IntroductionDuring week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region. These sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell). Other species have a number of additional placodes which form other sensory structures (fish, lateral line receptor). Note that their initial postion on the developing head is significantly different to their final position in the future sensory system (More? Sensory Notes). Otic placode- in the stage 13/14 embryo (shown below) the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst, which now lies beneath the surface surrounded by mesenchyme (mesoderm). The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth. Lens placode- lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens. Nasal placode- has 2 components (medial and lateral) and will form the nose olefactory epithelium.
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Stage 12 (dorsolateral view) showing otic placode |
Page Links: Introduction | Some Recent Findings | Otic Placode | Lens Placode | Nasal Placode | Development Overview | Molecular Factors | References | Glossary
Related Pages: Sensory Notes | Hearing and Balance | Vision | Smell
Streit A. The preplacodal region: an ectodermal domain with multipotential progenitors that contribute to sense organs and cranial sensory ganglia. Int J Dev Biol. 2007;51(6-7):447-61.
"At the base of placode induction lies the specification of a unique "placode field", termed the preplacodal region and acquisition of this "preplacodal state" is required for ectodermal cells to undergo otic development. Here I review the molecular mechanisms that sequentially subdivide the ectoderm to give rise to the placode territory."
Embryo (stage 13/14)- Otocyst Images below show a region within a series of cross-section slices in the head region showing the otocyst (otic vesicle). A structure formed from the otic placode pinching off from the embryo surface and sinking into the head mesenchyme, which will form components of the future inner ear. (More? Hearing and Balance)
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Al: Thin caudolateral walls of rhombencephalon. |
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A2: Otocyst (R). Apex of otocyst (primordium of L endolymphatic sac) |
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A3: Otocyst; surrounding mesenchyme = otic capsule. Note proximity of otocyst to wall of rhombencephalon. Superior glossopharyngeal ganglion. Vestibulo-cochlear-facial ganglion complex (R). Trigeminal ganglion (not in image excerpt). |
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A4: Indentation in rostral margin of otocyst = primordium of utriculosaccular canal. Note alignment of superior cardinal vein (LS) in relation to the nerve trunks (XS). Trigeminal ganglion (not in image excerpt). Note variation in height of cells of L otocyst wall. Dorsal end of 1st pharyngeal arch and groove (not in image excerpt). |
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A5: Facial ganglion (R). "Floor" of L otocyst. Note on R, the former position of the otocyst in relation to the 2nd pharyngeal arch. |
Embryo (stage 13/14)- Lens Placode Links below are to a series of cross-section slices in the head region showing the optic placode (lens placode). The optic placode will pinch off from the embryo surface forming a lens vesicle which will form the lens of the future eye. (More? Vision)
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B1: Developing diencephalon. Rathke's pouch. Lens pit (derived from thickened ectoderm of lens or "optic" placode). Optic nerve. Note extensive temporal region of retina. Primordium of nasolacrimal ducts. |
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B2: Optic cup with optic ventricle. Note unequal thickness of walls of optic cup (inner wall; sensory retina: outer wall; pigment epithelium). Lens pit. Note position of eye with respect to maxillary arch. Hyaloid vessels near ventral margin of optic cup. |
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B3: Optic cup with optic ventricle. Note unequal thickness of walls of optic cup (inner wall; sensory retina: outer wall; pigment epithelium). Note position of eye with respect to maxillary arch. Hyaloid vessels near ventral margin of optic cup. |
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B4: Junction of optic stalk to wall of prosencephalon (diencephalon). Note small branches of hyaloid blood vessels in region between lens pit and optic stalk. |
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B5: Continuity of optic ventricle and single forebrain ventricle via optic stalk. Note proximity of eye to nasal placode (R). |
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B6: Hyaloid vessels ventral to optic stalk. Optic mesenchyme (mesectoderm) arising directly from epithelium of dorsal surface of optic cup. Nasal placodes. |
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B7: Tangential section through optic cup, with surrounding optic mesenchyme. Nasal placode Interorbital ligament. Bridge of nose. |
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Animation of Eye Sections (B1-B7, 128 Kb) |
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Embryo (stage 13/14)- Nasal Placode Images below show a region within a series of cross-section slices in the head region showing the nasal placode. The nasal placode will remain on the embryo surface and will form the sensory epithelium of the future nose. (More? Smell )
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B5: Nasal placode, note its proximity to the eye. Note the large number of blood vessels lying in the mesenchyme directly behind placode. |
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B6: Nasal placodes. Note the lateral and medial portions of the placode. |
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B7: Nasal placode. |
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C1-C7: Nasal placodes (nasal pit forming on one side). Forebrain. note proximity of nasal pit to prosencephalon (future site of olfactory nerve). |
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C2-C4: Identify the medial and lateral nasal swellings. |
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Week 4 (Clinical Week 6)
Day |
Stage |
Event |
22 |
Stage 10 |
Neural Crest – differentiation at spinal cord level from day 22 until day 26 Neural – neural folds begin to fuse near the junction between brain and spinal cord, when Neural Crest cells are arising mainly from the neural ectoderm Neural Crest – trigeminal, facial, and postotic ganglia components visible (PMID: 17848161) Brain – rostral neural tube forms 3 primary brain vesicles (week 4) Respire – Week 4 - laryngotracheal groove forms on floor foregut. |
23 |
Heart – begins to beat in Humans by day 22-23, first functioning embryonic organ formed. |
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24 |
Stage 11 |
Thyroid – thyroid median endodermal thickening in the floor of pharynx Neural – rostral (or cephalic) neuropore closes within a few hours; closure is bidirectional, it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently. Optic ventricle appears |
25 |
Stage 12 |
Pituitary Week 4 – hypophysial pouch, Rathke’s pouch, diverticulum from roof GIT - Liver – septum transversum forming liver stroma and hepatic diverticulum forming hepatic trabeculae (PMID: 9407542) Neural – caudal neuropore takes a day to close (closure is approximately at future somitic pair 31/sacral vertebra 2) Neural – secondary neurulation begins Neural Crest – cardiac crest, neural crest from rhombomeres 6 and 7 that migrates to pharyngeal arch 3 and from there the truncus arteriosus (PMID: 17848161) |
26 |
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27 |
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28 |
Stage 13 |
Neural – the neural tube is normally completely closed, ventricular system now separated from amniotic fluid. Neural crest at spinal level is segregating, and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop. (PMID: 3354839) GIT - Liver – epithelial cord proliferation enmeshing stromal capillaries (PMID: 9407542) Sense - Smell – Crest comes from the nasal plates (PMID: 15604533) Skin – 4 weeks - simple ectoderm epithelium over mesenchyme Skin – 1-3 months ectoderm- germinative (basal) cell repeated division of generates stratified epithelium; mesoderm- differentiates into connective tissue and blood vessels |
Links: Textbooks | Online Textbooks | Search Textbooks | Reviews | Articles | Search PubMed | Glossary
Specific references can also be found on each notes page and Selected References 1999
Developmental Biology (6th ed) Gilbert, Scott F. Sunderland (MA): Sinauer Associates, Inc.; c2000. Early Mammalian Development
Search NLM Online Textbooks "sensory placode" : Developmental Biology | Endocrinology | Molecular Biology of the Cell | The Cell- A molecular Approach
Reviews
Streit A. The preplacodal region: an ectodermal domain with multipotential progenitors that contribute to sense organs and cranial sensory ganglia. Int J Dev Biol. 2007;51(6-7):447-61.
Schlosser G. Induction and specification of cranial placodes. Dev Biol. 2006 Jun 15;294(2):303-51.
Brugmann SA, Moody SA. Induction and specification of the vertebrate ectodermal placodes: precursors of the cranial sensory organs. Biol Cell. 2005 May;97(5):303-19.
Bhattacharyya S, Bronner-Fraser M. Hierarchy of regulatory events in sensory placode development. Curr Opin Genet Dev. 2004 Oct;14(5):520-6.
Articles
Pézeron G, Mourrain P, Courty S, Ghislain J, Becker TS, Rosa FM, David NB. Live analysis of endodermal layer formation identifies random walk as a novel gastrulation movement. Curr Biol. 2008 Feb 26;18(4):276-81.
Search Jun 2008 "sensory placode" 236 reference articles of which 35 were reviews.
Search PubMed: term = sensory placode | sensory placode development | week 4 sensory placode |
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Week 4 a series of paired surface specialized epithelial patches, sensory placodes, form in the head region. These placodes will contribute essential components of the future sensory systems. For more details on later development of these sensory placodes look at Sensory Notes
Please email Dr Mark Hill if you wish to make a comment about this current project.
