Musculature
The visceral musculature comprises circular and longitudinal fibres, which surround the entire intestinal tract, with the exception of the recurrent layer of the proventriculus (Fig. 4.1). The two systems of fibres differ in developmental origin: the circ
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Chapter 4 Musculature
4.1 Visceral Musculature The visceral musculature comprises circular and longitudinal fibres, which surround the entire intestinal tract, with the exception of the recurrent layer of the proventriculus (Fig. 4.1). The two systems of fibres differ in developmental origin: the circular fibres originate from a region of the mesodermal layer analogous to the splanchnopleura of other animals, i.e. a bilaterally symmetrical band of mesoderm cells extending continuously throughout most of the germ band (Fig. 4.2A). The longitudinal fibres derive from clusters of mesodermal cells which appear during stage 12 at the posterior end of the embryo and migrate anteriorly (Fig. 4.2A). At the beginning of germ band shortening in stage 12 (Fig. 2.21), the precursors of the circular fibres are organized like a palisade, consisting of a dorsal and a ventral row of tightly packed, slender cells (Fig. 4.2B, C). These cells adhere to both anterior and posterior midgut rudiments as germ band shortening progresses (Fig. 2.22). Further anteriorly and posteriorly, cells of the visceral mesoderm also surround the primordia of the foregut and hindgut. These terminal parts of the visceral mesoderm, which do not exhibit a regular, layered structure, are separated by wide gaps from the mesodermal cells associated with the midgut; these gaps will remain unchanged throughout development. The segment of the foregut that lacks visceral mesodermal cells, and thus visceral musculature, will become the recurrent layer of the proventriculus; the corresponding segment of the hindgut will develop into the region where the Malpighian tubules open into the hindgut (Figs. 4.1, 4.3). During closure of the midgut the precursor cells of the circular visceral muscles move dorsally and ventrally in association with the endodermal cells (Figs. 4.2B-D). This movement follows a highly stereotyped pattern. The visceral muscle precursors
J. A. Campos-Ortega et al., The Embryonic Development of Drosophila melanogaster © Springer-Verlag Berlin Heidelberg 1997
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Chapter 4
Musculature
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Ib sm ·... consist of ten distinct clusters of dorsal mesodermal cells. After fusion of the midgut primordia, the circular muscle cells form palisades, which will extend eventually to ensheath the midgut. Their nuclei separate into two rows (arrowheads) . The longitudinal visceral musculature (vrn /) originates from a group of mesodermal cells at the caudal end of the embryo (A). These cells later migrate along the developing circular muscle cells
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Musculature Fig. 4.3. Development of the visceral musculature. Longitudinal muscle fibres. A. Stage 11 embryo stained with an anti-~3 tubulin antibody. Stage 13 (B) and stage 14 (C) and stage 15 (E) embryos, carrying an enhancer trap insertion in the couchpotato gene that highlights the longitudinal visceral musculature. D Stage 16 embryo stained with an anti-myosin antibody. In A the primordia of the circular visceral muscles (vm c ) are or
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