lectures 22-23: neural crest cells
Neural Crest Cells: The Migratory Masters
Background and Introduction
Neural Crest Cells (NCCs): Renowned for their extraordinary migratory abilities and remarkable developmental plasticity, giving rise to an expansive array of cell types throughout the vertebrate body.
Textbook Reference: Chapters delve into the intricate processes encompassing the developmental origins, molecular specification, migratory pathways, and diverse fates of NCCs.
Learning Objectives for NCCs
Outcomes of NCCs:
Recall the various potential cell types and tissues derived from NCCs, as discussed in detail.
Explain how the anterior-posterior (A-P) axis location of NCC origin significantly influences their differential developmental outcomes and specific derivatives.
Specification and Potency:
Describe the molecular mechanisms underlying NCC specification and apply principles of lineage tracing techniques to understand and track cell potency and differentiation pathways in NCCs.
Development Mechanisms:
Explain the crucial roles of the extracellular matrix (ECM), various membrane proteins (including cell adhesion molecules and receptors), and the dynamic cytoskeleton in orchestrating NCC development, particularly during migration and differentiation.
Comparison of NCC Migration:
Compare and contrast the initiation cues, migratory patterns, and final destinations of NCCs based on their specific anatomical location of origin (e.g., cranial, trunk, vagal, sacral).
Formation and Development of Neural Crest Cells
Origin of NCCs:
NCCs originate from a transient population of cells known as neural plate border cells, located at the interface between the prospective neural plate and the non-neural ectoderm during the initial stages of neurulation.
Neurulation Process:
During primary neurulation, the neural plate folds inward to form the neural groove, which then fuses dorsally to create the neural tube.
Concurrently, the neural plate border cells undergo an epithelial-to-mesenchymal transition (EMT), detaching from the neural tube and transforming into migratory NCCs.
Post-Neurulation:
Following neural tube closure and their delamination, NCCs embark on extensive migratory journeys from the dorsal aspect of the neural tube to various distant locations throughout the embryo.
NCCs are also generated during secondary neurulation, contributing to caudal body structures.
Lineage Tracing of Neural Crest Cells
Definition:
A sophisticated biological technique designed to track the developmental lineage of specific cells, allowing researchers to observe their proliferation, differentiation, and migration from their origin to their final differentiated state.
Techniques:
Employ various sophisticated labeling strategies:
Sparse labeling: Involves introducing a limited number of distinct genetic or molecular labels into a small subset of cells, enabling individual cell tracking based on unique markers.
Barcoding: Utilizes a larger library of distinct, heritable labels (e.g., unique genetic sequences) to differentially tag numerous cells, allowing for high-resolution tracking of multiple lineages simultaneously.
Importance in NCC Studies:
Crucial for elucidating the incredible plasticity and multipotency of NCCs, enabling scientists to accurately trace their diverse developmental outcomes in adult tissues and unravel their contributions to various physiological and pathological conditions.
Variability in Neural Crest Cell Fates
Diverse Derivatives:
Peripheral Nervous System (PNS): Includes all peripheral neurons (e.g., sensory neurons of dorsal root ganglia, sympathetic and parasympathetic ganglia) and neuroglial cells (e.g., Schwann cells that myelinate peripheral nerves, satellite cells).
Endocrine Derivatives: Cells of the adrenal medulla (secreting catecholamines like epinephrine and norepinephrine), calcitonin-secreting parafollicular C cells of the thyroid gland.
Pigment Cells: Epidermal melanocytes, which produce melanin responsible for skin and hair color.
Structural Components: A vast range of craniofacial elements including parts of the facial cartilage, many bones of the skull and face (e.g., frontal bone, maxilla, mandible), connective tissues (e.g., dermis of the face and neck), odontoblasts (dentin-producing cells of teeth), and smooth muscle cells of large arteries.
Source: Derived from M. Jacobson () and multiple authoritative developmental biology sources.
A-P Axis Dependence and Multipotency
Location Influence:
The developmental fates of NCCs are profoundly influenced by their axial level of origin along the anterior-posterior (A-P) axis of the neural tube, indicating regional specification.
Cranial NCCs primarily form craniofacial structures, while trunk NCCs contribute to peripheral ganglia, melanocytes, and adrenal medulla cells. Trunk NCCs specifically reveal multipotent characteristics, with approximately of individually cultured trunk NCCs having the capability to differentiate into at least two or more distinct derivatives (e.g., neurons, glia, melanocytes).
EMT Process:
The epithelial-mesenchymal transition (EMT) is a fundamental cellular process critical for NCC delamination and migration. It involves the loss of apical-basal polarity, downregulation of epithelial adhesion molecules (like E-cadherin), and the acquisition of mesenchymal characteristics such as increased motility and invasiveness, enabling NCCs to leave the neural tube and migrate.
Specification of Neural Crest Cells
Trigger Events:
NCC specification is initiated and orchestrated by a convergence of signaling pathways, notably Wnt and BMP (Bone Morphogenetic Protein) signaling, at the neural plate border.
Wnt signaling establishes the neural plate border identity, leading to the activation of specific transcription factors (e.g., Pax3/7, Msx1/2, Zic1/2) that define the neural plate border cells.
A precise gradient of BMP signaling, with intermediate levels at the border, further refines the NCC specification program, triggering the expression of key NCC specifier genes such as Snail, FoxD3, and Sox9.
The activation of cadherin expression genes (specifically those relevant for migration rather than adhesion) coincides with NCC differentiation and delamination.
Cadherin Role in Neurulation:
Cadherins are vital cell adhesion molecules crucial for the proper morphogenetic movements during neurulation and NCC delamination.
E-cadherin is predominantly expressed in the epidermal ectoderm, maintaining its epithelial integrity.
N-cadherin is expressed in the neural plate and later the neural tube, facilitating its invagination and fusion.
During delamination, NCCs downregulate N-cadherin and upregulate other cadherins (e.g., Cad-6B, Cad-7) and integrins, allowing them to detach and migrate. Defined roles: E-cadherin helps stabilize actin filaments in epithelial junctions, while N-cadherin is involved in regulating actin dynamics and cell-cell interactions within the developing neural structure.
Mechanisms of NCC Migration
Contact Inhibition and Cues:
The dynamic interplay of Wnt and BMP signaling pathways leads to distinct patterns of cadherin expression across germ layers: E-cadherin in the epidermis and N-cadherin in the neural tube. During delamination, NCCs undergo a cadherin switch, decreasing expression of neural cadherins (N-cad, Cad-6B) and increasing expression of mesenchymal cadherins (e.g., Cad-7) and integrins, which promotes their detachment and migratory phenotype. This switch enables NCCs to migrate through diverse environments, often guided by both attractive and repulsive molecular cues in the extracellular matrix. Specifically, Cad-6B expression in NCCs plays a role in migration signaling through RhoA/Rac1 pathways, influencing their directionality and collective movement.
Migration Patterns:
NCCs migrate either as individual cells or as cohesive streams. Their migration is highly directional, influenced by physical barriers, chemoattractants (e.g., GFR1 for trunk NCCs), and chemorepellents (e.g., semaphorins, ephrins). The patterns are diverse: cranial NCCs migrate in distinct streams to form facial structures, while trunk NCCs follow either a ventral or dorsolateral pathway, each with specific molecular guidance.
GTPase Regulation in NCC Migration
Key GTPases:
RhoA: A small GTPase that, when activated, triggers the formation of stress fibers and focal adhesions, leading to actin contraction and cell body retraction. In NCC migration, RhoA is critical for generating the contractile forces required for rear retraction and maintaining cell cohesion within migratory streams.
Rac1: Another small GTPase that, when activated, stimulates actin polymerization at the cell periphery, promoting the formation of lamellipodia and filopodia, which are essential for cell protrusion and exploration of the migratory environment. Rac1 drives the leading edge extension crucial for directed cell movement.
Connection to Migration: The precise spatial and temporal activation of RhoA and Rac1, often in opposing gradients within the cell, is crucial for coordinating the dynamic changes in the actin cytoskeleton required for persistent cell migration. This coordinated activity is paramount during EMT and subsequent directed cell movement, as well as in axon outgrowth for neuronal differentiation from NCCs.
Migratory Paths of NCCs
Somite Interaction:
Somites, transient embryonic structures, provide two major migratory pathways for trunk NCCs, creating a segmented pattern of migration.
Ventral Pathway: Cells migrate through the anterior sclerotomes of the somites. This path is permissive due to the presence of attractive molecules like fibronectin and laminin, and the absence of inhibitory molecules. NCCs taking this route typically differentiate into neurons and glial cells of the dorsal root ganglia, sympathetic and parasympathetic ganglia, and chromaffin cells of the adrenal medulla.
Dorsolateral Pathway: Cells migrate between the epidermis and the dermis, dorsal to the somites. This pathway is utilized by NCCs that will ultimately differentiate primarily into epidermal pigment cells (melanocytes). This path is influenced by different molecular cues, such as Scatter Factor (SF)/Hepatocyte Growth Factor (HGF) and Kit signaling, that guide their movement away from the neural tube and into the skin.
Specifically, each pathway receives distinct molecular cues for movement. For instance, repulsive cues like ephrins and semaphorins in the posterior sclerotomes actively direct NCCs away, channeling them into the anterior sclerotomes via the ventral pathway.
Cardiac and Cranial Neural Crest Development
Cardiac NCCs: These cells originate from the caudal hindbrain region (rhombomeres -) and migrate to the pharyngeal arches, from where they significantly contribute to the development of the heart's outflow tract and septa. They are crucial for forming the conotruncal septa that divide the aorta and pulmonary artery, and for contributing to the septation between the atria and ventricles. Defects in cardiac NCC migration, proliferation, or differentiation can lead to severe congenital heart malformations such as persistent truncus arteriosus and tetralogy of Fallot.
Cranial NCCs: These predominantly develop into a wide array of craniofacial bones, cartilage, connective tissues (e.g., dermis of the face), and even odontoblasts (dentin-forming cells). Their migratory patterns are highly region-specific, involving distinct streams that populate the pharyngeal arches and facial prominences, indicating precise migratory patterns and outcomes.
Examples of Neural Crest Interaction: Contact inhibition, mediated by cell adhesion molecules like E-Cadherin and N-Cadherin, can enhance directionality during collective migration, where cells stop moving upon contact with another cell, pushing them forward in streams.
Phenomenon of Cell Migration in NCCs
Chase and Run Model:
This model describes scenarios where NCCs adopt specific migratory routes influenced by a sophisticated interplay of inhibitory and attractant signals from surrounding environments, such as those emanating from placodes (ectodermal thickenings that give rise to sensory structures). Some cells might 'chase' an attractant while others 'run' from a repellent.
Signaling Dynamics: The dynamic interaction of NCCs and placodes, along with other environmental cues, emphasizes the intricate industry's influence on migratory behavior and developmental patterning, ensuring proper tissue formation and organogenesis.
Conclusion
Neural Crest Cells: Serve essential roles in vertebrate development due to their diverse fates and dynamic migratory capabilities