Placodes

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

Other Placodes?
		Note that a second, later developing, form of ectodermal placode development occurs with the development of hair follicles and other integumentary specialisations. This topic does not directly relate to the specialised placodes of the head region covered here. (More? Hair Development | Integumentary_System Development)

Placode Links: placode | Week 4 | sensory | Otic Placode | Optic Placode | Nasal Placode | Category:Placode

Historic Embryology Placodes   1928 Sparrow Embryo | 1934 Auditory Placode Some Recent Findings Review - Transcriptional regulation of cranial sensory placode developmentCranial sensory placodes derive from discrete patches of the head ectoderm and give rise to numerous sensory structures. During gastrulation, a specialized "neural border zone" forms around the neural plate in response to interactions between the neural and nonneural ectoderm and signals from adjacent mesodermal and/or endodermal tissues. This zone subsequently gives rise to two distinct precursor populations of the peripheral nervous system: the neural crest and the preplacodal ectoderm (PPE). The PPE is a common field from which all cranial sensory placodes arise (adenohypophyseal, olfactory, lens, trigeminal, epibranchial, otic). Members of the Six family of transcription factors are major regulators of PPE specification, in partnership with cofactor proteins such as Eya. Six gene activity also maintains tissue boundaries between the PPE, neural crest, and epidermis by repressing genes that specify the fates of those adjacent ectodermally derived domains. As the embryo acquires anterior-posterior identity, the PPE becomes transcriptionally regionalized, and it subsequently becomes subdivided into specific placodes with distinct developmental fates in response to signaling from adjacent tissues. Each placode is characterized by a unique transcriptional program that leads to the differentiation of highly specialized cells, such as neurosecretory cells, sensory receptor cells, chemosensory neurons, peripheral glia, and supporting cells. In this review, we summarize the transcriptional and signaling factors that regulate key steps of placode development, influence subsequent sensory neuron specification, and discuss what is known about mutations in some of the essential PPE genes that underlie human congenital syndromes." Setting appropriate boundaries: Fate, patterning and competence at the neural plate border "The neural crest and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm into neural and non-neural regions, followed by the emergence of neural crest and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions." Graded levels of Pax2a and Pax8 regulate cell differentiation during sensory placode formation "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 "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 "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 "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 "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 "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." More recent papers This table allows an automated computer search of the external PubMed database using the listed "Search term" text link. This search now requires a manual link as the original PubMed extension has been disabled. The displayed list of references do not reflect any editorial selection of material based on content or relevance. References also appear on this list based upon the date of the actual page viewing. References listed on the rest of the content page and the associated discussion page (listed under the publication year sub-headings) do include some editorial selection based upon both relevance and availability. More? References | Discussion Page | Journal Searches | 2019 References | 2020 References Search term: Placode <pubmed limit=5>Placode</pubmed> Preplacodal Development Preplacodal development model 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) 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. 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. Stage 13 By week 5 Carnegie stage 13 the otic vesicle (otocyst) is completely formed and is no longer visible on the embryo surface. 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 A2L A3L A4L A5L Links: Inner Ear | Hearing and Balance Development Adenohypophyseal Placode Pituitary Development 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. Drosophila and mouse placode similarity 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 Zebrafish placode model 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 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 Online Textbooks Colloquium Series on Developmental Biology Induction and Segregation of the Vertebrate Cranial Placodes Park BY, Saint-Jeannet JP. San Rafael (CA): Morgan & Claypool Life Sciences; 2010. Developmental Biology (6th ed.) Gilbert, Scott F. Sunderland (MA): Sinauer Associates, Inc.; c2000. Chick embryo rhombomere neural crest cells | Some derivatives of the pharyngeal arches | Tissue Architecture of the Central Nervous System | Neuronal Types | Snapshot Summary: Central Nervous System and Epidermis Neuroscience Purves, Dale; Augustine, George J.; Fitzpatrick, David; Katz, Lawrence C.; LaMantia, Anthony-Samuel; McNamara, James O.; Williams, S. Mark. Sunderland (MA): Sinauer Associates, Inc. ; c2001 The Auditory System | The Inner Ear | Early Brain Development Clinical Methods 63. Cranial Nerves IX and X: The Glossopharyngeal and Vagus Nerves | The Tongue | 126. The Ear and Auditory System | An Overview of the Head and Neck - Ears and Hearing | Audiometry Eurekah Bioscience Collection Cranial Neural Crest and Development of the Head Skeleton 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

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