Final answer:
The three main domains of the limb bud are the apical ectodermal ridge, which stimulates limb outgrowth; the zone of polarizing activity, which directs digit identity; and the progress zone, where cells differentiate into limb structures. Limb buds become visible by the end of the fourth week, with limb rotation occurring in the seventh week to properly position the limbs.
Step-by-step explanation:
The three main domains of the limb bud during embryonic development are the apical ectodermal ridge (AER), the zone of polarizing activity (ZPA), and the progress zone. The AER is a thickened area of ectoderm at the tip of the limb bud which plays a critical role in directing outgrowth along the proximal-distal axis. Secondly, the ZPA is located at the posterior border of the limb bud and is responsible for anteroposterior patterning (digit identity). Lastly, the progress zone is the rapidly dividing area of mesenchyme just below the AER, which gives rise to the different structures of the limb as cells leave the progress zone and begin to differentiate.
During embryonic development, limb buds are observable at the end of the fourth week. The limb rotation is a crucial process that takes place during the seventh week in which the upper limbs rotate laterally and lower limbs rotate medially. This is an important part of the developmental process, positioning the limbs correctly for their final anatomical placement.