ectoderm germ layer - dr. michael belanich · 2020. 1. 26. · vegt and veg1 present in vegetal...
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Ectoderm Germ Layer
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Frog Fate Map
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Frog Fate Map
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Role of Organizer Chages in Late Frog Embryos
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Organizer forms three distinct regions
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Notochord formation in chick
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Beta-catenin localization
How does beta-catenin become localized to dorsal side?Beta-catenin evenly distributed before fertilization.Dsh and GBP translocated to dorsal side by kinesin
• Rides on microtubule tracks laid down after fertilization during cortical rotation
• Dorsal surface enriched in Dsh and GBP• Dsh and GBP inhibit GSK3
– Beta-catenin degrades on ventral side and is stabilized on dorsal side
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Beta-catenin activates other genes
Turns on goosecoid gene in organizer
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Goosecoid mRNA can induce new axis
Activates involution Determines dorsal mesoderm Represses Wnt8 Important in brain formation
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BMP Gradient Controls Dorsal/Ventral Axis
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VegT and Veg1
Present in vegetal cortical regionGenes required for endoderm and mesoderm
formationVegT antisence RNA causes epidermis only
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Xnr Gradient
High Xnr – Organizer, goosecoidMed Xnr – Lateral mesodermLow Xnr > high BMP4 + Xwnt8 – Ventral mesoderm
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Neural ectoderm induced by soluble factor?
Controlled by goosecoid and beta-catenin?
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Neural structures default – epidermis induced by BMP's
BMP – bone morphogenic proteinInduce ectoderm to become epidermisOrganizer secretes factors that block BMP from acting
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BMP inhibitors
Noggin, Chordin and FollistatinDiffusable proteinsInduced dorsal ectoderm to become neuralDorsalizes mesoderm cellsInhibits BMP'sFound in dorsal lip then in the notochord
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Noggin mRNA rescues Dorsal Structures
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Localization of Noggin
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Localization of Chordin mRNA
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Expression of paraxial protocadherin
Paraxial protocadherin expression
Blue=paraxial protocadherinRed=chordin
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How is the ant/pos axis determined?
During neurulation beta-catenin forms a gradientGreatest concentration at organizer
• Becomes posterior end
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BMP Gradient Controls Dorsal/Ventral Axis
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Wnt Gradient Controls Anterior/Posterior Axis
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Gastrulation Induces Nervous System
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Chick Hensen's Node Can Induce Gene Expression in Frog
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Development of Neural Crest
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Establishing Neural Cells• Competence
• Cell has the capacity to be induced to become neuroblasts
• Specification• Cell have been induced to be neuroblasts, but can
develop into other cells if signals change• Commitment (determination)
• Neuroblast has entered the neural differentiation pathway. Cannot be influenced by inhibitory signals.
• Differentiation• Neuroblast stop mitosis and express neuron
specific genes.
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Wnt/b-catenin
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Stem cells from posterior neural plate form spinal chord
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Chick Neural Tube Closure
Primary neurulationDirected by cells
adjacent to neural plate
AnteriorSecondary
neurulationSplit in medullary
chordPosterior
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Chick Neural Tube Closure
Starts near anterior endOpen at both ends until
fully closedAnterior and posterior
neuropore
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Primary Neurulation
Divides ectoderm into three types of cells• Neural tube - brain and spinal chord• Epidermis – skin• Neural crest cells – peripheral nerves, pigment
cells, glia etc...
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Primary Neurulation
MHPMedial neural
hinge pointDLHP
Dorsolateral hinge point
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Neural Tube Defects
Spina bifida – failure to close posterior neuroporeAnencephaly – failure to close anterior neuropore
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Folate Requirements During Pregnancy
Folate-binding protein expressed on neural folds of mouse embryos.
Causes of Folate Related Problems.Folate deficiency increases chance of neural tube
closure defects in humans.Most women bearing children with neural tube defects
have antibodies against folate-binding protein.Fungal contamination of corn produces fumonisin –
alters function of folate-binding protein.• Teratogen – substance that alters development.
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Role of Cadherins in Neurulation
Neural plate cells switch to N-cadherin expression just before neurulation.
Injection of E-cadherin mRNA into neural plate cells or injection of N-cadherin to adjacent cells alters development
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Secondary Neurulation
Medullary cord – condensed mesenchyme cellsSplits to form neural tube
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Brain Development
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Nervous System• Central Nervous System (CNS)
• Brain and spinal cord.• Both contain fluid-filled spaces which contain cerebrospinal fluid
(CSF).• The central canal of the spinal cord is continuous with the ventricles
of the brain.• White matter is composed of bundles of myelinated axons• Gray matter consists of unmyelinated axons, nuclei, and dendrites.
• Peripheral nervous system.• Everything outside the CNS.• Spinal and cranial nerves
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Somite formation
Derived from the paraxial mesoderm
Laid down during Hensen's node regression
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Order of somite formation is predetermined
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FGF and Retinoic Acid Gradients
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Retinoic Acid Gradient
• Posterior high => Anterior low• Retinoic acid production at posterior end
– Retinoic acid degraded at anterior end
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Pre-somatic mesoderm has positional identity
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Cdx genes
• Posterior end– Activated by retinoic acid
• Drosophila caudal gene homologue• Cdx in conjunctions with other morphogenic
factors activate Hox genes
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Hox Genes
• Homeotic gene paralogues– Have same basic functions of Drosophila
homeotic genes• Gene duplication caused multiple copies to
form• Expressed along dorsal axis
– Neural tube, neural crest, paraxial mesoderm, surface ectoderm
– Anterior boundary of hindbrain to end of tail
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Control of Hox Genes Expression
• Partially controlled by retinoic acid and Cdx genes– Transcriptional control
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Hom and Hox Gene Paralogues
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Segment Identity
Which Hox gene are expresses in a particular segment determines segment identity.
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How was this determined?
• Three types of experiments on Hox gene expression– Gene “knockout” (deletion)– Retinoic acid teratogenesis– Comparative anatomy
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Chick and Mouse Vertebral Pattern
Cervical Thoracic Lumbar SacralMouse 4 13 6 4Chick 14 7 12/13 5
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Comparative Anatomy of Bird and Mouse Vertebrae
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Mice exposed to retinoic acid
• A,C Normal; B, D Retinoic acid• Defects in facial and cranial skeleton• Missing vertebrae• Deformed pharyngeal arches
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Hox C-8 Deletion in Mouse• First lumbar vertebra
transformed into thoracic
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