Neural Development How do cells become nerve cells

Neural Development How do cells become nerve cells? – Environmental factors • Inductive events – Genetic factors • Competence Cell lineage Timing 1

Focus on the cortex 2

The cortex has 6 layers---how do they develop? superficial deep 3

Cortex Development: Formation of layers • neural progenitors rapidly divide in the ventricular zone • they keep dividing to produce more progenitors OR • they undergo terminal differentiation and migrate to final cortical destination • migrating neurons move along radial glia marginal zone cortical plate intermediate zone ventricular zone http: //www. stanford. edu/group/skmlab/movies/qtmovies/rad_migr. mov 4

At mitosis, precursor self-renews or differentiates Choice point for the precursor Cell cycle M G 2 G 1 precursor neuron precursor S 5

How do layers of the cortex form? Are there different kinds of progenitors? (layer 1 -specific progenitor) Does one progenitor make all cell layers? LINEAGE MODEL 1 2/3 4 5 6 6

Basic Techniques in Neurobiology: Neuronal Birthdating with 3 H-thymidine • 3 H-thymidine is incorporated into the DNA during replication • It marks all mitotic cells • It disappears in ~2 -4 generations. 3 H-thymidine time 7

Basic Techniques in Neurobiology: Lineage Tracing • Use retrovirus (incorporated into a cell’s genomic DNA but not infectious) • The virus DNA will be inherited by all the daughter cells (it doesn’t become diluted) virus time 8

Are there layer-specific progenitors? Expt: Use lineage tracing to label single neurons and follow their progeny. CP IZ VZ 1 2 3 4 5 6 Result: Single cells generate neurons in multiple layers. This result disqualifies the lineage model. 10

Different layers form at different developmental times “Inside First-Outside Last” development of cortical layers Born Last 1 Post-natal Day 2 (P 2) Dev Time 2/3 4 5 Born First 6 Embryonic Day 26 (E 26) 10

OBSERVATION: A single progenitor first produces layer 6 neurons, later produces layer 1 How does one progenitor produce different cell fates? 11

Does the environment instruct the layer fate? Expt: Transplant Progenitor cells from early stage to late stage What would they normally become? What do they become when the host environment is changed? E 36 = Early Most are layer 4 P 2 = Later Most are layer 2/3 Early into Later Most are layer 2/3 Early cells can take on later fate Model: Environment influences cell-fate 12

Does the environment instruct the layer fate? Expt: Transplant Progenitor cells from late stage to early stage What would they normally become? What do they become when the host environment is changed? Early Earlier Early in to earlier 13

Summary of many transplantation experiments Put early progenitor into late host environment, takes on late fate Put late progenitor into early host environment, takes on late fate Both the environment and the progenitor determine cell fate A) the age of the environment restricts cell-fate B) the age of the progenitor restricts cell-fate MODEL OF PROGRESSIVE RESTRICTION 14

Events inside the cell and outside influence layer decision

One extracellular cue that influences cortex formation Reeler mutant mice (missing Reelin protein) • Identified because of severe movement disorders • Mutation in an extracellular matrix protein (? ) • Cortex is built outside in • 26 human disorders associated with cortex malformation • One form of lisencephaly (smooth brain) is a Reelin mutant 16

Cortex development in Reeler mutant mice • Precursor cells never separate from the cortical plate • Grows Top down instead of Bottom up Normal Reeler Model: Reelin is an important extracellular cue that separates verticular zone from marginal zone 17

Decision point for the precursor Cell cycle M G 1 G 2 S neuron precursor 18

General Principle: An intracellular timing mechanism regulates cell fate decisions • At mitosis, precursor continues to self-renew or differentiates Choice point for the precursor Cell cycle M G 2 G 1 precursor neuron precursor S 5

A Timing Mechanism for Neural vs Glial Fate precursor 19

A single precursor cell will first produce neurons then glia Neural markers Glial markers 20

A single neuroblast clone will produce many different cells • In Drosophila, sequential expression of transcription factors induces different fates • Expression of transcription factors is controlled by cell cycle Transcription factors 22

Every cell in C elegans is determined by lineage Sydney Brenner 2002 Nobel Prize Genetics of Development in C elegans “In late 1962, Francis Crick and I began a long series of conversations about the next steps to be taken in our research. Both of us felt very strongly that most of the classical problems of molecular biology had been solved and that the future lay in tackling more complex biological problems. ” 23

Why C. elegans? • Reproduce sexually and asexually • Each worm makes 200 eggs • Develops from egg to larvae in 12 hrs • Total number of cells is 1000 • Total number of neurons is 300 24

Neurons 1 -25 in C. elegans Name ADAL ADAR ADEL ADER ADFL ADFR ADLL ADLR AFDL AFDR AIAL AIAR AIBL AIBR AIML AIMR AINL AINR AIYL AIYR AIZL AIZR ALA ALML ALMR Cell divisions AB. plapaaaapp AB. prapaaaapp AB. plapaaaapa ABprapaaaapa AB. alpppppaa AB. praaappaa AB. alppppaad AB. praaapaad AB. alpppapav AB. praaaapav ABplppaappa AB. prppaappa AB. plaapappa AB. praapappa AB. plpaapppa AB. prpaapppa AB. alaaaalal AB. alaapaaar AB. plpapaaap AB. prpapaaap AB. plapaaapav AB. prapaaapav AB. alapppaaa AB. arppaappa AB. arpppappa Cell fate Ring interneuron Anterior deirid, sensory neuron Amphid neuron Amphid finger cell Amphid interneuron Ring interneuron Amphid interneuron Neuron, sends processes laterally Anterior lateral microtubule cell 25

Lessons in Development 1. Lineage is an important factor in development (Different cell fates can be determined by cell division) 2. The cortex develops through the process of progressive restriction cues in the environment and within the cell determine its fate 3. The time in which you are born will determine what you become 26