2. Fertilization - union of sperm & egg cell --> 2n zygote...  [recombinant DNA]
  
           parts of a sperm* - acrosome, head, mito, & axoneme (flagella)  drawing* & SEM
  
                     sperm must penetrate......  1)  egg's outer, protective, jelly-like coat, 
                                                                 2) vitellin layer... (glycoproteins), & 
                              photo* by Lennart Nilsson          3)  egg's plasma membrane
 
           via ACROSOME REACTION...  requires specificity -  (invert)*,   (mammals)*,  
 
              monospermy:  fusion of a single sperm and egg nuclei.
              prevention of polyspermy:
                    plasma membrane & vitellin layer becomes depolarized;
                    Ca⁺⁺ is released from egg's E.R. and a wave of Ca* sweeps across egg;
                    high Ca leads to cortical reaction (= perivitellin space)
                    after fert, it hardens and separates... forming fertilization membrane* 
                    high Ca also leads to activation of egg = cell respiration & protein synthesis
     timeline of events*

 
   3. Cleavage* - rapid succession of cell divisions... without cell growth 
                              - no increase in size,  only an increase in cell numbers
                                                     (light microscope  pics-1* -  pics-2   video*  &  SEM-frog)
  
              forms hollow ball of cells called BLASTULA embryo  or  blastocyst*, 
              with internal fluid cavity is calledthe...   blastocoel         
                      General pattern*      pics sea urchin cleavage to blastula embryo*
                          depends                 frog*           &   pics frog cleavage*
                          upon yolk               chick*          &   pics chick cleavage*    
          
                      animal pole*   - portion of embryo housing primary tissues 
                      vegetal pole* - portion of embryo containing  "yolk" - that establish axes

                                        3 day old human embryo (8 cell stage)
                                        manipulation of human blastula embryo
                                        Sumanas, Inc. animation - Human embryonic stem cells

 Embryogenesis & Germ Cell Layers

        4. Gastrulation - period of cell migrations around blastopore ^{(an organizer)},
                                            which converts embryo from hollow ball of cells into
                                            a 3 layered stage called gastrula embryo*  &   pic
                                                                                 (invert*)  &   (frog*)   &   (chick*)
    
               forms 3 primary germ cell layers :
                          ectoderm - outer epidermal layers of organs, skin --> epithelial & nerve
                          endoderm - digestive tract tissue  --> gut & endocrine glands
                          mesoderm - fills in space in between --> notochord, muscle & connective
                                                                    tissues derived from the 3 embryonic germ layers of vertebrates*
   
                         gastrulation obliterates the blastocoel - forms new 
                                              cavity called archenteron - presumptive digestive cavity
               ►  period when new diploid genes of embryo becomes active          

 
   5. Organogenesis .... Organ Formation via   -   organizing centers*
                                              groups of cells that control fate of (organize) other cells
  
           ex:  neuralation* in frog - formation of nervous system via neural tube      
                             neural plate (ectoderm) - flat tissue surface that migrates
                             via cytoskeletal elements to to form tubes*
                                                    neural tube* = brain & spinal cord  
                                                    notochord (mesoderm) form cartilage-like backbone 
                       ►   almost all organs form from flat plates into tubes --> 3D shape
                             followed by segmented development via somites -  somites*
 
  
                                     Human Development:     6 weeks*   -   20 weeks*                      
                                                   Human development movie (slides of fertilization to fetus)   
                                                                             from  Robert J. Huskey, of University of Virginia                          
                                                          fish, chick, pig, & human embryo development movies (WGBH-Nova)
                                                         the 23 stages of human development  (Visible embryo & NIH)
                                                          the multidimensional human embryo - U of Michigan 

        Some key differences: comparing animal to plant development...
   
              major differences occur in mechanisms of
                  MORPHOGENEIS:                 the developmental mechanisms establishing
                                                               SHAPE/FORM and ORGANIZATION (body plan)
                  CELL DIFFERENTIATION :  processes by which cells becomes
                                                              SPECIALIZED in structure and function

                  in animals:  cell migrations - where cells create new shapes by migrating
                                                                    and form plates & tubes     fig 21.4a
                                & embryonic induction - where one group of cells influences development of 
                                                     an adjacent group of cells         ex : lens induction in eye
 
                  in plants:       cytokinesis  &  plane of Cell Division*   fig 21.4b

 
 Genetic Basis of Development - is Differential Gene Activity
                         cells become structurally, functionally, & biochemically different
                         by expressing different genes at different times in development
   
  1st must demonstrate all cells of an organism contain a complete genetic complement
                                     i.e., differentiation does not proceed by loss of genes with time
   
   Genomic Equivalency:  all cells contain same genes:  nuclear transplant experiments -
                    1.  F.C. Steward (1950) using carrots* grows whole plant from single cell
                    2.  Briggs & King (1952) & J.B. Gurdon (1974) demonstrate it in frogs* 
                    3. John Wilmut (1997) clones Dolly - cloning*
 
   Stem Cells:  unspecialized cells (in form & shape) that can reproduce indefinitely
                        under appropriate conditions --> differentiate into one or more cell types
                                  fertilized egg cells are totipotent (= all)  embryonic stem cells
                            vs. adult stem cells (pluripotent = many, but not all)- stem cells*
  
  Transcriptional Regulation: results in expression of Tissue Specific Proteins -fig 18.10*
                                                                             
                                                                                key concepts*