AN ELECTRON MICROSCOPE STUDY OF SARCOBLAST KARYOKLASIS DURING MYOGENESIS IN GRASSHOPPER EMBRYOS

Abstract

Striated muscle differentiation in Melanoplus differentialis differentialis Thomas (Grasshopper) embryo begins on the 8th day of post-diapause development. Upon the onset of myogenesis some sarcoblast nuclei show regional condensation of chromatin so that they appear to be in early prophase. In such a nucleus the envelope is lost over approximately half of its surface, and myofibrillar filaments appear in the nucleocytoplasmic junction oriented at 90[degree]] to the nucleus. Sometimes the filaments are in contact with the condensed chromatin of the nucleus where the portion of the nuclear envelope is absent. At intervals chromatin-like osmiophilic granules appear to be mobilized from the chromatin and become deposited onto the ends of the myofibrillar filaments, suggesting that these granules form the z bands. Such depositions occur simultaneously on several myofibrillae as the nucleus appears to be used up during myogenesis. Cytochemical tests indicate that the differentiating nuclei and the myofibrillae attached to the nucleus are rich in RNA. The DNA is restricted to the condensed nuclear chromatin and is present at the junction of the myofibrillae and the chromatin. Although the z band may contain DNA it was not possible to demonstrate unequivocally its presence or absence with the aid of the light microscope.