2. Fertilization - union of sperm & egg cell --> 2n zygote...  [forms recombinant DNA]
  
     parts*of a sperm - acrosome, head, mito, & axoneme (flagella)
                       "sperm is no more than a delevery vehicle for male DNA,
                        a genome with a hyperactive tail " 
                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 - (mammalian version)*.  
 
     monospermy:  fusion of a single sperm and egg nuclei.
     prevention of polyspermy (fusion by many sperm):
       in sea urchins plasma membrane & vitellin layer becomes depolarized, prevents fusion;
       also, Ca⁺⁺ is released from egg's E.R. and a wave of Ca* sweeps across egg...   Video*
       high Ca⁺⁺ leads to cortical reaction* creates perivitellin space as plasma membrane lifts
                      due to hyalin release - a glycoprotein that is attracts water to space.
         
         after fert, area 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 - fig*
           (pics echinoderm* - starfish-4-cell - invert* - invert-2 - SEM-frog* - Human*  )  
  
         forms hollow ball of cells i.e., BLASTULA embryo (video*) 
         outermost blastomeres join via tight junctions & seal interior cell mass, whose cells 
         pump Na into cavity favoring osmotic gradient into internal fluid cavity
         called the...   blastocoel.    mouse blastula w stem cells & Human postfertilization events
                  in mammals Blastocyst (or trophoblast) will become the placenta,
                  while the inner mass of cells* will form the embryo proper.  
     
      General clevage pattern           pics sea urchin cleavage to blastula embryo*  (video)
                                                          model animal = round worm (C. elegans) (video1  video2)
                          depends               frog*           &   pics frog cleavage*
                          upon yolk             chick*          &   pics chick cleavage    

                   animal pole   - is portion of embryo housing primary tissues 
                   vegetal pole - is portion of embryo containing  "yolk" - that establish axes

                       3 day old human embryo (8 cell stage)    &   injection of DNA to modify genome
                       manipulation of human blastula embryo   &   animation - Human embryonic stem cells  

 
 Embryogenesis & Germ Cell Layers

     4. Gastrulation - period of cell migrations around blastopore (an organizing center)...
                                           an opening into archenteron controlling cell migrations,
                                           that converts embryo from hollow ball of cells into
                                           a 3 layered stage called gastrula embryo*  &  pic   
      
             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
                                          (invert*  video*)   &   (frog* video )   &   (chick*)
   
                     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*    frog video
                                                    neural tube = brain & spinal cord  
                                                        notochord (mesoderm) forms cartilage-like backbone 
                       ►   almost all organs form from flat plates into tubes --> 3D shape
                       ►   following a segmented development pattern via somites

 ____________________________________________________________________________________________________________________________________________________________________________________
  
      Mouse embryo development: mouse vasculature
  
      Human development:             Human fertilization-clevage  &  Human blastula-gastrula
                                                 5, 14 and 20week Human embryos
                                                 slide show of Human development  
                                                 PBS video of Human development (with ad) 
                                                 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...
   
   1.  MORPHOGENEIS:     the developmental mechanisms establishing
                                           SHAPE/FORM and ORGANIZATION (body plan)
  
          in animals:  cell migrations - where cells create new morphogenic shapes 
                                                         by migrating cells forming plates & tubes    
                         & 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 21.4a & fig 21.4b

 
 Genetic Basis of Cell Differentiation - is Differential Gene Activity
                         cells become structurally, functionally, & biochemically different
                         by expressing different genes at different times in development
  Transcriptional Regulation: results in expression of Tissue Specific Proteins -fig 18.10*
 
  Do all cells posses a "full genetic complement" to express the whole organism???
             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 transplantation experiments.
             1.  F.C. Steward (1950) using carrots* grows whole plant from single cell
             2.  Briggs & King (1952) & J.B. Gurdon (1962) demonstrate totipotency in frogs* 
             3.  Ian Wilmut (1997) clones Dolly - cloning*        Clone your Pet
             4. S. Yamanaka (2002) 4 genes can reprogram adult mouse cells to pluripotent cells.
   Thus cells possess a complete genome to express whole organism: Genomic Equivalency
    
   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 ,i.e., all are embryonic stem cells
                           vs.   adult stem cells (pluripotent = many, but not all)- stem cells*
               key concepts*  

 back  copyright c2019     
 next   Charles Mallery,    Biology 150, Department of Biology, U. of Miami,  Coral Gables, FL 33124          
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