Chapter 2: Anatomy Flashcards
what is the shape of the lens?
binconvex
what are the components of the lens?
capsule, epithelium, cortex, nucleus
what is the axis that joins the anterior and posterior poles of the lens?
optic axis
what are meridians?
hypothetical lines on lens surface passing from one pole to the other
what are the functions of the lens?
maintain clarity, refract lights, accommodate in conjunction with CB/zonules
when does the lens lack blood supply and innervation? where does it get metabolic requirements after?
after fetal development; aqueous humour
what is the index of refraction of the lens?
1.4 centrally and 1.36 peripherally
how is the lens able to refract light?
its n is different from that of the aqueous and vitreous surrounding it
how much refractive power does the lens contribute in its nonaccomodative state?
20D
where does the rest of the convergent refractive power come from and how much?
40-45 D from air-cornea interface
what are the measurements and weight of birth vs adult lens?
birth: 6.4 mm equatorial; 3.5 mm AP; 90 mg.
adult: 9-10 mm equatorial; 5 mm AP; 255 mg.
what happens to the thickness, shape, and refractive index of the lens with aging?
- cortical thickness increases
- more curved
- decreased index of refraction
why does the index of refraction of the lens decrease with age?
increased insoluble proteins
what happens to the overall refraction with aging?
depends on the interplay between index of refraction decreasing and increasing thickness/curvature.
what is the lens capsule composed of?
elastic, transparent BM composed of type 4 collagen and other matrix proteins
what lays down the capsule?
epithelial cells
what part of the lens molds during accommodation?
capsule
what part of the lens capsule attaches to the zonules?
the outer layer, called the zonular lamella
what are the various thicknesses of the lens capsule?
- anterior 14 um; posterior 4 um.
- equator: 17 um.
- thickest in anterior (21) and posterior (23) pre-equatorial zones.
where do the lens zonules originate from?
basal lamina of the nonpigmented epithelium of the pars plana/plicata of the CB
what constitutes the zonular fibres?
microfibrils composed of elastic tissue
where do the zonules insert?
capsule; 1.5 mm anterior to equator and 1.25 mm posterior to equator
what happens to the zonules with age?
regress - leaving separate anterior and posterior layers that form triangle shape in cross sections
what are the dimensions of the zonules?
- 5-30 um diameter; microfibrils are 8-10 nm in diameter
what are the histo features of zonules?
PAS positive, eosinophilic
where does the lens epithelium lie?
immediately posterior to the anterior lens capsule
what is the function of the lens epithelium?
metabolically active - produces DNA, RNA, proteins, lipid, and adenosine triphosphate for energy
what is the location of the greatest activities of premitotic DNA synthesis in the lens epithelium?
ring around the anterior lens - germinative zone
describe lens differentiation in the epithelium.
newly formed cells migrate toward the equator –> elongate, lose organelles/gain protein mass –> differentiate into fibres (optically advantageous)
what meets the energy requirements of the lens fibres?
glycolysis
where do epithelial lens cells begin the process of terminal differentiation into fibres?
bow region (posterior pre-equatorial)
where is the germinative zone?
anterior pre-equatorial
what happens to the lens morphology as new lens fibres are laid down?
no cells are lost; new fibres crowd/compact the previously formed fibres –> older layers in centre, newer layers form the cortex
what are the oldest lens layers?
embryonic and fetal lens nuclei
what are lens sutures?
formed by interdigitation of anterior and posterior tips of spindle shaped fibres
why do multiple optical zones/Y-shaped sutures form?
strata of epithelial cells of differing optical densities are laid down throughout life; gradual transition occurs
