Mechanical causes of embryonic morphogenesis 

1. Mechanical causes of neurulation
Neurulation is  the formation of the neural tube by closure of the neural plate, which is directed by the underlying notochord. It goes through four temporary stages: 

(1) Formation of the neural plate

(2) Shaping of the neural plate

(3) Bending of the neural plate to form neural groove

(4) Closure of the neural groove to form neural tube

During the formation of neural tube, thin, fragile epithelial tissues must undergo precise, self-driven changes of shape.  Forces that drive these movements are generated within the embryonic tissue themselves. It is generally accepted that the cytoskeleton and other sub-cellular structural components drive these movements. 

  

(1) Neural plate formation
 

(2) Bending of neural groove
　

　

(3) Epithelial tissues folding at hinge point

　

(4) Closure of neural tube

　

　

 
The above photos (1) &(4) were obtained from  http://www.luc.edu/depts/biology/dev/, photos (2) &(3) were provided by Dr. Brodland.
2. Proposed systems biology approach

    Facing the new era of systems biology and bioinformatics, some researchers are now modeling and reconstructing the biological phenomena in silico.  Their numerical models are based on the experimental work and hopefully the numerical simulations can provide implications for future experimental work or better understanding of the correlation of the biological systems.  One schematic description of systems biology approach is shown in Fig 1 (Modified from Merks, 2005).

　

Fig 1.Schematic description of systems biology approach. (Modified from Merks, 2005)

    

     The next question will be what should be kept in the numerical model and what can be discarded.  This is the most important step before setting out a research project because it will finally determine if the outcome of the numerical analysis has real implication for the experimental work and if it has real appeal to other biologists.  Right now three levels of information are thought to control the function of a cell.  They are gene expression, signaling pathway at molecular level and cytoskeleton modifications.  For a particular cell function, only several gene expressions and a limited number of protein molecules are involved, which drive maybe only one or two cytoskeleton changes.  These cell functional paths are either parallel or intersect as shown in Fig 2.  For the research of a particular cell function, one can expect that the networking between parallel functional paths is limited and can be modeled.
　

Fig 2. Conceptual description of systems biology approach for neural tube closure problem

   

    Take the problem of neural tube closure in the early embryogenesis for example, some molecular level driving factors such as the expression of Shroom with the aid of GTPase Pap1 has been identified, and actin filament contraction has long been thought as the driving force for apical constriction at cytoskeleton level.  Therefore, a functional path can be set up from Shroom expression to actin filament contraction to apical constriction as shown in Fig 2.  In terms of numerical modeling, multi-algorithm driving engines have to be derived at different levels.  For example, biochemical engine will be used from Shroom expression to actin filament contraction and mechanical engine will be used from actin filament contraction to apical constriction.