Lecture 10: Nervous System Induction Flashcards
Steps in the formation of the notochord:
- Primitive node provides the source of cells that will form the notochord.
- Cellular precursors of the notochord migrate anteriorly and are then left behind as a rodlike aggregation as the primitive streak regresses.
- Notochordal cells fuse with flanking endodermal cells and temporarily form a neurenteric canal.
- Notochordal cells separate from the endoderm and aggregate to form a solid definitive notochord.
Inductive functions of the notochord:
- Important in the formation of many axial structures:
- i.e., Shh (signal)
- Induces overlying ectoderm to convert to neural ectoderm:
- i.e., Foxa-2 and Goosecoid (transcription factors)
- i.e., Noggin and Chordin (signals)
- Specifies identity of certain cells within the early nervous system.
- Transforms certain mesodermal cells of the somites into vertebral bodies.
- Stimulates earliest steps in the development of the pancreas.
Prechordal Plate
- Also referred to as the head organizer.
- Formed by early cells that pass through the primitive node.
- Source of signals such as shh involved in the ventral patterning of the forebrain.
- Source of signals important for the survival of neural crest cells that emigrate from the early forebrain.
Wnt3
Wnt3
Posterior epiblast
TARGET: Epiblast cells
Induces expression of nodal
Lefty 1 and Cer 1
Anterior hypoblast
Target: NODAL
Blocks expression of Nodal in anterior epiblast confines its expression to posterior epiblast
NODAL
Epiblast cells
Target: Multiple
Establishes a posterior signaling center which induces formation of the primitive streak
anteroposterior axis.
Note that expression of nodal in the posterior epiblast and blocking of nodal in the anterior epiblast results in the establishment of the anteroposterior axis.
Cerebrus-like
Origin: Ant. visceral endoderm
Effects: Inhibits development of posterior structures
DKK1
Origin: Ant. visceral endoderm
Target: Blocks Wnt
Effects: Inhibits development of posterior structures
Otx-2 Ant.
Lim-1
Target:Ant. visceral endoderm
Effects: Associated with induction of anterior CNS/head (See Figure 5.9)
Noggin
Origin: Primitive Node
Effect: Neural Inducer
Chordin
Origin: Primitive Node
Effect: Neural Inducer
Nodal
Origin: Primitive Streak
Effect: Induces primitive streak and helps to establish left-right symmetry
Goosecoid
Origin: Primitive Node
Target: chordin, noggin
Effect Establishment of primitive node; initiates notochord function; establishes midline structures cranial to node
Foxa-2
Origin: Primitive node
Target: Multiple
Effect: Establishment of primitive node; initiates notochord function; establishes midline structures cranial to node
Wnt-3
Origin: Primitive streak
Induces expression of nodal
Brachyury
Prim Streak
Notochord formation; normal movements of cells through primitive streak
Shh (late primitive streak stage)
Prechordal plate
multiple targets
Induces axial structures
Noggin
Chordin
Shh
Notochord
Effect: Neural Inducer
Goosecoid
Foxa-2
Origin: Prechordal Plate
Target: Chordin, noggin
Effect: activates genes
Explain why the amphibian dorsal lip is referred to as the organizer and explain its role in neural induction.
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Describe the role of the notochord as a primary organizer.
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Determination:
A process by which a cell or part of an embryo becomes restricted to a given developmental pathway. The point at which a cell becomes committed to a specific cell line.
Differentiation:
A complex of changes involved in the progressive specialization of structure and function; often resulting in the formation of luxury molecules.
Factors that determine asymmetry:
• First sign of asymmetry involves the beating of cilia around the primitive node:
• Asymmetrical beating results in a directional current and expression of TGF-β molecules :
• Nodal:
• Symmetry breaking molecule in the left side of the
embryo
• Lefty-1:
• Left side of the primitive streak
Asymmetrical current occurs between
2 and 6-somite stage in mouse.
Symmetry:
Nodal:
- Swept to left side of primitive node by ciliary current • Stimulates multiple gene expression pathways via:
- Pitx-2
Symmetry:
Lefty-1
- Expressed on left side of the embryo
* Blocks diffusion of molecules to the right side
Additional factors for symmetry:
- Shh
- Retinoic acid
- FGF-8
A review of cell adhesion molecules is given on pages 85-89. Review on own and review Figure 5.16.
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Be able to describe the distribution of CAMs in the early ectoderm (see Figure 5.17 and related text).
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