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Duke Elder - Anatomy 2: Structure of the eye > Lens and zonular apparatus > Flashcards

Lens and zonular apparatus Flashcards

1
Q

What is the lens?

A

highly organised system of specialised cells (so-called lens fibres) which constitutes an important component of the optical system of the eye

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2
Q

What is the overall function of the lens?

A

fulfils the function of altering the refractive index of light entering the eye to focus on the retina

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3
Q

What is the refractive power of the lens in dioptres? How does this compare with the cornea?

A

15 dioptres (less than cornea)

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4
Q

What property does the lens possess that the cornea doesn’t, despite its weaker refractive power?

A

can change shape under the influence of the ciliary muscle and thus alter its refractive power

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5
Q

What happens to the range of dioptric power of the lens with age?

A

diminishes

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6
Q

What is the transparency of the lens a result of?

A

the shape, arrangement, internal structure and biochemistry of the lens cells or lens fibres

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7
Q

What is decreased transparency of the lens konwn as?

A

cataract

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8
Q

What is the anatomical location of the lens?

A

lies behind the iris and in front of the vitreous body

encircled by the ciliary processes

held in place by the zonular fibres laterally, anterior vitreous face posteriorly (patellar fossa), and the iris anteriorly

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9
Q

What are three things that hold the lens in place?

A
  1. zonular fibres laterally
  2. anterior vitreous face posteriorly (patellar fossa)
  3. iris anteriorly
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10
Q

What process in fetal development leaves the lens transparent?

A

it is normally transparent and avascular following regression of the pupillary membrane and tunica vasculosa lentis late in fetal development

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11
Q

What provides nourishment to the lens?

A

the aqoueous and vitreous humours

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12
Q

What is the shape of the lens?

A

biconvex, ellipsoid structure with differing radius of curvature on the anterior and posterior surfaces

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13
Q

What is the anterior vs posterior curvature of the lens?

A
  • anterior: approx 10mm (range 8-14mm)
  • posterior: 6mm (range 4.5-7.5mm)
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14
Q

What are the centre points of the anterior and posterior surfaces of the lens known as?

A

anterior and posterior poles - connected by an imaginary axis

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15
Q

What distance does the anterior pole lie from the posterior corneal surface?

A

3mm

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16
Q

What separates the anterior and posterior surfaces of the lens?

A

the equator

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17
Q

What is the appearance of the equator of the lens and why?

A

ridges (indented) appearance caused by the zonular fibres

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18
Q

What is the diameter of the lens in the adult eye?

A

10mm

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19
Q

What is the axial length of the lens in the adult?

A

4mm

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20
Q

What changes happen to the lens through life?

A

it continues to grow (0.023mm per year) and alters shape throughout life; becomes rounder with age, especially after age 20 years

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21
Q

What are the 3 parts of the lens?

A
  1. capsule
  2. anterior or lens epithelium
  3. lens fibres
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22
Q

What is the lens capsule?

A

a thickened, smooth, basement membrane produced by the lens epithleium and lens fibres

completely envelops the lens and has regions of variable thickness

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23
Q

What are the 2 things which produce the lens capsule?

A
  1. lens epithelium
  2. lens fibres
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24
Q

What are the thickest parts of the lens capsule and what is its thickness here?

A

pre- and post-equatorially (17-28µm)

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25
Q

What are the thinnest parts of the lens capsule and what is its thickness here?

A

posterior pole (2-3µm) more so than anterior pole (9-14µm)

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26
Q

What is the appearance of the lens capsule under ultrastructural examination?

A

fibrillar or lamellar appearance

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27
Q

What are 2 things that the interfibrillary matrix of the lens capsule consists of?

A
  1. basement membrane glycoproteins (type IV collagen)
  2. sulphated GAGs responsible for period acid-Schiff-positive staining properties in histological sections
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28
Q

What type of collagen is present in the lens capsule interibrillary matrix?

A

collagen type IV

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29
Q

What property do the sulphated GAGs in the lens capsule responsible possess?

A

period acid-Schiff-positive staining properties in histological sections

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30
Q

What shape does the lens capsule cause the lens to assume when not under tension of the zonules, due to its elastic properties?

A

together with the cortex causes lens to assume a more rounded shape

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31
Q

Label the following on the image:

  • lens capsule
  • embryonic nucleus
  • fetal nucleus
  • adult nucleus
  • adult cortex
  • equator
  • central zone
  • germinative zone
  • transitional zone
  • lens bow
  • lens epithelium
A

good job!

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32
Q

What is cataract?

A

loss of the normal lens transparency, besides the normal age-related yellowing

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33
Q

What may cause cataract?

A

may be caused by cumulative damage by ultraviolet light or perturbations in lens fibre biochemistry

opacification may be the result of damage or disruption of the capsule, the lens fibre configuration or the lens epithelium

34
Q

What type of epithelial cells form the lens epithelium?

A

simple cuboidal epithelium

35
Q

Over which areas of the lens is the lens epithelium found?

A

restricted to the anterior surface of the lens

36
Q

What changes does the simple cuboidal epithelium of the lens undergo in different regions of the lens?

A

the cells become more columnar at the equator

as they elongate, the apical portion comes ot lie deeper to other, more anteriorly positioned, lens cells

37
Q

What are the elongated lens cells konwn as?

A

lens fibres

38
Q

How do the equatorial lens epithelial cells become transformed into lens fibres?

A

the cell nucleus and cell body sink deeper into the lens as further cells are laid down externally

39
Q

Where is mitotic activity greatest in the lens epithelium and what is this area konwn as?

A

pre-equatorial and equatorial lens epithelium

= germinative zone

40
Q

What are the shape and dimensions of lens fibres?

A

each lens fibre is 4 x 7 µm wide

hexagonal prismatic band in cross-section

up to 12mm in length (up to 1000x longer than its width)

41
Q

How are the lens fibres orientated in the lens?

A

the apical portion of the lens cell (or lens fibre) passes anteriorly, basal portion posteriorly

42
Q

What changes hapen to the lens cells for it to move position?

A

the cell nucleus migrates anteriorly as the cell is pushed deeper in the lens, hence creating the anteriorly oriented lens bow

meridionally oriented lens fibres extend the full length of the lens, meeting at anterior and posterior sutures

deeper (older) lens fibres are anucleate

43
Q

Where are the meridionally oriented lens fibres located?

A

extend the full length of the lens, meeting at the anterior and posterior sutures

44
Q

Which lens cells (lens fibres) in the lens are anucleate?

A

the deeper (and hence older) lens fibres

45
Q

What process allows continual growth of the lens?

A

addition fo superficial strips of new cells

46
Q

What is the arrangement of lens fibres within the lens due to continual growth?

A

series of concentrically arranged laminae, similar to the layesr of an onion

47
Q

How do different macroscopic regions of the lens differ in life?

A

the outer cortex of the lens has a softer consistency than the hard central nucleus

48
Q

How are lens fibres connected to adjacent lens fibres?

A

they are tightly packed with little intercellular space

neighbouring cells are linked by ball and socket cytoplasmic interdigitations and numerous gap junctions

49
Q

What is thought to e the purpose of the gap junctions between lens fibres?

A

they may aid maintenance of centrally positioned cells (via intercellular and molecular coupling or metabolic cooperation) some distance from the source of nutrition (aqueous humour)

50
Q

What are 3 cellular organelles that superficially located lens fibres are rich in?

A
  1. ribosomes
  2. polysomes
  3. rough endoplasmic reticulum
51
Q

What do the cell organelles oft he superifically located lens fibres (ribosomes, polysomes, RER) help them to achieve?

A

actively synthesise unique lens proteins, lens crystallins

52
Q

What cellular structures are rich in all lens fibres?

A

cytoskeletal elements oriented parallel to the long axis of the cell

53
Q

What can disturbances in the biochemistry of lens fibres / their arrangement lead to?

A

opacification known as cataract

54
Q

What holds the lens in position?

A

a complex 3D system of radially arranged zonules (zonsules of Zinn or the suspensory ligament of the lens)

55
Q

Where do the lens zonules attach to the lens capsule?

A

2mm anterior and 1mm posterior to the equator

present for a full 360 degrees of the lens circumference - resemble circular trampoline

56
Q

Where do the lens zonules arise and what is their course to reach the lens?

A

arise from the pars plana region of the ciliary epithelium and pass forward closely related to the lateral surfaces of the ciliary processes, then attach to the lens capsule

57
Q

What is the clinical importance of understanding the lens zonules’ distribution and the tension forces they are able to withstand?

A

important in modern day cataract surgery where the phacoemulsification probe exerts considerable downward force on the zonule

58
Q

What forces are normally transmitted by the zonules in humans and what function does this allow in humans?

A

the zonules normally transmit the radial forces that flatten the lens, allowing the eye to focus on distant objects

59
Q

What is thought to be the role of lens zonules which are present in non-accommodating species?

A

role in lens centration

60
Q

What is the structure of the lens zonules?

A

they consist of dense, glassy bundles 5 - 30 µm in diameter

each bundle consists of a series of fine fibres (0.35 - 1 µm in diameter), themselves composed of 10-12 nm diameter microfibrils

microfibrils → fine fibres → glassy bundles → zonules

61
Q

What do the 2 microscopic images show?

A

A = macroscopic view of ciliary processes (CP) and intervening lens zonules (arrowheads) inserting into lens capsule in a monkey eye. L=lens, PP=pars plana

B = Electron micrograph showing the close association of zonular fibres (F) to the non-pigmented epithelial cells (NPCE) of the ciliary processes. Note the material similar to zonular fibres beneath the basal lamina of the epithelium (arrowheads).

62
Q

What other human structure do the zonules most closely resemble?

A

biochemically unique but closest to the periodontal ligament of the teeth

63
Q

Which glycoprotein are zonules composed of principally?

A

fibrillin, a 350 kDa cysteine-rich glycoprotein

(although described as elastic fibres, do not contain significant amounts of elastin)

64
Q

What mutations in Marfan syndrome causes ocular manifestations of this condition?

A

mutations in the fibrillin-1 gene

65
Q

Which cells in the eye synthesise and maintain the zonular fibres that suspend the lens?

A

cells of the non-pigmented ciliary epithelium

66
Q

Where do the zonules fibres emerge from proximally and where are the distal portions connected?

A

proximal end: emerges from basal lamina of the ciliary epithelial cells

distal portions: connect to the lens capsule near the lens equator. in humans, fibres appear to insert directly into the capsule, tips terminate below the capsular surface

67
Q

What are the lens zonules synthesised and maintained by?

A

cells in the non-pigmented ciliary epithlium (same as the zonular fibres)

68
Q

At what stage is the lens zonule synthesised and when does it develop?

A

synthesised during embryonic and early postnatal development

likely zonular proteins turn over slowly, if at all

69
Q

What is the name of the condition of the lens that occurs in Marfan syndrome?

A

ectopis lentis

70
Q

What is Marfan syndrome?

A

condition characterised by disturbances in connective tissue and skeletal elements

71
Q

What is ectopis lentis?

A

the zonules are disrupted due to mutation in fibrillin-1 gene; the lens is dislocated or malpositioned within the eye

72
Q

What is the state of the ciliary body/ zonules in the non-accommodated state?

A

the ciliary body is relaxed and therefore maintains tension on the zonules

73
Q

What is the state of the ciliary body/ zonules in the accommodated state?

A

movement of the ciliary body causes slackening of the zonules, whereupon the lens assumes an increased anterior curvature, with resultant increase in refractive power (to focus on a near object), owing to elasticity of the lens capsule and the outer cortical layers

74
Q

What do some authorities believe are the two types of lens zonules?

A

main zonules and ‘tension’ zonules

tension zonules are placed under tension during accommodation

75
Q

What is presbyopia?

A

ageing changes in the eye - condition develops around age 40-50 years when elasticity of the lens markedly decreases and there is associated atrophy of the ciliary muscle fibres

lens fails to change shape sufficiently during accommodation

becomes evidence as decreasing ability to read i.e. use near vision

76
Q

At what age does presbyopia typically develop?

A

age 40-50 years

77
Q

What are the 2 key physical changes in the eye in presbyopia?

A
  1. decreased elasticity of the lens
  2. atrophy of ciliary muscle fibres
78
Q

How does presbyopia become evident through symptoms?

A

decreasing ability to read i.e. use near vision

79
Q

What structures are the posterior lens zonules associated with?

A

collagenous material of the anterior hyaloid membrane

80
Q

What is the role of the zonules running perpendicular to the main zonule stream?

A

form circumferential bands near the base of the ciliary processes or in the pars plana (posterior zonular girdle) and over the apices of the ciliary processes (anterior ciliary girdle)

81
Q

What is the anterior zonular girdle?

A

where zonules runing perpendicular to the main zonule stream form circumferential bands over the apices of the ciliary processes

82
Q

What is the posterior zonular girdle?

A

where zonules runing perpendicular to the main zonule stream form circumferential bands near the base of the ciliary processes or in the pars plana

Duke Elder - Anatomy 2: Structure of the eye (14 decks)

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