Striated visceral muscle cells are scattered singly or in small groups at the base of the intestinal cells of the mid-gut of Drosophila melanogaster larvae. Fibers less than 1 , U in diameter, designated as small, contain a single myofibril, few, if any, dyads and few mitochondria. Fibers of somewhat larger diameter contain dyads and more mitochondria. Both types of fiber have a perforate Z band which appears as discontinuous bodies in longitudinal sections and as a perforate sheet of dense rims and clear perforations in transverse sections. The Z rims contain filaments, 30 to 50 A, oriented in the transverse direction. The number and arrangement of myofilaments and the ultrastructure of the Z band are consistent with the function of these muscles.Dipteran visceral fibers that perform slow contractions resembling those of the functionally comparable smooth muscle of vertebrate intestine are striated (Schaeffer et al., '67). The general organization of these striated visceral fibers, the cellular membranes and organelles, and the myofilament array are all characteristic of other slowly contracting invertebrate striated muscles. In addition, these visceral fibers contain separate clumps of Z material or Z bodies. This incomplete Z band is present in striated muscles which can supercontract to less than 50% of rest length (Leyton and Ullrick, '70; Osborne, '67).Although the ultrastructure of visceral muscles in several other insects has been reported (Kawaguti, '67; Smith et al., '66; Schaeffer et al., '67; Smith, '68), it seemed desirable to analyze the myofilament arrangement and fine structure of Z bands in greater detail and to explore additionally the assumption that these muscles can undergo supercontraction. The larval midgut from Drosophiln melunogaster was used in the investigation.
MATERIALS AND METHODSIntact third-instar (85 to 90 hours) of Drosophila melanognster were placed in 4% paraformaldehyded% glutaraldehyde fixative, (Karnovsky, '65) for one hour and then diced and fixed for an additional 90 J. MORPH., 134: 315-334.minutes. The tissue was rinsed in buffer for two hours and then post-fixed in 1% osmium tetroxide. After rapid dehydration in increasing concentrations of ethanol, the tissue was embedded in Epon (Luft, '61)? and thin sections were cut with a diamond knife on a Porter Blum MT-2 ultramicrotome. The sections were stained with a saturated solution of uranyl acetate in 50% ethanol, post-stained with lead citrate (Reynolds, '63) and examined in a Siemen's Elmiskop I A.
RESULTSThe mid-gut epithelium consisted primarily of columnar absorptive cells with apical microvilli and numerous infoldings of the basal cell membrane ( figs. 2-3). Small undifferentiated cells without basal infoldings and containing many free ribosomes constituted adjacent replacements ( fig. 2). Cells constituting visceral muscle were arranged in circular and longitudinal arrays ( fig. 1 ). Striated muscle cells were scattered singly or in small groups in the extracellular matrix at the base of the intestinal cell...
