Introduction to Human General Embryology Developmental Genetics

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  Damage from external sources, including viral infections, x-rays and other radation, and poor nutrition
  

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  Depending on the stage of development at which the exposure to the teratogen takes place, a variation of developmental malformations may occur.
  

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  Stunting of the fingers and toes due to damage from external sources (teratogens) including viral infections such as rubella, x-rays and other radiation, and poor nutrition by week 8.
  

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  In Week 3 we see the formation of the heart, the beginning development of the brain and spinal cord, and the beginning of the gastrointestinal tract.
  

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  Teratogens introduced during this period may cause severe problems such as the absence of one or more limbs or a heart that is outside of the chest cavity at birth.
  

Factors influencing infertility in males include:

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  Infection
  
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  Hernia
  
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  Vas Deferens
  
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  Androgen Deficiency
  
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  Testicular mass
  
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  Varicocele
  

This is the series of mitotic division induced in the zygote immediately after fertilization. The following facts are associated with it:

1. 
   Occurs after fertilization
   

2. 
   It is the first step in embryonic development
   

3. 
   It is associated with the division of fertilized egg into a large number of smaller cells (blastomeres).
   

4. 
   It Leads to development of a hollow blastocyst
   

5. 
   At 12 -16 cell – stage, the zygote becomes the morula. It is described as a mulberry-shaped solid cell mass
   

6. 
   As cleavage goes on, the mass of the embryo does not increase.
   

7. 
   At 8 –cell stage, compaction occurs. It is associated with the cell polarization, change in shape of the embryo, flattening of the blastomeres against each other and formation of tight junctions.
   

8. 
   Development of the blastocyst with a blastocoel cavity and two masses of cells called the inner mass cells (embryoblast) and the outer mass cells (Trophoblast). This is associated with cell adhesion which is critical in cell differentiation and the changes in form (morphogenesis) of the blastocyst. Without cell adhesion, the cells die.
   

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