Ocular Anatomy and Physiology Flashcards

The foveal avascular zone (FAZ) receives its oxygen supply primarily through which mechanism?

Answer: Diffusion from the underlying choriocapillaris

The FAZ is devoid of retinal capillaries by definition. The foveal photoreceptors — the most metabolically demanding cells in the retina — rely entirely on diffusion of oxygen and nutrients from the choriocapillaris across Bruch's membrane and the retinal pigment epithelium. This is why macular degeneration, which disrupts the choriocapillaris, is so devastating to central vision.

Which of the following best describes the functional significance of the zonula occludens (tight junctions) in the retinal pigment epithelium?

Answer: They form the outer blood-retinal barrier, restricting paracellular transport

Tight junctions (zonula occludens) between RPE cells constitute the outer blood-retinal barrier (oBRB). This barrier prevents unregulated passage of molecules from the fenestrated choriocapillaris into the subretinal space, maintaining the precisely controlled ionic and molecular environment required for photoreceptor function. Breakdown of this barrier is implicated in conditions such as central serous chorioretinopathy.

A patient's corneal endothelium shows a cell density of 1,200 cells/mm². Below which threshold does the endothelium typically lose its ability to maintain adequate corneal dehydration, leading to corneal decompensation?

Answer: 1,000 cells/mm²

Corneal decompensation typically occurs when endothelial cell density falls below approximately 500–1,000 cells/mm², with most clinicians using ~500 cells/mm² as the critical lower threshold. However, 1,000 cells/mm² is the widely accepted clinical warning threshold at which pump reserve is severely compromised and decompensation risk becomes imminent. At 1,200 cells/mm², this patient is near that danger zone.

During dark adaptation, the Purkinje shift refers to which phenomenon?

Answer: A shift in peak spectral sensitivity from ~555 nm (photopic) to ~507 nm (scotopic) as illumination decreases

The Purkinje shift describes the change in the eye's peak spectral sensitivity from approximately 555 nm (green-yellow, cone-mediated photopic vision) to approximately 507 nm (blue-green, rod-mediated scotopic vision) as luminance decreases. This is why red objects appear relatively brighter in daylight but blue objects appear relatively brighter at dusk — rods are more sensitive to shorter wavelengths than cones.

The ciliary body's pars plana is the preferred site for surgical entry (vitrectomy, intravitreal injections) for all of the following reasons EXCEPT:

Answer: It contains the majority of aqueous-secreting non-pigmented epithelial cells

The non-pigmented epithelial cells responsible for aqueous humor secretion are concentrated in the pars plicata (the anterior, ridged portion of the ciliary body with ~70 ciliary processes), NOT the pars plana. The pars plana is the relatively flat, avascular posterior portion that provides a safe surgical corridor — approximately 3.5–4 mm posterior to the limbus — where entry avoids the lens, major ciliary vessels, and the anterior vitreous base.

Which layer of the cornea contains type VII collagen anchoring fibrils that attach the basement membrane to the underlying stroma?

Answer: The epithelial basement membrane zone

Type VII collagen forms anchoring fibrils that span from the epithelial basement membrane into the anterior stroma (anchoring plaques), securing the epithelium to Bowman's layer and stroma. These fibrils are specifically associated with the sub-epithelial basement membrane zone. Mutations in COL7A1 cause dystrophic epidermolysis bullosa, and disruption of these fibrils explains why corneal epithelial erosions occur in anterior basement membrane dystrophy (ABMD/map-dot-fingerprint dystrophy).