Stereo-electron microscopy of nucleoli, Balbiani rings and endoplasmic reticulum in Chironomus salivary gland cells.

Employing stereo-electron microscopy on sections (ca. 0.1 micrometer thick) of isolated salivary glands from Chironomus tentans, we have formulated structural models for the organization of the peripheral nucleolar strands, the Balbiani ring (BR) transcription axes, and the tubular rough endoplasmic reticulum (RER). (i) The ca. 67 nm wide peripheral nucleolar strands display a higher-order organization of ca. 13.6 nm granules connected to a central axis, possibly representing a storage of pre-ribosomal RNP. (ii) The BR transcription axes were visualized in three-dimensions, and the density of lateral RNP fibrils was estimated (ca. 36 RNP fibrils/micrometer transcription axis). This estimate of the density of nascent RNP fibrils, suggests that in situ the DNA packing ratio of transcribing regions in intermediate between extended DNA and a ca. 10 nm polynucleosomal fiber. (iii) The tubular RER appears to reveal extended helical arrays of polyribosomes. The helices have an overall diameter of ca. 130 nm and exhibit a mean of 12 ribosomes per helical turn. We suggest that this arrangement of membrane-bound ribosomes represents a functional specialization of the cell, facilitating the translation of the very large BR mRNA of ca. 37 kb into a secretory product.