Cytoarchitecture of the fetal murine soleus muscle

Abstract

The organogenesis of the soleus muscle of the 129 ReJ mouse (a mixed muscle, which in the adult contains approximately equal numbers of slow-twitch oxidative and fast-twitch oxidative-glycolytic myofibers) was studied in spaced, serial transverse, and longitudinal sections of muscles of 14-, 16-, and 18-day in utero and 1- and 5-day postnatal mice. A discrete soleus muscle was distinguished by 14 days in utero. It consisted of groups of closely apposed primary myotubes displaying junctional complexes and a pleomorphic population of mononucleated cells. Between 14 and 16 days in utero there was little de novo myotube formation. At 16 days in utero, basal lamina surrounded groups of primary myotubes; and primitive motor endplates were found on these myotubes. At 18 days in utero, the basal-lamina-enclosed groups of primary myotubes were no longer present. At this stage, basal lamina surrounded clusters (consisting of one primary myotube and one or more secondary myotubes) or independent myotubes (single myotubes surrounded by their own basal lamina). Cluster formation and cluster dispersal occurred concurrently, beginning at 18 days in utero and extending until birth. At birth, there was still a substantial population of immature, secondary myotubes that interdigitated with larger, more mature primary myofibers. At this stage, intermuscular axons had begun to myelinate, and postsynaptic specialization of the motor endplates had begun. Cluster dispersal and myonuclear migration was completed during the first 5 days postnatally with the muscle taking on adult characteristics. Beginning at 16 days in utero and extending into the neonatal period, there was evidence of myotube death in the soleus muscle.

Ultrastructural analysis of the smooth‐to‐striated transition zone in the developing mouse esophagus: Emphasis on apoptosis of smooth and origin and differentiation of striated muscle cells

“…Using the same sampling principle at ultrastructural level, we could work out in the present study a detailed analysis of the striated myogenesis from skeletal muscle progenitor cells to mature striated muscle fibers in the P4 esophagus. Striated myogenesis in esophagus ultrastructurally in broad outline was very similar to skeletal muscle (Kelly and Zacks, 1969;Platzer, 1978;Ontell and Kozeka, 1984;Ontell et al, 1988), but we found remarkable differences in detail.…”

Modulation of contractile protein gene expression in fetal murine crural muscles: Emergence of muscle diversity

“…During the early stages of limb muscle development (-11.5-14 days gestation), only primary myotubes are present in the crural muscles (Ontell and Kozeka, 1984a;Ontell et al, 1988a). It has been demonstrated that these primary myotubes are versatile in that transcripts for genes encoding a-cardiac and a-skeletal actin, MLCIA, MLCIF, and MLC3F; MHC,,b, MHCp/,lo,, and MHC,, can be demonstrated with in situ hybridization (Ontell et al, 1993).…”

“…) During the fetal period, the developing myofibers come under the influence of motor neurons. A few primitive myoneural junctions are present in the extensor digitorum longus muscle at 14 days gestation (Ontell and Kozeka, 1984a), but none is found in the soleus muscle until 16 days gestation (Ontell et al, 1988a). Innervation continues to occur in these muscles as new myofibers form, and the myoneural junctions undergo their initial maturation during the fetal period.…”

Differences in the histogenesis of EDL and diaphragm in rat

“…d) In both muscles, primary myotubes make contacts with each other before secondary myotubes appear. The apparent discrepancy between rat EDL, in which contacts exist between primary myotubes, and mouse EDL, in which primary myotubes are completely separated from each other (Ontell et al, 1988>, may be due to a small difference in the developmental stage at which the muscles were examined.…”