UNSW Embryology

Week 3 - Neuralation

© Dr Mark Hill (2011)

Acknowledgements

Introduction

Within the conceptus, gastrulation converts the bilaminar embryo into the trilaminar embryo (ectoderm, mesoderm, endoderm).

Key early steps embryo development have begun in: axial process formation (notochord), mesoderm differentiation (somites, heart, vascular) ectoderm differentiation (neuralation).

The axial process, which will form the notochord or axial mesoderm has an important role in "patterning" the overlying neural plate as it folds to become the neural tube.

The extraembryonic spaces (chorionic, amniotic, yolk) and intraembryonic spaces (coeloms) also continue to develop and the embryonic disc begins to fold.

Note that some tissue development during week 3 are covered in System Development notes.

Overview of early Neural Tube Formation

These images are from frog (Xenopus) development showing three phases of neural tube formation.

			Ectoderm Mesoderm (containing notocord) Endoderm
neural plate	neural groove	neural tube	(Images- Lance Davidson)

Developmental Overview of Nervous System

Ectoderm

• 2 parts
• midline neural plate
  • columnar
• lateral surface ectoderm
  • cuboidal
  • sensory placodes
  • epidermis of skin, hair, glands, ant. pituitary, teeth enamel

Neural Plate

• extends from buccopharyngeal membrane to primitive node
• forms above notochord and paraxial mesoderm
• neuroectodermal cells
  • broad brain plate
  • narrower spinal cord
• 3 components form
  • floor plate
  • neural plate
  • neural crest

Neural Determination- Neural Plate

• neuronal populations are specified before plate folds
  • signals from notochord and mesoderm
  • secrete noggin, chordin,follistatin
• all factors bind BMP-4 an inhibitor of neuralation
  • bone morphogenic protein acts through membrane receptor

Neural Determination- Neural populations

• neuronal populations are als specified before plate folds
• lateral inhibition generates at spinal cord level 3 strips of cells
• expression of delta inhibits nearby cells, which express notch receptor, from becoming neurons
  • Delta-Notch
  • generates Neural strips

Neural Groove

• forms in the midline of the neural plate (day 18-19)
• either side of which are the neural folds
• continues to deepen until about week 4
• neural folds begins to fuse
• at 4th somite level

Neural Plate

Neural Tube

Neural Tube

• fusion of neural groove extends rostrally and caudally
• begins at level of 4th somite
• "zips up" neural groove
• leaves 2 openings at either end- Neuropores
• forms the brain and spinal cord

Secondary Neuralation

• caudal end of neural tube formed by secondary neuralation
• develops from primitive streak region
• solid cord canalized by extension of neural canal
• mesodermal caudal eminence

Neuropores

• cranial neuropore closes before caudal
• failure to close- Neural Tube Defects
• severity dependent upon level
  • spina bifida

Neural Tube Defects (NTD)

• found that supplementation of maternal diet with folate reduces incidence of NTD
  • Women who have one infant with a neural tube defect have a significantly increased risk of recurrence (40-50 per thousand compared with 2 per thousand for all births).
  • A randomised controlled trial conducted by the Medical Research Council of the United Kingdom demonstrated a 72% reduction in risk of recurrence by periconceptional (ie before and after conception) folic acid supplementation (4mg daily).

Australian Neural Defects

• Neural tube defects
• Fetal Alcohol Syndrome

Neural Crest

• where neural folds fuse a special population of cells arise
• Neural Crest
• dorsal to the neural tube, as a pair of streaks
• pluripotential
  • forms many different types of cells
• cells migrate throughout the embryo
  • studied by quail-chick chimeras
  • transplanted quail cells have obvious nucleoli compared with chicken
• Neural Crest Derivitives
• Cells formed
  • dorsal root ganglia
  • autonomic ganglia
  • adrenal medulla
  • drg sheath cells, glia
  • pia-arachnoid sheath
  • skin melanocytes
  • connective tissue of cardiac outflow
  • thyroid parafollicular cells
  • craniofacial skeleton
  • teeth odontoblasts

Early Brain Structure

• rostral neural tube forms 3 primary brain vesicles (week 4)
• prosencephalon (forebrain)
• mesencephalon (midbrain)
• rhombencephalon (hindbrain)

from the 3 vesicle 5 secondary vesicles form

• prosencephalon
  • telencephalon (endbrain, forms cerebral hemispheres)
  • diencephalon (betweenbrain, forms optic outgrowth)
• mesencephalon
• rhombencephalon
  • metencephalon (behindbrain)
  • myelencephalon (medullabrain)

cavity within tube forms ventricles

Stage 13/14 Embryo

Brain Flexures

• rapid growth folds the neural tube forming 3 flexures
• Brain Flexures
  • cervical flexure
    • between brain stem and spinal cord
  • midbrain flexure
    • pushes mesencephalon upwards
  • pontine flexure
    • generates 4th ventricle

Neural Layers

• neuroblasts arise in ventricular layer
• migrate along radial gial
• stop at cortical plate

Transverse Section SC

Spinal Cord

Axes

Ventral

• Experimental manipulation of interactions.
• Initial experiments looked at how isolated tissues may influence the development of the spinal cord.
• Repositionining of specific tissues both in vivo and in vitro
• specific markers of or alteration of differentiation.

Notocord Induction

Ventral- Sonic Hedgehog

• notochord secretes sonic hedgehog
• Gene expression studies (ISH) showed shh gene expression occured in a subset of inducing tissues
• has a patterning role elsewhere
  • limb, sclerotome, lung
• 2 signaling activities acting
  • locally and at a distance

Ventral- Sonic Hedgehog

• Binds to cell surface receptor patched
• without shh, patched (Ptc) binds smoothened (Smo)
• with shh shh-Ptc releases Smo activating G protein pathway

Gene Diseases

• shh Human mutation- holoprosencephaly 3
• characteristic facies of the severe form of HPE which included a single fused eye (cyclopia) and a nose-like structure (proboscis) above the eye
• Downstream targets of Sonic hedgehog signalling
  • transcription factors like Gli3, responsible for Greigs polycephalosyndactyly in humans
  • d Hoxd13, responsible for polysyndactyly

Dorsal- Dorsalin

• growth factor controls patterning in embryonic mesoderm (frog)
• Transforming Growth factor beta, (TGF b)
  • related factors BMP-2, BMP-4, BMP-7, radar
  • flies related protein determines dorsoventral
• homology search of vertebrate library identified protein of same family.
• dorsalin-1 (dsl-1)
  • Basler, Cell 73, p687, 1993

Dorsalin-1

• From overlying ectoderm
• Naming comes from the obvious reason that it promotes the differentiation of neural crest cells.
• Also signal for dorsal signal of neural tube.
• Inhibits the differentiation of motoneurons.
• Implication is that dsl-1 and shh act antagonistically, or competitively to establish d-v axis of neural tube.

Rostro-Caudal Axis

• What about the third pattern axis?
• Brain rostro-caudal axis is generated by differential expression of Hox genes.
• Transcriptional activators.
  • corresponding to genetic order on chromosome.
  • Wilkinson, Nature, 341, p405, 1989.

Hox Genes

• A family of transcription factors
• Discovered in flies and conserved between all species. antennapedia
• Expressed in sequence along the embryo rostro-caudal axis.
• Regulate many other aspects of development.

Hox Genes

• Stands for Homeobox domain Genes
• 180aa region binds DNA and regulate gene expression
• large family of genes organized and expressed in sequence on the chromosome
• Nkx-2.2 first detected at 1 somite stage
• Lim hox gene expressed at spinal cord level

Hox Expression in Spinal cord

• Summary
  • LIM Expression & Motoneuron Populations
    • MMC- Medial Motor Column (trunk)
    • MMCm (medial) axial near vertebra
    • MMCl (lateral) ventral body wall
    • LMC- Lateral Motor Column (limbs)
    • LMCm (ventral)
    • LMCl (dorsal)
    • CT- Column of Terni (visceral)
  • LIM Expression

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Quick Links

Week 3 Pages:

Introduction | Abnormalities | Gastrulation | Trilaminar Embryo | Neural Tube Formation | Early Somite Formation | References | Text only page | WWW Links |

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- All Systems Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Renal Genital Musculoskeletal Integumentary

Movies ----------------- Movies Quicktime Flash Audio Week 1 Week 2 Week 3 Stage 13 Stage 22 Neural Heart Gastrointestinal Renal Genital Other Embryos

Other Embryos ----------------- All Animals Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

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