DIFFERENTIATION  4/30/03  See Chpt 9

*Cells of embryo acquire distinct characteristics and specialized 
functions
     *Identity of cells distinct and persist in adult 
     *Condition normally stable and irreversible

*Changes in gene expression lead to production of characteristic cell 
proteins

*Precursor cells become determined as their developmental potential 
becomes reduced
    *Once determined, a cell passes that determined state to its progeny
    *Process is gradual
    *In most instances proliferation does not occur during the final 
      stages  of differentiation

*Mechanism for regulation
  *Complex genetic control mechanisms and sites along chromosome
     * binding sites for transcription factors
     * interaction among regulatory molecules
     * whether ts factor is phosphorylated     

 * Morphogens such as transcription factors and other regulatory
     proteins in the cytoplasm can regulate gene activity; 
     requires that nucleus is receptive and responsive to these   

What determines the particular pattern of gene activity in a 
differentiated cell?

  Profound permanent alteration        OR      Changes in 
   in genetic material                      chromatin or regulation 
                                             by regulatory proteins

Experimental approach - select nuclei from differentiated cells and place 
in a different cytoplasmic environment

  Classic work in amphibians using irradiated egg as source of cytoplasm
    Nuclei from younger developmental stages most successful in 
       supporting devel.  F 9.3
    Nuclei from adult tissues which retain mitotic activity or larval 
       stages also can support development;  lower % and infertile [F 9.2]

  Cell fusion experiments - nuclei from different cells share common 
       cytoplasm
    Human liver cell in rat muscle cytoplasm - liver-specific genes 
        repressed while human muscle proteins are synthesized [F 9.4]  
    Chick red blood cell in human cancer cell - new array of chick 
         proteins produced

       Demonstrates that no irreversible alteration of genetic material 
         has occurred.  
       The genes required for development are still present and can 
         function; repressed genes can be reactivated.

      Also indicates that transcription factors and other regulatory 
        proteins in the cytoplasm can regulate gene activity and 
        that nucleus is receptive and responsive to these

Transdifferentiation - one cell type changes into another
     Regeneration - newt RPE recruited to form lens; can be mimiced in 
       culture  [F 9.5]
     Cultured chromaffin cells will transdifferentiate into sympathetic 
       neurons when exposed to  different environmental cues [F 9.6]

How is pattern of gene activity controlled?  How is a specific pattern 
transmitted over many cell cycles?  fibroblasts, liver cells, gut 
epithelium, etc

Chemical or structural alteration of chromosome
     heterochromatin - Barr body
     methylation

Master genes may code for protein that serves as transcription factor for 
     many other genes [F9.9]; may stimulate or repress transcription 
     (remember this in relation to induction?)

Complex genetic control mechanisms and sites along chromosome [F9.15]
     binding sites for transcription factors
     interaction between ts factors
     interaction among regulatory molecules
     whether ts factor is phosphorylated

External factors
     environmental cues alter gene expression
     signal transduction - action of peptide hormones and growth factors 
     steroid hormones enter cell/nuc - bind to receptor and to DNA

Continued presence of regulatory molecules may be required