MusFIG 4: Muscle arm development.

A & B. Head and neck muscle arm development. (Light green) primordial pharynx cells; (dark green) myoblasts; (red) nascent nerve ring; (orange) basal lamina; (beige) hypodermis. A. At about 350 min after first cell cleavage, head and neck muscles directly surround the pharynx primordium where the nerve ring will form. Shortly after, concurrent with amphid axon outgrowth (not shown), muscle cells are thought to migrate to the periphery, towards the hypodermis. B. As muscle cells migrate, they may leave attachments behind next to the pharynx, which later become the muscle arms. These muscle arms may then be guided towards their muscle plate positions around the nerve ring by the GLR cells (not shown).
C & D. Body wall muscle development. It is suggested that muscle arm development from body wall muscles occurs in 2 phases: embryonic and larval. C. Muscle arms in the L2-stage larva. During mid embryogenesis myoblasts either move away or become displaced from motor neurons (not shown). As they move, they are thought to leave an arm behind, attached to the neurons. As a result, in newly hatched larva each muscle cell contains one to two muscle arms (dark green arms). Concomitant with post-embryonic motor neuron growth, additional larval muscle arms (light green arms) grow from these muscle cells towards the motor neurons (red) by active outgrowth (white arrow). D. In the adult, each muscle cell contains three to six arms.

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