Placodes

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Introduction

Human Embryo stage 14 sensory placodes

Placodes are ectodermal thickenings which have important roles in development of special sensory systems.

In human development, during week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region.

Recent research suggests that all sensory placodes may arise from common panplacodal primordium origin around the neural plate, and then differentiate to eventually have different developmental fates. These sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell). Note that their initial postion on the developing head is significantly different to their final position in the future sensory system.

  • Adenohypophyseal placode
  • Otic placodes - the first placodes visible on the surface of the embryo.
  • Olfactory (Nasal) placodes - has 2 components (medial and lateral) and will form the nose olfactory epithelium.
  • Optic (Lens) placodes - lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.
  • Profundal/trigeminal placodes


Other species have a number of other placodes which form additional sensory structures (fish, lateral line receptor).

  • Epibranchial placodes
  • Lateral line placodes
  • Hypobranchial placodes
Links: Week 4 | Otic Placode | Optic Placode | Nasal Placode | Sensory System Development

Some Recent Findings

  • Graded levels of Pax2a and Pax8 regulate cell differentiation during sensory placode formation[1] "Pax gene haploinsufficiency causes a variety of congenital defects. Renal-coloboma syndrome, resulting from mutations in Pax2, is characterized by kidney hypoplasia, optic nerve malformation, and hearing loss. ..We sho.w that differential levels of zebrafish Pax2a and Pax8 modulate commitment and behavior in cells that eventually contribute to the otic vesicle and epibranchial placodes."
  • Mutual repression between Gbx2 and Otx2 in sensory placodes reveals a general mechanism for ectodermal patterning[2] "In the vertebrate head, central and peripheral components of the sensory nervous system have different embryonic origins, the neural plate and sensory placodes. This raises the question of how they develop in register to form functional sense organs and sensory circuits. Here we show that mutual repression between the homeobox transcription factors Gbx2 and Otx2 patterns the placode territory by influencing regional identity and by segregating inner ear and trigeminal progenitors. Activation of Otx2 targets is necessary for anterior olfactory, lens and trigeminal character, while Gbx2 function is required for the formation of the posterior otic placode. Thus, like in the neural plate antagonistic interaction between Otx2 and Gbx2 establishes positional information thus providing a general mechanism for rostro-caudal patterning of the ectoderm."
  • An effective assay for high cellular resolution time-lapse imaging of sensory placode formation and morphogenesis[3] "This new imaging assay provides a powerful method to analyze directly development of placode-derived sensory neurons and subsequent ganglia formation for the first time in amniotes. Viewing placode development in a head cross-section provides a vantage point from which it is possible to study comprehensive events in placode formation, from differentiation, cell ingression to ganglion assembly. Understanding how placodal neurons form may reveal a new mechanism of neurogenesis distinct from that in the central nervous system and provide new insight into how cells acquire motility from a stationary epithelial cell type."
  • Epibranchial Placodes[4] "The inner ear and the epibranchial ganglia constitute much of the sensory system in the caudal vertebrate head. ...However, recent studies indicate that both systems arise from a morphologically distinct common precursor domain: the posterior placodal area. This review summarises recent studies into the induction, morphogenesis and innervation of these systems and discusses lineage restriction and cell specification in the context of their common origin."
  • Otic Placode[5] "The inner ear epithelium, with its complex array of sensory, non-sensory, and neuronal cell types necessary for hearing and balance, is derived from a thickened patch of head ectoderm called the otic placode. ...Collectively, our results suggest that Wnt8a provides the link between FGF-induced formation of the pre-otic field and restriction of the otic placode to ectoderm adjacent to the hindbrain."
  • Postotic Placode[6] "The (zebrafish) embryonic line originates from a postotic placode that produces both a migrating sensory primordium and afferent neurons. Nothing is known about the origin and innervation of the larval lines. Here we show that a "secondary" placode can be detected at 24 h postfertilization (hpf), shortly after the primary placode has given rise to the embryonic primordium and ganglion."

Preplacodal Development

Preplacodal development model.jpg

Preplacodal development model[7]

Late Blastula Stage
  • Bmp acts as a morphogen that specifies neural crest (NC) within a narrow but low range of signalling.
  • Higher levels of Bmp signaling establish the non-neural ectoderm as a broad zone of uncommitted cells with potential to form epidermal or preplacodal ectoderm (PPE).
  • Within the non neural ectoderm
    • changing levels of Bmp do not distinguish preplacodal from epidermal potential.
    • preplacodal competence factors are uniformly induced throughout this domain.
  • expression of tfap2a/c overlaps with the lateral edges of the neural plate where, perhaps in combination with neural markers, they cell-autonomously specify NC fate.
Late Gastrula Stage (9–10 hpf)
  • preplacodal ectoderm (PPE) fate is specified in competent cells near the neural-nonneural border by dorsally expressed Bmp antagonists, Fgf and Pdgf.
  • Complete attenuation of Bmp is required for PPE specification.


Relevant markers for each ectodermal domain are shown.

Experiments carried out in zebrafish.

(Above text from figure legend[7])

Otic Placode

The otic placode is the first of the sensory placodes visible on the surface of the developing human embryo. This placode will differentiate to contribute almost entirely the components of the inner ear. The images below show the first appearance on the embryo surface during week 4 and the eventual disappearance from the surface by week 5. This is only the beginning of the complex development of this structure, influenced by the surrounding epidermis, neural tube and neural crest.

Stage 11

The scanning EM of the week 4 human embryo Carnegie stage 11 shown below is a superior dorsal view of the paired otic placodes sinking into the surface at the level of the hindbrain between day 24 and day 25.

Stage11 sem13.jpg Stage11 sem20.jpg

Stage 12

By Carnegie stage 12 26 days, only a small opening of the developing otic vesicle (otocyst) remains visible on the embryo surface located behind the second pharyngeal arch.

Stage12 sem4.jpg Stage12 sem5.jpg

Stage 13

By week 5 Carnegie stage 13 the otic vesicle (otocyst) is completely formed and is no longer visible on the embryo surface.

Stage13 sem1.jpg

Cross-sections of the embryo head at this stage show the otocyst now lies within the embryo as a hollow fluid-filled epithelial "ball", located between the epidermis and the neural tube (hindbrain).

Stage 13 serial labeled images
Stage 13 image 051.jpg Stage 13 image 052.jpg Stage 13 image 053.jpg Stage 13 image 054.jpg
A2L A3L A4L A5L


Links: Inner Ear | Hearing and Balance Development

Adenohypophyseal Placode

The hypophysis, or pituitary, is an endocrine gland that links the brain to peripheral endocrine organs and systems of the body through several specific hormones. The developmental origin of the hypophysis is unique, with epithelial origins from neural ectoderm (posterior) and from surface ectoderm (anterior) the adenohypophyseal placode.


In the mouse, gonadotropin-releasing hormone-1 neurones control the release of gonadotropins from the anterior pituitary and were thought to originate from the adenohypophyseal placed. A recent study has shown that they are really associated early with the formation of the nasal placode.[8]

Drosophila and mouse placode similarity.jpg

Drosophila and mouse placode similarity[9]


Links: Pituitary Development

Olfactory Placodes

(Nasal)

Optic Placodes

Human lens development (Carnegie stage 22, Week 8)

Optic placodes (Lens) lie on the embryo surface, adjacent to the out-pocketing of the nervous system (forms the retina) and will form the lens.

surface ectoderm -> lens placode -> lens pit -> lens vesicle -> lens fibres -> lens capsule and embryonic/fetal nucleus.

Links: Lens Development | Vision Development

Trigeminal Placodes

(Profundal)


Links: Profundal/trigeminal placodes

Epibranchial Placodes

Epibranchial ganglia sensory neurons formed by the facial, glossopharyngeal, and vagal placodal regions. These ganglia neurons relay from the sensory organs such as gustatory taste buds, heart baroreceptors, gut sensory enteric nerves.

Sensory System

Hearing cartoon.jpg
Senses Links: Introduction | placode | Hearing and Balance hearing | balance | vision | smell | taste | touch | Stage 22 | Category:Sensory
| original Sensory page
Hearing Links: Introduction | inner ear | middle ear | outer ear | balance | placode | hearing neural | Science Lecture | Lecture Movie | Medicine Lecture | Stage 22 | hearing abnormalities | hearing test | sensory | Student project

  Categories: Hearing | Outer Ear | Middle Ear | Inner Ear | Balance

Historic Embryology - Hearing 
Historic Embryology: 1880 Platypus cochlea | 1892 Vertebrate Ear | 1902 Development of Hearing | 1906 Membranous Labyrinth | 1910 Auditory Nerve | 1913 Tectorial Membrane | 1918 Human Embryo Otic Capsule | 1918 Cochlea | 1918 Grays Anatomy | 1922 Human Auricle | 1922 Otic Primordia | 1931 Internal Ear Scalae | 1932 Otic Capsule 1 | 1933 Otic Capsule 2 | 1936 Otic Capsule 3 | 1933 Endolymphatic Sac | 1934 Otic Vesicle | 1934 Membranous Labyrinth | 1934 External Ear | 1938 Stapes - 7 to 21 weeks | 1938 Stapes - Term to Adult | 1940 Stapes | 1942 Stapes - Embryo 6.7 to 50 mm | 1943 Stapes - Fetus 75 to 150 mm | 1946 Aquaductus cochleae and periotic (perilymphatic) duct | 1946 aquaeductus cochleae | 1948 Fissula ante fenestram | 1948 Stapes - Fetus 160 mm to term | 1959 Auditory Ossicles | 1963 Human Otocyst | Historic Disclaimer

Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9

Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline

References

  1. <pubmed>22745314</pubmed>
  2. <pubmed>22564795</pubmed>
  3. <pubmed>2155472</pubmed>| BMC Neurosci.
  4. <pubmed>20460364</pubmed>
  5. <pubmed>20171206</pubmed>
  6. <pubmed>20554875</pubmed>
  7. 7.0 7.1 <pubmed>20885782</pubmed>| PLoS Genet.
  8. <pubmed>20008041</pubmed>
  9. <pubmed>18056636</pubmed>| PMC2148390

Online Textbooks


Search Bookshelf placode development

Reviews

<pubmed>20801420</pubmed> <pubmed>20460364</pubmed> <pubmed>9927591</pubmed>

Articles

17205191 15380243 10906460

Search Pubmed

June 2010 "placode development" All (852) Review (90) Free Full Text (285)

Search Pubmed placode development | otic placode development | optic placode development | nasal placode development | adenohypophyseal placode development

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Cite this page: Hill, M.A. (2024, March 29) Embryology Placodes. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Placodes

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G