Ectoderm Germ Layer

Frog Fate Map

Frog Fate Map

Role of Organizer Chages in Late Frog Embryos

Organizer forms three distinct regions

Notochord formation in chick

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

Beta-catenin activates other genes

Turns on goosecoid gene in organizer

Goosecoid mRNA can induce new axis

Activates involution Determines dorsal mesoderm Represses Wnt8 Important in brain formation

BMP Gradient Controls Dorsal/Ventral Axis

VegT and Veg1

Present in vegetal cortical regionGenes required for endoderm and mesoderm

formationVegT antisence RNA causes epidermis only

Xnr Gradient

High Xnr – Organizer, goosecoidMed Xnr – Lateral mesodermLow Xnr > high BMP4 + Xwnt8 – Ventral mesoderm

Neural ectoderm induced by soluble factor?

Controlled by goosecoid and beta-catenin?

Neural structures default – epidermis induced by BMP's

BMP – bone morphogenic proteinInduce ectoderm to become epidermisOrganizer secretes factors that block BMP from acting

BMP inhibitors

Noggin, Chordin and FollistatinDiffusable proteinsInduced dorsal ectoderm to become neuralDorsalizes mesoderm cellsInhibits BMP'sFound in dorsal lip then in the notochord

Noggin mRNA rescues Dorsal Structures

Localization of Noggin

Localization of Chordin mRNA

Expression of paraxial protocadherin

Paraxial protocadherin expression

Blue=paraxial protocadherinRed=chordin

How is the ant/pos axis determined?

During neurulation beta-catenin forms a gradientGreatest concentration at organizer

• Becomes posterior end

BMP Gradient Controls Dorsal/Ventral Axis

Wnt Gradient Controls Anterior/Posterior Axis

Gastrulation Induces Nervous System

Chick Hensen's Node Can Induce Gene Expression in Frog

Development of Neural Crest

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.

Wnt/b-catenin

Stem cells from posterior neural plate form spinal chord

Chick Neural Tube Closure

Primary neurulationDirected by cells

adjacent to neural plate

AnteriorSecondary

neurulationSplit in medullary

chordPosterior

Chick Neural Tube Closure

Starts near anterior endOpen at both ends until

fully closedAnterior and posterior

neuropore

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...

Primary Neurulation

MHPMedial neural

hinge pointDLHP

Dorsolateral hinge point

Neural Tube Defects

Spina bifida – failure to close posterior neuroporeAnencephaly – failure to close anterior neuropore

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.

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

Secondary Neurulation

Medullary cord – condensed mesenchyme cellsSplits to form neural tube

Brain Development

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

Somite formation

Derived from the paraxial mesoderm

Laid down during Hensen's node regression

Order of somite formation is predetermined

FGF and Retinoic Acid Gradients

Retinoic Acid Gradient

• Posterior high => Anterior low• Retinoic acid production at posterior end

– Retinoic acid degraded at anterior end

Pre-somatic mesoderm has positional identity

Cdx genes

• Posterior end– Activated by retinoic acid

• Drosophila caudal gene homologue• Cdx in conjunctions with other morphogenic

factors activate Hox genes

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

Control of Hox Genes Expression

• Partially controlled by retinoic acid and Cdx genes– Transcriptional control

Hom and Hox Gene Paralogues

Segment Identity

Which Hox gene are expresses in a particular segment determines segment identity.

How was this determined?

• Three types of experiments on Hox gene expression– Gene “knockout” (deletion)– Retinoic acid teratogenesis– Comparative anatomy

Chick and Mouse Vertebral Pattern

Cervical Thoracic Lumbar SacralMouse 4 13 6 4Chick 14 7 12/13 5

Comparative Anatomy of Bird and Mouse Vertebrae

Mice exposed to retinoic acid

• A,C Normal; B, D Retinoic acid• Defects in facial and cranial skeleton• Missing vertebrae• Deformed pharyngeal arches

Hox C-8 Deletion in Mouse• First lumbar vertebra

transformed into thoracic

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