Lecture 7 - Neural Induction

What is developmental plasticity in early embryos?

The ability of early embryonic cells to change their developmental fate and give rise to different cell types

How does developmental potential change as embryonic development progresses?

Most cells (except stem cells) become increasingly restricted in their developmental potential

Is genetic information lost as cells differentiate during development?

No, genetic information is not lost during differentiation

What experiment in frogs demonstrates that differentiated cells retain all genetic information?

Transplanting the nucleus of a differentiated cell into an enucleated egg can result in the development of a tadpole

What is differential gene expression?

The process by which cells turn specific genes on or off during development to acquire specialised functions

What types of information do cells integrate to determine their fate?

Internal information (asymmetrically distributed factors) and external information (signalling molecules)

What is neural induction?

The process by which ectodermal embryonic cells commit to a neural fate instead of becoming epidermis or mesoderm

What is gastrulation in Xenopus development?

The process by which a simple blastula reorganises into a multilayered embryo with three germ layers: ectoderm, mesoderm, and endoderm

Where does Xenopus development begin after fertilisation?

With a small involution of cells on the side opposite the sperm entry site

What is the Involuting Marginal Zone (IMZ)?

The region where cells begin to involute during gastrulation; also known as the Dorsal Lip of the Blastopore

What is another name for the Involuting Marginal Zone?

The Dorsal Lip of the Blastopore (DLB)

What role does the dorsal lip play in development?

It acts as the organiser, directing cell movements and tissue patterning

Where is mesoderm established during development?

Beneath the future neural (neurogenic) region

What structures does the mesoderm give rise to?

Muscles, bones, and other internal structures

Why is interaction between mesoderm and ectoderm important?

It is critical for neural induction

How does mesoderm influence neural development?

Mesodermal signals transform the overlying ectoderm in the neurogenic region into the neural plate

What happens to the neural plate after it forms?

It undergoes morphogenic movements to form the neural tube

Who conducted the classic organiser experiment in newts?

Hans Spemann and Hilde Mangold

What organism was used in Mangold and Spemann’s experiment?

Newt embryos (Triturus species)

What was the main goal of Mangold and Spemann’s experiment?

To understand how cells in the early embryo control development of surrounding tissues, particularly body axis formation and neural tissue specification

What embryonic structure was removed for transplantation?

The dorsal lip of the blastopore

What is the significance of the dorsal lip of the blastopore?

It is the future organiser region of the embryo

Where was the dorsal lip transplanted?

Into the ventral side of another embryo, a normally non-neural region

What was observed after transplantation of the dorsal lip?

The host embryo developed a secondary body axis, resulting in embryos with two heads

What structures formed as part of the secondary body axis?

The neural tube (brain and spinal cord) and somites (muscle precursors)

Did the transplanted dorsal lip form all of the new tissues?

No, the transplanted cells contributed partially, but most of the new structures were derived from host tissue

What key developmental principle did this experiment demonstrate?

The organiser can induce surrounding host cells to change fate and form new body structures

What role does the dorsal lip of the blastopore play in development?

It acts as an organiser that instructs neighbouring cells to adopt specific developmental fates

What key concept about cell fate was demonstrated by organiser experiments?

Cell fate is not determined solely by lineage; cell–cell interactions are crucial for development

Who conducted the classic organiser experiments in Hydra?

Ethel Harvey

What organism was used in Ethel Harvey’s experiments?

Hydra

What type of tissue did Harvey remove from Hydra?

Body regions such as the head or foot

What was done with the removed Hydra tissue?

It was transplanted to a different position on another Hydra

What was Harvey observing in these transplantation experiments?

Which structures regenerated and how the host tissue responded

What happened when head tissue was transplanted to the mid-body region?

It induced formation of a secondary axis (a new head) from surrounding host cells

How did foot tissue compare in inductive ability?

Foot tissue had more limited inductive ability than head tissue

Did the transplanted tissue form all the new structures?

No, host tissue contributed significantly to the new structures

What does positional information mean in Hydra development?

Cells know their position along the body axis and respond accordingly

What organiser-like property was demonstrated in Hydra?

Certain regions, such as the head, can act as organisers that direct surrounding cells to form specific structures

What broader developmental principle did Harvey’s experiments demonstrate?

Organiser activity is not restricted to vertebrates

What is epiboly in zebrafish development?

The spreading and thinning of cell layers to enclose the yolk

What tissue spreads over the yolk during zebrafish epiboly?

The blastoderm (cells on top of the yolk)

What are the three layers involved in zebrafish epiboly?

The enveloping layer (EVL), deep cells, and yolk syncytial layer (YSL)

What is the enveloping layer (EVL)?

The outermost epithelial layer of the blastoderm

What is the fate of the deep cells during zebrafish development?

They give rise to the embryo proper

What is the yolk syncytial layer (YSL)?

An extra-embryonic layer that helps pull the blastoderm over the yolk

What cellular mechanisms drive epiboly?

Cell flattening and spreading, intercalation of deep cells, and actomyosin constriction at the blastoderm margin

What is convergence–extension?

A cell rearrangement movement where tissue narrows along one axis (convergence) and elongates along another (extension)

What signalling pathway mediates convergence–extension?

Planar cell polarity (PCP) signalling

What cellular behaviours underlie convergence–extension movements?

Cell intercalation and changes in cell shape

What is the developmental outcome of convergence–extension in zebrafish?

Formation of a long, narrow body axis required for proper embryonic patterning

What characterises the 6–10 somite stage in zebrafish?

Early segmentation, somites begin forming along the anterior–posterior axis, and the neural plate starts folding

At what time post-fertilisation does the 6–10 somite stage occur?

Approximately 12–14 hpf

What happens during the 10–14 somite stage?

Neural tube closure begins, and somites elongate and differentiate

At what time post-fertilisation does the 10–14 somite stage occur?

Approximately 14–16 hpf

What are the key features of the 17–18 somite stage?

Advanced somite formation, continued neural tube closure, and early differentiation of muscles and the notochord

At what time post-fertilisation does the 17–18 somite stage occur?

Around 18 hpf

What defines the 30 somite stage in zebrafish?

Neurulation is complete, the neural tube is fully formed, and somites are clearly segmented and patterned

At what time post-fertilisation does the 30 somite stage occur?

Approximately 24 hpf

What organisers are involved in zebrafish body axis formation?

The dorsal organiser (the shield) and the tail organiser

What is the role of the tail organiser in zebrafish development?

It controls tail formation and posterior body patterning

Why is a single organiser insufficient to form a complete body axis in zebrafish?

Axis-forming information is distributed across multiple organisers, including the tail organiser

What did shield transplant experiments reveal about axis formation?

Transplanting the shield alone never produced a complete axis because posterior (tail) information is controlled by a separate organiser

What major tissues arise from the ectoderm?

Skin and the nervous system

What structures are derived from the mesoderm?

Musculoskeletal system, vasculature, reproductive system, and excretory system

What organs develop from the endoderm?

Gut, lungs, liver, and thyroid

Are germ layer fates conserved across frog, fish, and mouse?

Yes, the basic germ layer derivatives are conserved across these vertebrates

What is Krox20?

A zinc-finger transcription factor expressed in developing hindbrain regions (rhombomeres 3 and 5)

What is the main function of Krox20 in the hindbrain?

It specifies segment identity and guides neuronal differentiation and patterning

In which species is Krox20 expressed in similar hindbrain segments?

Fish, amphibians, birds, and mammals

What does the conserved expression of Krox20 indicate about organiser activity?

That organiser activity leads to similar gene expression patterns across species, demonstrating cross-species conservation

Which hindbrain segments express Krox20?

Rhombomeres 3 and 5

When does the neural tube appear in human embryos?

During the 4th week after fertilisation

From which germ layer is the central nervous system (CNS) derived?

Ectoderm

From which structure is the peripheral nervous system (PNS) derived?

Neural crest cells

What do somites from the mesoderm become?

Bone and muscle

How is the neural plate specified in humans?

Chemical signals instruct the ectoderm to become neural plate

How does the neural plate form the neural tube?

The ectoderm folds into a tubular structure called the neural tube

From what part of the neural tube does the CNS develop?

The walls of the neural tube

What are neuropores?

Open ends of the neural tube

When do the neuropores close?

Before the end of the 4th week

Which part of the neural tube closes first?

The middle closes first

What directions do the anterior and posterior neuropores correspond to?

• Anterior → rostral/dorsal

• Posterior → caudal/ventral

How is human neural tube formation similar to frogs?

In both, the ectoderm folds to form a neural tube

What can result from failure of neural tube closure?

Serious developmental conditions, known as neural tube defects (NTDs)

What is anencephaly?

Absence of the cerebrum due to failure of neural tube closure

What is spina bifida?

A defect in the spinal cord caused by incomplete neural tube closure

What is hydranencephaly?

A condition where only the brainstem develops; most of the cerebrum is absent

Which gene in zebrafish is associated with neural tube defects?

VANGL1

How does VANGL1 affect neural tube development?

It regulates convergent-extension movements and modulates Wnt/β-catenin signalling

What developmental process is disrupted in VANGL1 mutants in zebrafish?

Convergent-extension movements necessary for proper neural tube formation

What happens if animal caps (dorsal ectoderm) are isolated before gastrulation?

They give rise to epidermis, not neural tissue

What happens if the same dorsal ectoderm tissue is isolated during gastrulation?

It gives rise to neuronal tissue

Why does dorsal ectoderm acquire neural fate during gastrulation?

Because the dorsal mesoderm (organiser) comes into close contact with the overlying ectoderm

How does the dorsal mesoderm induce neural fate in ectoderm?

It releases inductive signals that instruct the ectoderm to become neural tissue

What does the animal pole of a Xenopus embryo give rise to?

Ectoderm (epidermis, or neural tissue if induced)

What does the vegetal pole of a Xenopus embryo give rise to?

Endoderm (gut)

What does the intermediate zone of a Xenopus embryo give rise to?

Mesoderm (blood, bone, muscle)

When does mesoderm form in Xenopus?

Only when animal pole cells interact with vegetal pole cells

How is mesoderm induced by vegetal cells?

Vegetal cells release a diffusible signal that induces mesoderm in adjacent animal cells