Lens (vertebrate anatomy)

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.

Accommodation in humans is well studied and allows artificial means of supplementing our focus, such as glasses, for correction of sight as we age.

In a human adult, the lens is typically about 10mm in diameter and 4mm thick, though its shape changes with accommodation and its size grows throughout a person's lifetime.

[6] 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.

The lens fiber cytoplasms are linked together via gap junctions, intercellular bridges and interdigitations of the cells that resemble "ball and socket" forms.

With 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.

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.

[4] Several proteins control the embryonic development of the lens though PAX6 is considered the master regulator gene of this organ.

[21] In many aquatic vertebrates, the lens is considerably thicker, almost spherical resulting in increased light refraction.

[24][25] Even among terrestrial animals the lens of primates such as humans is unusually flat going some way to explain why our vision, unlike diving birds, is particularly blurry under water.

[26] In humans the widely quoted Helmholtz mechanism of focusing, also called accommodation, is often referred to as a "model".

Normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball.

At short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power.

While not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance.

[33] The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body.

In a 1911 Nobel lecture Allvar Gullstrand spoke on "How I found the intracapsular mechanism of accommodation" and this aspect of lens focusing continues to be investigated.

Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence.

[45][46][47] Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it.

These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals.

[26] In cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens.

In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects.

While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal.

There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea.

It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals.

β and γ crystallins are found primarily in the lens, while subunits of α -crystallin have been isolated from other parts of the eye and the body.

[60] The chaperone functions of α-crystallin may also help maintain the lens proteins, which must last a human for their entire lifetime.

[64] People lacking a lens (a condition known as aphakia) perceive ultraviolet light as whitish blue or whitish-violet.

[67] By nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery.

3D lens model from sheep with parts labeled and images of cells from different parts overlayed
Sheep eye lens para-formaldehyde fixed front view. Small lenses are about 1cm in diameter. Small bumps at edge are remnants of suspensory ligaments
Sheep lens fixed side view. Note the largest lens has damaged capsule and iris attached
Microscope image of lens cell types and capsule
Sheep lens capsule removed. Decapsulation leads to a nearly formless blob.
Eye lens micrographs and diagram of growth region of the capsule.
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-Sutures
Left 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 fibers
Similar to a human, this is a lens forming in a chicken eye
Pattern of lens fibers (anterior and lateral aspect)
Bony fish eye. Note the spherical lens and muscle to pull the lens backward
An image that is partially in focus, but mostly out of focus in varying degrees.
Eye and detailed ray path including one intraocular lens layer
3D reconstruction of lens in a living 20 year old human male focusing from 0 dioptres (infinity) to 4.85 dioptres (26mm) side & back views
Two horse lenses suspended on water by cling wrap with 4 approximately parallel lasers directed through them. The 1 cm spaced grid indicates an accommodated, i.e. relaxed, near focus, focal length of around 6cm
Schachar model of lens focus
Tracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant
Wrinkled lens fibers in picture below compared to straight fibers above
Diving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.
Bony fish eye. Note the spherical lens and muscle to pull the lens backward
Graph showing optical density (OD) of the human crystalline lens for newborn, 30-year-old, and 65-year-old from wavelengths 300-1400 nm.
Channels regulate lens transport.
1:posterior segment 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:Schlemm's canal 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscule 15:inferior rectus muscule 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina 1: posterior segment 2: ora serrata 3: ciliary muscle 4: ciliary zonules 5: Schlemm's canal 6: pupil 7: anterior chamber 8: cornea 9: iris 10: lens cortex 11: lens nucleus 12: ciliary process 13: conjunctiva 14: inferior oblique muscule 15: inferior rectus muscule 16: medial rectus muscle 17: retinal arteries and veins 18: optic disc 19: dura mater 20: central retinal artery 21: central retinal vein 22: optic nerve 23: vorticose vein 24: bulbar sheath 25: macula 26: fovea 27: sclera 28: choroid 29: superior rectus muscle 30: retina
1:posterior segment 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:Schlemm's canal 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscule 15:inferior rectus muscule 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina