Which embryologic event marks the beginning of lens formation?

• A. Thickening of surface ectoderm to form the lens placode
• B. Invagination of neural ectoderm
• C. Regression of hyaloid artery
• D. Formation of optic cup

Answer: (A)

Lens formation begins when the surface ectoderm over the developing eye thickens to form the lens placode, driven by signals from the adjacent optic vesicle. This placode is the first recognizable lens-specific structure, and it then invaginates to create the lens pit and eventually the lens vesicle, setting the stage for the differentiation of the lens fibers.

The other events belong to different parts of ocular development: invagination of neural ectoderm forms the neural tube; formation of the optic cup comes from the invaginated optic vesicle and contributes to the retina; regression of the hyaloid artery is a later vascular change rather than the initiation of lens formation.