The embryonic origin of the abdominal (ventrolateral) musculature in the albino rat

Development of the amniote ventrolateral body wall

In vertebrates, the trunk consists of the musculoskeletal structures of the back and the ventrolateral body wall, which together enclose the internal organs of the circulatory, digestive, respiratory and urogenital systems. This review gives an overview on the development of the thoracic and abdominal wall during amniote embryogenesis. Specifically, I briefly summarize relevant historical concepts and the present knowledge on the early embryonic development of ribs, sternum, intercostal muscles and abdominal muscles with respect to anatomical bauplan, origin and specification of precursor cells, initial steps of pattern formation, and cellular and molecular regulation of morphogenesis.

Lbx1 is required for muscle precursor migration along a lateral pathway into the limb

In mammalian embryos, myogenic precursor cells emigrate from the ventral lip of the dermomyotome and colonize the limbs, tongue and diaphragm where they differentiate and form skeletal muscle. Previous studies have shown that Pax3, together with the c-Met receptor tyrosine kinase and its ligand Scatter Factor (SF) are necessary for the migration of hypaxial muscle precursors in mice. Lbx1 and Pax3 are co-expressed in all migrating hypaxial muscle precursors, raising the possibility that Lbx1 regulates their migration. To examine the function of Lbx1 in muscle development, we inactivated the Lbx1 gene by homologous recombination. Mice lacking Lbx1 exhibit an extensive loss of limb muscles, although some forelimb and hindlimb muscles are still present. The pattern of muscle loss suggests that Lbx1 is not required for the specification of particular limb muscles, and the muscle defects that occur in Lbx1(−/−) mice can be solely attributed to changes in muscle precursor migration. c-Met is expressed in Lbx1 mutant mice and limb muscle precursors delaminate from the ventral dermomyotome but fail to migrate laterally into the limb. Muscle precursors still migrate ventrally and give rise to tongue, diaphragm and some limb muscles, demonstrating Lbx1 is necessary for the lateral, but not ventral, migration of hypaxial muscle precursors. These results suggest that Lbx1 regulates responsiveness to a lateral migration signal which emanates from the developing limb.

Development and innervation of the abdominal muscle in embryonic Xenopus laevis

The morphogenesis and innervation of the ventral abdominal musculature in Xenopus embryos was examined using microscopic techniques. Muscle development begins at Nieuwkoop and Faber Stage 31, when aggregates of undifferentiated cells form on the ventrolateral margins of rostral trunk myotomes. During subsequent stages, aggregates form and detach from progressively more caudal myotomes to form a series of seven discrete cell clusters (anlagen). The anlagen migrate ventrally in a cell-free space between the epidermis and a subepidermal layer of pigment cells. Extracellular aggregates of 30-nm granules are evident transiently between the migrating anlagen and the epidermis. During stages 39 and 40, each anlage transforms into a sheet of myotubes which attaches rostrally and caudally to adjacent sheets to form a seven-segmented muscle. The series of broad segments, approximately one fiber thick, extends from the pericardium to the level of the proctodeum. The embryonic muscle is innervated by the ventral rami of spinal nerves 2 to 9. The major nerve trunks to the muscle develop between stages 35/36 and 40. Axons initially grow ventrally along the paths taken by the muscle anlagen. When the anlagen become muscle segments, the nerves are deep to the narrow boundaries between the segments. Spinal nerve 2 ramifies in the first muscle segment and sends fibers rostrally to the geniohyoid muscle. The findings represent the first description of the development of this muscle in Xenopus and the first account of the development of the abdominal motor nerves in an amphibian embryo.

On the origin and development of the ventrolateral abdominal muscles in the avian embryo. An experimental and ultrastructural study

In avian embryos the formation of ventrolateral abdominal muscles was studied by (1) heterospecific grafting experiments between chick and quail embryos and (2) ultrastructural examinations of cells having part in this process. The results demonstrate that the muscle cells are of somitic origin while the connective tissue derives from the somatopleure. Somatopleural cells do not differentiate into myocytes, and somite cells which have entered the ventrolateral abdominal wall, do not contribute to the connective tissue. It is concluded that both dermatome and myotome cells undergo muscular differentiation. The formation of muscles is found to take place in four characteristic steps. During the 4th day of development, epithelially structured ventral somite buds enter the somatopleure. The light cells of the inner myotome layer are elongated in a cranio-caudal direction and contain randomly distributed microfilaments. On the 5th day, the buds lose their epithelial arrangement and change into compact processes in which cells intermingle. The myotome cells show short bundles of thin and thick microfilaments. The third step can be characterized by the appearance of intercellular spaces and the disaggregation of processes becoming invaded by somatopleural cells. Thus, subdivision in single muscle blastemata begins to occur. In 7-day embryos, the muscle anlagen are distinctly separated and the first myotubes containing regularly arranged myofibrils are found. Coincidentally, signs of cell death are observed. Up to the 10th day, the tendons being of somatopleural origin become plainly outlined and the muscle anlagen move to their definitive positions. It is assumed that the formation of muscle pattern is controlled by the somatopleure.