Human lens

Outline

• Introduction

• Embryology

• Lens suture

• Lens anatomy

Introduction

• Bi convex structure• Behind posterior

chamber and pupil• Cornea and lens are

principle refractive element of eye

• Divides eye into anterior and posterior segments

Embryology

• Primary fiber formation

• Secondary fiber formation

• Fiber elongation

Primary fiber formation

• The thickening of surface ectodermal cells to form the lens placode (yellow).

• The invagination of the lens placode toward the developing optic cup (blue). .

• The elongation of posterior lens vesicle cells as they terminally differentiate to form primary lens fibers.

Secondary fiber formation

Zones Area of anterior lens

Role

Central zone 80% Not differentiated into fibers

Pre germinative zone 5% Daughter add to the lens epithelial population

Germinative zone 10% Daughter cells are selected to terminally differentiate into additional fibers.(secondary fibers)

Transistional zone 5% Nascent fibers

The lens epithelium- basal layer

Fiber elongation

• Lens epithelial cells differentiate into secondary fibers.

• As these cells migrate posteriorly, their shape changes from low cuboidal to high columnar and finally to elongate, crescent fiber.

• However, it is most important to note that the in addition to a crescent curvature, the ends of most secondary fibers have additional curvature in opposite directions but within growth shells.

Fiber elongation

• A view through a lens split along its anteroposterior or visual axis reveals concentric growth shells and/or radial cell columns

• All primary and secondary fibers formed are retained and must be supported for a lifetime.

• Primary lens fibres—forms emryonic nucleus

• Secondary lens fibres - includes all other nucleus

• Lens capsule-produced by anterior epithelial cells

• Lens zonules—from neuroectoderm in ciliary area(3rd – 5th month)

Lens suture

• Y suture of fetal nucleus

• Star sutures of juvenile and adult nuclei and cortex

• Sutures and optical lens quality

Y suture of the fetal nucleus

• End of fibres latitudinal arc segments

Formation of Y sutures during fetal development. •After the primary fiber mass (dark gold embryonic nucleus) has been formed, six straight fibers normally positioned equidistantly around the equator, separate growth shells into equal sextants composed of S-shaped fibers. •The ends of the S-shaped fibers touch and overlap to form suture branches (blue lines) that extend to confluence at the poles.•The end result is an upright Y anterior suture and an inverted posterior Y suture

The three anterior suture branches are normally oriented at 120 longitudinal degrees to one another to form a Y suture pattern. The three posterior suture branches are also normally oriented at 120 degrees to one another but because of opposite-end curvature are offset 60 degrees to the anterior suture branches to form an inverted Y suture pattern.

Anterior sutureErect Y suture

Posterior sutureInverted Y suture

STAR SUTURES OF THE JUVENILE AND ADULT NUCLEI AND CORTEX

Structural elements in the production of a primate lens six branch star suture formed from birth through infancy •At birth there are three Y suture branches.•In successive growth shells, additional straight fibers form additional suture branches. •By the end of the infantile period, 12 straight fibers, positioned equidistantly around the equator, separate growth shells into equal groups of S-shaped fibers with ends that touch and overlap to form a simple star suture.

Key structural elements in the production of discontinuous suture planes in human lenses. •From birth through infancy no uniform shape•12 suture branches form “discontinuous” suture planes throughout the juvenile nucleus. •18 suture branches constituting the star suture of the adult nucleus continue to form discontinuous suture.•the 24 suture branches constituting the complex star suture of the cortex continue to form discontinuous suture planes extending from the adult nucleus to the lens periphery.

SUTURES AND LENS OPTICAL QUALITY

• In the Y suture light rays passing would repeatedly encounter fiber membrane, cytoplasm with crystallins, extracellular space, causing a reduction in lens sharpness of focus.

• However, as lenses grow and age and light rays pass through the lens, they encounter fewer above mentioned structures.

Lens anatomy

• The radius of curvature of anterior surface is 10 mm and that of posterior surface is 6 mm..

• Most fibers are hexagonal in cross section with two broad and four narrow faces.

• The relative sizes of the cortex and nucleus in a newborn (upper) and middle-aged (lower) human lens..

Topography

Location

Refractive power

Structure of lens

• Histologicaly composed of three structures

• Lens capsule

• lens epithelium

• Lens fibers

Lens capsule

• Transparent covering that surround the entire lens.

• Histologically it is a basement membrane.• The capsule is produced anteriorly by the

lens epithelium and posteriorly by the elongating fiber cells.

• It is composed of type IV collagen fibers and sulphated glycosaminoglycans.

• It is highly elastic in nature because of lamellar or fibrillar arrangement of fibers.

• Lens capsule is thickest near equator and thinnest at posterior pole.

This extreme thinness of the posterior capsule makes it more vulnerable for posterior capsular tear or rent during cataract surgery.

Lens epithelium

• Simple cuboidal epithelium and is found only in the anterior surface of the lens.

• secrete the anterior lens capsule throughout the life.

• Near the equator it becomes columnar• There is no posterior lens epithelium

because the cells originally located there have elongated into primary fibers of the lens

Lens fibers

• Transitional zone cells continue to elongate and differentiate, they turn meridionally

• The apical end of these cells pass anteriorly towards the anterior pole and the basal end are pushed posteriorly towards the posterior pole.

• New superficial lens fibers are added in a concentrically arranged lamina, like the layers of an onion.

Zones of lens

• Nucleus occupies 84% of the lens and cortex occupies 16%.

• The nucleus is further subdivided into embryonic, fetal, infantile, and adult nuclei.

• Epinucleus is formed by the zone between foetal nucleus and cortex.

HydroseparationsHydrodissection is the separation of lens from its capsule whereas hydrodelineation is achieved by injecting fluid between epinucleus and nucleus.

Lens cortex

• A cortex can be defined as the outer part or external layers of an internal organ

• The initial growth shells of secondary fibers comprise the initial external layers of the lens and are, thus, the original cortical fibers.

Lens nucleus

• The embryonic nucleus is comprised solely of primary fibers.

• The fetal nucleus is comprised of all the secondary fibers formed until birth.

• The fibers formed after birth and through sexual maturation are juvenile nuclear fibers.

• The adult lens nucleus is comprised of all the secondary fibers formed after sexual maturation minus the fibers of the cortex.

Lens cortex and nucleus

• A normal adult human lens

• The human adult lens nucleus has a uniform yellow coloration clearly distinct from the colorless cortex.

• Based on the diameter of the chromatic region, the average thickness of the cortex in adult human lenses (age range 49 to 73 years) is 1.13 mm ± 0.15 mm.

• The thickness of the lens cortex does not appear to change with age.

Zonules of lens

• Zonules or suspensory ligament of lens are a group of radially arranged,thread like fibres which helps the lens to held in position

Cilliary zonules

Pars orbicularis- part over pars plana

Zonular plexus- part in between cilliary process and pars plicata

Zonular fork- part which consolidates to zonular bundles

Zonular limbs

Anterior zonular limb

Equatorial zonular limb

Posterior zonular limb

Depending on their attachment

Accomodation • The eye has the capacity to adjust its focus from distance to

near objects because of the ability of the lens to change shape, a phenomenon known as accommodation.

• Zonular fibers on the lens capsule is controlled by the action of the parasympathetically innervated ciliary muscle.

• When cilliary muscles contracts, relaxation of zonular tension occurs. The lens then assumes a more spherical shape, resulting in increased dioptric power which helps to bring nearer objects into focus.

• Ciliary muscle relaxation causes the zonular tension to increase. As a result, lens flattens, which helps in bringing more distant objects into view.

Presbyopia- at the age of 40-50 yrs, the elasticity of the lens diminishes. The contractility of the ciliary muscle also diminishes due to the structural changes in the muscle. As a result lens fails to change its shape sufficiently during accommodation.

LENS CELL HOMEOSTASIS

• The size of any tissue cell population is determined by the cell birth rate(KB) and the cell loss rate(KL).

• The lens grows throughout life because KB is greater than KL.

• From fetal development, and continuing throughout life, the lens produces more cortical fibers that eventually become nuclear fibers.

LENS EPITHELIAL CELL HOMEOSTASIS

• GZ KB is greater than GZ KL, and, therefore, the GZ is a growing cell population throughout life.

• Consequently, although CZ KB is greater than CZ KL from birth through adulthood, it is less from adulthood through old age

• Although the CZ is a growing cell population throughout adulthood, it is a regressing cell population for the rest of life.

LENS EPITHELIAL CELL APOPTOSIS

• The lens epithelium eliminates some cells by apoptosis throughout life.

• The increasingly large and prominent lysosomal bodies likely represent breakdown of apoptotic cells and fragments.

Lens composition

Lens disorder

Lens coloboma Cataract

Recap

• Introduction

• Embryology

• Lens suture

• Lens anatomy

Thank you

References

• Fundamentals and principles of Ophthalmology,section 2,2014-2015, American Academy of Ophthalmology

• Duane's Foundations of Clinical Ophthalmology, Foundation volume 1

• Jack J Kanski, Brad Bowling, Clinical Ophthalmology, seventh edition 2011

• M.J. Roper- Hall, Stallard’s Eye Surgery, Seventh Edition, 1989

• Parsons’ Diseases of the Eye, Twentieth Edition 2007

• Internet eophtha