Lens anatomy and physiology Flashcards
Lens vesicle
During gestation, a single layer of epithelial cells forms a hollow sphere
Posterior cells elongate to become ___, which fills the lens vesicle and forms the ___ nucleus.
primary lens fibers –> Embryonic nucleus.
PPE!!!!
Anterior cells near the equator divide to produce new epithelial cells. These cells elongate to become ____ and form the ___ nucleus.
Secondary lens fibers and form the fetal nucleus.
As cells reach the sutures, nuclei and organelle are degraded. Cells consist only of ___
A cell membrane, cytoskeleton, and crystallines (proteins)
After birth, do anterior or posterior cells continue to divide and elongate?
Only anterior cells/secondary lens fibers. This forms the adult nucleus.
The basement membrane of the outer epithelial cells is called the
Lens capsule. Surrounds the entire lens.
The lens capsule is composed of primarily type __ collagen and GAGs.
Type 4 collagen and GAGs. The collagen makes the lens capsule highly elastic.
Where do zonules insert into the lens? Where did the zonules originate?
Insert into the capsule in the equator region of the lens. Zonules originated from the Bm of the non-pigmented epithelium of the ciliary body.
Chromophores (yellow pigment) in the lens absorb what type of UV?
UVA (320-400nm). Prevents it from reaching the retina. However, this absorption can lead to the production of free radicals that cause DNA damage and protein aggregation.
Ciliary muscle contracts (stands)
Zonules relax. Lens equator diameter decreases. Anterior lens radius of curvature decreases/anterior lens surface curvature steepens. Lens refraction power increases to greater than 20D.
With age, ability for the lens to change is curvature is impaired due to which 3 things?
ciliary body moves progressively inward towards the lens.
Lens thickens.
Lens rigidity increases– Nucleus becomes stuffer than the cortex.
Contributions to lens transparency (6)
Lack of blood vessels
Orderly arrangement of lens fibers with uniform spacing between them. Cells are hexagonal.
Few cellular organelles.
High concentration of crystallines. Water soluble proteins that comprise over 90% of total cellular protein. Alpha function as molecular chaperones to stabilize proteins/prevent aggregation.
Na+/K+ ATPase in epithelial cell membranes. Removes excess water by pumping Na+ into the anterior chamber.
Presence of glutathione and ascorbic acid (antioxidants that neutralize free radicals)
Cyrsallines
Water soluble proteins the comprise over 90% of total cellular protein. Alpha function as molecular chaperones to stabilize proteins and prevent aggregation.
Two types of antioxidants in the lens
Glutathione and ascorbic acid.
They neutralize free radicals and prevent DNA damage/protein denaturation.
With age, what contributes to the loss of transparency? (4)
Chromophore concentration increases = more yellowing, DNA damage, and protein aggregation.
Glutathione concentration decreases. = DNA damage, and protein aggregation.
A crystallin concretion decreases = Leads to protein aggregation. By age 45, no a crystallines evident in the lens.
Epithelial cell membrane permeability increases. Allows more solutes and water to enter the epithelial cells = swelling. Disrupts orderly atonement and uniform spacing of lens fibers.
