Abstract
The formation of a perfect vertebrate body plan poses many questions that thrill developmental biologists. Special attention has been given to the symmetric segmental patterning that allows the formation of the vertebrae and skeletal muscles. These segmented structures derive from bilaterally symmetric units called somites, which are formed under the control of a segmentation clock. At the same time that these symmetric units are being formed, asymmetric signals are establishing laterality in nearby embryonic tissues, allowing the asymmetric placement of the internal organs. More recently, a “shield” that protects symmetric segmentation from the influence of laterality cues was uncovered. Here we review the mechanisms that control symmetric versus asymmetric development along the left-right axis among vertebrates. We also discuss the impact that these studies might have in the understanding of human congenital disorders characterized by congenital vertebral malformations and abnormal laterality phenotypes.
Highlights
The formation of a perfect vertebrate body plan poses many questions that thrill developmental biologists
This finding sets the ground for the possibility that in mammals LR embryonic patterning may be set during the cleavage stages to what has been described in snails, where manipulation of the first cleavages resulted in reverted shell coiling and visceral situs inversus [87]
RA binds to heterodimers of retinoic acid receptor (RAR) and retinoid X receptor (RXR) and activates transcription of RA-responsive genes upon binding to specific DNA sequences known as retinoic acid response elements (RAREs) [89]
Summary
The bilateral symmetric appearance of the vertebrate body plan is largely due to the symmetric organization of the skeleton and its associated muscles. The origin of this symmetry can be traced back. Symmetry 2010, 2 to early developmental stages when the somites–embryonic structures that will differentiate into the axial skeleton (vertebrae, intervertebral discs and ribs) and skeletal muscles–are being formed. Each new pair of somites is progressively laid down from the rostral region of the presomitic mesoderm (PSM) in a bilateral symmetric way on both sides of the axial structures i.e., neural tube and notochord. Every 4–5 hours a new pair of somites is formed in the human embryo, every 120 minutes in the mouse, every 90 minutes in the chick and every 30 minutes in the zebrafish [1]
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