Publications

Using a computer-based system for model building and analysis, three-dimensional models of 24 Drosophila melanogaster salivary gland nuclei have been constructed from optically or physically sectioned glands, allowing several generalizations about chromosome folding and packaging in these nuclei. First and most surprising, the prominent coiling of the chromosomes is strongly chiral, with right-handed gyres predominating. Second, high frequency appositions between certain loci and the nuclear envelope appear almost exclusively at positions of intercalary heterochromatin; in addition, the chromocenter is always apposed to the envelope. Third, chromosomes are invariably separated into mutually exclusive spatial domains while usually extending across the nucleus in a polarized (Rabl) orientation. Fourth, the arms of each autosome are almost always juxtaposed, but no other relative arm positions are strongly favored. Finally, despite these nonrandom structural features, each chromosome is found to fold into a wide variety of different configurations. In addition, a set of nuclei has been analyzed in which the normally aggregrated centromeric regions of the chromosomes are located far apart from one another. These nuclei have the same architectural motifs seen in normal nuclei. This implies that such characteristics as separate chromosome domains and specific chromosome-nuclear envelope contacts are largely independent of the relative placement of the different chromosomes within the nucleus.

Tissue samples obtained from human gingiva with soft tissue calcification were processed for light and transmission electron microscopy. The stroma in these specimens revealed numerous foci of calcification in a matrix that consisted of closely packed branching microfibrils, 12 nm in diameter and a maximum of 2.5 micron in length. Calcospherites, 0.25-1.11 micron in diameter, were present within the matrix. They were constructed of needle-like units shown by high resolution electron microscopy and energy dispersive X-ray microanalysis to be apatite. Larger calcified masses were composed of calcospherites which were fused together. Matrix vesicles or other forms of membraneous material could not be found. The relationship between the mineral and non-collagenous microfibrils may suggest a role for the latter in the onset of calcification in this ectopic site.