Heterogeneous Organelles Research Articles

Coiled bodies without coilin.

Nuclei assembled in vitro in Xenopus egg extract contain coiled bodies that have components from three different RNA processing pathways: pre-mRNA splicing, pre-rRNA processing, and histone pre-mRNA 3'-end formation. In addition, they contain SPH-1, the Xenopus homologue of p80-coilin, a protein characteristic of coiled bodies. To determine whether coilin is an essential structural component of the coiled body, we removed it from the egg extract by immunoprecipitation. We showed that nuclei with bodies morphologically identical to coiled bodies (at the light microscope level) formed in such coilin-depleted extract. As expected, these bodies did not stain with antibodies against coilin. Moreover, they failed to stain with an antibody against the Sm proteins, although Sm proteins associated with snRNAs were still present in the extract. Staining of the coilin- and Sm-depleted coiled bodies was normal with antibodies against two nucleolar proteins, fibrillarin and nucleolin. Similar results were observed when Sm proteins were depleted from egg extract: staining of the coiled bodies with antibodies against the Sm proteins and coilin was markedly reduced but bright nucleolin and fibrillarin staining remained. These immunodepletion experiments demonstrate an interdependence between coilin and Sm snRNPs and suggest that neither is essential for assembly of nucleolar components in coiled bodies. We propose that coiled bodies are structurally heterogeneous organelles in which the components of the three RNA processing pathways may occur in separate compartments.

Heterogeneity of lysosomes in human fibroblasts.

Lysosomes are defined traditionally with the marker enzyme acid phosphatase. We showed recently that lysosomes from human fibroblasts can be separated into a light and dense fraction as well as prelysosomal population. We now provide evidence that although acid phosphatase is enriched in all three fractions, the marker enzyme in the prelysosomal compartment is qualitatively distinct from that of the lysosomes. Ultrastructural analysis showed that the acid phosphatase in the prelysosomal vesicles deposited an extremely electron-dense reaction product, entirely obliterating the lumen of the vesicle, in contrast to that of the light and dense lysosomes which deposited a fine and diffuse product scattered throughout the luminal space. Biochemical analysis showed that only 51% of the acid phosphatase in the prelysosomes was inhibited by tartrate, while 80% of that in the lysosomes was tartrate-inhibitable. Immunoprecipitation with antibodies specific for various isozymes of acid phosphatase showed that 39% of the acid phosphatase in the prelysosomes was of the 'lysosomal' type whereas over 50% of the acid phosphatase in the lysosomes was of this type. These results showed that acid phosphatase in the prelysosomes of human cultured fibroblasts can be distinguished from that of the lysosomes cytochemically, biochemically, and immunologically and that lysosomes, as marked by acid phosphatase, are a heterogeneous organelle.

WHEN IS A CHROMOPLAST?

Chromoplasts are heterogeneous organelles and their carotenoids can be associated with several different structural elements including globules, tubules and membranes. In the flower petals of some species, carotenoids are absent or present only in trace amounts. Nevertheless, the plastids in such flowers can closely resemble or even be indistinguishable in structure from chromoplasts.

The internal reticular apparatus of Camillo Golgi: a complex, heterogeneous organelle, enriched in acid, neutral, and alkaline phosphatases, and involved in glycosylation, secretion, membrane flow, lysosome formation, and intracellular digestion.

Its topography is one of the most characteristic features of the Golgi apparatus and the reticular nature of this organelle is evident in Golgi's first drawings, in light microscopic enzyme cytochemical preparations, and in high voltage electron micrographs of thick sections. Although individual components of the Golgi apparatus may differ in staining characteristics, morphology, contents, and enzymatic activities, they are integrated into a dynamic topographical and functional unit that is closely associated with the endoplasmic reticulum. Modulation of enzymatic activities and morphological and enzymatic heterogeneity are not surprising in an organelle that is the site of both synthetic and digestive events, including glycosylation, sulfation, formation of secretory granules and lysosomes, and the degradation of endocytized material.

OSMIUM IMPREGNATION OF THE GOLGI APPARATUS.

AbstractThe Golgi complex consists of a heterogeneous assemblage of small vesicles, larger vacuoles and characteristic packets of flattened cisternae morphologically distinct from other membranous organelles of the cytoplasm. It is polarized with respect to its position in the cell and also shows recognizable polarity in the arrangement of structural components within the complex. This polarity holds for cells in which the Golgi complex seems to have quite different functions, such as in the epithelial cells of mouse epididymis and of Brunner's gland, and plasma cells in the lamina propria of the intestine examined in this study. The present investigation employs a classical method for impregnation of the Golgi apparatus with the objective of more accurately localizing with the electron microscope the sites of osmium reduction within this heterogeneous organelle and of comparing the distribution of these sites with morphological indications of polarization within the Golgi lamellar systems. The osmium impregnation procedure generally used in this study consisted of fixing the tissues in 1.3% osmium tetroxide buffered with s‐collidine followed by osmication in 2% aqueous osmium tetroxide for 40 hours at 40°C. After osmication the three cell types studied consistently exhibited a selective localization of osmium in the cisternae and vesicles on one side of the Golgi complex. The observations that only the outer vesicles and cisternae contain osmium while other cytologically similar elements under the same conditions do not, emphasizes the heterogeneous, bipolar nature of the Golgi apparatus. The substances responsible for osmium reduction are quite dependent on the local environment provided are apparently formed during the prolonged phase of the exposure of the tissue to osmium and are most likely localized at the inner face of the membrane of the outer cisternae and vesicles.