On the quantitative stereo-morphology of microbodies in rat hepatocytes

Immunohistochemical Localization of Peroxisomal Enzymes During Rat Embryonic Development

Peroxisomes are cytoplasmic organelles involved in a variety of metabolic pathways. Thus far, the morphological and biochemical features of peroxisomes have been extensively characterized in adult tissues. However, the existence of congenital peroxisomal disorders, primarily affecting tissue differentiation, emphasizes the importance of these organelles in the early stages of organogenesis. We investigated the occurrence and tissue distribution of three peroxisomal enzymes in rat embryos at various developmental stages. By means of a highly sensitive biotinyl-tyramide protocol, catalase, acyl-CoA oxidase, and ketoacyl-CoA thiolase were detected in embryonic tissues where peroxisomes had not thus far been recognized, i.e., adrenal and pancreatic parenchyma, choroid plexus, neuroblasts of cranial and spinal ganglia and myenteric plexus, and chondroblasts of certain skeletal structures. In other tissues, i.e., gut epithelium and neuroblasts of some CNS areas, they were identified earlier than previously. In select CNS areas, ultrastructural catalase cytochemistry allowed identification of actively proliferating organelles at early developmental stages in several cell types. Our data show that in most organs maturation of peroxisomes parallels the acquirement of specific functions, mainly related to lipid metabolism, thus supporting an involvement of the organelles in tissue differentiation.

Tubular peroxisomes in HepG2 cells: selective induction by growth factors and arachidonic acid

We showed recently the plasticity of the peroxisomal compartment in the human hepatoblastoma cell line HepG2 as evidenced by the presence of elongated tubular peroxisomes measuring up to 5 microm next to much smaller spherical or rod-shaped ones (0.1-0.3 microm). Since the occurrence of tubular peroxisomes in a given cell in culture is synchronized, with neighboring cells containing either small spherical or elongated tubular peroxisomes, cell counting of immunofluorescence preparations stained for catalase was used for the quantitative assessment of the dynamics of the peroxisomal compartment and the factors regulating this process. Initial studies revealed that the formation of tubular peroxisomes is primarily influenced by the cell density as well as by lipid- and protein-factors in fetal calf serum, being independent of an intact microtubular network. Biochemical studies showed that the occurrence of tubular peroxisomes correlated with the expression of the mRNA for 70 kDa peroxisomal membrane protein (PMP70), but not with that of matrix proteins. By cultivation of cells in serum- and protein-free media specific factors were identified which influenced the formation of tubular peroxisomes. Among several growth factors tested, nerve growth factor (NGF) was the most potent one inducing tubular peroxisomes and its effect was blocked by K252b, a specific inhibitor of neurotrophin receptor pathway, suggesting the involvement of signal transduction in this process. Furthermore, from several polyunsaturated fatty acids (PUFA) which all induced tubular peroxisomes, the arachidonic acid (AA) was the most potent one. Our observations suggest that tubular peroxisomes are transient structures in the process of rapid expansion of the peroxisomal compartment which are induced either by specific growth factors or by polyunsaturated fatty acids both of which are involved in intracellular signaling.

Biogenesis of peroxisomes in fetal liver

Peroxisomes are single membrane-limited cell organelles that are involved in numerous metabolic functions. Peroxisomes do not contain DNA; the matrix and membrane proteins are encoded by the nuclear genome. It is assumed that new peroxisomes are formed by division of existing organelles. The present article gives an overview of microscopic studies and recent unpublished results dealing with peroxisome biogenesis in mammalian fetal liver and presents data on peroxisomes in oocytes. Cytochemical (catalase and D-aminoacid oxidase activity) and immunocytochemical data in rat and human liver (antigens of catalase, the three peroxisomal beta-oxidation enzymes, alanine: glyoxylate aminotransferase, peroxisomal membrane proteins with molecular weights of 42 and 70 kDa) indicate that during embryonic and fetal development the peroxisomal population undergoes a differentiation with respect to the composition of the matrix and to the size and number of the organelles. In the youngest stages, rare and small peroxisomes are present, into which the matrix components are imported in a sequential way. The import seems asynchronous in peroxisomes of the same hepatocyte. The size and number of the peroxisomes increase during liver development. In rat and human liver, no morphological or immunocytochemical evidence for an elaborate network of interconnected peroxisomes ("reticulum") was found. Instead, peroxisomes presented as individual organelles, which occasionally show membrane extensions. The importance of the metabolic functions of peroxisomes in human liver is emphasized by the peroxisomal disorders. In the liver of affected fetuses, the microscopic features associated with the defect can already be recognized; i.e., either catalase containing peroxisomes are absent and catalase is localized in the cytoplasm (in fetuses affected with Zellweger syndrome or with infantile Refsum disease) or peroxisomes are present but they are abnormally enlarged (e.g., a fetus affected with acyl-CoA oxidase deficiency). In the quail ovary, numerous peroxisomes are observed in the oocyte and in the granulosa cells during follicle maturation, but not in the full-grown egg. Thus, the mechanism of peroxisome inheritance remains unresolved.

Ultrastructural aspects of the biogenesis of peroxisomes in rat liver

A model summarizing our current concepts on the ultrastructural basis of the biogenesis of peroxisomes is presented. Accordingly, the initial stage of de novo build-up of peroxisomes is characterized by the formation of myelin-like figures and membranous attachments onto the surface of pre-existing peroxisomes. Such membranous structures may provide the appropriate lipid environment for the incorporation of peroxisomal membrane proteins and subsequently become the preferential sites for import of newly synthesized matrix proteins. After the import the membranous structures develop into small peroxisomes which may remain attached briefly to the larger particles but eventually separate to become new peroxisomes. Whereas some matrix proteins such as catalase are distributed in all newly formed peroxisomes, other ones like urate oxidase and D-amino acid oxidase are compartmentalized only in some of them, giving rise to heterogeneity of peroxisomes.

Application of automatic image analysis for quantitative morphological studies of peroxisomes in rat liver in conjunction with cytochemical staining with 3-3'-diaminobenzidine and immunocytochemistry

We describe the application of automatic image analysis for quantitative morphological studies of peroxisomes in rat liver. For automatic detection by light and electron microscopy peroxisomes must be stained with the alkaline DAB procedure for catalase. There is a good agreement between the results obtained by conventional morphometric techniques and by automatic image analysis of DAB-stained electron microscopic preparations. Moreover, the image analyzer may be used in conjunction with a light microscope for evaluation of semithin sections (1-0.25 microns), provided the section thickness factor is taken into consideration. This latter approach has proven highly efficient in estimation of peroxisome proliferation. The limitations of this method and the relevance of volume density as a reliable morphometric parameter for evaluation of peroxisome proliferation are discussed. In the second part of this study we present the application of image analysis for quantitation of alterations of individual peroxisomal enzyme proteins after treatment with bezafibrate in immunogold stained ultrathin sections. There is good agreement between the results of quantitative immunocytochemistry and Western (immuno) blot analysis of highly purified peroxisomal fractions. In our experience quantitative immunoelectron microscopy provides a versatile, highly sensitive, and efficient method for detection of modulations of various proteins in peroxisomes. Finally the limitations and prospects of quantitative immunocytochemistry for investigation of peroxisomal proteins are discussed.

Peroxisomes in guinea pig liver: their peculiar morphological features may reflect certain aspects of lipoprotein metabolism in this species

We have studied the ultrastructural characteristics and the distribution of peroxisomes in guinea pig liver using electron-microscopic cytochemistry for catalase and morphometry. By light microscopy, peroxisomes appear as dark 0.2-0.5 microns granules in the cytoplasm of liver parenchymal cells, often forming large clusters that measure up to 5 microns across. Rows of single peroxisomes or their aggregates line the sinusoidal surface of hepatocytes. Electron microscopy reveals that clusters of up to 25 individual peroxisomes are usually located in the subsinusoidal region of parenchymal cells. The mean diameter and the volume density of peroxisomes are larger in pericentral than in periportal regions of the liver lobule. Whereas large amounts of lipoprotein particles with a mean diameter of 160 nm (chylomicrons) are present in the Disse space, the cytoplasm of parenchymal cells contains multivesicular bodies and abundant lipid droplets. In addition, the Golgi complexes show distended lipoprotein-filled vesicles suggesting active biosynthesis of lipoproteins. We propose that the unique features of peroxisomes in guinea pig liver, such as cluster formation and alignment along the sinusoidal surface, may be related to the high levels of lipoproteins in the portal circulation and their hepatic catabolism in this species.

Three-dimensional reconstruction of a peroxisomal reticulum in regenerating rat liver: evidence of interconnections between heterogeneous segments

The three-dimensional (3-D) form and the interrelationship of peroxisomes (Po) in the model of regenerating rat liver after partial hepatectomy were studied by computer-assisted 3-D reconstruction of serial ultrathin sections. Po were labeled cytochemically for either catalase, which stains them all uniformly, or for D-amino acid oxidase (DAA-OX), which gives a heterogeneous reaction with lightly and darkly stained PO. In regenerating rat liver, Po exhibit marked pleomorphism with some budding forms and dumbbell-shaped ones. The 3-D reconstruction revealed many single spherical Po measuring 0.15-0.8 micron in diameter. In addition, two to five Po were found interconnected with each other via narrow 30-50-nm hourglass-shaped bridges forming a reticulum. Such aggregates of Po measured 1.5-2.5 microns across. Whereas all segments of this reticulum stained homogeneously for catalase, they exhibited a marked difference in the intensity of the DAA-OX reaction. These observations are consistent with the view of peroxisomal proliferation by budding or fragmentation from preexisting ones. Under such conditions of rapid growth as in regenerating liver, Po may be interconnected forming a reticulum. The interconnections between Po with differing DAA-OX activities suggest that they originate from the same parent organelle.

Application of automatic image analysis for morphometric studies of peroxisomes stained cytochemically for catalase. II. Light-microscopic application

The feasibility of the application of a television-based image analyzer, the Texture Analysis System (TAS, Leitz Wetzlar, FRG) in conjunction with a light microscope for morphometric studies of hepatic peroxisomes has been investigated. Rat liver peroxisomes were stained with the alkaline-DAB method for localization of catalase and semithin (0.25 and 1 micron) sections of plastic-embedded material were examined under an oil immersion objective. The TAS detected the peroxisomal profiles selectively and determined their morphometric parameters automatically. The same parameters were obtained also by morphometric analysis of electron micrographs from the same material. The volume density of peroxisomes determined by TAS in semithin sections of normal liver, after correction for section thickness, is quite close to the corresponding value obtained by morphometry of electron micrographs. The difference is approximately 20%. In animals treated with the hypolipidemic drug bezafibrate, which causes proliferation of peroxisomes, TAS detected readily the increase in volume density of peroxisomes in semithin sections. In comparison with electron microscopy, however, the light-microscopic approach seems to underestimate the proliferation. The lower resolution of the light microscope and overlapping of neighbouring particles in relatively thick sections used for light-microscopic analysis may account for the differences. The present study has demonstrated the usefulness of automatic image analysis in conjunction with selective cytochemical staining of peroxisomes for morphometry of this organelle in rat liver. The light-microscopic approach is not only faster but is also extremely economical by obviating the use of an electron microscope.

Serial section analysis of mouse hepatic peroxisomes

The ultrastructure and organization of mouse hepatic peroxisomes were investigated using serial thin sections and the alkaline diaminobenzidine technique for visualization of the peroxidatic activity of catalase. Mouse periportal hepatocytes exhibit three classes of peroxisomes which display morphological and cytochemical heterogeneity: 1) large, circular to ovoid organelles containing a crystalline nucleoid, 2) small, circular to elongate, anucleoid particles, and 3) tail-like extensions which are devoid of both catalase activity (only traces of reaction deposits) and a crystalline core. Serial section analysis reveals that these profiles correspond to three diverse interconnecting peroxisomal segments which constitute a highly complex organelle. In particular, the large nucleoid-containing peroxisomal segment exhibits an intimate relationship to the endoplasmic reticulum. However, direct membrane continuities between the two compartments are never observed. With respect to the complex structure of the organelle the following conclusions can be drawn concerning biochemical studies on liver peroxisomes: 1) During homogenization and subcellular fractionation procedures, fragmentation of peroxisomes into particles of different size classes should be expected. 2) These peroxisomal fragments are inhomogeneous with respect to their matrix contents and possess at least one rupture site on their membrane surface. 3) Soluble matrix and, to a lesser degree, membrane components of peroxisomes contribute to the soluble fraction. 4) Crude microsomal fractions are regularly contaminated by peroxisomal membrane fragments.

Peroxisomes in sebaceous glands. V. Complex peroxisomes in the mouse preputial gland: serial sectioning and three-dimensional reconstruction studies

The ultrastructure of peroxisomes in partially differentiated cells of the mouse preputial gland was investigated using serial thin sections and three-dimensional reconstruction as well as the alkaline diaminobenzidine technique for visualization of the peroxidatic activity of catalase. An analysis of serial sections indicates that the different types of intensely stained peroxisomal profiles, classified according to their shape, represent random planes through highly complex peroxisomes. These organelles exceed 4 micron in length and exhibit a focal heterogeneity with respect to their size, shape and enzyme distribution. The graphical three-dimensional reconstruction demonstrates that the most intricate peroxisomes are characterized by tortuous, elongate, and branched tubular segments of varying diameter equipped with enlarged terminal hollow-spherical structures which engulf areas of cytoplasm. A close spatial relationship is established between adjacent peroxisomes and peroxisomes and mitochondria, the latter two of which synchronously develop into highly complex structures. A close association is also observed between peroxisomes and the endoplasmic reticulum, whereby membrane continuities between the two compartments cannot be demonstrated. These observations are inconsistent with traditional concepts concerning peroxisomal shape and size, the number per cell, as well as their biogenesis from the endoplasmic reticulum. The functional significance of individual highly complex peroxisomes and their assemblage forming an extensive net-like membraneous system throughout the cell is discussed with respect to intracellular energy transport and transmembrane electron exchange.

The proliferative response of hepatic peroxidomes of neonatal rats to treatment with SU-13 437 (nafenopin)

The repeated administration of the hypolipidemic agent Su-13 437 (nafenopin) to neonatal rats roughly doubled the number of peroxisomes in the liver tissue and caused a sixfold volumetric expansion of the peroxisomal compartment. During the proliferative response, the size-distribution of the peroxisomes was reversibly altered, enlarged particles appearing in numbers varying according to the dose given. By means of a new method for quantitative autoradiography, it was shown that (a) the concentration of silver grains over peroxisomes was comparable to that found over the endoplasmic reticulum; (b) the peak incorporation of [3H]arginine into the peroxisomes was dealyed in comparison with that into the endoplasmic reticulum; (c) the label, once incorporated into the expanding peroxisomal compartment, displayed the same shift to large particles as did the whole population. These results are compatible with the biosynthetic pathway for peroxisomal catalase proposed earlier (cf. reference 12), and with the notion that the drug-induced size-shift might have resulted from progressive growth of a particular class of peroxisomes formed in the presence of the agent. Evidence is presented to show that during the recovery period the larger peroxisomes are removed preferentially.