Characterization of the early development of specific hypaxial muscles from the ventrolateral myotome

We have previously found that the myotome is formed by a first wave of pion eer cells generated along the medial epithelial somite and a second wave em anating from the dorsomedial lip (DML), rostral and caudal edges of the der momyotome (Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998a) Mech. Dev. 74, 59-73; Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998b) Development 125, 4259-4271). In this study, we have addressed the development and precise fa te of the ventrolateral lip (VLL) in non-limb regions of the axis. To this end, fluorescent vital dyes were iontophoretically injected in the center o f the VLL and the translocation of labeled cells was followed by confocal m icroscopy. VLL-derived cells colonized the ventrolateral portion of the myo tome, This occurred following an early longitudinal cell translocation alon g the medial boundary until reaching the rostral or caudal dermomyotome lip s from which fibers emerged into the myotome, Thus, the behavior of VLL cel ls parallels that of their DML counterparts which colonize the opposite, do rsomedial portion of the myotome. To precisely understand the way the myotome expands, we addressed the early generation of hypaxial intercostal muscles, We found that intercostal musc les were formed by VLL-derived fibers that intermingled with fibers emergin g from the ventrolateral aspect of both rostral and caudal edges of the der momyotome. Notably, hypaxial intercostal muscles also contained pioneer myo fibers (first wave) showing for the first time that lateral myotome-derived muscles contain a fundamental component of fibers generated in the medial domain of the somite. In addition, we show that during myotome growth and e volution into muscle, second-wave myofibers progressively intercalate betwe en the pioneer fibers, suggesting a constant mode of myotomal expansion in its dorsomedial to ventrolateral extent, This further suggests that specifi c hypaxial muscles develop following a consistent ventral expansion of a 'c ompound myotome' into the somatopleure.