Differentiation of the Neural Tube Gilbert - Chapter 12

Today’s Goals Realize that Nervous system is patterned on A-P and D-V axis Describe mechanisms by which this pattern is set-up –Identify important signaling pathways involved

Ectoderm Surface Ectoderm: –Epidermis, hair, nails, olfactory epithelium, mouth epithelium, tooth enamel, lens, cornea Neural Crest: –PNS (sympathetic and parasympathetic), adrenal medulla, melanocytes, facial cartilage, dentine of teeth Neural Tube: –Brain, neural pituitary, spinal cord, motor neurons, retina

Differentiation of Neural Tube The actual gross morphological changes, making each portion of the neural tube special –Specialized on A-P axis –Specialized on D-V axis –Specialized cell types form Neurons, glia, neural tissues (brain, spinal cord)

Neural Tube Differentiation Differentiation into various regions of the CNS Happens in 3 ways, simultaneously –Gross anatomical changes - bulging and constriction of Neural tube to form chambers of brain and Spinal Cord –Tissue-level Changes - Cells rearrange into functional regions of the brain –Cellular-level Changes - Cells differentiate into various neural cell types Neurons (nerve cells) Glia (supporting cells)

The A-P Axis of the Nervous System The anterior portion of Neural tube is organized into various regions along the A-P axis (Will form regions of the brain) Early Neural Tube –Forebrain (Prosencephalon) –Midbrain (Mesencephalon) –Hindbrain (Rhombencepalon)

Anterior-Posterior Neural specialization: mammalian brain - See figure 12.9

A-P axis Controlled by a variety of genes –Those expressed during gastrulation Chordin, noggin, etc. –Hox genes - give global A-P identity We’ll discuss further later in the semester

Dorsal-Ventral Neural Patterning Dorsal spinal cord - sensory neurons Ventral spinal cord - motor neurons –What’s the difference? Interneurons –Relay info between the above ***D-V polarity of neural tube relies on signals from the notochord (mesoderm) and overlying ectoderm (epidermis)***

Ventralizing Signals A signaling molecule is released from the notochord SHH induces the ventral neural tube to become “floor plate” –This induces the floor plate to secrete SHH –Result is a gradient of SHH expression from ventral to dorsal

Expression of SHH mRNA in a 4 day old Chick embryo

SHH Member of the Hedgehog Family of Proteins Example of “Paracrine” signaling Used in many places during embryonic development to influence cell fate

How does it work? (a simplified look) SHH is released from a inducer cell Binds to its receptor -Patched protein –On the surface of nearby responder cell This causes a second protein - Smoothened - To become activated Causes Phosphorylation of “Fused” protein Activation of a transcription factor that turns on expression of new genes

Hedgehog Intracellular Signaling Cascade

SHH Also important in: –Somite Development –Limb Development –Neural Differentiation –Mutants in SHH have drastic phenotypes, including Cyclopia

Ex. Of Cyclopia in a Lamb (Not a SHH Mutant)

Dorsalizing Signals Overlying ectoderm secretes proteins in the TGF-ß family of proteins (Transforming growth factor) –BMP’s (bone morphogenetic protein) BMP4, BMP7 This induces the dorsal neural tube to form the “roof plate” –Roof plate begins to express BMP4 protein –Again a gradient of expression is established

TGF-ß Signaling Pathway Ligand (Ex. BMP-4) Binds to receptor –Receptors are Heterodimers –Transmembrane proteins When ligand binds, triggers cascade of phosphorylation events Phosphorylates SMAD4 transcription factor Enters nucleus and activates gene expression

Gradients Set up Unique Cell Identities Depending on signals received, cells turn on different transcription factors Cells that receive high levels of SHH –Turn on Nkx6.1, Nkx2.2 –Become ventral neurons Cells that receive less SHH and more TGF- ß’s –Turn on Nkx6.1, Pax6 –Become motor neurons

Each Cell gets its own address!

Dorsalin (TGF-ß family) Sonic Hedgehog Motor neurons

Notochord/Sonic Hedgehog Protein Necessary for ventral floor plate cell formation –Remove notochord, no floor plate cells! Notochord, and particularly SHH are sufficient to induce floor plate –Notochord transplant can induce ectopic floor plate –Pellet of cells that express SHH induce ectopic floor plate –2nd floor plate makes 2nd set of motor neurons

Dev. Of Human Spinal Cord

Adult Neural Stem Cells Until recently - believed we could not replace neurons after the first few years of life Recent studies suggest that adult mammalian brains are capable of producing new neurons Studies in rats, other mammals

Adult Neural Stem Cells Post-mortem studies of patients treated injected with a chemical that traces new cell division (marker for cancer cells, and any other cell that is dividing) –Showed NEW neurons, although FEW Well established for some regions of the brain, controversial in others (cortex) How they are maintained is not well understood

Xenopus Neurulation Lab Examine stages of Xenopus undergoing neurulation –4 Stages –Draw what you see and label: Neural Plate Neural Groove Neural Folds Neural Tube 15 points - Lab Activity Grade