Cell Birth and Death Chapter Three
Neurogenesis All neurons and glial cells begin in the neural tube Differentiated into neurons rather than ectoderm based on factors we have already discussed If these cells are not part of spinal cord have to migrate elsewhere Brain is complex three dimensional object migration within brain regions
Controlled by: Cellular clocks: Number of cells Limit the number of cell divisions allowed for the progenitor cell Mitogen signals: Extra cellular signals that send cell into cell division Mitotic inhibitors: Block cell division Programmed Cell Death: Apoptosis signals
Cell Lineage Lineage of a cell refers to: Which cells will divide Where the cells will migrate to Type of neurons that will be generated from a precursor Therefore genesis of type of neuron and migration of neuron come from one signal
Number of Cells in Drosophila Depends of Cell Lineage In Drosophila, neurogenesis involves delamination: cells from the ventral epidermis become enlarged and are squeezed out of the epidermis. Delamination of the Drosophila embryo results in neuroblast formation Neuroblast cells are the progenitors of nervous tissue (glia and neurons). First division of neuroblast cells generate a pair of ganglion mother cell (GMC). Each GMC generates one pair of neurons
Drosophila neurons Lineage and order: 1. Cells delaminate from ventral-lateral ectoderm Forming neural precursors (Neuroblasts) 2. Neuroblasts are organized into columns and rows 3. Asymmetrical divisions: 1. One neuroblast keeps dividing 2. One ganglion mother cell forms neurons
Neurogenesis in Drosophila Migrating neurons form the segmental ganglia of the ventral nerve cord
Vertebrate Neurogenesis Neural tube begins only a single layer thick Multiple cell divisions make it a much thicker tube Tube differentiates into different cell types
Vertebrate Neurogenesis Ependymal zone now known as Ventricular Zone
Interkinetic nuclear migration Mitosis always happens on the innermost Ventricular zone S phase always happens at the outermost Ventricular zone now defined as: Innermost section of neural tube Where the M phase is
Neurogenesis in Vertebrates Neural tube neurons migrate From the ependymal zone (on the inside = ventricular surface) Ventricular zone Mantle zone Marginal zone To the mantle zone (in the middle of the neural tube)
How Can We Study Neurogenesis? Thymidine and BromodeoxyUridine can be used to label newly-generated cells during neurogenesis Tritiated thymidine use autoradiography to visualize label BromodeoxyUridine use immunocytochemistry to visualize label Cells incorporate thymidine or BrdU during the S phase (active DNA replication)
Migration Up and Down Labeled cells (red) are only dividing when they are in the ventricular zone (down) When the cells are in S phase they are within mantle zone (up) Then migrate back down to ventricular before dividing again
Up & Down Movement of Progenitors Cells Nuclei During Neurogenesis Marginal zone Ventricular zone S-phase cell M Cell cycle length Mitotic cell G2 G1 S S Neurogenesis and migration in neural tube Thymidine labeling reveals the cell cycle length of neuronal progenitors
Up & Down Movement of Progenitors Cells Nuclei During Neurogenesis
Cell Divisions Can be either symmetrical or asymmetrical Symmetrical division Producing two progenitor daughter cells Both of which can divide further Asymmetrical division Producing one progenitor keeps dividing One post-mitotic neuron Later in development more asymmetrical divisions and less symmetrical ones
Length of Cell cycle The length of the cell cycle increases as embryogenesis proceeds Early neural tube Shorter cell cycles More cell divisions More symmetrical divisions Later neural tube Longer cell cycles Less division more asymmetrical
Length of Cell cycle Length of cell cycle increases due to a longer G1 phase
Number of Cells Divisions Depends on: 1. Stage of development: Embryonic vs. adult Early on = Expansion phase More divisions (shorter G1 phase) More symmetrical divisions Later on Neurogenesis almost stops 2. Physical location of progenitor cells
Cell Divisions No more Mitosis
Thymidine Birthdating Injection of thymidine at various developmental stages point to the location of different cell populations in the developing cortex Cells that stop dividing shortly after administration will show stronger label Compared to cells that keep dividing
Thymidine Birthdating Showed that neurogenesis is incredibly well ordered Example: Cerebral cortex always develops medially to laterally
Retroviral Vectors - GFP Retroviral vectors can also be used to label dividing progenitor nerve cells with GFP GFP has the advantage that labeling does not fade with everincreasing number of cell divisions Only original cell and its daughter cells will be labeled
Retroviral Vectors - GFP This study labeled one neuroblast This one neuroblast formed: 1. Both neurons and glial cells 2. The descendent cells were spread out in different areas of the brain
Generalizations Distinct neurons originate in a fairly invariant time table Sometimes the entire population of one neuron may become post-mitotic within a short period of time Large neurons are developed before smaller ones Pattern of development is consistent with evolution of neurons Less evolved regions develop 1st
Length of Cell cycle G1 phase is lengthening It is believed that both intrinsic and extrinsic factors are involve in the lengthening of the G1 phase..how? Cell cycle control genes
Cell cycle control The genes that control the cell cycle control the amount of proliferation of all cells Including neuroblasts Cyclins activate kinases Kinases phosphorylate substrate proteins either activate or deactivate Different cyclin/kinase pairs activate different parts of the cell cycle
Control of Cell Cycle Progression to next phase of cell cycle depends on phosphorylation of specific substrates by CDK Rb = retinoblastoma protein For example: Binding of cyclind to CDK4 (or 6) causes cells to go into S-phase Binding of cyclinb to CDK2 cause cells to go into M- phase
Cell-Cell Interactions Control Number of Generated Cells TGFβ (restrains the cell cycle) TGFb dephosphorylates the tumor inhibitor, retinoblastoma protein (rb), and prevents cell from entering S phase CyclinD (promote cell cycle) CyclinD phosphorylates the tumor inhibitor, retinoblastoma protein (rb), and permits cell to enter S phase
Cell-Cell Interactions Control Number of Generated Cells Several factors called growth factors or mitogens stimulate cell cycle and proliferation Examples: Fibroblast growth factor (FGF) promote proliferation of fibroblasts Epidermal growth factor (EGF) stimulate growth of epidermal cells These factors most likely reduce length of cells in the G1-phase. During development the G1-phase lengthening may be due to reduction of these factors
Programmed Cell Death Unchecked cell growth leads to cancer Neuroblastomas or gliomas Cell cycle genes control by stopping proliferation Other genes control by killing off cell through apoptosis Examples: p53, p27, p21
p27 gene p27 binds up CyclinD1 Therefore p27 mutant has opposite phenotype of CyclinD1 mutant Example with retinal cells:
CyclinD and p27 mutants A = wild type retina B = CyclinD-/- C = p27-/- D = Double mutant CyclinD-/-; p27-/-
Growth Factors The very same growth factors can regulate proliferation AND determine which cell types will be formed FGF Patterns anterior types Shh Patterns ventral types Wnt Patterns dorsal types
Shh Shh can act as a mitogen Released from Purkinje neurons Remember Purkinje cells generated early Certain neuroblasts have Shh receptors Patched and Smoothened These neuroblasts respond to Shh signal and proliferate: Add Shh more granule cells (maybe cancer) Block Shh fewer granule cells
A Variety of Growth Factors Control Neurogenesis and Gliogenesis GGF controls the transformation of astrocytes into radial glial cells GGF = glial growth factor PDGF = platelet-derived growth factor CNTF = ciliary neurotrophic factor Progenitor cells (MP) can acquire a variety of fates. For example, MP cells exposed to NT3 and FGF becomes neuronal cells, MP cells treated with CNTF become astroglia, and PDGF induces formation of oligodendrocytes
Neurons or Glial Cells? Most progenitor cells can form either neurons or glial cells How do they make this choice? As development continues progenitor cells become specialized so that they can only form one or the other What factors control the tight ratio of neurons to glial cells that exists?
Neurons or Glial Cells How is this choice determined?
Neuron or Glial Cell Depends on these factors: Region of the nervous system where progenitor exists Age of the embryo being examined Expression of specific extra cellular signaling factors Some are proneural Some are proglial
Examples FGF2 and neurotrophin3 neurons CNTF and EGF astrocytes (glial) Notch: inhibits neural formation If Notch is activated glial If Notch is blocked overrun with proneural genes neurons TGFβ neurons in certain conditions TGFβ glial in other conditions
Overall Pathway:
Proneural vs. hes hes is activated by Notch Antagonizes proneural genes
Optic Nerve Neurons do not form within the optic nerve Only glial cells Special case where any of the growth factors will promote glial cells only
Any Questions? Read Chapter Three