Paraxial Mesoderm and Somites Overview

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Questions and Answers

What is the primary function of somites during development?

To form connective tissues and muscles (correct)

To develop the circulatory system

To define the body cavity

To create the dermis only

What does the paraxial mesoderm primarily give rise to?

The urogenital system

Extra-embryonic membranes

Somites and head mesoderm (correct)

The circulatory system

How do vertebrate species compare in terms of somite number?

All vertebrates have the same number of somites

Somites are only present in mammals

Somite number is highly variable among species (correct)

Somite number is irrelevant to body segmentation

What segment variation is observed in different vertebrate species?

Variation in segment size and composition Signup and view all the answers

Which type of mesoderm forms the kidneys and gonads?

Intermediate mesoderm Signup and view all the answers

What unique feature do thoracic vertebrae have?

They possess ribs that protect organs Signup and view all the answers

During which stage of development does mesoderm formation occur?

Gastrulation Signup and view all the answers

What is a main characteristic of paraxial mesoderm?

It creates somites which are transitory epithelial blocks Signup and view all the answers

What is the role of Notch-Delta signaling in somitogenesis?

Controls the timing and order of Hox gene expression. Signup and view all the answers

How does the Lunatic fringe (Lfng) gene affect vertebral malformations?

Its loss leads to severe vertebral malformations. Signup and view all the answers

What significant difference is noted in the oscillations of Lunatic fringe expression between corn snakes and mice?

Corn snakes exhibit threefold more oscillations than mice. Signup and view all the answers

What is the effect of retinoic acid on somite formation?

It represses Wnt and Fgf8, affecting the segmentation process. Signup and view all the answers

What mechanism maintains posterior growth during somitogenesis?

Fgf8 feedback represses retinoic acid through Cyp26A1. Signup and view all the answers

What is the primary gene expressed in sclerotome progenitors?

Pax1 Signup and view all the answers

Which process enables the sclerotome cells to migrate towards the neural tube?

Epithelial to mesenchymal transition (EMT) Signup and view all the answers

What do the signaling centers of the notochord and floor plate secrete?

Shh and Noggin Signup and view all the answers

What is the function of tendons formed from the syndetome?

Connect muscle and bone Signup and view all the answers

Which factor signals the most dorsal cells of the sclerotome to become syndetome?

FGF from myotome Signup and view all the answers

What is a notable role of Syndetome cells after their migration?

Differentiate into tendons Signup and view all the answers

Which domain does the primaxial myotome derive from?

Dorsal neural tube Signup and view all the answers

What does the term 'resegmentation' refer to in sclerotome development?

Fusing of adjacent sclerotome segments Signup and view all the answers

What role does Noggin play in mesoderm specification?

It blocks BMP signaling to promote paraxial mesoderm formation. Signup and view all the answers

How do Hox genes influence somite development?

They control the identity and specification of each somite. Signup and view all the answers

What initiates the mesenchymal-to-epithelia transition (MET) in somite formation?

Activation of Mesp transcription factor. Signup and view all the answers

What is the primary function of Eph-ephrin signaling in somite boundary formation?

Regulates epithelialization and establishes distinct somite boundaries. Signup and view all the answers

What occurs during somitogenesis?

Somites differentiate sequentially in a head-to-tail arrangement. Signup and view all the answers

Which of the following describes the epithelial boundaries in somites?

They define individual somites during development. Signup and view all the answers

How many somites do humans typically have?

38-45 Signup and view all the answers

What does the expression pattern of Hox genes on chromosomes reflect?

The anterior-posterior positioning of the somites. Signup and view all the answers

Study Notes

Paraxial Mesoderm: Somites and Their Derivatives

Segmentation is a highly conserved feature in vertebrates, crucial for the evolution of specialized functions.
Vertebrate species have similar segment numbers, but segment size and function vary based on environmental needs.
Thoracic segments are unique in having ribs which protect organs, and their number varies greatly among species.
Segment number/size is determined by mesodermal division along the anterior-posterior axis; snakes have ~300 segments while humans have ~38.
Mesoderm forms between the endoderm and ectoderm synchronously with neural tube development.
Presomitic mesoderm (PSM) forms somites, vital for structuring vertebrae, muscle, and skeleton.

Major Mesoderm Lineages

Chordamesoderm (axial mesoderm): Forms the notocord and intervertebral discs.
Paraxial mesoderm (somitic mesoderm): Forms somites and head mesoderm; somites are transitory epithelial blocks of cells near the neural tube, and differentiate into muscle and connective tissues of the back, as well as face and skull tissues.
Intermediate mesoderm: Forms the urogenital system (kidneys, gonads, and ducts).
Lateral plate mesoderm: Forms circulatory system, body cavity linings, pelvic/limb bones, and extra-embryonic membranes.

Gastrulation and Neurulation (Chick Embryo, Mesodermal Perspective)

Mesoderm formation during gastrulation occurs between the endoderm and ectoderm.
Sequential stages show the formation of a notochord, paraxial mesoderm, and lateral plate mesoderm.
Somite formation and other mesodermal development processes are shown in detail.
Stages are depicted (Embryonic day 24-48).

Specification of Somites

Mesodermal subtypes (chordamesoderm, paraxial, intermediate, and lateral plate) development is specified along the mediolateral axis due to varying BMPs level, with higher BMP4 expression in more lateral cells.
Noggin (BMP inhibitor) helps shape the BMP gradient. Adding Noggin to lateral plate mesoderm reshapes it into paraxial mesoderm.
Hox genes on chromosomes are patterned to reflect their expression along the antero-posterior axis; Hox gene expression determines somite identity and specification.

Somitogenesis

Somitogenesis progresses from head to tail direction.
Presomitic mesoderm forms somitomeres (cell groups, precursors to somites).
Epithelial boundaries define each somite.
Somite counts vary across species (humans: 38-45; mice: 65; snakes: >500).

Eph-Ephrine Signaling & Somite Formation

Somites transition from mesenchymal to epithelial cells (MET) through cell conversion.
Mesodermal posterior (Mesp) transcription factors initiate MET, but they are restricted to anterior somite halves.
Eph-ephrin signaling regulates epithelialization, leading to distinct somite boundaries.
Boundary formation involves Cdc42 repression and integrin x5-fibronectin interactions for fissure formation/separation.

Notch Delta Signaling in Somitogenesis

Loss of Notch target Lunatic Fringe (Lfng) or its partner Distal-less-3 (Dll3) in mice causes severe vertebral malformations.
Similar vertebral defects are observed in humans with Lunatic Fringe gene mutations.
This highlights conserved roles in vertebral segmentation across different species.

How Many Somites Does a Snake Have?

Corn snake embryos at different stages show increasing somite formation.
Corn snakes have a threefold more oscillation of Lunatic fringe expression than mice at similar stages.
Four vertebrate species exhibit distinct somite formation patterns.

Regulatory Mechanisms Governing Somitogenesis

Segmentation clock involves Notch-Delta signaling and the timing/order of Hox gene expression.
Hox genes repress Wnt signaling and indirectly affect Fgf8 expression, which influences segmentation.
Retinoic acid inhibits posterior signals (Fgf8, Wnt3a), promoting anterior somite formation.
Fgf8 feedback represses retinoic acid through Cyp26A1 while maintaining posterior growth.
Signaling balances (retinoic acid, Wnt, and Fgf8) establish a determining front, for anterior somite formation before posterior.

Sclerotome Development

Sclerotome progenitors express Pax1.
They transition from epithelial to mesenchymal cells (EMT)
Migrate towards the neural tube.

Syndetome Development

Forms tendons; portion of sclerotome.
Expresses Scleraxis gene.
Functions to connect muscle and bone.
Located in the most dorsal segment of the sclerotome.

Induction of Scleraxis

FGF from the myotome signals the most dorsal sclerotome cells to differentiate as syndetomes.
Syndetome cells migrate and differentiate into tendons.

Primaxial and Abaxial Domains of Vertebrate Mesoderm

Lateral Somitic Frontier separates primaxial and abaxial muscles (somite derived vs lateral plate dermis).
Satellite cells- undifferentiated myoblasts, stay around mature muscles, as skeletal muscle stem cells, crucial for growth and repair.
Day-9/13 chick embryo's mesoderm displays Prox1 gene expression in the abaxial region, showcasing the lateral somitic frontier dividing regions.

Differential Gene Expression in Myotome

Primaxial myotome development is driven by Wnt1/Wnt3a (dorsal neural tube) and low Shh.
Pax3 and Wnt expression is key for myogenin and MRF4 activation, leading to muscle formation.
BMP4 inhibition by Noggin allows Wnt from the epidermis to induce abaxial myotome formation.

Ablating Noggin-Secreting Epiblast Cells

Control Embryos have normal muscle formation with strong myosin presence.
Noggin-ablated embryos display significant muscle loss, eye defects and abdominal wall herniation.

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Description

This quiz explores the characteristics and functions of paraxial mesoderm and somites in vertebrate development. Understand the significance of segmentation and how mesodermal differentiation influences the structure of vertebrae, muscles, and skeletons. Test your knowledge about the evolution and variance of somite structures across different species.