Basic Sciences and Ocular Anatomy/Physiology
This domain tests the anatomy, histology, physiology, embryology, and basic pathology that underpin every clinical station. Expect corneal layers, aqueous dynamics, retinal structure, visual pathway anatomy, and autonomic control of the pupil.
Core anatomy and physiology
- Cornea has five layers: epithelium (regenerates from limbal stem cells), Bowman layer (acellular, does not regenerate), stroma (~90% of thickness; type I collagen), Descemet membrane (regenerates; type IV collagen), and endothelium (non-replicating monolayer, ~2,500 cells/mm² at birth, lost ~0.6%/year). Endothelial Na+/K+-ATPase keeps the stroma deturgesced.
- Aqueous is secreted by ciliary epithelium (carbonic anhydrase, beta receptors) and drains mainly through trabecular meshwork into Schlemm canal (~80–90%); uveoscleral outflow is minor and prostaglandin-responsive.
- Retina has 10 layers; the fovea is cone-only and avascular; rod density peaks about 20° from fixation. Phototransduction hyperpolarizes photoreceptors.
- Visual pathway: nasal retinal fibers cross at the chiasm; temporal fibers stay ipsilateral. Chiasmal lesions give bitemporal hemianopia; optic tract lesions give homonymous defects.
- Pupil: parasympathetic (CN III, ciliary ganglion) constricts; sympathetic (superior cervical chain) dilates.
- Embryology: optic vesicle from forebrain; lens from surface ectoderm; corneal endothelium/stroma and uveal melanocytes from neural crest.
Worked example
A 72-year-old has corneal edema after phacoemulsification. Reasoning: the endothelium is a non-dividing monolayer; surgical trauma reduces pump and barrier function, so stromal water rises. If specular microscopy shows fewer than 1,000 cells/mm², recovery is slow and hypertonic saline may help. The edema is not caused by Bowman layer, which is acellular.
Traps
Bowman layer does not regenerate; Descemet does. Nasal fibers cross, not temporal. Aqueous outflow is mainly trabecular. The fovea has no rods.