- The difference between cells that undergo apical constriction (lateral cells) and those that experience apical expansion (medial cells) is determined by their proximity to Shh signaling 
- This is based on existing patterns of Shh signaling, which is high at the midline, and decreases gradually as it reaches the lateral points 
- This is further supported by previous studies that altered Shh signaling, and found that the midline cells faced issues in forming its normal cell shape 
- Therefore, high levels of Shh produce short and expanded cell shape, while low levels of Shh produce tall and constricted cell shape 

Influence of Sonic Hedgehog on Apical Constriction 

- Recent research has elucidated some of the mechanisms that may be involved in cranial neural tube closure, which includes apical constriction of lateral cells within the midbrain, while there is sustained apical expansion for midline cells 
- These processes are regulated by Shh signaling, and is therefore a vital morphogen factor in cranial neural tube cell remodeling, and subsequent closure  

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- Mechanism that precedes/ prepares for neural tube closure
- Changes rectangular epithelial cells into wedge shaped-cells by forcing the cells to contract (via protein actomyosin) 
- Ultimately leads to bending of tissue, creating "hinge points" that are required for neural tube closure 
- While apical constriction is a required prerequisite for neural tube closure, questions remain about whether this process is needed during the act of neural tube closure itself 
- Knowing that many of the processes that drive cranial neural tube closure are unknown, apical constriction is being investigated for its possible role in this process

Apical Constriction 

- A molecular signaling molecule that specifies ventral cell fates in the neural tube when in high quantities 
- When in moderate quantities, Shh specifies more median cell fates 
- Involved in cell fate decisions in limb, and gut, as well as the neural tube 

Sonic Hedgehog (Shh) 

Brooks, E. R., Islam, M. T., Anderson, K. V., & Zallen, J. A. (2020). Sonic hedgehog signaling directs patterned cell remodeling during cranial neural tube closure. eLife, 9, e60234. https://doi.org/10.7554/eLife.60234

Sonic hedgehog signaling directs patterned cell remodeling during cranial neural tube closure

Mechanisms of Cranial Neural Tube Closure 

Expression of Shh has been known to influence the following: 
- Axon guidance 
- Cell Migration 
- Mesenchymal cell clustering 
- Changes to epithelial cell structure 

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