Placode

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(embryology) A platelike epithelial thickening, frequently marking, in the embryo, the anlage of an organ or part.

Thick, flat sections of embryonic ectoderm from which the sense organs develop.

An area of thickening in the embryonic epithelial layer from which some organ or structure later develops.

A platelike structure, especially a thickening of the ectoderm marking the site of future development in the early embryo of an organ of special sense, e.g. the auditory placode (ear), lens placode (eye) and olfactory placode (nose).

• lens p. — the ectodermal thickening which develops into the lens vesicle and later the lens.
• nasal p. — one of a pair of ectodermal thickenings which are the forerunners of the external nares and nasopharyngeal epithelium.
• olfactory p. — an ectodermal thickening of the embryo which ultimately provides the sensory nerves for the olfactory region of the nasal mucosa.
• otic p. — one of the pair of ectodermal thickenings in the vertebrate embryo which invaginates and is a major contributor to the internal ear.

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 Adenohypophyseal placode

• 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 olfactory placode (or nasal placode^[1]) gives rise to the olfactory epithelium of the nose.

• The trigeminal placode consists of opthalmic and maxillomandibular parts.

• The epibranchial placodes consist of the geniculate, petrosal, and nodose placodes.

Non human species

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

• The profundal placodes

References

1. ^ hednk-027 — Embryology at UNC

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