Scientific theory in vertebrate development
Axial twist Schema of the proposed development of the axial twist. Developmental phases are (from top to bottom): (1) the embryo turns on its left side; (2) the anterior head grows in the same direction, but the rest of the body grows oppositely into a twist. So that ultimately (3) external bilateral symmetry is regained. Note that there is no evolutionary pressure for internal symmetry so the heart (and other organs) remain asymmetric. Details System vertebrate body plan Anatomical terminology [edit on Wikidata]
The axial twist theory (a.k.a. axial twist hypothesis) is a proposed scientific theory to explain a range of unusual aspects of the body plan of vertebrates (including humans).[1] It states that the rostral part of the head is "turned around" regarding the rest of the body.[2] This end-part consists of the face (eyes, nose, and mouth) as well as part of the brain (cerebrum and thalamus). According to the theory, the vertebrate body has a left-handed chirality.
The axial twist theory competes with a number of other proposals that focus on more limited, specific aspects, most of which explain contralateral forebrain organization, the phenomenon that the left side of the brain mainly controls the right side of the body and vice versa.[3] None of the proposed theories explaining this phenomenon, including axial twist theory, have gained general recognition.[4] The genetic basis underlying the proposed developmental twist is not yet understood.
The axial twist theory would explain various anatomical phenomena, and addresses how and when the proposed twist between the end of the head and the rest of the body develops. It also addresses the possible evolutionary history. One prediction of the theory was the aurofacial asymmetry, which was then found empirically,[5] albeit by one of the authors of the original theory.
Phenomena the theory can explain include:
According to the axial twist developmental model, the anterior part of the head turns against the rest of the body, except for the inner organs. Due to this twist, the forebrain and face are turned around such that left and right, but also anterior and posterior are flipped in the adult vertebrate.
History [ edit ]
In the end of the 19th century, the famous neuroscientist and Nobel Prize winner Santiago Ramón y Cajal proposed a theory to explain the contralateral organization of the forebrain that was rapidly and widely accepted.[6][4] This theory, the visual map theory, proposes that the optic chiasm restores the retinal image on the visual cortex,[7] Cajal's theory remained virtually undisputed for more than a century.[8][9] However, more ideas have been put forward (see section #Relation to other theories and hypotheses).
Embryology [ edit ]
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