Gastrulation & Segmentation in Vertebrate Embryos

Part A

Overview

This section discusses the critical processes of gastrulation and segmentation in vertebrate embryos, exploring how these processes establish the basic body plan of vertebrates.

Key Elements of Lecture

Anterior-Posterior Body Axis Formation
Basic Processes of Gastrulation
Formation of Three Germ Layers
Sequential and Periodic Formation of Somites

Bilateral Symmetry in Vertebrates

Vertebrates exhibit bilateral symmetry, which is defined as having a distinct anterior-posterior (AP) body axis.
They have a segmented body design characterized by the serial repetition of parts along the long body axis.
The definition of segmentation is crucial: it allows for diverse body parts to develop from similar basic structures during embryonic development.

Gastrulation and Germ Layer Formation

Gastrulation is a pivotal phase in early embryonic development where cells of the epiblast migrate into the primitive streak.
During this process, cells undergo an epithelial-to-mesenchymal transition (EMT) allowing them to migrate beneath the epiblast, resulting in the formation of three primary germ layers:
- Endoderm (internal layer)
- Mesoderm (middle layer)
- Ectoderm (outer layer)

Three Germ Layers and Their Derivatives

The ectoderm gives rise to:
- Central nervous system
- Epidermal cells of the skin
- Neural crest cells
The mesoderm forms:
- Notochord
- Somites (which develop into vertebrae and muscle)
- Paraxial, intermediate, and lateral plate mesoderm
The endoderm differentiates into:
- Digestive tube
- Lung cells (alveolar)
- Forms the male and female germ cells

Structure of the Embryo

The embryo is defined by layers and structures, including:
- Lateral plate mesoderm
- Paraxial mesoderm
- Neural tube
- Details about orientation: anterior, posterior, lateral plate, and coronal directions are considered.

Somitogenesis and Segmentation

Somites are formed from unsegmented mesoderm and represent balls of epithelial tissue that develop sequentially from the unsegmented paraxial mesoderm.
Somite formation begins at the anterior end of the body, with new somites being added posteriorly. A regular periodicity is observed in somite formation (1 every 90 minutes in chickens, and 1 every 2 hours in mice).
The axial skeleton (except the skull) arises from somites.

Clock and Wavefront Model

Fibroblast Growth Factors (FGFs) are critical in controlling phenomena such as cell motility and elongation of the embryo:
- FGF8, for instance, establishes a gradient in the presomitic mesoderm (PSM), regulating segmentation.
High concentrations of FGF at the posterior end inhibit somite formation, while lower concentrations at the anterior allow it to occur.
Hes1, a transcription factor expressed in a cyclic manner in the PSM, acts as a component of the segmentation clock, indicating that the oscillatory expression of transcription factors contributes to segmentation precision.

Segmentation Summary

The segmentation mechanism involves a produced array of tissues arranged along the embryonic axis, regulated through a ‘clock and wavefront’ process which generates periodic pulses of segmental activity.

Part B

Key Elements of Lecture

Grouping of Vertebrae into Anatomical Domains
Cell Identity and Position Along the Axes
Role of Hox Genes in Segment Identity

Segmental Identity

Each embryonic segment possesses unique identities that influence the types of tissues formed. A clear example can be seen in vertebrae: cervical vertebrae are characterized by a lack of rib attachments.
These segment patterns are highly conserved across different species.

Determination of Segmental Identity

The identity of segmental tissues is pre-determined before segmentation begins. For example, mesoderm grafted to another location retains its identity (e.g., thoracic mesoderm will still generate thoracic vertebrae).

Hox Genes and Morphogenesis

Hox genes encode transcription factors that are crucial in regulating morphogenesis. These genes are evolutionarily conserved and play vital roles in establishing morphological identities of somites and vertebrae.

Comparison of Hox Genes

There exists a significant conservation of Hox genes across diverse animal taxa, such as in Drosophila and Mus musculus (mice). This similarity illustrates a common evolutionary mechanism for defining body regions in animals.

Functional Activity of Hox Genes in Embryos

The expression of Hox genes in the developing mouse embryo is time and spatially regulated, forming distinct expression domains that correlate with the vertebral formula:
- For instance, regions designated for cervical, thoracic, and lumbar vertebrae display specific patterns of Hox gene expression.
Such patterns can explain deviations in vertebrae identity across species.

Role of Mutations in Hox Genes

Knockout studies in mice have illustrated that mutating specific Hox genes leads to identifiable changes in vertebra identity, such as transforming lumbar vertebrae to thoracic due to loss of Hox 10 function or changes from sacral to lumbar resulting from loss of Hox 11 function.

Linking Gastrulation and Axis Patterning

Differential expression of Hox genes is evident in the primitive streak during gastrulation. Cells expressing anterior Hox genes localize to the anterior mesoderm while posterior Hox genes are expressed for longer periods in the streak.
The spatial and temporal dynamics of Hox gene expression are intricately tied to axial identity and segmentation outcomes in vertebrates.

Conclusion Summary

The organization of the anterior-posterior axis in vertebrates is segmented, with segments formed through the addition of somites in a sequential manner.
This developmental identity is established during gastrulation, evidenced by cell emergence from the primitive streak.

Inspirational Quote

"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." – Lewis Wolpert (19 October 1929 – 28 January 2021)