organiser and neural induction

organiser and neural induction

1. role of the organiser

   • crucial for neural induction and organisation of anteroom-prosterior axis
   • amphibians - spemann organiser
   • the shield in zebrafish
   • hensens node in the chick
     • region at the anterior end of the primitive streak i chick blastoderm
     • contributes cell to the prechordal plate mesoderm, notochord somites and gut endoderm.
     • produces inducing signals
     • notochord froms int the dorsal midline anterior to the Hensens node
       • neural plate from above
       • an somites are formed from mesoderm on either side of the notochord
   • can induce a complete body axis is transplanted into another embryo at an appropriate stage
   • able to organise both doors-ventral and anteroom-prosterior body plans
   • induce neural tissue from the endoderm
   • during gastrulation the ectoderm of the dorsal region of the embryo becomes speified as neuroectoderm - the neural plate.
     • during neurulation the neural plate from the neural tube
       • eventually differentiates into the nervous system, brain and spinal cord
       • also gives rise to some non-neuronal structures
         • throws of neural crest cells
           • migrate through the body to give rise to the sensory and autonomic parts of the peripheral NS
         • some tissues of the face and jaws - such as bone and cartilage
           • in other parts of the body are derived from mesoderm
         • example of one germ layer giving rise to tissue characteristics of another
   • patterning of the NS must be linked to that of the mesoderm, cocordinated via the organiser
     • NS must develop in correct relationship with other body structures

2. inductive capacity of the organiser changes during gastrulation

   • primary embryonic induction
   • located in the dorsal lip of the blastopore
   • if an early gastrula is grafted to the ventral side of another gastrula
     • can give rise to a complete second embryo with a well defined head, NS, trunk region and tail!
   • during gastrulation, the antero-prosterior axis of the embryo becomes specified
     • so cell that make up the organiser induce only posterior structures later in gastrulation
     • also the quantity and nature of inducing signals produced by the organiser changes over time

3. the neural plate is induced in the ectoderm

   • first indicated by the organiser transplant experiment in frogs
   • in a secondary embryo that forms at site of transplantation
     • NS develops that would normally have formed the ventral epidermis
       • suggests that Neural tissue can be developed from un specified ectoderm
         • via signals emitted from organiser mesoderm
   • formation of the NS is dependent on an inductive signal
   • BMP inhibitor NOGGIN - can induce neural differentiation in ectoderm
     • BMPs are expressed throughout the ectoderm but lost in neural plate
     • neural plate can only develop if BMP is absent
     • organiser lifts block on neural tissue development by expressing proteins to inhibit BMP activity
     • cerberus protein
       • antagonises BMP, Wnt and Nodal signals

4. generation of the spinal cord

   • in chicks and mice
     • the spinal crd is generated from a small region of proliferating stem cells that develops in the ectoderm
       • at the posterior end of the neural plate
         • on both sides of the node and primitive streak
           • m zone becomes distinct just before somite formation begins and moves posteriorly down the node
             • leaves behind neural progenitor cells that produce the spinal cord
   • in frogs
     • elongation of the posterior neural plate is part of the general elongation of the embryo
       • along the anteroom-prosterior neural plate axis
   • the spinal cord is formed from a zone of stem cells that arises in the posterior neural plate

5. The NS is patterned by signals from the mesoderm

   • pieces of anterior mesoderm can pattern a head with a brain
   • pieces of posterior mesoderm can pattern a trunk and spinal cord.

6. neural crest cells arise from borders of the neural plate

   • induced at borders of the neural plate
   • in neuralation- rise up to form neural folds and the dorsal portion of the neural tube
     • migrate to give rise to range of tissues and cell types
   • neural crest cells are the origin of:
     • neurons and glia of the autonomic NS
     • glial schwann cells of PNS
     • melanocytes of the skin
     • adrenaline producing cells of the adrenal gland
     • much of the bone, cartilage and connective tissue of the face
   • forms branchial arches from hindbrain