STEREO-ELECTRON MICROSCOPY OF NUCLEOLI, BALBIANI RINGS AND ENDOPLASMIC-RETICULUM IN CHIRONOMUS SALIVARY-GLAND CELLS

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  • 1 January 1980
    • journal article
    • research article
    • Vol. 22  (2) , 714-723
Abstract
Employing stereo-electron microscopy on sections (approximately 0.1 .mu.m thick) of isolated salivary glands from C. tentans, structural models for the organization of the peripheral nucleolar strands, the Balbiani ring (BR) transcription axes, and the tubular rough endoplasmic reticulum (RER) were formulated. The approximately 6.7 nm wide peripheral nucleolar strands display a higher-order organization of approximately 13.6 nm granules connected to a central axis, possibly representing a storage of pre-ribosomal RNP [ribonucleoprotein]. The BR transcription axes were visualized in 3-dimensions, and the density of lateral RNP fibrils was estimated (approximately 36 RNP fibrils/.mu.m transcription axis). This estimate of the density of nascent RNP fibrils suggests that in situ the DNA packing ratio of transcribing regions is intermediate between extended DNA and a polynucleosomal fiber of approximately 10 nm. The tubular RER appears to reveal extended helical arrays of polyribosomes. The helices have an overall diameter of approximately 130 nm, and exhibit a mean of 12 ribosomes per helical turn. This arrangement of membrane-bound ribosomes probably represents a functional specialization of the cell, facilitating the translation of the very large BR mRNA of approximately 37 kb into a secretory product.