Question

Explain how left-right asymmetry is established in the mammalian
embryo, including cellular and molecular detail in your answer. What are cilia and what patterning defects might you expect in embryos that fail to form cilia?(500words)

Answer 1

Cytoskeletal re-arrangements following fertilization set up microfilaments or microtubules which are oriented along the newly established LR axis. (C) Motor proteins (such as dynein [LRD] and kinesin [KIF-3B]) translocate along these tracks and result in an asymmetric localization of certain mRNAs. (D) These mRNAs are translated, the resulting proteins perhaps targeted to correct regions and held in place by ankyrin proteins such as inv, and thus initiate ion flux. (E) The differential ion flux results in LR-asymmetric gradients of pH and voltage. In particular, cells across the ventral midline possess significantly different membrane potential levels. (F) The system of gap-junctional communication is set up, featuring junctional isolation across the ventral midline and a path of GJC circumferentially around it. (G) The voltage gradient between the L and R sides imposes a unidirectional net movement of as-yet-uncharacterized small signaling molecules. This results in accumulation on one side of the midline from an initially random (homogenous) distribution. (H) The accumulation of these small molecule morphogens on one side induces gene expression in conventional ways.