The placodes correspond to thickenings of the rostral surface ectoderm. Each placode consists of a cell group that advances as a result of an induction by a neighboring section of the neural tube into the mesenchym lying just below it and thus develops specific organs (sensory cells and peripheral neurons). Analogously, one can consider the neural plate as a very large placode that separates out from the ectoderm and develops into the neural tube!
From the sensory placodes emerge the receptors of the olfactory (1st cranial nerve) and vestibulocochlear nerves (VIIIth cranial nerves) as well as the eye lenses. With the exception of the eyes in which the light sensitive cells sit in the neuroepithelium and not in the ectodermal placode, the signal converters for external stimuli arise in the placode itself. With this the placodes deliver decisive elements for the development of the higher sensory organs (vision, hearing, balance, smell, taste).
- The olfactory placode induced by the neuroepithelium of the prosencephalons leads to the formation of the fila olfactoria (nervus olfactorius I) as well as of the olfactory bulb (bulbus olfactorius).
- The lens placodes induced by the optic vesicle (neuroepithelium of the diencephalons) develop into the eye lenses
- The auditory placodes induced by the neuroepithelium of the rhombencephalon, pinch themselve off and form the anlage of the innen ear as well as of the nervus vestibulocochlearis (VIIIth cranial nerve).
- Olfactory placode
- Bulbus olfactorius
- Lens placode
- Optic vesicle
- Otic or auditory vesicle
- The epipharyngeal placodes contribute to the formation of the taste buds. Their development is induced by the end branchings of the n. facialis VII, n. glossopharyngeus IX and N. vagus X on the tongue and in the pharynx. These buds consist of secondary sensory cells (and thus are not equipped with an axon), supporting cells and basal cells. They develop mainly between the 5th and 7th months.