The lens capsule is a component of the

globe A globe is a spherical Earth, spherical Model#Physical model, model of Earth, of some other astronomical object, celestial body, or of the celestial sphere. Globes serve purposes similar to maps, but, unlike maps, they do not distort the surface ...

of the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

. It is a clear elastic

basement membrane The basement membrane, also known as base membrane, is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tis ...

similar in composition to other basement membranes in the body. The capsule is a very thick basement membrane and the thickness varies in different areas on the lens surface and with the age of the animal. It is composed of various types of fibers such as collagen IV,

laminin Laminins are a family of glycoproteins of the extracellular matrix of all animals. They are major constituents of the basement membrane, namely the basal lamina (the protein network foundation for most cells and organs). Laminins are vital to bi ...

, etc. and these help it stay under constant tension. The capsule is attached to the surrounding eye by numerous suspensory ligaments and in turn suspends the rest of the lens in an appropriate position. As the lens grows throughout life so must the capsule. Due to the shape of the capsule, the

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

naturally tends towards a rounder or more globular configuration, a shape it must assume for the eye to

focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...

at a near distance. Tension on the capsule is varied to allow the lens to subtly change shape to allow the eye to focus in a process called accommodation. Early in embryonic development the lens capsule is highly vascularized, but later during embryo development becomes avascular and transparent, serving as a diffusion barrier helping to protect the lens. It is permeable to low

molecular weight A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

compounds, but restricts the movement of larger things like bacteria, viruses and large

colloidal particle Particle size is a notion introduced for comparing dimensions of solid particles ('' flecks''), liquid particles (''droplets''), or gaseous particles ('' bubbles''). The notion of particle size applies to particles in colloids, in ecology, in gr ...

s. As the capsule contains the lens, it is clinically significant in regard to surgery of the lens. For example, it is used to contain new artificial lenses implanted after cataract surgery.

Anatomy

The lens capsule is a transparent membrane that surrounds the entire lens. The capsule is thinnest at the posterior pole with approximate thickness of 3.5μm. Average thickness at the equator is 7μm. Anterior pole thickness increases with age from 11-15μm. The thickest portion is the annular region surrounding the anterior pole. This will also increase with age (from 13.5-16μm). The ligaments suspending the lens form attachments in the equatorial area and more so just to the front and back of the equator. There are tens of thousands of these ligaments in a mouse lens and for the most part they appear to connect directly to the lens capsule. As the lens grows throughout the life of most vertebrates, the capsule is required to grow as well. As shown in the accompanying micrographs and diagrams, equatorial cells can have periodic cellular processes penetrating the capsule. Ligament club attachments

The structures in the images are consistent with the laying down of new capsular material required for growth. Even though the capsule is a highly elastic structure, it contains no elastic fibers. Elasticity is because of the thick lamellar arrangement of the collagen fibers.

Function

The capsule helps give the lens its more spherical shape in aquatic vertebrates such as fish and more ellipsoidal shape in land based vertebrates such as sheep. In humans the lens ellipsoid becomes more flattened with age. The capsule is the basement membrane for the epithelial cells at the front of the lens and the rapidly growing more flexible fiber cells of the back of the lens and below the epithelium at the front. Without the capsule substrate forming a tense support, these cells lose their form as in the picture of a decapsulated sheep lens.

Accommodation

Normally, when

ciliary muscle The ciliary muscle is an intrinsic muscle of the eye formed as a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer, the uvea ( vascular layer). It controls accommodation for vie ...

s are in a relaxed state, the

zonules The zonule of Zinn () (Zinn's membrane, ciliary zonule) (after Johann Gottfried Zinn) is a ring of fibrous strands forming a wikt:zonule#Noun, zonule (little band) that connects the ciliary body with the crystalline lens (anatomy), lens of the eye ...

will pull the capsule. Due to this zonular tension anterior lens surface is flatter resulting in more distant objects being in focus. When ciliary muscles contract, the zonular tension will reduce allowing lens to assume more spherical shape. This shape change increases the focusing power of the eye allowing closer objects to come into focus. The process of changing the lens's focusing power to see closer objects more clearly is known as accommodation.

Embryology

The lens vesicle is developed from

surface ectoderm The surface ectoderm, AKA external ectoderm, is one of the two early embryonic divisions of the ectoderm. The other early division of the ectoderm is the neuroectoderm. The surface ectoderm develops into the following structures: *Skin (only th ...

. It will separate from surface ectoderm at approximately day 33 in a human and only 3 days for a chicken. Lens capsule developed from basal lamina of lens vesicle will cover early lens fibers. Capsule is evident at 5 weeks of human gestation and begins its role in protecting and supporting the lens interior.

Lens protection

Early embryologic development of the lens capsule gives the lens material an immune privilege. It will also help protect the lens from viruses, bacteria and parasites.

Vascular lens capsule

During

fetal development Prenatal development () involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal deve ...

vascular lens capsule (tunica vasculosa lentis) develops from the

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

that surrounds the lens. It receives arterial blood supply from the

hyaloid artery The hyaloid artery is a branch of the ophthalmic artery, which is itself a branch of the internal carotid artery. It is contained within the optic stalk of the eye and extends from the optic disc through the vitreous humor to the lens. Usually ...

. This blood supply slowly regresses and the vascular capsule disappears before birth. The disappearance of the anterior vascular capsule of the lens is useful in estimating the

gestational age In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method, if available. Such metho ...

. While the vascularization disappears during gestation, the micrographs in this article show cells still active on the lens exterior after vascular regression. These cells may be the avascular portion of the original mesenchyme that surrounded the lens.

Clinical significance

In intra-capsular cataract extraction (ICCE), the whole lens including the anterior part of the capsule is removed. During more common extra capsular

cataract surgery Cataract surgery, also called lens replacement surgery, is the removal of the natural lens (anatomy), lens of the human eye, eye that has developed a cataract, an opaque or cloudy area. The eye's natural lens is usually replaced with an artific ...

procedures like micro incision cataract surgery,

phacoemulsification Phacoemulsification is a cataract surgery method in which the internal lens of the eye which has developed a cataract is emulsified with the tip of an ultrasonic handpiece and aspirated from the eye. Aspirated fluids are replaced with irrigation ...

etc., the clouded lens is removed through an opening made in the anterior lens capsule. The

intraocular lens An intraocular lens (IOL) is a lens (optics), lens implanted in the human eye, eye usually as part of a treatment for cataracts or for correcting other vision problems such as myopia, near-sightedness (myopia) and farsightedness, far-sightednes ...

is then inserted into the lens capsule which is capable of rapid healing. The best place for intraocular lens implantation is within the capsular bag. Posterior capsular opacification and posterior capsule rupture are common complications of cataract surgery.

References

{{DEFAULTSORT:Capsule Of Lens Human eye anatomy