|dc.description.abstract||The locomotor function of the caudal muscle cells of ascidian larvae
is identical with that of lower vertebrate somatic striated (skeletal) muscle
fibers, but other features, including the presence of transverse myomuscular
junctions, an active Golgi apparatus, a single nucleus, and partial innervation,
are characteristic of vertebrate myocardial cells.
Seven stages in the development of the compound ascidian Distaplia occidentalis
were selected for an ultrastructural study of caudal myogenesis. A timetable
of development and differentiation was obtained from cultures of isolated embryos
The myoblasts of the neurulating embryo are yolky, undifferentiated cells.
They are arranged in two bands between the epidermis and the notochord in the
caudal rudiment and are actively engaged in mitosis.
Myoblasts of the caudate embryo continue to divide and rearrange themselves
into longitudinal rows so that each cell simultaneously adjoins the epidermis and
the notochord. The formation of secretory granules by the Golgi apparatus coincides
with the onset of proteid-yolk degradation and the accumulation of glycogen
in the ground cytoplasm.
Randomly oriented networks of thick and thin myofilaments appear in the peripheral
sarcoplasm of the muscle cells of the comma embryo. Bridges interconnect
the thick and thin myofilaments (actomyosin bridges) and the thick myofilaments
(H-bridges), but no banding patterns are evident. The sarcoplasmic
reticulum (SR), derived from evaginations of the nuclear envelope, forms intimate
associations (peripheral couplings) with the sarcolemma.
Precursory Z-lines are interposed between the networks of myofilaments in the
uesicutate embryo, and the nascent myofibrils become predominantly oriented
parallel to the long axis of the muscle cell.
Muscle cells of the papittate embryo contain a single row of cortical myofibrils.
Myofibrils, already spanning the length of the cell, grow only in diameter by the
apposition of myofilaments. The formation of transverse myomuscular junctions
begins at this stage, but the differentiating junctions are frequently oriented
obliquely rather than orthogonally to the primary axes of the myofibrils.
With the appearance of H-bands and M-lines, a single perforated sheet of
sarcoplasmic reticulum is found centered on the Z-line and embracing the I-band.
The sheet of SR establishes peripheral couplings with the sarcolemma.
In the prehatching tadpole, a second collar of SR, centered on the M-line and
extending laterally to the boundaries with the A-bands, is formed. A single perforated
sheet surrounds the myofibril but is discontinuous at the side of the myofibril
most distant from the sarcolemma. To produce the intricate architecture of the
fully differentiated collar in the swimming tadpole (J. Morph., 138: 349, 1972), the free ends of the sheet must elevate from the surface of the myofibril, recurve,
and extend peripherally toward the sarcolemma to establish peripheral couplings.
Morphological changes in the nucleus, nucleolus, mitochondria, and Golgi
bodies are described, as well as changes in the ground cytoplasmic content of
yolk, glycogen, and ribosomes.
The volume of the differentiating cells, calculated from the mean cellular dimensions,
and analyses of cellular shape are presented, along with schematic diagrams
of cells in each stage of caudal myogenesis. In an attempt to quantify the
differences observed ultrastructurally, calculations of the cytoplasmic volume
occupied by the mqjor classes of organelles are included.
Comparison is made with published accounts on differentiating vertebrate
somatic striated and cardiac muscles.||en