens epithelial cells located at the lens equator. these cells lengthen towards the front and back wrapping around fibers already laid down. the new fibers need to be longer to cover earlier fibers but as the lens gets larger the ends of the newer fibers no longer reach as far towards the front and back of the lens. the lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. these seams being less crystalline then the bulk of the lens are more visible and are termed "sutures". the suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens. the lens continues to grow after birth, with the new secondary fibers being added as outer layers. new lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the "germinative zone" and "bow region". the lens epithelial cells elongate, lose contact with the capsule and epithelium at the back and front of the lens, synthesize crystallin, and then finally lose their nuclei (enucleate) as they become mature lens fibers. in humans, as the lens grows by laying down more fibers through to early adulthood, the lens becomes more ellipsoid in shape. after about age 20 the lens grows rounder again and the iris is very important for this development. several proteins control the embryonic development of the lens though pax6 is considered the master regulator gene of this organ. other effectors of proper lens development include the wnt signaling components bcl9 and pygo2. the whole process of differentiation of the epithelial cells into crystallin filled fiber cells without organelles occurs within the confines of the lens capsule. older cells cannot be shed and are instead internalized towards the center of the lens. this process results in a complete temporally layered record of the differentiation process from the start at the lens surface to the end at the lens center. the lens is therefore valuable to scientists studying the process of cell differentiation. variations in lens structure bony fish eye. note the spherical lens and muscle to pull the lens backwardin many aquatic vertebrates, the lens is considerably thicker, almost spherical resulting in increased light refraction. this difference helps compensate for the smaller angle of refraction between the eye's cornea and the watery environment, as they have more similar refractive indices than cornea and air. the fiber cells of fish are generally considerably thinner than those of land vertebrates and it appears crystalin proteins are transported to the organelle free cells at