|Gives rise to||cranial ganglia, peripheral nervous system|
Placodes are embryonic structures that give rise to structures such as hair follicles, feathers and teeth. The term "neurogenic placode" generally refers to cranial placodes that have neurogenic potential - i.e. those that give rise to neurons associated with the special senses and cranial ganglia. Cranial placodes include a diverse range of structures found across chordates, but the neurogenic placodes found in vertebrates arose later in evolution.
The cranial placodes that have neurogenic potential (i.e. give rise to neurons) can be divided into two groups, the dorsolateral placodes and the epibranchial placodes.
- dorsolateral placodes includes:
- The epibranchial or epipharyngeal placodes generate the distal portion of the ganglia of cranial nerves VII, IX and X:
- The geniculate placode, associated with the first pharyngeal groove, generates the geniculate ganglion and distal parts of cranial nerve VII
- The petrosal placode, associated with the second pharyngeal groove, generates the inferior ganglion of glossopharyngeal nerve and distal parts of cranial nerve IX
- The nodosal placode, associated with the third branchial cleft, generates the nodose ganglion and distal parts of cranial nerve X
- The olfactory placode (or nasal placode) gives rise to the olfactory epithelium of the nose.
- The cranial placodes that do not give rise to neurons are:
- The profundal placode, corresponding to the ophthalmic lobe of the trigeminal complex. In Xenopus this remains partly unfused.
- In aquatic vertebrates, the lateral line placodes, which give rise to the lateral line system.
- The hypobranchial placodes, a neurogenic placode found in some amphibians of unknown function
Other ectodermal placodes
The term placode or ectodermal placode is sometimes used to refer specifically to cranial or neurogenic placodes, but is also used for areas of the ectoderm that give rise to structures such as mammary glands, feathers and hair.
- Park, Saint-Jeannet (2010). Induction and Segregation of the Vertebrate Cranial Placodes. Morgan & Claypool Life Sciences.
- hednk-027—Embryo Images at University of North Carolina