DEVELOPMENT of the HEAD and NECK PLACODES Localized

Home , Lens placode, Otic pit, Otic placode

DEVELOPMENT OF THE Innovations in the early HEAD AND NECK evolution of vertebrates z Development of organs of special sense z Development of a large neural circuitry (the • Placodes and the development of brain) to integrate input and responses organs of special sense z Development of an effective feeding apparatus (jaws) • L. Moss-Salentijn z Development of an improved respiratory apparatus (gills)

Phylogeny of early deuterostomes

Analogues of organs of special sense may develop in lower chordates.

PLACODES NEURAL PLATEPLATE Localized thickened areas of specialized ectoderm, lateral to the neural crest, at

the border between neural NEURAL GROOVEGROOVE plate and the future epidermis.

Different kinds of placodes

z Contributing to organs of special sense: – Olfactory – Lens (only placode that does not have neural fate) – Otic z Contributing to distal ganglia ofof branchiomeric nerves: Example: otic placode. – Trigeminal (Ophthalmic,V1) – Epibranchial (4) z Hypobranchial (2) (contribute toto hypobranchial ganglia -- frogfrog only;only; notnot in chick,chick, mouse,mouse, zebrafish)

Distribution of placodes at 3 developmental stages Development of placodes: similarities

z Under influence of surrounding tissues – no evidence for role of neural crest in this process z All express one or more members of Pax family as transcription factors early in development

A. Initial inductioninduction ofof placodes in pre-pre-placodal ectoderm fieldfield B. Olfactory placodal cells areare incorporatedincorporated in in outer outer folds folds of of anterior neuralneural ridge ridge

Development of placodes - Location of placodes differences z Epibranchial placodes: pharyngeal endodermendoderm (BMP-7 signal), Pax2 and Sox3 (BMP-7 signal), Pax2 and Sox3 z Near forebrain : z Ophthalmic placode of V: neurectoderm of mesencephalon (diffusible signalsignal ?),?), Pax3Pax3 – Olfactory placode z Otic placode: initially axialaxial andand non-axialnon-axial – Lens placode mesoderm, Pax 8; later hindbrain (FGF-3,-8,-10 signals), Pax2, Sox3, Notch z Lens placode: forebrain && anterioranterior mesodermmesoderm (BMP-4, later BMP-7 signals),signals), Pax6,Pax6, laterlater Pax2Pax2 z Olfactory placode: anterior mesodermmesoderm (and(and forebrain? – no signal identified as yet), Pax6Pax6

Location of placodes

z Dorsolateral : Otic placode: related to (= evolved from or having common origin with) lateral line system

Location of placodes

z Intermediate between otic placode and epibranchial placodes : Ophthalmic component of trigeminal placode

Location of placodes z Epibranchial series – dorsal ends of 1st – 4th pharyngeal grooves z Hypobranchial series in frogs – ventral 1. Olfactory ends of 2nd – 3rd pharyngeal grooves ? 2. Otic 3. Trigeminal (V) 4. Facial (VII) 5. Glossopharyngeal (IX) 6. Vagal (X)

Branchiomeric nerves: origins and axon projection patterns

Fates of placodes Development of organs of special sense

Olfactory epithelium: development of the nose Olfactory epithelium: development of the nose

Transient pioneer neurons set up scaffold

Olfactory epithelium

68-84 days Olfactory placode gives rise to:

z Sensory receptor cells of olfactory epithelium of the nose (odorant sensing) z Sensory receptor cells of vomeronasal epithelium (pheromone sensing) z Basal cells and support cells (olfactory ensheathing cells - glia) 46-54 days

Development of the eye : 1. evagination of forebrain (optic vesicle) 2. invagination of lens placode

Optic vesicle formsforms optic optic cup cup under influenceinfluence ofof lens lens primordium . BetweenBetween developingdeveloping lens vesiclevesicle andand opticoptic cup cup : : primary vitreousvitreous body. body.

Hyaloid A.: terminal branch ofof ophthalmic A. (future central artery of retina)

In lenslens vesiclevesicle posteriorposterior cells cells elongate toto formform primary primary lens lens fibers. InIn thirdthird month month anterior epitheliumepithelium elongates elongates to formform secondarysecondary lens lens fibers fibers (most ofof maturemature lens)lens)

Optic cup: NC derived mesenchyme around Inner layerlayer neural retina the optic cup: Outer layer pigment retina Thin inner choroidchoroid Optic stalk: Outer fibrous sclera sclera Axons from neuralneural retina retina grow grow through thethe choroidal fissure toto NC derived mesenchyme anterior brain optic nerve to lens: Anterior layerlayer contributes toto cornea cornea Posterior layerlayer pupillary membrane Between anterioranterior and and posterior posterior layers: anterioranterior chamber chamber of of eye eye Behind posteriorposterior layer: layer: posterior posterior chamber.

Otic placode invagination: Development of inner ear otic pit

Otic pit to otic vesicle Otic pit to otic vesicle Differential growth of otic vesicle

Saccule: ventral, willwill givegive riserise toto mature mature saccule and cochlea. Some placodal cells migratemigrate outout of of vesicular vesicular wall: statoacousticstatoacoustic ganglion ofof CNCN VIIIVIII Utricle: dorsal, willwill givegive riserise to to mature mature utricle, utricle, semicircular semicircular canals andand endolymphatic duct.

Otic capsule: future petrous part of temporal bone