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Lecture 4

BIOC19H3 Lecture Notes - Lecture 4: Neurulation, Lens Placode, Hans Spemann


Department
Biological Sciences
Course Code
BIOC19H3
Professor
Ian Brown
Lecture
4

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BIOC19
Lec 4
Sept 27, 2017
Lens Development
Lens development as a model system
Characteristics of the differentiated lens as an important component of the eye
Developmental Biology of the differentiation of the lens in the eye
An example of the differentiation of a highly specialized cell type exhibiting well defined
morphological features and packed full of a unique tissue-specific protein necessary for
the functioning of the lens of the eye
Eye structure- location of the lens
- Lens
- At cornea is where light comes through the eye
- Light goes into little hole area = pupil of eye formed by iris muscular structure but
also gives color of eye
- Just behind iris, lens held by zonular ligament or fibers attached to ciliary body
composed of involuntary muscle cells when they contract pull on zonular ligament and
can change its shape
- Vitreous humor gel like
- Retina light comes through cornea pupil opening and focused on lens at the retinal
cells at the back affects photo receptors sends signals to optic nerve to create
sensation of visual image
- 2 capsule or coating layers: choroid (inner) and sclera (outside) puff layer to which
voluntary muscles of eye attached
- Enable you to change aiming of eyeball looking left/right or up/down (sclera)
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- Changing shape of lens by involuntary muscles
Characteristics of the lens of the eye
Transparent, biconvex structure
Composed of individual lens cells
Polar structure: anterior (front) surface is less curved than posterior
Function: focuses incoming light onto the retina
In most animals, the lens is fully developed at birth but continues to grow through life
Suspended in place by zonule fibres/ligaments to the muscles of the ciliary body
ACCOMODATION: shifting the focus of the eye from distant objects to close objects
Young human eye can change focus from distance to 7 cm from the eye in milliseconds
Accommodation ability declines with age
Distant Vision vs. Close vision
- Shifting focus of eye from distance object to a closer object
- Distant vision (look at prof) vs. close vision (holding up pen and looking at it)
- Distant vision lens flat
- Close vision shape of lens changed front part of lens much more curved
- Lens has to change shape depending on how close/distant human object is controlled
by involuntary muscles on cilia body
Zonule fibers and ciliary body
Smooth muscles in ciliary body contract and relax, tugging on the zonule fibers
Result: change in the shape of the lens
Flat frot o les for far fousig
‘ouded frot o les for ear focusing
Lens nutrition
Avascular structure (no blood vessels) composed of individual cells, relies on diffusion of
nutrients and water to maintain growth and transparency
Relative to other tissues, the lens has lower energy requirements
The lens obtains energy by glycolysis
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Cellular layers of the lens
3 main parts:
1. Lens capsule - extracellular matrix makes the capsule of the lens
2. Lens epithelium - Single cell layer
3. Lens fibre cells - Formed upon layer upon layer of fiber cells like layers of an onion
Lens capsule
A smooth transparent layer that completely surrounds the lens
An extracellular matrix composed of collagen and glycosaminoglycans
The lens capsule is synthesized by the underlying lens epithelial layer
Lens epithelium
Single layer of cuboidal cells that possess strong intercellular adhesion properties
Firmly attached to the overlying lens capsule layer
Maintains lens osmolarity and volume
Lens fiber cells
Form the bulk of the lens
Long, thin, transparent cells
Lose their nuclei and organelles with maturation
Packed with gamma crystalline
Gives lens optical properties
- Cells more recent closer to the lens epithelium
- Nuclei becomes condensed
- deeper in fiber cells
Why is lens development a good model system?
1. Avascular tissue
2. Classical object studied in experimental embryology
3. Lens-specific protein: crystallin
4. Simple system
5. Easy separation of developmental stages
- No blood vessel system
- Can pop lens out of eye and put it in a media, in contrast if you take out a heart or
kidney separate organ from blood vessel system and put it into culture medium
- Good for experimental
- 3. Crystallin builds levels in mature lens cells bc 95% of cellular protein- can use as
marker for diff lens specific proteins
- 3 crystallin genes
- Alpha, beta and gamma
- Gene that can encode crystallin = product of gamma crystallin gene
- 4. just composed of lens cells, heart has diff cell types e.g. muscle cells etc.
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