Three-dimensional ultrastructural analysis of peroxisomes in HepG2 cells. Absence of peroxisomal reticulum but evidence of close spatial association with the endoplasmic reticulum.

Peroxisomes in the human hepatoblastoma cell line, HepG2, exhibit distinct alterations of shape, size, and distribution, dependent on culture conditions (cell density, duration in culture, and presence of specific growth factors). Although many cells with elongated tubular peroxisomes are present in thinly seeded cultures, spherical particles forming large focal clusters are found in confluent cultures. The authors have analyzed the ultrastructure and the spatial relationship of peroxisomes of HepG2 cells at different stages of differentiation, using three-dimensional (3D)-reconstruction of ultrathin serial sections, and electronic image processing. Cells were prepared for immunofluorescence using different antibodies against peroxisomal matrix and membrane proteins, as well as for electron microscopy after the alkaline 3,3'-diaminobenzidine staining for catalase. The results indicate that the tubular peroxisomes, which can reach a length of several microns, are consistently isolated, and never form an interconnected peroxisomal reticulum. At the time of disappearance of tubular peroxisomes, rows of spherical peroxisomes, arranged like beads on a string, are observed, suggesting fission of tubular ones. In differentiated confluent cultures, clusters of several peroxisomes are seen, which, by immunofluorescence, appear as large aggregates, but after 3D reconstruction consist of single spherical and angular peroxisomes without interconnections. The majority of such mature spherical peroxisomes (but not the tubular ones) exhibit tail-like, small tubular and vesicular attachments to their surface, suggesting a close functional interaction with neighboring organelles, particularly the endoplasmic reticulum, which is often observed in close vicinity of such peroxisomes.

An optimized, fully automated system for fast and accurate identification of chromosomal rearrangements by multiplex-FISH (M-FISH).

Multiplex-FISH (M-FISH) is a recently developed technique by which each of the two dozen human chromosomes-the 22 autosomes and the X and Y sex chromosomes-can be stained or "painted" with uniquely distinctive colors. Using a combinatorial labeling technique and a specially designed filter set, each DNA probe can be identified by its unique spectral signature. Here we present several significant optimizations of the M-FISH technology. First, a new strategy for labeling the probes is described which allows for easy and fast production of the complex M-FISH probe mix. Second, a newly developed, completely motorized microscope equipped with an eight-position filter wheel and a new generation of filter sets is presented that allows fully automatic imaging of a complete metaphase spread within seconds. Third, to determine the characteristic spectral signatures for all different combinations of fluorochromes, we developed a novel multichannel image analysis method. The spectral analysis is solely guided by the image information itself and does not require any user interaction. A complete analysis of a metaphase spread can be accomplished in less than 3 min. Sophisticated built-in quality controls were developed, and the value of visual inspection of M-FISH images as a simple means of controlling the computer-generated chromosome classification are illustrated. In addition, we discuss advantages of adding new fluorochromes to the traditionally used five fluorochromes.

Evidence against a looped structure of the inactive human X-chromosome territory.

Multicolor fluorescence in situ hybridization with a whole chromosome composite probe for the X-chromosome and microdissection probes for the Xp and Xq arms, as well as for the Xp terminal, Xq terminal, and X centromer specific subregional probes, was applied to three-dimensional (3D) preserved human female amniotic fluid cell nuclei. Confocal laser scanning microscopy and three-dimensional image analysis demonstrated distinctly separated Xp arm and Xq arm domains. 3D distance measurements revealed a high variability of intrachromosomal distances between Xpter, Xcen, and Xqter specific probes within both X territories. A 3D distance measurement error of +/- 70 nm was found in control experiments using quartz glass microspheres labeled with different fluorochromes. Our data argue against the hypothesis of Walker et al. (1991, Proc. Natl. Acad. Sci. USA 88, 6191-6195) that a looped structure of the inactive X territory is formed by tight telomere-telomere associations.