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limb bud
The limb bud is a structure formed early in limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development 〔( Gilbert, Scott F. "Developmental Biology". 9th ed., 2010 )〕 as mesenchymal cells from the lateral plate mesoderm and the somites begin to proliferate to the point where they create a bulge under the ectodermal cells above. The limb bud remains active throughout much of limb development, and its signaling stimulates formation of another signaling center, the apical ectodermal ridge (AER) as well as formation of the zone of polarizing activity (ZPA) within the mesenchyme. The mesenchymal cells of the limb bud, which stimulate AER formation as well as maintain AER activity, determine what type of limb will form. ZPA signaling will establish polarity of the limb, as well as sustain proper AER activity.
== Position and formation 〔==
Limb buds form from cells located in the greater limb field region. The Hox genes, which define features along the anterior-posterior axis, dictate at which point along that axis limb buds will form. Though limbs emerge at different locations in different species, their position always correlates with the level of Hox gene expression along the anterior-posterior axis.
The upper limb buds, which are destined to become forelimbs (arms in humans), form at a location between the lower cervical vertebrae and upper thoracic vertebrae. The lower limb buds, which will become hindlimbs (legs in humans), form a few days after the upper limb buds near the lumbar vertebrae and upper sacral vertebrae. All limb buds must rely on other signaling factors to obtain their identity. Hox gene expression influences expression of Tbx proteins that in turn dictate limb identity. In humans, Tbx4 specifies hindlimb status, while Tbx5 specifies forelimb status.
In mice, however, both hindlimbs and forelimbs can develop in the presence of either Tbx4 or Tbx5. Here, Pitx1 appears to be necessary for specification of the hindlimb, whereas its absence results in forelimb development. Tbx4 and Tbx5 appear to be important for limb outgrowth, but not necessarily their identity in mice.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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