Fine Structure and Differentiation of Ascidian Muscle I. DIFFERENTIATED CAUDAL MUSCULATURE OF DlSTAPLlA OCClDENTALlS TADPOLES
Date
1972
Authors
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Journal ISSN
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Publisher
John Wiley & Sons, Inc.
Abstract
The structure of the caudal muscle in the tadpole larva of the
compound ascidian Distaplia occidentalis has been investigated with light and
electron microscopy. The two muscle bands are composed of about 1500 flattened
cells arranged in longitudinal rows between the epidermis and the
notochord. The muscle cells are mononucleate and contain numerous mitochondria,
a small Golgi apparatus, lysosomes, proteid-yolk inclusions, and large
amounts of glycogen. The myofibrils and sarcoplasmic reticulum are confined
to the peripheral sarcoplasm.
Myofibrils are discrete along most of their length but branch qear the tapered
ends of the muscle cell, producing a Felderstmktur. The myofibrils originate
and terminate at specialized intercellular junctional complexes. These myomuscular
junctions are normal to the primary axes of the myofibrils and resemble
the intercalated disks of vertebrate cardiac muscle. The myofibrils insert
at the myomuscular junction near the level of a Z-line. Thin filaments (presumably
actin) extend from the terminal Z-line and make contact with the
sarcolemma. These thin filaments frequently appear to be continuous with
filaments in the extracellular junctional space, but other evidence suggests that
the extracellular filaments are not myofilaments.
A T-system is absent, but numerous peripheral couplings between the sarcolemma
and cisternae of the sarcoplasmic reticulum (SR) are present on all cell
surfaces. Cisternae coupled to the sarcolemma are continuous with transverse
components of SR which encircle the myofibrils at each I-band and H-band.
The transverse component over the I-band consists of anastomosing tubules
applied as a single layer to the surface of the myofibril. The transverse component
over the H-band is also composed of anastomosing tubules, but the myofibrils
are invested by a double or triple layer. Two or three tubules of sarcoplasmic
reticulum interconnect consecutive transverse components.
Each muscle band is surrounded by a thin external lamina. The external
lamina does not parallel the irregular cell contours nor does it penetrate the
extracellular space between cells. In contracted muscle, the sarcolemmata at the
epidermal and notochordal boundaries indent to the level of each Z-line, and
peripheral couplings are located at the base of the indentations. The external
lamina and basal lamina of the epidermis are displaced toward the indentations.
The location, function, and neuromuscular junctions of larval ascidian caudal
muscle are similar to vertebrate somatic striated muscle. Other attributes,
including the mononucleate condition, transverse myomuscular junctions, prolific
gap junctions, active Golgi apparatus, and incomplete nervous innervation
are characteristic of vertebrate cardiac muscle cells.
Description
Keywords
Biology
Citation
MICHAEL J. CAVEY AND RICHARD A. CLONEY "Fine Structure and Differentiation of Ascidian Muscle: I. DIFFERENTIATED CAUDAL MUSCULATURE OF DlSTAPLlA OCClDENTALlS TADPOLES" J. MORPH., 138: 349-374.