A neurogenic placode is an area of thickening of the

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

in the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic head

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

layer that gives rise to neurons and other structures of the

sensory nervous system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved i ...

. Placodes are embryonic structures that give rise to structures such as

hair follicles The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

feathers Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier e ...

and

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

. 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 In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them: * vision (the eye) * hearing and balance (the ear, which includes the auditory system and vestibular system) * smell (the nose) * ta ...

and cranial ganglia. Cranial placodes include a diverse range of structures found across

chordates A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...

, but the neurogenic placodes found in

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

arose later in evolution.

In humans

The cranial placodes that have neurogenic potential (i.e. give rise to

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

) can be divided into two groups, the dorsolateral placodes and the epibranchial placodes. * dorsolateral placodes includes: ** The trigeminal placode, which consists of ophthalmic and maxillomandibular parts, and gives rise to the cells of the

trigeminal ganglion A trigeminal ganglion (or Gasserian ganglion, or semilunar ganglion, or Gasser's ganglion) is the sensory ganglion at the base of each of the two trigeminal nerves (CN V), occupying a cavity ( Meckel's cave) in the dura mater, covering the trige ...

** The

otic placode In embryology, the otic placode is a thickening of the ectoderm on the outer surface of a developing embryo from which the ear develops. The ear, including both the vestibular system and the auditory system, develops from the otic placode beginnin ...

forms the

otic pit The auditory pit, also known as the otic pit, is the first rudiment of the internal ear. It appears shortly after that of the eye, in the form of a patch of thickened ectoderm, the auditory plate, over the region of the hind-brain The hindbrai ...

and the

otic vesicle Otic vesicle, or auditory vesicle, consists of either of the two sac-like invaginations formed and subsequently closed off during embryonic development. It is part of the neural ectoderm, which will develop into the membranous labyrinth of the in ...

, giving rise eventually to organs of hearing and equilibrium. * The epibranchial or ''epipharyngeal'' placodes generate the distal portion of the ganglia of cranial nerves VII, IX and X: ** The

geniculate placode The geniculate ganglion (from Latin ''genu'', for "knee") is a collection of pseudounipolar sensory neurons of the facial nerve located in the facial canal of the head. It receives fibers from the facial nerve. It sends fibers that supply the la ...

, associated with the first

pharyngeal groove A pharyngeal groove (or branchial groove, or pharyngeal cleft) is made up of ectoderm unlike its counterpart the pharyngeal pouch on the endodermal side. The first pharyngeal groove produces the external auditory meatus (ear canal). The rest (2, ...

, generates the

geniculate ganglion The geniculate ganglion (from Latin ''genu'', for "knee") is a collection of pseudounipolar sensory neurons of the facial nerve located in the facial canal of the head. It receives fibers from the facial nerve. It sends fibers that supply the lac ...

and distal parts of

cranial nerve VII The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...

** The

petrosal placode Petrosal may refer to: * Inferior petrosal sinus * Petrosal process * Petrosal nerve (disambiguation) * Petrous ganglion * Petrous part of the temporal bone The petrous part of the temporal bone is pyramid-shaped and is wedged in at the base of ...

, associated with the second pharyngeal groove, generates the inferior ganglion of glossopharyngeal nerve and distal parts of

cranial nerve IX The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper Medulla oblongata, medulla, just anterior (closer to the nose) to t ...

** The nodosal placode, associated with the third

branchial cleft A pharyngeal groove (or branchial groove, or pharyngeal cleft) is made up of ectoderm unlike its counterpart the pharyngeal pouch on the endodermal side. The first pharyngeal groove produces the external auditory meatus (ear canal). The rest (2, ...

, generates the

nodose ganglion The inferior ganglion of the vagus nerve, (nodose ganglion) is a sensory ganglion of the peripheral nervous system. It is located within the jugular foramen where the vagus nerve exits the skull. It is larger than and below the superior ganglion ...

and distal parts of

cranial nerve X The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right v ...

* The

olfactory placode The nasal placode (or olfactory placode) gives rise to the olfactory epithelium of the nose. Two nasal placodes arise as thickened ectoderm from the frontonasal process. They give rise to the nose, the philtrum of the upper lip, and the primary pa ...

(or ''nasal placode)'' gives rise to the

olfactory epithelium The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory ...

of the

nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes th ...

. * The cranial placodes that do not give rise to neurons are: ** The

lens placode The Lens placode is a thickened portion of ectoderm which serves as the precursor to the lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piec ...

under the direction of the

optic vesicle The eyes begin to develop as a pair of diverticula (pouches) from the lateral aspects of the forebrain. These diverticula make their appearance before the closure of the anterior end of the neural tube; after the closure of the tube around the 4th ...

gives rise to the

lens of the eye The lens, or crystalline lens, is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it ca ...

. ** The adenohypophyseal placode, which forms the

anterior lobe of the pituitary gland A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis or pars anterior) is the glandular, anterior lobe that together with the posterior lobe (posterior pituitary, or the neurohypophysis) makes up the p ...

Other animals

* The profundal placode, corresponding to the ophthalmic lobe of the trigeminal complex. In

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...

this remains partly unfused. * In fish and larval amphibians, the lateral line placodes, which give rise to the

lateral line system The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

. * The

hypobranchial placodes Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arc ...

, a neurogenic placode found in some

amphibians Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbore ...

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

that give rise to structures such as

mammary glands A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in prima ...

feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...

s and

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

References

{{reflist Embryology of nervous system