Driever Neural Developement

• critical process in early embryonic developement where undifferentiated cells are directed to become neural cells, ultimately forming the central nervous system (CNS)

• induction begins with initiation of the neural plate

  —> important signaling: BMP / Smad1 inhibition achieved by Chordin / Noggin, but also maternal ß-catenin and also FGF (FGF in low conc. also directly induces neural fate

  —> the organizer region (Spemann) is crucial for controlling the cells fates

1. Formation of neural plate (tickening of ectoderm)

2. Formation of the neural folds (at edges of neural plate / epidermis)

3. Elevation of the neural folds (middle anchored to notochord)

4. closing to form neural tube (Anterior Neuropore becomes brain, Posterior spinal chord)

   —> As the neural tube closes, some cells at the neural crest, the region where the neural folds meet, undergo an epithelial-to-mesenchymal transition —> neural crest cells

• the neural anlage, which refers to the early neural tissue or precursor to the central nervous system (CNS), initially does not have distinct anterior or posterior characteristics. Neural induction sets the stage for the development of the neural plate, and the subsequent processes of neurulation and regionalization further define the anterior-posterior (A-P) axis

• WNT signaling (and frzb as WNT inhibition) affect A-P patterning

Anteroposterior patterning in the neural plate:

—> (Spemann) Organizer - planar signals (patterning along a single layer or plane, often determining the anterior-posterior axis e.g. WNT pathway)

—> axial mesoderm - vertical signals (patterning along the vertical axis, specifying dorsal-ventral identity in tissues like the neural tube e.g. Shh specifying ventral fate)

1. Initiation of Neural Induction:

   • The Spemann Organizer secretes signaling molecules, including proteins like Noggin and Chordin, which inhibit the activity of BMP (Bone Morphogenetic Protein). BMP is involved in promoting epidermal and non-neural fates. Inhibition of BMP is a key step in neural induction, as it allows the ectodermal cells to adopt a neural fate and form the neural plate, a precursor to the CNS.

2. Establishment of Dorsal-Ventral Axis:

   • The Spemann Organizer contributes to the establishment of the dorsal-ventral axis by releasing signals that counteract the ventralizing effects of BMP. This axis is crucial for the subsequent patterning of the neural tube, with different regions of the spinal cord acquiring distinct dorsal or ventral identities.

3. Formation of the Notochord:

   • The notochord, a rod-like structure that forms along the midline of the embryo, is induced by the Spemann Organizer. The notochord releases signals like Sonic Hedgehog (Shh), which plays a key role in ventral patterning of the neural tube.

4. Neural Crest Induction:

   • The Spemann Organizer is also involved in the induction of neural crest cells. These cells arise at the border between the neural plate and the non-neural ectoderm, and they play a crucial role in the development of various tissues, including parts of the peripheral nervous system.

5. Pattern Formation in the Neural Tube:

   • Through its influence on BMP and other signaling pathways, the Spemann Organizer contributes to the regionalization of the neural tube. This process results in the formation of different regions along the anterior-posterior and dorsal-ventral axes, leading to the development of distinct structures in the brain and spinal cord.

• Notochord forms the floor plate of neural tube —> Ventral fate by Shh signaling

• BMP signalling (dorsalin) from roof plate of neural tube —> dorsal fate

• intermediate signaling in the zone where both gradients meet form diverse groups of interneuron —> Combinational expression or cross-repressive interactions

  
  

• removal or transplantation of the notochord leads to no formation of floor plate and no Shh signaling and transplatation to a second one and more ectopic motor neurons

• WNT activity high on posterior side

• Anti WNTs (competitors e.g. frzb) gradient high on anterior not active on posterior —> Absence of Wnt activity required for forebrain, increasing Wnt conc. specify more caudal CNS domains

• Retinoic acid gradient along A-P axis of embryo or FGF signaling promoting posterior cell fates

• Interaction with organizer

• Reporter Constructs and Transgenic Animals: —> WNT reporter mouse

  • Genetic engineering techniques can be used to generate transgenic animals with reporter constructs. These reporter constructs typically consist of a promoter region responsive to Wnt signaling, driving the expression of a fluorescent or bioluminescent protein.

  • When Wnt signaling is active, the reporter gene is expressed, and the fluorescence or bioluminescence emitted by the protein can be detected and visualized.

  • Examples of reporter genes include green fluorescent protein (GFP), red fluorescent protein (RFP), or luciferase.

• Immunohistochemistry and Immunofluorescence (antibody to WNT pathway molecules e.g. catenin)

• Fluorescent Wnt Ligands, microscopy and live imaging

• Anterior neural border (ANB)

• Zona Limitans Intrathalamica (ZLI): Boundary between thalamus and prethalamus in forebrain, crucial for forebrain regionalization.

• Midbrain-Hindbrain Boundary: Isthmus region organizing midbrain and hindbrain development.

Initially also:

1. Spemann Organizer: Dorsal blastopore lip region releasing Noggin and Chordin to inhibit BMP, crucial for dorsal-ventral patterning.

2. Notochord: Rod-like structure along embryo midline secreting Shh, influencing ventral patterning in the neural tube (floor plate).

3. Floor Plate: Ventral midline of neural tube releasing Shh, specifying motor neurons and ventral interneurons.

4. Roof Plate: Dorsal midline of neural tube contributing to dorsal-ventral patterning and specifying dorsal cell fates.

• midbrain Patterning and anterior hindbrain patterning (regionalization and boundaries)

• control of neuronal differentiation (cell identity)

• control of proliferation

• neural stem cells

• morphogenesis

• establishes polarity in the Tectum

• axon guidance of the retinotectal projektion

YES, the functional and morphological organization of the rhombencephalon (later spinal chord) is segmental Rhombomeres (segmental units): Each rhombomere will give rise to a unique set of motor neurons that control different muscles in the head

Segmentation —> clonal restriction (descendants of a cell will not cross rhombomere boundaries)

• labeling of neuroepithelial cells at early and late stages and mapping the clones at 2 days

• early labels will be visual accross boundaries of segment, late labels only in a segment because injected after rhombomere formation

• Retinoic acid treatment causes loss of anterior neural structures!

• Retinol transported in cell —> enzymes dehydrogenate it to retinal —> it then acts on rezeptors (all-trans RA binds mostly) in the nucleus RAR / RXR which are transcriptionfactors

• The combined synthesis and degradation established a stable retinoic acid gradient

• mesoderm produces RA degrading enzyme which act on neural plate forebrain and midbrain and overlaps to the RA dehydrogenase enzyme RALDH2 at hindbrain, in the spinal chord the RA gradient is high

• Different RA gradient shape and functions early and late

• by interactions with many gradient forming molecules the pre-RHB and the rhombomeric hindbrain (RHB) can establish r1-7 segments interaction molecules are Hoxa1-4 from RA side (spinal chord side) and iro7 or Krox20 from brain side

Definition:

➢ Neurogenesis is the process by which a multitude of postmitotic neurons are generated from a relatively few neural stem cells/progenitors in the neural epithelium.

➢ Neural stem cells are self-renewing and multipotent, and have the ability to generate complex lineages in a fixed temporal order

➢ Some daughter cells remain as dividing progenitors while others are committed to become neurons or glia (gliogenesis).

➢ Neurogenesis needs to balance proliferation and commitment to differentiation

➢ Neurogenesis is responsible for populating the growing brain and spinal cord with neurons during development

• Lateral Inhibition: Cells receiving less (or no) Delta signal initiate neuronal differentiation

• by expressing more Delta it inhibits Delta in adjacent cells so only that one cell becomes neuron —> “one cell wins”

• Inhibition of Neural Differentiation by Notch (downstream inhibiting neurogenin and by that neuroD = cell differentiation)

• by expressing more Delta (activating the differentiation genes neurogenin and neuroD) it inhibits Delta in adjacent cells so only that one cell becomes neuron —> “one cell wins”

• the inhibited cells remain precurser cell or stem cells until adult age, other will become glia cells during gliogenesis

• by cell proliferation and neuro/gliogenesis

• Axon, dendrite growth

• Synaptogenesis

• Myelination

• Neural circuits are refined based on sensory input and interactions with the environment

more recent research has provided evidence that neurogenesis can occur in specific regions of the adult mammalian brain, a phenomenon known as adult neurogenesis

—> subventricular zone or hippocampus (where stem cell niches are)

neural stem cell (NSC) niche is a specialized microenvironment within the brain that provides essential signals and support for the maintenance, self-renewal, and differentiation of neural stem cells, it is connected to ECM, Vascular network and recieves niche specific factors and signals (long range or short)

Neuron

• Neurotransmitter phenotype

• Chemical phenotype (receptors?)

• Morphology of soma, axon and dendrites

• Axonal projection phenotype

• Electrical phenotype

How is the neuronal phenotype determined?

• “Prepattern” (embyogenesis) • Signals • cell zycle • “Lineage” • & …. Other Mechanisms

• Dorsovenral patterning determines neuronal identify in the spinal cord

• Transcription factor „prepattern“ in the telencephalon

• Local patterning determines neurotransmitter phenotype - followed by migration of precursors to target areas to achieve regional complexity