A limb bud is a small, protruding structure that forms during the early stages of embryonic development in vertebrates. It is the precursor to limbs, such as arms and legs, and is composed of mesodermal tissue. Limb buds begin to develop around the fourth week of gestation in humans and are essential for the formation of the skeletal and muscular systems. As the limb bud grows, it undergoes a process called morphogenesis, where it differentiates into specific parts like bones, muscles, and skin. The development of limb buds is influenced by various signaling pathways, including Sonic Hedgehog and Fibroblast Growth Factors, which guide the proper formation and patterning of limbs.