Placode

A placode is an area of thickening in the embryonic epithelial layer where some organ or structure later develops. The term usually refers to cranial placodes, peripheral nervous system structures associated with the special senses and cranial ganglia. Placodes also develop in the formation of other structures, such as teeth and feathers.

Cranial Placodes

The placodes associated with the special senses are:

• The otic placode forms the otic pit and the otic vesicle, giving rise eventually to organs of hearing and equilibrium.

• The lens placode under the direction of the optic vesicle gives rise to the lens of the eye.

• The nasal placode (or olfactory placode) gives rise to the olfactory epithelium of the nose.

• The lateral line placodes (in aquatic vertebrates) give rise to the lateral line system.