DEVELOPMENT OF THE HEAD AND NECK


	Placodes and the development of organs of special sense

	L. Moss-Salentijn

Innovations in the early evolution of vertebrates


	Development of organs of special sense
	Development of a large neural circuitry (the brain) to integrate input and responses
	Development of an effective feeding apparatus (jaws)
	Development of an improved respiratory apparatus (gills)

Phylogeny of early deuterostomes

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PLACODES
Localized thickened areas of specialized ectoderm, lateral to the neural crest, at the border between neural plate and the future epidermis.

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Different kinds of placodes


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

Distribution of placodes at 3 developmental stages

Development of placodes: similarities


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

Development of placodes - differences


	Epibranchial placodes: pharyngeal endoderm (BMP-7 signal), Pax2 and Sox3
	Ophthalmic placode of V: neurectoderm of mesencephalon (diffusible signal ?), Pax3
	Otic placode: initially axial and non-axial mesoderm, Pax 8; later hindbrain (FGF-3,-8,-10 signals), Pax2, Sox3, Notch
	Lens placode: forebrain & anterior mesoderm (BMP-4, later BMP-7 signals), Pax6, later Pax2
	Olfactory placode: anterior mesoderm (and forebrain? – no signal identified as yet), Pax6

Location of placodes

Location of placodes


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

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Location of placodes


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

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Location of placodes


	Epibranchial series – dorsal ends of 1^st – 4^th pharyngeal grooves
	Hypobranchial series in frogs – ventral ends of 2^nd – 3^rd pharyngeal grooves ?

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Branchiomeric nerves: origins and axon projection patterns

Fates of placodes

Development of organs of special sense

Olfactory epithelium: development of the nose

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Olfactory placode gives rise to:


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

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