By Levi Clancy for Student Reader on
Vertebrates are bilaterally symmetric about the midline for many structures, like eyes, nasal passages, and limbs. However, most internal organs (including endodermal derivatives) are arranged asymmetrically in the abdominal cavity. For example, the heart is on the ventral side. The lung has fewer lobes on this side to provide space for the heart. Similarly, endodermal organs are invariantly placed within the abdominal cavity so there is enough space for each organ.
Shh Induces Nodal Induces Asymmetry
Left/right asymmetry is controlled early in development by the node. The node expresses Shh only on its left side. Shh then induces expression of nodal, a secreted protein. If the pattern of nodal expression is made symmetric by implanting a pellet of cells expressing Shh on the right side of the node, then organ asymmetry is lost, and organs are randomly placed in the body.
Cilia Cause a Directional Flow
Cells in the node contain cilia that cause a directional flow of a signaling molecule involved in left/right asymmetry. Mutations in left-right dynein (LRD) cause randomization of organ placement. Dyneins are motor proteins that move along microtubules and drive cilia movement. Individuals with Kartagener's Syndrome have immotile cilia and their handedness is thus random.