Fundamental Morphological Changes in Human Olivary Hypertrophy

Morphological features of neurons in human inferior olivary nuclei were studied in 6 autopsied patients with dentato olivary pathway lesions, who had survived for between 6 days and 15 months. Central chromatolysis like neuronal enlargements were already present in the acute cases. Electron microscopy revealed round, homogeneous and electron dense granules, varying in diameter from 0.15 μm to 2.5 μm, occuring frequently within the rough endoplasmic reticulum (ER) in the chromatolytic neurons of all 6 patients. No similar granules were observed in 6 controls. The vacuoles were due to dilatation of the rough ER, and often contained floccular materials. Neurofilamentous hyperplasia in the neurons was more frequently seen in the chronic cases. These findings suggest that the fundamental changes in the neurons in olivary hypertrophy occur within the rough ER.     

The ultrastructure of the olivary pretectal nucleus in rats. A tracing and GABA immunohistochemical study

 The olivary pretectal nucleus is a primary visual centre, involved in the pupillary light reflex. In the present study an ultrastructural analysis was made of the olivary pretectal nucleus by means of separate, anterograde and retrograde tracing techniques and immunohistochemistry of gamma-aminobutyric acid. Large-projection neurons and two types of gamma-aminobutyric acid-immunoreactive (GABA-ir) neurons are observed in the olivary pretectal nucleus. The primary dendrites of the projection neurons have a dichotomous appearance, the secondary dendrites a multipolar appearance. At the ultrastructural level the projection neurons have well-developed Golgi fields, abundant rough endoplasmic reticulum and the nucleus is always heavily indented. Numerous small GABA-ir neurons and a few medium-sized GABA-ir neurons are found. The small GABA-ir neurons contain a few stacks of rough endoplasmic reticulum and the nucleus is oval-shaped. The medium-sized GABA-ir neurons have well-developed Golgi fields, a moderate number of rough endoplasmic reticulum stacks and an indented nucleus. GABA-positive dendritic profiles containing vesicles also are observed. In the neuropil of the olivary pretectal nucleus, retinal terminals are found that contain round clear vesicles and electron-lucent mitochondria. They make asymmetric synaptic contacts (Gray type I) with dendritic profiles and with profiles containing vesicles. Terminals originating from the contralateral olivary pretectal nucleus exhibit small, round clear vesicles, electron-dense mitochondria and make asymmetric synaptic contacts (Gray type I) mainly with dendritic profiles. Two types of GABA-ir terminals were found. One type is incorporated in glomerulus-like arrangements, whereas the other type is not. GABA-ir terminals contain pleomorphic vesicles, electron-dense mitochondria and make symmetric synaptic contacts (Gray type II). Retinal terminals, terminals originating from the contralateral olivary pretectal nucleus and GABA-ir terminals are organized in glomerulus-like structures, in which dendrites of the large projection neurons form the central elements. Triadic arrangements are observed in these structures; a retinal terminal contacts a dendrite and a GABA-ir terminal and the GABA-ir terminal also contacts the dendrite. The complexity of the synaptic organization and the abundancy of inhibitory elements in the olivary pretectal nucleus suggest that the olivary pretectal nucleus is strongly involved in processing visual information in the pupillary light reflex arc. Received: 17 July 1996 / Accepted: 24 September 1996     

Ultrastructural and morphometric studies of normal rat hepatocytes

To investigate the arrangement of organelles in normal rat liver hepatocytes, we examined liver tissues by electron microscopy. We also analyzed hepatocyte organelles by morphometry using the NIH image software. The smooth ER area appears as a network of branching tubules and contains many glycogen rosettes. The rough ER area presents lamellar plates and contains free polyribosomes. The Golgi complex area comprises the parallel smooth surface of associated vesicles and contains secretory granules, lysosomes and a few glycogens. Ratio of the cross-sectional area of hepatocytes shows the smooth ER area (60 +/- SD 1.5%), the rough ER area (35 +/- SD 1.4%) and the Golgi complex area (4 +/- SD 1.7%) of the total cytoplasm. The rough ER shows a dendrite pattern in a hepatocyte. Many mitochondria are spread between the smooth ER and the rough ER area. The smooth ER directly communicates with the rough ER, the Golgi complex cisternae at a forming face and the nuclear envelope. Anastomosing tubules with polyribosomes between the smooth ER and rough ER, between smooth ER and the nuclear envelope frequently. These results provide a basics at the cellular level for better understanding the experimentally induced changes and for improving the pathological hepatic diagnosis.     

Chondroid chordoma of the skull base: immunohistochemical and ultrastructural study of two cases with special reference to microtubules within rough-surfaced endoplasmic reticulum

Two cases of skull base chordoma (case 1, a 57-yearold woman; case 2, a 69-year-old woman) were investigated immunohistochemically and ultrastructurally. The tumors showed histopathological features typical of chondroid chordoma and contained both classical chordomatous and hyaline cartilaginous components. Tumor cells were immunoreactive for cytokeratin, vimentin, and S-100 protein, but negative for microtubule-associated protein 2 and class III beta-tubulin (tub-B3). Tumor cells of case 2 were immunoreactive for tauprotein and class II beta-tubulin (tub-B2), whereas those of case 1 were negative. Ultrastructurally, tumor cells in both cases showed the presence of abundant glycogen granules, well-developed intracellular organelles, and desmosome-like junctions. In case 2, several microtubules were closely packed and ran parallel or in random directions within the dilated cisterns of rough-surfaced endoplasmic reticulum (rough ER). “Microtubules within rough ER” has been described in several neoplasms, including classical and chondroid chordomas. Although previous reports documented the tub-B3 immunoreactivity in chordomas, our results suggested that, in our case 2, the predominant isoform of beta-tubulin in microtubules within rough ER was not tub-B3 but tub-B2.     

Fine structural analysis of calcitonin gene-related peptide in the mouse inferior olivary complex

Climbing fiber afferents to the cerebellum, from the inferior olivary complex, have a powerful excitatory effect on Purkinje cells. Changes in the responsiveness of olivary neurons to their afferent inputs, leading to changes in the firing rate or pattern of activation in climbing fibers, have a significant effect on the activation of cerebellar neurons and ultimately on cerebellar function. Several neuropeptides have been localized in both varicosities and cell bodies of the mouse inferior olivary complex, one of which, calcitonin gene related peptide (CGRP), has been shown to modulate the activity of olivary neurons. The purpose of the present study was to investigate the synaptic relationships of CGRP-containing components of the caudal medial accessory olive and the principal olive of adult mice, using immunohistochemistry and electron microscopy. The vast majority of immunoreactive profiles were dendrites and dendritic spines within and outside the glial boundaries of synaptic glomeruli (clusters). Both received synaptic inputs from non-CGRP labeled axon terminals. CGRP was also present within the somata of olivary neurons as well as in profiles that had cytological characteristics of axons, some of which were filled with synaptic vesicles. These swellings infrequently formed synaptic contacts. At the LM level, few, if any, CGRP-immunoreactive climbing fibers, were seen, suggesting that CGRP is compartmentalized within the somata and dendrites of olivary neurons and is not transported to their axon terminals. Thus, in addition to previously identified extrinsic sources of CGRP, the widespread distribution of CGRP within olivary somata and dendrites identifies an intrinsic source of the peptide suggesting the possibility of dendritic release and a subsequent autocrine or paracrine function for this peptide within olivary circuits.     

Prolactin-secreting adenomas. A light and electron microscopical study.

The microscopical and ultrastructural findings in twenty-five prolactin-secreting pituitary adenomas removed by transsphenoidal surgery have been analyzed and compared with previously published cases. Under the light microscope, these adenomas could easily be confused with chromophobe adenomas. However, in eight cases a small number of cells contained erythrosinophilic granules of the type found in prolactin cells. Electron microscopical study of the tumor cells showed scarce secretory granules frequently undergoing exocytosis, a prominent rough endoplasmic reticulum, and a large Golgi area containing immature granules. Cytoplasmic bundles and aggregates of filaments, as well as myelin figures and glycogen within the mitochondria were occasionally found. Comparison with prolactin cells during lactation, in pituitary autografts, and under in vitro stimulation shows that the tumor cells are actively secreting.     

Desending and intrinsic inputs to dorsal cochlear nucleus of cats: A horseradish peroxidase study

Horseradish peroxidase was injected unilaterally into the dorsal cochlear nucleus of adult cats in efforts to find neurons innervating the dorsal cochlear nucleus from (1) higher auditory nuclei or (2) other subdivisions of the cochlear nucleus. Following horseradish peroxidase injections and short survival periods, reactive neurons were most common in the dorsal and ventral nuclei of the lateral lemniscus and in the superior olivary complex of both sides of the brain stem. In the superior olivary complex, most neurons of the medial segment and border cells of the lateral segment reacted as did periolivary cells of the ventrolateral, dorsomedial, and preolivary areas, but not in the medial nucleus of the trapezoid body. Hilus neurons of the lateral superior olive reacted contralateral to the injection site. Although inferior colliculus neurons contained lightly stained granules bilaterally, more reactive neurons (including unusually large tripolar neurons) contained heavily stained granules in the contralateral colliculus. Intrinsic reactive neurons mainly included ipsilateral octopus cells, multipolar neurons of the nerve root regions, and stellate cells of the more rostra] anteroventral cochlear nucleus. All findings were confirmed by comparison to control animals.Our findings of specific neuronal types projecting to the cat dorsal cochlear nucleus suggest a relatively greater input from the nuclei of the lateral lemnisci of both sides than previously believed. Also, our results showed an unusually heavy input from the nearby superior olivary complex to the dorsal cochlear nucleus as well as inputs from specific cell types of the ipsilateral antero- and postero-ventral cochlear nucleus. By correlating these findings with those of other types of studies, we concluded that (1) too much emphasis has been placed upon inputs to the dorsal cochlear nucleus from the inferior colliculus relative to the descending pontine inputs and that (2) a new circuit involving the ventral cochlear nucleus, the dorsal cochlear nucleus and the medial superior olive may provide binaural information to large dorsal cochlear nucleus cells that terminate in their own unique areas of higher auditory nuclei.     

The site of origin of calcitonin gene-related peptide-like immunoreactive afferents to the inferior olivary complex of the mouse.

The intent of the present study is to define the brainstem nuclei which give rise to CGRP-immunolabeled afferents to the inferior olivary complex of the mouse. A technique which combines retrograde transport of fluorescent microspheres with immunohistochemistry was used to address this question. In the present study, intensely labeled CGRP neurons were localized within several cranial nerve nuclei including the hypoglossal, facial, oculomotor, motor nucleus of the trigeminal nerve and nucleus ambiguus, as well as in the parabrachial nucleus, locus coeruleus and medullary and pontine reticular formation. In addition, lightly labeled CGRP neurons were identified within the deep cerebellar nuclei, the inferior olivary complex, lateral reticular nucleus, medial and lateral vestibular nuclei, nucleus Darkschewitsch, interstitial nucleus of Cajal, the central gray area adjacent to the third ventricle, and the zona incerta. The origin of the projection to the inferior olivary complex primarily arises from the deep cerebellar nuclei, the locus coeruleus, and the central gray matter of the mesodiencephalic area. In addition, a small CGRP input is derived from the superior and lateral vestibular nuclei as well as the zona incerta. In conclusion, we have identified several extrinsic sources of CGRP to the inferior olivary complex and have localized it within afferents that have been shown to have either excitatory (mesodiencephalic nuclei) or inhibitory (cerebellar nuclei) effects on olivary circuits. The presence of CGRP in these functionally diverse brainstem and cerebellar afferents suggests that the peptide may act as a co-transmitter to modulate the activity of olivary neurons.     

Characteristics of neurons from the anterior region of the rat amygdaloid body by electron microscopy

Dark and light neurons with morphological signs of secretory activity are described within one of the major sexually dimorphic zones of brain amygdaloid nucleus (anterior cortical nucleus). Dark neurons are of medium size, they have large nuclei and well developed perikarya. The latter contain numerous free ribosomes and mitochondria, dilated rough endoplasmic reticulum cisterns, hypertrophied Golgi complex with forming secretory granules. Elementary neurosecretory granules (ENG) 75-300 nm in size were found in light neurosecretory cells, which seem to be similar to dark cells but are at different phase of functional activity. Synapses of all types known, containing small light vesicles, vesicles with dense core and ENG, were found in active neuropil.     

Rhythmogenesis in a hybrid system--interconnecting an olivary neuron to an analog network of coupled oscillators

A hybrid system in which an olivary neuron was interconnected to an analog simulator was used to study possible mechanisms by which the combined behavior of individual olivary neurons evokes synchronized membrane potential oscillations in a large population of neurons. The analog system was composed of four identical, interconnected oscillating units, each of which was capable of producing a damped sinusoidal oscillation in response to a trigger signal. When the units were coupled to each other, a single trigger pulse to one unit could evoke sustained oscillations. The integrity of the connections within the system was required to maintain these oscillations. In the hybrid system the analog system was reciprocally connected to an olivary neuron in a slice preparation. As in the analog system, the hybrid system could generate sustained oscillations following a trigger pulse to one of the units, as well as following a low threshold Ca spike in the neuron. Activation of the low threshold Ca conductance in the olivary neuron was necessary to achieve both gain and in-phase activity within the hybrid system, and thereby sustain the oscillations. The ability of the hybrid system to generate sustained oscillation is frequency dependent. Sustained oscillations were readily obtained at a "preferred frequency" of 5.2 Hz (n = 7) which was independent on the parameters used by the simulator, or on the membrane potential of the neurons. These results, which demonstrate the advantage of a new experimental approach developed to study rhythmogenesis in inferior olivary neurons, support the hypothesis that the inferior olivary nucleus, acting as an interconnected network of oscillating units, can generate an accurate subthreshold oscillation that serves as an internal time reference.     

The effect of lipofuscin on cellular function

The neurons of the supraoptic nucleus of male C57BL/Icrfat mice at 6 or 28 months of age were examined from normally hydrated, osmotically loaded and osmotically loaded-rehydrated animals. Using quantitative morphological techniques, a reduction in the concentration of lipofuscin in the neurons was observed in osmotically loaded mice at both ages, and these levels were restored to control values during rehydration. In addition, there was a significant difference in the pattern of response of lipofuscin levels between the two age groups during the experiment. The concentration of hormone containing neurosecretory granules in the neurons of the supraoptic nucleus did not differ significantly between the two age groups during the experiment. However, the surface area of rough endoplasmic reticulum per unit volume of the supraoptic nucleus cell did differ significantly between the two age groups over the course of the experiment. It is concluded that increasing concentrations of lipofuscin do not affect the ability of the cell to control the concentration of neurosecretory granules or rough endoplasmic reticulum. The simplistic view that lipofuscin accumulates with age to the detriment of cell function must be revised.     

Characterization of the rhesus monkey superior olivary complex by calcium binding proteins and synaptophysin

This study was performed in order to characterize the main nuclei of the rhesus monkey superior olivary complex by means of antibodies against the calcium binding proteins parvalbumin, calbindin and calretinin and the synaptic vesicle protein synaptophysin. These markers revealed the neuronal morphology and organization of nuclei located within the rhesus monkey superior olivary complex. The architectural details included the distribution of axonal terminals on neurons. The medial superior olivary nucleus was present as a column of neurons. No clear segregation of calretinin-positive terminals was noticed on the medial and lateral dendritic fields of these neurons. The lateral superior olivary nucleus was characterized by a distinct nuclear shape. Calretinin-, parvalbumin- or calbindin-positive terminals contacted somata and dendrites. The medial nucleus of trapezoid body could be clearly differentiated as a distinct region in the rhesus monkey superior olivary complex. Somata of that nucleus showed calbindin- and parvalbumin-labelling whereas somatic calyces of Held were reavealed by calretinin and synaptophysin labelling. The results are discussed with respect to the processing of acoustic information in primate species and their ability to hear high and low frequencies, which is reflected by anatomical correlates.     

Excitatory synaptic potentials in neurons of the deep nuclei in olivo-cerebellar slice cultures.

Excitatory postsynaptic potentials evoked in neurons of the deep cerebellar nuclei, either by electrical stimulation within the nuclei in cerebellar slice cultures or by electrical stimulation of olivary explants in olivo-cerebellar co-cultures, were investigated in the rat by means of intracellular recordings. In neurons of the deep cerebellar nuclei, stimulation of the nuclear tissue, as well as stimulation of the olivary tissue, induced a fast rising excitatory postsynaptic potential, followed by an inhibitory postsynaptic potential and a long-lasting excitation. The fast rising excitatory postsynaptic potential and the following inhibitory postsynaptic potential were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The remaining depolarization was abolished by D-(-)-2-amino-5-phosphonovalerate, suggesting that this potential was mediated by N-methyl-D-aspartate receptors. With only D-(-)-2-amino-5-phosphonovalerate added to the bath, the slow excitation was depressed, whereas the fast excitatory and inhibitory postsynaptic potentials were not affected. In the presence of bicuculline, the 6-cyano-7-nitroquinoxaline-2,3-dione- and the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials elicited by stimulation of the olivary tissue had the same latency, and were both graded with stimulation strength. The time-to-peak and the duration of the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials were considerably longer than those of the 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Topographical gradient in expression of R2D5 antigen in superior olivary nuclei and hippocampal dentate gyrus of the cat.

An immunohistochemical analysis of the cat central nervous system revealed that a monoclonal antibody which recognizes a soluble cytosolic protein, R2D5, bound two regions in a prominent spatial gradient. In the medial and lateral superior olivary nuclei of the brainstem, R2D5 immunoreactivity appeared as a gradient across a population of topographically ordered principal neurons. The spatial gradient corresponded to the tonotopic organization in the superior olivary nuclei: i.e., R2D5 immunoreactivity tended to occur more frequently and intensely in low-frequency neurons than in high-frequency neurons. Granule cells in the hippocampal dentate gyrus also had a pronounced spatial gradient in R2D5 immunoreactivity expression, and this gradient corresponded to the septotemporal axis of the hippocampus. Granule cells of the temporal (ventral) portions of the hippocampus were labeled intensely with R2D5 antibody, while those located in progressively more septal (dorsal) portions had gradually less immunoreactivity. These results suggest that in both the superior olivary nuclei and the hippocampal dentate gyrus, neurons differ in intrinsic properties by their position along specific axes. They suggest also that the hippocampus has an intrinsic functional organization related to the spatial gradient along its septotemporal axis.     

Ultrastructural evidence of sexual dimorphism in supraoptic neurons: a morphometric study

Summary We have recently shown that in spite of the absence of receptors for gonadal steroids in the supraoptic nucleus (SON) of the rat hypothalamus, the volume of the nucleus and the size of its neurons are larger in males than in females, and that these differences between male and female rats are correlated with body weight and dependent on the vasopressinergic neurons. As supraoptic neurons and their organelles enlarge when they are engaged in active peptidergic secretion we have carried out a morphometric ultrastructural analysis to determine if cell structures involved in the synthesis and storage of neurosecretory material also display weight-dependent sex dimorphism. Groups of six male and six female rats aged 30,60 and 180 days were used. Nucleoli, rough endoplasmic reticulum and neurosecretory granules were analysed and we estimated their volume or surface densities and the total volume of nucleoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecretory granules, the densities of the organelles did not differ among the groups studied, but total values were higher in males. These differences were found to be weight-dependent. Since the organelles studied are regarded as reliable indicators of the neurosecretory activity of supraoptic neurons, our data fully support the view that the weight-dependent sexual dimorphism observed in this nucleus reflects greater synthetic activity of its vasopressinergic neurons associated with the need to maintain water balance in larger bodies.     

Ultrastructural Study of Two Gonadotrophs in Rats Bearing an Adrenocortical Carcinoma (Snell 494)

Two morphologically distinct gonadotrophs have been identified in the anterior pituitary of rats bearing the Snell 494 tumor. Type 1 gonadotrophs first showed ultrastructural alterations at 4 weeks after implantation of the tumor. Numerous large 300 to 600 mμ granules appeared in type 1 cells. The opacity of the granules was variable, and they sometimes contained non-membrane-bounded spherical inclusions. These granules were dispersed among more electron-opaque, 200-mμ granules usually seen in gonadotrophs. A third granule approximately 500 to 600 mμ in diameter was apparently a lysosome-derived structure. Type 2 gonadotrophs first showed changes after 4 weeks, but the alterations were especially prominent by 8 to 10 weeks. No large granules characteristic of type 1 cells were seen in type 2 cells. By 8 to 10 weeks, very few 200-mμ granules were observed in the cytoplasm of type 2 cells. The endoplasmic reticulum was dilated in most type 2 cells; frequently, the cisternae were dilated considerably, forming large lake-like areas. Hypertrophic rough endoplasmic reticulum and the presence of intracisternal granules within cisternae of the rough reticulum suggest that type 2 cells were stimulated by the tumor. Changes in gonadotrophs were undoubtedly produced by the large amount of corticosterone secreted by the tumor. The corticosterone secreted by the tumor. The corticosterone may well act directly on the hypothalamo-hypophyseal axis or it could act indirectly on the ovary; ovarian steroids would then exert a feedback effect. The ovary was atrophic and did not contain maturing Graafian follicles or corpora lutea. In somatotrophs, the number of secretory granules increased, probably reflecting suppression of hormonal release by corticosterone.     

The superior olivary complex of the hamster has multiple periods of cholinergic neuron development

Cholinergic neurons of the superior olivary complex share a common embryological and phylogenetic origin with brainstem motor neurons and serve as the major descending efferent pathway either to the cochlea as part of the olivocochlear system or to the cochlear nucleus. In this study, we investigated the developmental expression patterns of choline acetyltransferase (ChAT) and its co-localization with calcitonin gene-related peptide within the superior olivary complex and neighboring brainstem motor nuclei. At embryonic day 12, neurons in the ventral nucleus of the trapezoid body were first to express ChAT. The temporal expression pattern of both ChAT mRNA and immunoreactivity in this periolivary region mimicked motor neurons in the facial and trigeminal motor nuclei. Just before birth, shell neurons surrounding the lateral superior olive expressed ChAT. Neither ChAT-positive periolivary neurons nor shell neurons co-expressed calcitonin gene-related peptide during development or in the adult. Immediately following birth, intrinsic neurons within the lateral superior olive expressed ChAT but not calcitonin gene-related peptide. However, a transient increase in the number of ChAT-positive neurons in the lateral superior olive coincided with the onset of the calcitonin gene-related peptide co-expression within these neurons. We conclude that ChAT expression appears first in periolivary regions containing medial olivocochlear neurons, precedes the expression of calcitonin gene-related peptide in the superior olivary complex, and is co-expressed with calcitonin gene-related peptide within the lateral superior olive containing lateral olivocochlear neurons. These data suggest that the lateral olivocochlear system co-expresses ChAT and calcitonin gene-related peptide, whereas the medial olivocochlear system does not.     

Presence of Tissue Transglutaminase in Granular Endoplasmic Reticulum is Characteristic of Melanized Neurons in Parkinson's Disease Brain

Parkinson's disease (PD) is characterized by the accumulation of α‐synuclein aggregates and degeneration of melanized neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross‐linking. In PD brain, tTG‐induced cross‐links have been identified in α‐synuclein monomers, oligomers and α‐synuclein aggregates. However, whether tTG and α‐synuclein occur together in PD affected neurons remains to be established. Interestingly, using immunohistochemistry, we observed a granular distribution pattern of tTG, characteristic of melanized neurons in PD brain. Apart from tTG, these granules were also positive for typical endoplasmic reticulum (ER)‐resident chaperones, that is, protein disulphide isomerase, ERp57 and calreticulin, suggesting a direct link to the ER. Additionally, we observed the presence of phosphorylated pancreatic ER kinase (pPERK), a classical ER stress marker, in tTG granule positive neurons in PD brain, although no subcellular colocalization of tTG and pPERK was found. Our data therefore suggest that tTG localization to granular ER compartments is specific for stressed melanized neurons in PD brain. Moreover, as also α‐synuclein aggregates were observed in tTG granule positive neurons, these results provide a clue to the cellular site of interaction between α‐synuclein and tTG.     

Immunocytochemical and morphometric analysis of acinar zymogen granules in human acute pancreatitis

Summary In the present study fine structural changes of acinar zymogen granules were investigated in human acute pancreatitis. Pancreatic tissue was obtained at surgery from 6 patients, prepared for ultrastructural analysis, and stained immunocytochemically for trypsinogen. Stereological parameters of zymogen granules were evaluated. The density of the immunocytochemical labelling for trypsinogen was estimated over zymogen granules, the rough endoplasmic reticulum, Golgi apparatus and the acinar lumina. In acute pancreatitis the number of zymogen granules was diminished and their size reduced. The density of the labelling for trypsinogen was unchanged over zymogen granules but showed a significant reduction over the rough endoplasmic reticulum, Golgi apparatus, and the acinar lumina. In general the integrity of zymogen granules was well preserved. Focally degenerative changes of zymogen granules and large autophagosomes were observed. From the immunogold labelling a disturbance of enzyme synthesis and secretion was suggested. Evidence is given that a disruption of the zymogen granule membranes and a fusion with lysosomal bodies might contribute to the pathogenesis of human acute pancreatitis.