Anatomy, Physiology and Embryology of the Eye
Core concepts
Know the layers of the cornea (epithelium, Bowman, stroma, Descemet, endothelium) and that the endothelium is neural-crest derived and non-regenerating in humans. Trace the aqueous pathway: ciliary epithelium → posterior chamber → pupil → anterior chamber → trabecular meshwork → Schlemm canal → collector channels → episcleral veins; the non-pigmented ciliary epithelium is the secretory layer. Understand optic nerve head anatomy and the blood supply of the retina (central retinal artery supplies inner retina; choroid supplies outer retina).
Embryology is heavily tested: the optic vesicle invaginates to form the optic cup and stalk; surface ectoderm forms the lens vesicle; neural crest forms corneal stroma/endothelium, sclera, uveal melanocytes, and orbital bones. Recall that the hyaloid artery regresses to form Cloquet canal, and that the fetal fissure closes around week 7.
Traps
- Confusing which layer regenerates (epithelium yes, endothelium no).
- Mislabelling the ora serrata versus pars plana.
- Forgetting that the ciliary body produces aqueous but the trabecular meshwork drains it.
- Pupillary light reflex pathway: afferent is optic nerve, efferent is oculomotor (parasympathetic).
Rule of thumb
If a structure arises from neural crest, suspect it in anterior segment dysgenesis syndromes (e.g., Axenfeld-Rieger). If it arises from neuroectoderm, suspect it in retinal/uveal disease.
Worked example
A question asks which layer prevents corneal edema. Answer: the endothelium, via its Na/K-ATPase pump; its failure causes stromal edema and later epithelial bullae.