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0V015 Biology & Pathology > L46 The Lens > Flashcards

L46 The Lens Flashcards

1
Q

What does the uvea consist of?

A

3 structures: Iris ciliary body Choroid At front its made up of iris which then merges with the ciliary body which then merges with the choroid.

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2
Q

What is the word uvea derived from in greek?

A

Uva - meaning grape

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3
Q

Why is the uvea called the uvea?

A

If you took off the sclera and held the eye by the optic nerve and wiggled it about, it would resemble a grape.

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4
Q

Describe the lens in situ

A

The rear surface (posterior surface) is more curved than the anterior surface.

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5
Q

How is the lens attached to the ciliary body?

A

By suspensory ligaments/ zonular fibres.

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6
Q

Where does the anterior pole of the lens lie in relation to the cornea?

A

It lies about 3mm posterior to the cornea.

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7
Q

What is the anterior chamber?

A

The space between the iris and the cornea.

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8
Q

What is the posterior chamber?

A

Between the iris and the lens. (behind the iris and infront of the lens).

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9
Q

How does the weight of the lens change as you age?

A

As you age the lens grows thus weight increases.

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10
Q

During what time period does most of the growth of the lens occur?

A

During the ages of 0-10 years old.

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11
Q

What does it mean to be crenulated?

A

Basically means to not be smooth. You can think lumpy.

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12
Q

Define morphology

A
  1. the study of the forms of things.
  2. a particular form, shape, or structure.
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13
Q

What is the surface morphology of the lens?

A

Shows crenulations due to the insertion of suspensory ligaments.

(this mainly happens at the equator)

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14
Q

Define concentric

A

of or denoting circles, arcs, or other shapes which share the same centre, the larger often completely surrounding the smaller.

(Basically like a structure that resembles a target where circles are in circles)

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15
Q

How many concentric layers does the lens consist of and what are they?

A

The lens consists of three concentric layers.

These are the capsule (outer coating), epithelium (single layer) and stroma (bulk of the lens - made up of lens fibres).

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16
Q

Is the capsule uniform all the way around the lens?

A

No. The thickness of the capsule can be shown by Fincham’s model.

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17
Q

What are the limitations of Fincham’s model?

A

It only depicts the thickness of the capsule in an adult lens. Recent research has shown that capsuler thickness varies with age.

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18
Q

What happens to capsuler thickness with age?

A

Initially you start off with thickness lying at the equator of the lens, however, as you age thickness of the capsule moves towards the poles.

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19
Q

Describe the structure of the capsule

A

Although at high magnification, the capsule apears to lack structure it is actually made up of (around 40) lamellae of superhelical collagen. (basically collagen arranged in a coil fashion - think kinky curls)

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20
Q

Exam tip - if you are asked what something is made up of and you have no clue what the right answer is , what should be your educated guess?

A

COLLAGEN - collagen is the most abundant material in the body and especially the eye.

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21
Q

What is the lens capsule made of?

A

Collagen

22
Q

How come the capsule is elastic if it is made up of collagen (which is an inelastic material)?

A

It is made up of lamellae of superhelical collagen (basically collagen arranged in a coil fashion - think kinky curls) - this allows for that elastic property. (Great analogy for this is a slinky toy - it is metal but also has an elasticated property because of its structure)

23
Q

What is the 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.

[It also plays an important role in cataract surgery - this is not a function but is something we humans use the capsule for].

24
Q

What happens in cataract surgery?

A

In cataract surgery the lens stoma is ‘liquefied’ by an ultrasound probe and sucked out, leaving the capsule behind like an empty bag, which is then filled with an artificial lens.

25
Q

What is the lens capsule in relation to the lens epithelium?

A

Is actually the basement membrane of the underlying epithelium.

(the epithelium actually produces the lens capsule).

26
Q

Where is the lens epithelium found?

A

It is only found on the anterior surface of the lens and the equator (you don’t find it at the back).

27
Q

What is the lens epithelium present as at the anterior pole of the lens?

A

They are flattened squamous cells.

28
Q

As you move towards the equator (from the anterior pole) how is the lens epithelium present?

A

It is present as cuboidal epithelia

29
Q

At the equator how is the lens epithelium present?

A

At the equator, lens epithelia are almost columnar.

30
Q

How are lens epithelial cells joined to each other and underlying lens fibres?

A

By a number of junctions (desmosomes).

31
Q

Why are lens epithelial cells joined to each other and underlying fibres via desmosomes?

A

Because desmosomes are very strong junctions and because the lens is continually changing shape during accommodation, you need to have these strong junctions to ensure epithelial cells don’t fall apart during accommodation.

32
Q

What is special about gap junctions?

A

They have gaps allowing nutrients to get from one place to another.

33
Q

True or false- Back surface of the lens doesn’t have an epithelium.

A

True

34
Q

What are lens fibres made up of?

A

long thin cells that have a hexagonal structure (NOT collagen).

it looks like they tesselate

35
Q

What do we call the place at which lens fibres join one another?

A

Sutures

36
Q

How do sutures in young and old lenses differ?

A

Sutures in young children are ‘Y’ shaped ( on anterior side and inverted on the posterior side of the lens).

Sutures in old people are often referred to as being the shape of a ‘nine-pointed star’ (despite not really looking like a nine-pointed star or even always having nine points!).

37
Q

Why do older individuals have ‘nine-pointed star’ shaped sutures?

A

As you get older the lens runs out of space so you can have these beautifully regular Y shaped sutures instead you have to cram all these lens fibres in leading to these ‘nine-pointed star’ shaped sutures.

38
Q

What’s special about lens fibres?

A

Lens fibres have few organelles. The nucleus, for example, is lost soon after fibre formation. (this is partially due to making the lens transparent).

They have the highest protein content of any cell in the body (>90% of their wet weight); alpha, beta and gamma crystallins ( crystallines are what these protein are called) are what give the lens its high refractive index.

Lens fibres are joined by a variety of junctions

39
Q

What are the three types of junctions that can occur between cells?

A

1) Sticky glycoproteins on cell surface – which act as a “glue” [these arent present in the lens]
2) Mechanical junctions:
- ball and socket
- tongue and groove
3) Specialised cell-cell junctions:
- occluding (tight)
- anchoring (desmosomes)
- gap

A cell can posses more than one of these types of junction

40
Q

Which types of mechanical junctions are found in the lens?

A

Ball and socket joints are found between lens fibres.

Tongue and groove joints are also found between cells within the cells.

41
Q

Where is each type of mechanical junction most abundant in the lens?

A

Ball and socket junctions are most abundant in the outer lens, while tongue and groove junctions lie deeper.

42
Q

What is an occluding junction (AKA a tight junction - also known as zonulae occludentes) and what does it do?

A

Adjacent protein molecules in opposing plasma membranes fuse, sealing/occluding the space between cells. This prevents the passage of molecules between cells

43
Q

What is an anchoring junction or desmosome and why is it used?

A

These are adhering junctions, or desmosomes, where adjacent cells are joined by fibres from the cell’s cytoskeleton (filaments).

They are very robust and are most common in tissues subject to severe mechanical stress e.g. skin epithelium, cardiac muscle, uterus.

44
Q

Where are desmosomes found?

A

They are most common in tissues subject to severe mechanical stress e.g. skin epithelium, cardiac muscle, uterus.

45
Q

What are gap junctions and what do they allow?

A

Gap junctions are where adjacent cells are joined by their membrane proteins, however, these proteins allow cytoplasmic continuity between the cells. This means certain molecules/ions can pass directly from one cell to the next

46
Q

What are the junctions between membrane proteins in gap junctions known as?

A

Connexons

47
Q

Why are the cells that make up the lens joined together by gap junctions?

A

The lens has no blood supply. It receives its nutrients from the aqueous humour. The cells that make up the lens are therefore joined by gap junctions, so nutrients can diffuse from the lens surface to cells in its interior

48
Q

The size /shape of lens epithelial cells vary depending on location- why?

A

The reason for this variation is because of the way lens fibres are formed.

There is a region between the pole and the equator - where the cells are cuboidal known as the germinative zone. Epithelial cells in this zone are highly mitotically active, displacing cells towards the equator. Epithelial cells already at the equator are pushed inwards by the new arrivals, forming new lens fibres.

49
Q

What do newly formed lens fibres do?

A

Lose their organelles, they start destroying all their endoplasmic reticule and their nuclei start to disintegrate.

And then they start to produce lots of crystallines and of course are making lots of membranes as they elongate and go towards the front and rear pole.

50
Q

What is the nuclear bow of the lens?

A

As we know when lens fibres are newly formed their nuclei disintegrates, however, this doesn’t happen instantaneously. So you will have outer lens fibres still having their nuclei and they hang around a while before migrating to the equator and rear pole thus forming the nuclear bow.

51
Q

Does the lens continuously grow all our lives?

A

Yes (all cells accumulate with time).

52
Q

What are the subdivisions of the lens?

A

Embryonic nucleus - also referred to as the nucleus.

Cortex is basically all the lens fibres added after you were born.

[Its a bit extra to split into foetal and adult nucleus but people do it so just be aware]

0V015 Biology & Pathology (57 decks)

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