introduction to the uvea and structure of the lens Flashcards
1
Q
what is the adnexa of eye ?
A
- eyelid( muscles and glands )
- lacrimal gland
- tear film
2
Q
what forms the outer coat of the eye ?
A
sclera and cornea
3
Q
what does uvea consist of ?
A
- choroid - middle coat of eye sandwiched between sclera and retina
- ciliary body
- iris
4
Q
what is the lens attached to ?
A
lens is attached to ciliary body by thin fibres called suspensory ligaments/zonules fibres
5
Q
what two surfaces does lens have ?
A
anterior and posterior surfaces which meet at equator
6
Q
what is centre of anterior surface of lens known as ?
A
anterior pole- 3mm behind cornea
less convex than posterior
7
Q
what is centre of posterior surface of lens known as ?
A
posterior pole
8
Q
what is anterior chamber ?
A
vitreous filled cavity between cornea and front surface of lens
9
Q
what does anterior of iris touch ?
A
anterior chamber
10
Q
what is posterior to iris but anterior to lens ?
A
posterior chamber
11
Q
what is posterior chamber ?
A
filled with aqueous humour
12
Q
what is behind lens ?
A
vitreous chamber
13
Q
what is axial diameter of lens ?
A
3.5 to 4mm at birth increase to 5mm in old age
14
Q
describe the equator of lens ?
A
- not smooth
15
Q
how many layers does lens have ?
A
the lens consists of 3 concentric layers
- thin elastic capsule on the outside
- simple epithelium
- stroma ( made of lens fibres)
16
Q
what is the lens capsule ?
A
- surrounds the lens
- basement membrane of the cells epithelial cells which lie inside of it
- produced by epithelial cells
17
Q
explain the structure of lens capsule ?
A
- appears to lack structure
- made up of lamellae ( 40 layers ) of superhelical collagen
- very elastic ( no elastic tissue gets its elasticity from collagen )
- thickness varies
depending on location within the lens - thicker anteriorly
- 16um at anterior pole
- less than 3um at posterior pole
- at age 22 capsule is thickest at equator
- as we age maximal thickness is between pole and equator
18
Q
explain Fincham lens capsule structure ?
A
- at age 22 capsule is thickest at equator
- as we age maximal thickness is between pole and equator
19
Q
what is lens capsule function ?
A
- it’s elasticity plays an important role in accommodation
- the capsule also forms a barrier stopping large particulate matter such as inflammatory cells entering the lens, while allowing smaller necessary metabolites through
- plays an important role in cataract surgery
20
Q
what happens in cataract surgery ?
A
- the lens stroma is liquefied by an ultrasound probe and sucked out leaving the capsule empty
- empty capsule is filled by an artificial lens
21
Q
what is layer inside lens capsule ?
A
lens epithelium
22
Q
how does the shape of lens epithelium vary ?
A
- the shape of lens epithelium varies depending on where you are in lens
- anterior pole - epithelial cells are squamous
- between equator and pole - epithelial cells are cuboidal
- at equator - cells are columnar
23
Q
what are adjacent epithelial cells joined by in the lens ?
A
desmosomes- strong junction
gap junctions
24
Q
what is function of desmosomes in lens ?
A
connect epithelial cells to underlying lens fibres
25
what other junction do you get between adjacent epithelial cells ?
gap junctions - allow ionic continuity from one epithelial cell to other
26
what is different about posterior surface of lens ?
posterior lens surface has no epithelial cells
27
what is lens stroma made off ?
made of lens fibres - long elongated cells which appear hexagonal
28
what is fibre in lens stroma ?
long cells elongated cells which appear hexagonal
29
what is significant about hexagonal shape ?
they tessellate together with no gaps
30
where do lens fibres run from ?
lens fibres run from front surface of lens to back surface and join to one another at sutures
31
how are sutures in young lens ?
in young lens sutures form a shape of a y
32
what is suture ?
where lens fibres join to one another
33
how are sutures in old lens ?
you have star like arrangement
| - less space for sutures to from a y shape
34
do lens fibers have organelles ?
- lens fibres have few organelles
| - nucleus is lost after fibre formation
35
do lens fibers have proteins ?
- lens fibres have the highest protein content of any cell in the body (>90% of their wet weight )
- alpha , beta and gamma crystallins are what give the lens its high refractive index
36
what are lens fibres joined by ?
- ball and socket junctions are most abundant in the outer lens
- tongue and groove junctions lie deeper
37
what are different types of junctions between cells ?
1. sticky glycoprotein on cell surface
2. mechanical junctions
- ball and socket
- tongue and groove
3. specialised cell-cell junction
- occluding (tight)
- anchoring ( desmosomes)
- gap
38
what is a ball and socket junction ?
. membrane of one cell bludges out to form a ball
. membrane of adjacent cell bludges in to form a socket
. ball fits into socket joining two cells together
39
what is example of ball and socket ?
- bone and hip
| - between adjacent lens fibres
40
what is tongue and groove junction ?
- tongue and groove
| - this what we know from woodwork
41
what is example of tongue and groove ?
this is junction found between the cells within the lens
42
why do we need many mechanical junctions within the lens ?
- lens is only transparent because of regular arrangement of lens fibres which fit tightly together
- lens is always changing shape , so lens fibres are always squeezed and pulled and they have to be tightly held together and forming gaps and loosing transparency of lens
43
what are occluding junctions ? are they important in lens
- adjacent protein molecules in opposing plasma membrane fuse , sealing the space between cells . this prevents the passage of molecules
- occluding junction are not very important in lens
44
what are desmosomes ? and where are they found in lens
- adjacent cells are joined by fibres which form part of cytoskeleton
- very robust and found in tissue to subject to severe mechanical stress
e. g. lens epithelium
45
what are gap junctions ?where are they found within lens
- adjacent cells are joined by membrane protein, these allow cytoplasmic continuity between cells . this means that certain molecules/ions pass directly from one cell to next
- very important in lens epithelium and lens fibres
46
what are junctions between membrane proteins in gap junctions known as ?
connexons
47
where does lens receive its nutrients from ?
lens has no blood supply . it receives its nutrients from the aqueous humour
48
how do nutrients flow from aqueous humour to the centre of lens ?
via gap junctions
49
what happens to epithelial cells in lens at the germinative zone ?
- at the germinative zone epithelial cells undergo mitosis displacing cells towards the lens equator
- when cells get to equator they become columnar
- then they get pushed to lens stroma where they elongate and turn into lens fibres
50
where do lens fibres that make up stroma of lens originate from ?
they originate from epithelium
51
what do can be seen at lens equator ?
- nuclear bow which is made up of residual nuclei of the most recently formed lens fibres
52
how many new lens fibres shell do we lay every year ?
- 5 layers every year
- the older you get the more layers of cells you get
-
53
what is the lens nucleus ?
lens nucleus contains the oldest cells
54
what are the subdivisions of the lens ?
. embryonal nucleus
. foetal nucleus
. adult nucleus
. cortex
55
why is lens unusual ?
you never loose any cells
