Developmental Biology: Somite Formation and Ephrin B Function

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Questions and Answers

What is the name of the process that presomitic mesoderm undergoes?

Somitogenesis

Which of the following genes are cyclically activated in the presomitic mesoderm?

Notch and WNT

What is the name of the type of mesoderm involved in somite formation?

Paraxial mesoderm

What is the role of the Notch signaling pathway in somitogenesis?

To regulate the segmentation clock Signup and view all the answers

What is the name of the gradient established by the interaction of WNT and RA in the cranio-caudal axis?

Cranio-caudal gradient Signup and view all the answers

What is the name of the protein involved in the regulation of the segmentation clock?

Notch protein Signup and view all the answers

What is the term for the repeating pattern of gene expression in the presomitic mesoderm?

Segmentation clock Signup and view all the answers

What is the signaling pathway involved in the establishment of the opposing gradient in the cranio-caudal axis?

WNT/FGF signaling pathway Signup and view all the answers

What concentration of RA and FGF/Wnt is required for somitogenesis to initiate?

High RA and low FGF/Wnt Signup and view all the answers

What is the role of Notch signaling in somite differentiation?

Activation of Lunatic Fringe gene and Transcription factor HES1 Signup and view all the answers

Why do somites secrete ephrins B proteins?

To avoid sticking together Signup and view all the answers

What is the name of the model that describes the criteria for somitogenesis initiation?

Wave front model Signup and view all the answers

What is the result of FGF8 expression and Wnt transcription factor activation?

Activation of Notch signaling pathway Signup and view all the answers

What is the role of FGF and Wnt in the mesoderm?

They regulate segmentation clock activity Signup and view all the answers

What is the name of the transcription factor activated by Notch signaling in somite differentiation?

Transcription factor HES1 Signup and view all the answers

What is the purpose of the Notch signaling pathway in vertebrate somitogenesis?

To activate transcription factors in somite differentiation Signup and view all the answers

What is the role of ephrin B in somite formation?

It prevents neighboring somites from sticking to each other Signup and view all the answers

What is the result of high Wnt and low Snail levels in the somite?

Mesenchymal cells become epithelial-like tissue Signup and view all the answers

What is the characteristic of somites and their sclerotomes?

They are identical but have different HOX codes Signup and view all the answers

What is the role of the ectoderm-derived Wnt in somite formation?

It turns off the expression of Snail genes Signup and view all the answers

What is the significance of the HOX code in somite development?

It determines the segment identity of somites Signup and view all the answers

What is the characteristic of the transition from mesenchymal to epithelial-like tissue in the paraxial mesoderm?

It is a change in the nature of the mesenchymal tissue Signup and view all the answers

What is the outcome of the mesenchymal-epithelial conversion in the somite?

The formation of epithelial-like tissue Signup and view all the answers

What is activated during the mesenchymal-epithelial conversion?

The genes for the components of the extracellular matrix and CAM Signup and view all the answers

Study Notes

Somite Formation and Segmentation

Ephrin B, secreted by pre-formed somites, binds to the EphA4 receptor on almost complete somites, preventing neighboring somites from sticking together.
Ephrin B creates gaps between somitomers, forming the intersomitic space.
The ectoderm-derived Wnt signal penetrates the nearly completed somite, turning off the Snail gene, and inducing mesenchymal-epithelial conversion.

Mesenchymal-Epithelial Conversion

High Wnt and low Snail levels trigger the conversion of mesenchymal cells into epithelial-like tissue.
The genes for fibronectin and N-cadherin, components of the extracellular matrix and CAM, are activated during this process.

Somite Specification and HOX Code

Somites develop sequentially, with each somite having a unique identity determined by the HOX code.
The HOX code is a differentiated profile of HOX gene expression, which describes a series of rules regarding segment identity.

Segmentation of Presomitic Mesoderm

The segmentation of the presomitic mesoderm depends on the segmentation clock and wave front.
Notch and WNT genes are cyclically activated in the presomitic mesoderm, establishing an opposing gradient in the cranio-caudal axis.

Regulation of Segmentation Clock

FGF8 and WNT vs RA establish an opposing gradient in the cranio-caudal axis, regulating the segmentation clock and Notch pathway activity.
Notch signaling is responsible for activating transcription factors, such as Lunatic Fringe and HES1, in somite differentiation.

Entry of Paraxial Mesoderm into Somitogenesis

A region with a high concentration of RA and a low concentration of FGF and Wnt can initiate somitogenesis, according to the wave front model.
Activation of FGF expression and the Wnt transcription factor leads to the activation of the Notch signaling pathway.

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