[Morpho functional state of the peripheral organs of the immune system in rats after the hypokinesia and in the period of rehabilitation]

Using the quantitative methods, the remodeling of the cytoarchitectonics of the morpho-functional zones in the grouped lymphoid nodules (GLN) or Peyer's patches and in the mesenteric lymph nodes (MLN) were studied in 30 rats after 30-day-long exposure to hypokinesia and during the period of rehabilitation (30 days after hypokinesia discontinuation). It was found that following the hypokinesia the germinal centers in lymphoid nodules in GLN retained the lymphocytopoiesis, while in the internodular zone the proportion of immature cells was increased and plasma cells appeared. In the similar structural zones of MLN, the complete suppression of lymphocytopoiesis and T-cell maturation was noted. During the rehabilitation period, the cytoarchitectonic indexes recovery was more pronounced in GLN than in MLN. However, the quantitative parameters of their cellular composition did not reach the values found in the group of intact of animals.

The chirality of gut rotation derives from left-right asymmetric changes in the architecture of the dorsal mesentery

We have investigated the structural basis by which the counterclockwise direction of the amniote gut is established. The chirality of midgut looping is determined by left-right asymmetries in the cellular architecture of the dorsal mesentery, the structure that connects the primitive gut tube to the body wall. The mesenchymal cells of the dorsal mesentery are more condensed on the left side than on the right and, additionally, the overlying epithelium on the left side exhibits a columnar morphology, in contrast to a cuboidal morphology on the right. These properties are instructed by a set of transcription factors: Pitx2 and Isl1 specifically expressed on the left side, and Tbx18 expressed on the right, regulated downstream of the secreted protein Nodal which is present exclusively on the left side. The resultant differences in cellular organization cause the mesentery to assume a trapezoidal shape, tilting the primitive gut tube leftward.

Optical monitoring of microlymphatic disturbances during experimental lymphedema

BACKGROUND

Rat mesentery has been widely used to study microvascular functions. The goal of this work is to extend this animal model to monitor blood and lymph microvessel function during lymphedema.

METHODS AND RESULTS

Lymphedema is created by microsurgical removal of regional lymph nodes (lymphadenectomy) or ligation of the collecting vein. Water volume in mesenteric tissue, microvessel diameters, phasic contraction, valve function, lymph flow velocity, and cell migration were analyzed during lymphedema development. Dynamic observation of water amount after lymphadenectomy revealed increasing edema from 30 min to 1 week; greatest degree of edema at one week, and gradual decrease in edema from 1 to 11 weeks. These effects were accompanied by acute constriction of lymph vessels and slowing of lymph flow velocity, switching to dilation and appearance of new blood capillaries at week 1, progressing to dilation and degenerative changes of the microlymphatic wall at week 4, and, finally, leading to lymphatic fibrosis and lymphangiogenesis at week 11. Acute venous insufficiency (30 min after vein ligation) led to significant edema, decreasing blood flow velocity to stasis, and output of erythrocytes from venules to interstitium, with further movement to microlymphatics and regional lymph nodes.

CONCLUSIONS

Rat mesentery as an animal model in combination with an advanced optical imaging system is valuable in studying microlymphatic disturbances in mesentery during the development of experimental lymphedema from latent period to chronic stages, including monitoring of individual cell dislocation with high resolution optical imaging.

Derivation of muscles of the Aristotle’s lantern from coelomic epithelia

Transmission electron microscopy was employed to study structural changes in the lantern muscles occurring during the transition from young to adult in the sea urchin Strongylocentrotus nudus. A comparative examination of four major lantern muscles (compass depressors, compass elevators, protractors and retractors) suggests that myogenesis involves four consecutive stages. At the initial stage, the muscles show the organization of a mesentery delimited by pseudostratified coelomic epithelia, which are composed of peritoneal cells spanning the whole height of each epithelium, and myoepithelial cells, which are clustered together to fill the interstices between the basal processes of the peritoneal cells. During the next stage, the clusters of myoepithelial cells partly "sink" into the underlying connective tissue. At the third stage of muscularization, the myoepithelial cells increase in size and further invade the underlying connective tissue so that the myoepithelium splits into an apical peritoneal layer and a deeper mass of myoepithelial cells immersed in the connective tissue. However, these two layers are connected by a continuous basal lamina. This is thus the first description of an intermediate developmental stage between pseudostratified myoepithelim and genuine echinoderm muscles. For such a myoepithelium, we propose the term "immersed myoepithelium". At the most advanced stage of myogenesis, the myocytes detach completely from the epithelium to form subepithelial muscle bundles. Myogenesis in the sea urchin takes a long time during which continuous myogenic differentiation occurs in the coelomic epithelium and the newly formed myocytes and associated neurons penetrate into the underlying connective tissue.

In vivo photothermal flow cytometry: imaging and detection of individual cells in blood and lymph flow

Flow cytometry is a well-established, powerful technique for studying cells in artificial flow in vitro. This review covers a new potential application of this technique for studying normal and abnormal cells in their native condition in blood or lymph flow in vivo. Specifically, the capabilities of the label-free photothermal (PT) technique for detecting and imaging cells in the microvessel network of rat mesentery are analyzed from the point of view of overcoming the problems of flow cytometry in vivo. These problems include, among others, the influences of light scattering and absorption in vessel walls and surrounding tissues, instability of cell velocity, and cells numbers and positions in a vessel's cross-section. The potential applications of this new approach in cell biochemistry and medicine are discussed, including molecular imaging; studying the metabolism and pathogenesis of many diseases at a cellular level; and monitoring and quantifying metastatic and apoptotic cells, and/or their responses to therapeutic interventions (e.g., drug or radiation), in natural biological environments.

Contribution of mesenterial muscle dedifferentiation to intestine regeneration in the sea cucumber Holothuria glaberrima

Holothurians (sea cucumbers) have been known from ancient times to have the capacity to regenerate their internal organs. In the species Holothuria glaberrima, intestinal regeneration involves the formation of thickenings along the free mesentery edge; these thickenings will later give rise to the regenerated organ. We have previously documented that a remodeling of the extracellular matrix and changes in the muscle layer occur during the formation of the intestinal primordium. In order to analyze these changes in depth, we have now used immunocytochemical techniques and transmission electron microscopy. Our results show a striking disorganization of the muscle layer together with myocyte dedifferentiation. This dedifferentiation involves nucleic activation, disruptions of intercellular junctions, and the disappearance of cell projections, but more prominently, the loss of the contractile apparatus by the formation and elimination of spindle-like structures. Muscle dedifferentiation can be seen as early as 2 days following evisceration and continues during the next 2 weeks of the regeneration process. Dedifferentiation of myocytes might result in cells that proliferate and give rise to new myocytes. Alternatively, dedifferentiating myocytes could give rise to cells with high nuclear-to-cytoplasmic ratios, with some being eliminated by apoptosis. Our results, together with those in other regeneration models, show that myocyte dedifferentiation is a common event in regeneration processes and that the dedifferentiated cells might play an important role in the formation of the new tissues or organs.

Platelet-activating factor and kinin-dependent vascular leakage as a novel functional activity of the soluble terminal complement complex

The infrequent occurrence of septic shock in patients with inherited deficiencies of the terminal complement components experiencing meningococcal disease led us to suspect that the terminal complement complex is involved in vascular leakage. To this end, the permeabilizing effect of the cytolytically inactive soluble terminal complement complex (SC5b-9) was tested in a Transwell system measuring the amount of fluorescein-labeled BSA (FITC-BSA) leaked through a monolayer of endothelial cells. The complex caused increased permeability to FITC-BSA after 15 min as opposed to the prompt response to bradykinin (BK). The effect of SC5b-9 was partially reduced by HOE-140 or CV-3988, two selective antagonists of BK B2 and platelet-activating factor receptors, respectively, and was completely neutralized by the mixture of the two antagonists. Also, DX-88, a specific inhibitor of kallikrein, partially inhibited the activity of SC5b-9. The permeabilizing factor(s) released after 30 min of incubation of endothelial cells with SC5b-9 caused a prompt leakage of albumin like BK. Intravital microscopy confirmed both the extravasation of circulating FITC-BSA across mesenteric microvessels 15 min after topical application of SC5b-9 and the complete neutralization by the mixture of HOE-140 and CV-3988. SC5b-9 induced opening of interendothelial junctions in mesenteric endothelium documented by transmission electron microscopy.

[Organization of the muscular component of the lymphangion wall in different parts of the lymphatic bed]

Complex comparative analysis of the organization of smooth muscle (SM) forming the wall of lymphatic vessels in bovine small intestinal mesenterium was performed using the methods of morphometry, quantitative histochemistry (including the analysis of nuclear DNA content, and cytoplasmic protein content) and electron microscopy. SM cells (SMC) isolated by dissociation were studied and were found to possess various levels of differentiation, associated with specific morphometric and metabolic characteristics. The structure of SMC population was shown to vary in both different parts of lymphatic bed and within the wall of an individual lymphangion. The results obtained indicate the cellular heteromorphism of lymphatic bed SM. The peculiarities of SM organization in lymphatic vessels are functionally dependent and are determined not only by the level of SM representation in their wall but also by the proportions of different SMC types.

A comparison of the anti-inflammatory activities of conjugated estrogens and 17-? estradiol

OBJECTIVE AND DESIGN

Unregulated chronic inflammatory process partly due to an estrogen deficiency may render postmenopausal women vulnerable to degenerative conditions such as arthritis, osteoporosis, atherosclerosis, and Alzheimer's disease. Current confusion regarding therapeutic efficacy of estrogen replacement therapy may be due to different estrogen formulations used, short term therapy, as well as advanced stage of the disease.

MATERIALS AND METHODS

We compared anti-inflammatory activities of two major estrogen preparations, conjugated equine estrogen (CEE) and 17-beta estradiol, using an animal model (rat mesentery) of in vivo inflammatory reaction to intravenously infused amyloid-beta, examined by video recording and subsequently analyzed by transmission electron microscopy. Cellular markers of inflammation were monitored: leukocyte migration, platelet activation, mast cell activation/degranulation, and endothelial disruption.

RESULTS

Low doses of CEE (0.3 mg/kg for 3 weeks) demonstrated significant anti-inflammatory activity, whereas even at high doses (2.0 mg) 17-beta estradiol had only minimal activity.

CONCLUSION

CEE, a mixture of several compounds, may have some component(s) with significant anti-inflammatory activity. The anti-inflammatory activity of CEE may have a role in prevention of several degenerative diseases associated with menopause.

Kit-like immunopositive cells in sheep mesenteric lymphatic vessels

Recent electrophysiological studies have suggested that there is a subpopulation of cells in lymphatic vessels which act as pacemakers controlling the characteristic spontaneous contractile activity in this tissue. In this study, electron microscopy and immunohistochemical techniques were used on sheep mesenteric lymphatic vessels to investigate the morphology of the cells comprising the lymphatic wall. The smooth muscle cells were not orientated in circular and longitudinal layers as is seen in the gastrointestinal tract, but were arranged in bundles which interlock and cross over in a basket-weave fashion. Antibodies to Kit and vimentin, which are widely used to label specialised pacemaking cells in the gastrointestinal tract (known as interstitial cells of Cajal), demonstrated the existence of an axially orientated subpopulation of cells lying between the endothelium and the bulk of the smooth muscle. Examination of this area using electron microscopy showed cells which were electron dense compared to the underlying smooth muscle and contained caveolae, Golgi complexes, mitochondria, 10-nm filaments, a well-developed endoplasmic reticulum and a basal lamina. The smooth muscle cells typically contained caveolae, dense bodies, mitochondria, abundant filaments, sER and basal laminae. Cells dispersed for patch-clamp studies were also stained for vimentin and myosin. Myosin-staining cells had the typical spindle appearance of smooth muscle cells whereas the vimentin-positive cells could either be branched or more closely resemble the smooth muscle cells. The present study provides the first morphological evidence that specialised cells exist within the vascular system which have the ultrastructural characteristics of pacemaker cells in other tissues and are vimentin and Kit positive.

Effects of temperature on the wall strength and compliance of frog mesenteric microvessels

In single perfused mesenteric microvessels of pithed frogs, we assessed wall strength from the critical pressure, PB, which has to be applied within the vessel in order to induce openings in the walls through which fluid and cells can extravasate. PB was determined in capillaries and venules of tissues at 12-20 The P(B) (mean +/- S.E.M.) in 22 vessels between 12 and 20 degrees C, P(B) was 92.0 +/- 7.40 cm H2O which was significantly higher than at room temperatures (P<0.001). The compliance of the vessel wall was estimated using both the red cell method and the oil meniscus technique. There was no measurable effect of temperature on wall compliance. The compliance of vessels from which the cells had been removed by previous perfusion with detergent solutions was very similar to that of intact vessels between 12 and 20 degrees C and between 0 and 5 degrees C. The negligible effects of temperature upon compliance suggest that microvessel walls have to be distended to a greater extent in cold tissue before P(B) is reached. This, together with their rapid closure, is consistent with the hypothesis that pressure-induced openings in microvascular walls are dependent on an active response of the endothelium rather than being the result of stress failure of the basement membrane.

Different pathways of macromolecule extravasation from hyperpermeable tumor vessels

Tumor microvessels are hyperpermeable to plasma proteins, a consequence of tumor cell-secreted vascular permeability factor/vascular endothelial growth factor (VPF/VEGF). However, the pathways by which macromolecules extravasate from tumor vessels have been little investigated. To characterize tumor vessels more precisely and to elucidate the pathways by which macromolecules extravasated from them, we studied two well-defined, VPF/VEGF-secreting murine carcinomas, MOT and TA3/St. Whether grown in ascites or solid form, MOT tumors induced large, pericyte-poor "mother" vessels whose lining endothelium developed fenestrae that involved 1.8-5.6% of the surface. Fenestrae developed in parallel with markedly reduced endothelial cell vesiculo-vacuolar organelles (VVOs). TA3/St tumors, which secreted more VPF/VEGF than MOT tumors, elicited mother vessels with unchanged VVOs and without fenestrae. In both tumors, a plasma protein tracer, ferritin, extravasated through VVOs and in MOT tumors ferritin also extravasated through fenestrae. Endothelial gaps were not observed in either tumor. Thus, not all VPF/VEGF-secreting tumors induce fenestrated endothelium. Also, VVOs provide an internal store of membrane that can be transferred to the endothelial cell surface to provide the substantial increase in plasma membrane necessary for mother vessel formation in MOT tumors. Such transfer was apparently unnecessary in TA3/St tumors in which extensive early endothelial cell division provided the increased plasma membrane necessary for forming mother vessels.

[Structure of the rat lymphoid organs after prenatal exposure to indomethacin during antigenic stimulation]

Primary immune response of thymus and mesenteric lymph nodes to prenatal action of indomethacin was studied in one month rats. Indexes of cell reactions in the experimental rat thymus were found to coincide with control during all phases although blast transformation intensity was slightly reduced. In mesenteric lymph nodes primary immune response was suppressed which was connected with lymphocyte functional insufficiency. This is manifested through less intensive intercellular cooperation in the first phase, suppression of blast transformation in B lymphocytes in the second and decrease of functional activity of the node B zones in the third ones.

[Pathways of tissue fluid transport]

Mesenteries, cornea, lens and costal cartilages, nourished, as it is commonly believed, diffusely, were investigated by dye injection into vascular bed, and using luminescent and electron microscopy. The presence of regular nonvascular microcirculation pathways (interstitial canals) providing fluid passage from blood vessels into interstitium and consequently through the opened interendothelial contacts in the lymph capillaries into the lymphatic vessels was found when examining jejunal mesentery. Findings on the existence of extravasal paths of mobile tissue fluid were obtained after examination of avascular tissue of cartilage, cornea and lens as well. Their presence adds to reported data on lymph formation and allows to reconsider such unclear items as nonvascular organ nutrition, deficiency of primary lymph in prenodular lymph vessels and experimental remodelling of elephantiasis.

Type VI collagen is associated with microfibrils and oxytalan fibers in the extracellular matrix of periodontium, mesenterium and periosteum

Type VI collagen was immunolocalized in several soft connective tissues at the light and electron microscopic level. Positive labeling was found in all tissues examined, periodontal ligament, gingiva, mesenterium and periosteum. The labeled structures could be divided into 2 categories: microfibrils intermingling with collagen fibrils, and those that formed bundles (oxytalan fibres and elastin-associated microfibrils). Control sections incubated with antibody preabsorbed to purified type VI collagen, or with non-immune antibody, proved to be negative. Our observations indicate that the structural organization of type VI collagen varies from small microfibrillar structures associated with the collagen and elastin fibre systems to highly ordered parallel arrays of oxytalan bundles.

Pathology of experimental Ebola virus infection in African green monkeys. Involvement of fibroblastic reticular cells

BACKGROUND

Ebola virus has been responsible for explosive lethal outbreaks of hemorrhagic fever in both humans and nonhuman primates. Previous studies showed a predilection of Ebola virus for cells of the mononuclear phagocyte system and endothelial cells.

OBJECTIVE

To examine the distribution of lesions and Ebola virus antigen in the tissues of six adult male African green monkeys (Cercopithecus aethiops) that died 6 to 7 days after intraperitoneal inoculation of Ebola-Zaire (Mayinga) virus.

METHODS

Tissues were examined histologically, immunohistochemically, and ultrastructurally.

RESULTS

A major novel finding of this study was that fibroblastic reticular cells were immunohistochemically and ultrastructurally identified as targets of Ebola virus infection.

CONCLUSIONS

The role of Ebola virus-infected fibroblastic reticular cells in the pathogenesis of Ebola hemorrhagic fever warrants further investigation. This is especially important because of recent observations indicating that fibroblastic reticular cells, along with the reticular fibers they produce, maximize the efficiency of the immune response.

In situ detection of constitutive superoxide anion production in granules of mast cells

3,3'-Diaminobenzidine, in the presence of manganese and cobalt ions, was applied for the detection of superoxide anions in unfixed cryostat sections of rat oesophagus, trachea, skin and intact mesenterium. In all connective tissues, a blue final reaction product was found in a granular form in mast cells. The amount of final reaction product formed after incubation with diaminobenzidine and cobalt ions was increased by the addition of manganese ions. Electron microscopical analysis revealed that the electron-dense final reaction product was exclusively present in the granules of mast cells and on elastin fibres. It was found that the constitutive spontaneous formation of final reaction product in mast cells was enzymatic and dependent on the presence of oxygen in the medium. Of all the enzyme inhibitors and free radical scavengers tested, only azide strongly reduced the amount of final reaction product. It was concluded that the reaction was partly caused by peroxidase activity, but that superoxide anions are also constitutively and spontaneously produced in mast cell granules. The exact enzymatic source could not be established. Whether this property of mast cell granules plays an antimicrobial role in connective tissues can only be speculated.

Transcellular openings through frog microvascular endothelium

Reconstructions from serial ultrathin sections of microvascular endothelium suggest that gaps, which are induced by a range of stimuli, may pass through endothelial cells as well as between them. To address the possibility that the transcellular gaps are not artefacts of aldehyde fixation, we have reconstructed fourteen gaps, induced by the ionophore A23187, in frog mesenteric microvessels where the primary fixative was osmium tetroxide. All fourteen gaps were transcellular. The different actions of osmium tetroxide and glutaraldehyde lead us to consider that it is highly unlikely that transcellular gaps are fixation artefacts.

NPY- and CGRP-like immunoreactive nerve fibers in the testis and mesorchium of the toad (Bufo arenarum)

The presence and distribution of peptidergic nerve fibers were studied in the testis and mesorchium of the toad by means of immunohistochemistry. Cryostat sections of the testis and whole-mount preparations of mesorchia were immunostained with antisera to calcitonin gene-related peptide (CGRP) and neuropeptide tyrosine (NPY). After leaving the mesorchium CGRP-immunoreactive (IR) fibers were seen predominantly running in between the seminiferous tubules. In addition, a small population of CGRP-IR nerve fibers formed thin plexuses around blood vessels. Conversely, NPY-like immunoreactivity predominated in nerve fibers that formed dense plexuses around vessels both in the mesorchium and testis. Additionally, some single NPY-IR nerve fibers could be seen in both structures studied. The functional significance of these peptidergic systems in the testis of the toad remains to be analyzed.

Lymphatic drainage from the distal small intestine in sheep

Lymphatic drainage from the wall of the distal small intestine, important especially in young sheep as a major site of gut-associated lymphoid tissue, begins with a series of longitudinally oriented subserosal vessels. These vessels convey lymph to the mesenteric border of the intestinal wall and unite to form larger vessels which course through the mesentery to the mesenteric lymph nodes. These nodes lie towards the periphery of a broad, fan-shaped mesentery, adjacent to major arteries and veins. Mesenteric vessels convey lymph from the jejunum and part of the proximal ileum to the jejunal nodes. Lymph from most of the ileum is conveyed to caecal nodes. The larger mesenteric lymphatics have well formed smooth muscle and connective tissue layers surrounding the endothelium. They are often adjacent to, but rarely within, the connective tissue band anchoring the major arteries and veins to one or both lamellae of mesentery. Few anastomoses occur between vessels from opposing sides of the gut wall or the mesentery. Afferent lymphatics enter the subcapsular and trabecular sinuses of the nodes over most surfaces apart from the hilar region. Lymph flows through cortical tissue to the medulla, which occupies most of the node. In the medulla, sinuses occur within medullary cords as well as between them. Initial efferent lymphatics occur throughout medullary tissue. Efferent vessels emerge at a hilus then coalesce and drain into the jejunal or ileal trunk. The hilus of the node varies from a flat, poorly defined area on the lesser curvature, to a depression or groove. The latter commonly occurs in elongated jejunal nodes.(ABSTRACT TRUNCATED AT 250 WORDS)

A double-embedding technique for thin tissue membranes

A new double-embedding technique for thin tissue membranes is presented. This technique is useful for thin membranes such as mesenteric membranes from rodents, which usually measure only 10 microns in thickness. Several membranes are fixed and mounted on four needles located at the bottom of a plastic box. The box is filled with agarose at 50 C and then allowed to solidify. The agarose block is then removed, dehydrated in alcohol, cleared with HistoPetrol (isoparaffin hydrocarbons), permeated with paraffin and sectioned. The morphology is comparable to that obtained with methacrylate plastic embedding but is less time-consuming, less hazardous since no plastic hardener and activator are used and makes immunohistochemical studies easier.

Connective tissue repair in zinc deficiency. An ultrastructural morphometric study in perforated mesentery in rats

OBJECTIVE

To quantify measures of healing in zinc-deficient and healthy rats.

DESIGN

Randomized study.

MATERIAL

30 male Sprague-Dawley rats.

INTERVENTIONS

Zinc deficiency was induced in half the rats. All rats underwent laparotomy and standard perforations were made in the small intestinal mesentery with a scalpel. At 1, 3, 5, 7 and 10 days after operation 6 rats were killed by overdose of anaesthetic agents and the specimens of the mesentery were fixed.

MAIN OUTCOME MEASURES

Measurement of cellular volume density, surface density of the rough endoplasmic reticulum, and surface density of the plasma membrane.

RESULTS

Perforations started to close on day 4, and most were closed by day 10. Cellular volume density reached its peak between days 3 and 5, as did surface density of rough endoplasmic reticulum. There were no significant differences between the two groups for either measurement. The surface density of the rough endoplasmic reticulum, however, was significantly higher in controls than in zinc deficient animals on days 3-10 (p less than 0.001). The surface density of the plasma membrane was significantly higher in zinc-deficient animals on days 1-3 (p less than 0.04), and in control animals on days 5-10 (p less than 0.01).

CONCLUSIONS

Protein synthesis and formation of scar tissue were slightly lower in the zinc-deficient animals, and the higher plasma membrane surface density implies that contraction may be an important part of healing in the small intestinal mesentery in rats.

The endothelial vesicle system in cryofixed frog mesenteric capillaries analysed by ultrathin serial sectioning

Conventional EM sections of chemically fixed capillary endothelial cells reveal numerous apparently free smooth plasmalemmal vesicles. However, the method of ultrathin (less than 150 A) serial sectioning has shown that the smooth vesicle profiles arise merely as a result of the EM thin sectioning of two sets of complex vesicular invaginations from the luminal and abluminal cell surfaces, which end blindly in the cytoplasm. While 50-70% of the total population of vesicular profiles appear to lack connections to the cell surface in conventional (500-700 A thick) EM thin sections less than 1% truly free vesicles can be found by the ultrathin serial section analyses. In the present study it is examined whether similar conclusions apply to endothelial cells which were directly frozen by slam-freezing and subsequently freeze-substituted. The three-dimensional organization of the plasmalemmal vesicular system was analyzed in four series of 19, 18, 13, and 10 ultrathin sections (approximately 110 A thick) of capillaries from frog mesenteries quickly excised from decapitated frogs (Rana pipiens). None of 920 vesicular profiles (diameter 500-1,200 A) which appeared free in individual thin sections of the series represented free vesicles; all profiles either communicated with other vesicles, the cell surface, or in rare cases turned out to be part of cytoplasmic tubular membrane structures. It is concluded that free smooth plasmalemmal vesicles are very rare in rapidly frozen as well as in directly fixed frog capillary endothelium. The volume density of profiles (13-15%), the proportion of apparently free vesicle profiles (70%), and interconnected profiles (20%) were similar to the picture previously found in single EM sections of frog mesenteric capillaries. No transendothelial channels were found in the four series of ultrathin sections of capillaries. However, continuities between the luminal and abluminal cell surfaces were seen in the endothelium of venules. Furthermore, in the ultrathin series of the capillaries, vesicular units belonging to the two sets of invaginations and cytoplasmic tubular membrane structures were in more cases found in very close contact-as fused to share one unit membrane. If this finding is representative for the in vivo situation, it may reflect that the vesicular system represents a highly dynamic system with possibilities for mixing of membranes, cellular traffic of lipid, membrane proteins, and receptors between internal compartments and the cell surfaces, as well as occasional exchange of macromolecules between blood and tissue through rare temporary connections between the two sets of surface invaginations, without actually moving vesicles.

Light and electron microscopy of the structural organization of the tissue-lymphatic fluid drainage system in the mesentery: an experimental study

Supplementing vital microscopy and histophysiology, we examined using combined light and electron microscopy the tissue fluid-lymphatic drainage system of the mesentery isolated from guinea pigs, rabbits, and tree shrews. In silver impregnated tissue, different types of lymphatics and blood vessels were able to be distinguished along with argyrophilic and argyrophobic structures in the connective tissue. Some initial lymphatic pathways were interrupted by non-endothelialized tissue zones thus forming separate but discrete vascular "islands". After carbon labeling of the lymphatic collectors, carbon particles were seen to escape from the initial lymphatic lumen at various sites. Electron microscopy revealed wide apertures in the lymphatic endothelial cells of these microvessels. These morphological findings support the concept of an "open" prelymphatic-lymphatic system in the mesentery. The special histometrical features exhibited by a flat membranous organ like the mesentery are discussed in terms of physiologic function of mesenteric tissue fluid transport.

The ultrastructure of specialized contacts between small intensely fluorescent (SIF) cells in the inferior mesenteric ganglion of the guinea pig

The plasma membranes of small intensely fluorescent (SIF) cells in the inferior mesenteric ganglion of guinea pig have been investigated with thin section electron microscopy and freeze-fracture technique. Gap junctional elements and tight junctional strands were found between adjacent cells in the freeze-fractured material.

Distribution of extracutaneous melanin pigment in Sparus auratus, Mugil cephalus, and Dicertranchus labrax (Pisces, Teleostei)

The morphological and biochemical characteristics of pigment accumulations found in the kidney, liver, spleen, and mesentery of three different species of teleost fishes have been studied. There are significant differences in number, distribution, and morphology of pigment accumulations in different organs of the three species. Biochemical studies have shown the existence of tyrosinase activity in the mesentery of Mugil cephalus and in the kidney and mesentery of Sparus auratus. No tyrosinase activity was found in any internal organs of Dicertranchus labrax. That activity was assayed using three methods: tyrosine hidroxylation, dopa oxidation, and melanin formation. The morphological and biochemical observations are in agreement. In those organs in which we have demonstrated melanin synthetic activity, the pigment cells are morphologically and like melanophores, while in the organs that show no melanin synthetic activity, the pigment cells resemble macrophages.

Permeability of frog mesenteric capillaries after partial pronase digestion of the endothelial glycocalyx

1. The proteolytic enzyme pronase, which degrades the endothelial cell glycocalyx, was perfused through single capillaries of frog mesentery. Hydraulic conductivity (Lp) of each vessel was determined before and after pronase perfusion. In three vessels in which Lp increased, the ultrastructure of interendothelial clefts was examined. In a separate group of frogs the effect of pronase on the endothelial glycocalyx was assessed by using cationized ferritin to label the capillary luminal surface. 2. Control Lp was 2.0 x 10(-7) cm s-1 cmH2O-1 (10 mg ml-1 bovine serum albumin, BSA, in frog Ringer solution). Vessels were then perfused with a solution containing 0.1 mg ml-1 pronase and 10 mg ml-1 BSA for 1 min. Lp measured in the same eleven vessels increased to 4.9 x 10(-7) cm s-1 cmH2O-1 (P less than 0.005). 3. Transverse sections of three of these vessels were examined by transmission electron microscopy at eight sites along each vessel. In these sections a total of 156 interendothelial cell clefts were found and photographed. No morphological features, such as fenestrations, transendothelial channels, or intercellular gaps associated with inflammation, were found which might account for the increases in Lp. 4. Measurement of cleft dimensions yielded a harmonic mean cleft depth (delta x) of 0.32 microns and an arithmetic mean cleft depth of 0.64 microns. Mean width (w) of the clefts outside the tight regions was 0.012 microns and the cleft length per unit area (L) was 1330 cm-1. The mean fractional pore area of vessel wall per unit cleft depth, Ap/delta x, calculated as Lw/delta x, was 48.7 cm-1. 5. There was less cationic ferritin (CF) labelling of the luminal glycocalyx in pronase-perfused than in control capillaries. On average, the proportion of the luminal surface covered by CF was 85% in controls and 42% in pronase-treated capillaries (P less than 0.01). In some vessels the CF pattern was greatly disrupted, indicating large changes in the glycocalyx structure. 6. It is concluded that the moderate increases in Lp induced by pronase perfusion are associated with partial digestion of the endothelial glycocalyx but are not accompanied by changes in the dimensions of the intercellular cleft. These observations support the fibre matrix hypothesis of capillary permeability and suggest that the endothelial glycocalyx contributes as much as 60% of the hydraulic resistance of the capillary wall.

Comparison of mesenteric with antimesenteric crypt and villus populations in the mouse jejunal epithelium

Variation in the size and composition of crypts and villi along the length of the intestinal tract is well known. Here we investigate possible variation around the circumference of the intestine. This is a concern because most studies have ignored potential circumferential variation and its implications for experimental design in cell kinetic studies. We compared the crypt and villus populations of the mesenteric half with those of the antimesenteric half of proximal mouse jejunum. The branching crypt index and crypt and villus dimensions were measured. We found no evidence of differences in the branching crypt index, in the mean crypt and villus size, nor in the distribution of crypt and villus sizes between these two populations.

An ultrastructural study of dynorphin-immunoreactive nerve fibers and terminals in the celiac-superior mesenteric ganglion of the guinea pig

Dynorphin-immunoreactive nerve fibers and terminals were identified in the celiac-superior mesenteric ganglion of the guinea pig at the ultrastructural level with the peroxidase-antiperoxidase technique. The immunostained material was localized in the large dense core vesicles of the terminals but was also present diffusely in the axoplasm. The terminals formed numerous axodendritic and a few axosomatic contacts, interpreted as synapses, with the principal ganglion cells. These findings suggest that dynorphin plays a role as a neurotransmitter or neuromodulator in the ganglion and, taken together with earlier findings, indicate an involvement of dynorphin neurons in the intestino-intestinal inhibitory reflex.

Induction of peritoneal adhesions with small intestinal ischaemia and distention in the foal

Twenty-two foals were divided into groups of intestinal distension and intestinal ischaemia as methods to induce peritoneal adhesions. In the first group, the lumen of a segment of distal small intestine was occluded without extramural vascular compromise and distended with lactated Ringer's solution to a constant pressure of 25 cm H2O for 2 h within the abdomen. The ischaemic group underwent 70 mins total vascular occlusion of identical segments of bowel. Serosal biopsies were obtained before and after each experimental procedure and following 60 mins of reperfusion. Similar biopsies were harvested from a control group of foals with no bowel occlusions. The foals were destroyed 10 days after surgery and tissues collected for histological and ultrastructural evaluation. Experimental and control mesothelial surfaces were denuded histologically immediately after experimental occlusions. Serosal oedema and cellular infiltration were observed following reperfusion of the ischaemic segments but were present immediately after 2 h of distension. All foals had developed bowel-to-bowel and bowel-to-mesentery adhesions of the experimental segments. Control foals under 30 days old exhibited mesenteric contraction and thickening of the isolated segment whereas those older than 30 days had little or no mesenteric thickening or contraction. Histologically, in the experimental segments, fibrous tissue had formed on the outer boundary of the original serosa, and new mesothelial-like cells were present on the surface of fibrous tissue in some areas. Some serosal fibrosis was also seen in most of the control segments.

Idiopathic retractile (sclerosing) mesenteritis and its differential diagnosis

We report a case of retractile mesenteritis presenting as an abdominal mass with incomplete small-bowel obstruction. Histological features included fat necrosis, fibrosis, elastosis, dystrophic calcification, and chronic inflammation. Lymphatic obstruction resulted in the accumulation of lipid-laden macrophages in the ileal mucosa. Ultrastructurally, myofibroblasts were the principal cells present. The differential diagnosis of retractile mesenteritis is discussed with particular attention to myofibroblastic disorders such as inflammatory pseudotumors, desmoids, retroperitoneal fibrosis, and other uncommon conditions that appear to be morphologically or clinically distinguishable although the etiology and pathogenesis are obscure.

Loss of mesothelial electronegative fixed charges during murine septic peritonitis

Previous studies showed that murine septic peritonitis induced a substantial reduction of the anionic site density distribution in mesenteric and diaphragmatic microvessels. The present study shows that acute experimental septic peritonitis induces a severe reduction of the anionic site density distribution along the submesothelial basement membrane. Five days after induction of peritonitis, there was a partial recovery of anionic sites which even at 13 days was not completed. This observation suggests that the increased protein losses observed during peritonitis are the consequence of increased microvascular and mesothelial permeability to anionic plasma proteins secondary to neutralization and/or disappearance of the anionic sites located in the microvascular wall and in the mesothelial layer.

Phosphatidylcholine synthesis by peritoneal mesothelium: its implications for peritoneal dialysis

This study investigated the possibility that the peritoneum is capable of synthesizing phosphatidylcholine (PC), a lubricant surfactant, in an amount similar to that produced by pulmonary alveoli. The synthesis of PC by rat lung (positive control), liver (negative control), and transparent mesentery (test tissue) was determined by in vitro incubation of these tissues in the presence of (methyl-14C) choline chloride for three hours at 37 degrees C in Warburg flasks. All lipid material present in tissue and incubation media was extracted by the Folch technique. Carrier egg PC was added to each sample and total PC was isolated using high performance thin-layer chromatography. The PC fractions were counted for total radioactivity. No statistically significant difference was observed between the mean radioactive value for mesenteric tissue compared with lung tissue. The mean radioactive value for liver when compared with mesenteric and lung tissue was significantly lower (P less than 0.001). Thus, under the conditions of the experiment, we have demonstrated for the first time that peritoneal tissue is capable of synthesizing PC in amounts similar to that produced by the lung. Electron microscopy of transparent mesentery (test tissue) showed that mesothelium constituted the bulk of the cell population. Therefore, it is most likely that the PC that has been detected in peritoneal fluid is produced by the mesothelial cell. These findings are of significant relevance to developing concepts on the role of surfactant phospholipids in the physiology of peritoneal dialysis.

Ultrastructural study of 4 cases of Ki-1 positive large anaplastic cell malignant lymphoma

The ultrastructural morphology of 4 cases of large anaplastic cell malignant lymphoma (Ana ML) is reported. Three cases were primary Ana ML and one pleomorphic large T cell lymphoma with some Ki-1 positive cells. All were confirmed by immunohistochemistry on frozen and paraffin sections. The Ki-1 and EMA positive tumour cells had an abundant cytoplasm, with no differentiation and large pale nuclei with multiple compact or dispersed nucleoli. The morphology is that of an activated cell engaged in protein synthesis and/or in the mitotic cycle. These tumour cells resemble to the Hodgkin's and monolobated Reed-Sternberg cells described in Hodgkin's disease.

Specializations of plasma membranes in Pacinian corpuscles: implications for mechano-electric transduction

Pacinian corpuscles of cat mesentery were studied with freeze-fracture and thin sectioning methods after chemical fixation. Intramembranous particles (IMPs) exhibit differences in both density and pattern of distribution between the axolemma of the smooth short axis (x-axis) region and that of the axonal spine region of the long axis (y-axis) of the axon terminal. The axolemma of the x-axis has IMPs at a density of 2687 +/- 581 per micron2 (mean +/- S.E.M.), and these particles are 9.0 +/- 1.7 nm (mean +/- S.D.) in diameter. In contrast, the axolemma of the y-axis has a higher density of IMPs (3607 +/- 612 per micron2) which are larger (diameter, 10.0 +/- 1.7 nm). The particle distribution is not homogeneous in x-axis membranes as there are small patchy areas devoid of particles scattered throughout the entire surface. The E-face of the axolemma has a low density of IMPs (approximately 200 per micron2 in both x- and y-axes). However, IMPs in the E-face are smaller (approximately 9 nm) in the x-axis than in the y-axis (approximately 10 nm). The inner core lamellar cells have IMPs at a density of 3276 +/- 739 per micron2 and 553 +/- 169 per micron2 in the P- and E-faces, respectively. The particles are about 10 nm in diameter in both faces. Many gap junctions occur between lamellar cells especially near the clefts, suggesting that hemilamellae of each inner core half are kept at the same electrotonic potential. The outer core lamellar cells have IMPs at a density of 2239 +/- 403 per micron2 and 536 +/- 123 per micron2 in their P- and E-faces, respectively. The particles are approximately 10 nm in diameter in both faces. A noteworthy finding is that tight junctions are prominent at cell-to-cell appositions within individual lamellae, especially in the first and second (or sometimes third) innermost lamellae of the outer core. These tight junctions are considered to be a barrier to the leakage of fluid and/or ions between interlamellar spaces as well as between inner and outer cores. An intermediate cell layer is identified between the inner and outer cores. The connective tissue space of this cell layer corresponds to the endoneurium, indicating that intermediate layer cells are comparable to endoneurial fibroblasts.(ABSTRACT TRUNCATED AT 400 WORDS)

A morphologic and histochemical study of the mesentery in the guinea pig

The guinea pig mesentery is a uniform, continuous, thin (18 micron) sheet of connective tissue covered by a single layer of flattened mesothelial cells on both surfaces. Tight and gap junctions provide for cell-to-cell adhesion among mesothelial cells. These cells possess numerous micropinocytotic vesicles; a conspicuous basal lamina separates the mesothelium from the underlying connective tissue. Most of the material found between the two serous coverings consisted of a three-dimensional meshwork of abundant collagenous fibers intermingled with a sparse net of very thin (0.4 micron) elastic fibers. Two distinct populations of collagen fibrils are segregated into different compartments of the mesentery. One population is formed of thick (56 nm) fibrils which associate to form closely packed fibers. The second population, composed of loosely arranged thin (38 nm) fibrils which do not become assembled into fibers, is found underlying the basal lamina that separates the mesothelium from the connective tissue. These observations strongly suggest that the mesentery contains both collagens type I and type III. The guinea pig mesentery contains 6.8 mg of sulfated glycosaminoglycans/g dry weight. Most of these glycosaminoglycans (78%) were identified as dermatan sulfate, whilst the rest (22%) corresponded to heparan sulfate.

Morphology and fibrinolytic activity of canine autogenous mesothelium used as venous substitute

Autogenous mesothelium was used as venous substitute in ten dogs. Patches of mesothelium of three different origins were grafted into the anterior wall of the common iliac veins (CIV): peritoneum taken from and including the posterior rectus sheath (PRS), simple peritoneum (P) and mesentery (M). Animals were killed after 2, 4, 8, and 16 days and after 3 months. The segments of CIV, including the patches, were removed for study. On light microscopy, the PRS grafts showed a normal mesothelium but marked submesothelial fibrosis. The M and P grafts showed normal mesothelium and only mild fibrous thickening. On scanning electron microscopy, there was a perfect continuity of the mesothelial cells and the normal endothelium at the suture line. In the center of the graft, the cells had become elongated along the axis of blood flow. Fibrinolytic activity (FA) was measured by a standardized fibrin plate technique and quantitated in tissue activator units per gram of tissue (TAU/g). The mean FA of iliac vein specimens was 1101.7 +/- 133.3 TAU/g (mean +/- SEM). The mean FA determined before grafting for each kind of mesothelium was the following: PRS = 418.8 +/- 26.9 TAU/g; P = 873.0 +/- 107.1 TAU/g; M = 1142.3 +/- 91.4 TAU/g where only PRS showed values significantly lower than iliac vein mean FA (P less than 0.001). Postoperatively, the mesothelial FA, after an initial reduction, increased on day 4 and reached values significantly higher than the control values (1445.7 +/- 204.1 TAU/g tissue vs 853.1 +/- 62.3 TAU/g tissue; P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural evidence for contacts between migrating L5222 rat leukemia cells and extracellular matrix components of the rat mesentery

The nature of interactions between cells migrating through tissues and their structural surroundings are largely unknown. We have therefore examined the ultrastructural relationship between L5222 rat leukemia cells, moving through the loose connective tissue of the mesentery, and components of the extracellular matrix (ECM). Ultrathin tissue sections, fixed in the presence of ruthenium hexammine trichloride (RHT), revealed the following: Constitutents of fibrillar and nonfibrillar elements of the ECM are in contact with the plasma membrane of L5222 cells. Linear nonfibrillar ECM elements contact the plasma membrane at point-like sites, often associated with root-like structures present within the submembraneous microfilament mesh. Aggregates of ECM material are connected to patch-like cell membrane sites, associated with a condensed, plate-like part of the microfilament mesh. Point-like and patch-like contacts are more numerous at the anterior part of polarized migrating L5222 cells than on the posterior end. In round resting leukemia cells they are evenly distributed around the cell periphery. We suggest that the ECM-cell membrane contacts represent tissue adhesion sites. We therefore hypothesize that in migrating cells a coordinate interaction occurs between the contact sites and the continuous microfilament meshwork which results in a simultaneous backward movement of ECM-membrane contacts on the cell body and in a net forward movement of the whole cell. Since Dembo et al. (1981) present a similar mechanism for in vitro locomotion of granulocytes, we assume that blood cell locomotion in vivo and in vitro depends on similar molecular mechanisms: force generation by the cell, transmembraneous linkage between cytoskeletal and ECM elements, and membrane fluidity. The major difference in blood cell locomotion through a three-dimensional tissue or on a plane substratum would then be given by the distribution of contact sites, occurring around the cell periphery or limited to the ventral cell surface, respectively.

Tumor-associated desmoplasia in the rabbit mesentery characterized by morphological, biochemical and cytophotometric methods

Intraperitoneal implantation of V2 carcinoma cells in the rabbit leads to invasion of the mesentery and to structural tissue alterations which are concomitantly of a destructive and a desmoplastic type. In this report, we describe the desmoplastic changes which are characterized by the increased formation of collagen and of proteoglycans resulting in an increased thickness of the membrane. Biochemical data indicate that the total amount of collagen increases with time after implantation, whereas the relative amount per unit of dry weight, as well as the contributions of type I (15-25%) and type III (6-8%), stay within the same range. The increased synthesis of extracellular matrix is accompanied by a change in the appearance of the fibroblasts which now show the morphologic features of synthesizing cells. Also, an appreciable number have entered the S-phase. We propose that the desmoplastic changes are tumor-associated, since implantation of epithelial cells from normal rabbit liver does not result in similar alteration. Our findings are discussed in view of the role played by tumor and/or host cells in the increased production of extracellular matrix, of possible factor(s) elaborated by the tumor cells, and of the general significance of desmoplastia for tumor spread.

[Electron microscopic characteristics of plant peroxidase transport pathways in the microcirculatory bed of the rabbit peritoneum]

The dynamics of exogenic peroxidase transfer from blood into the roots of the rabbit mesenteric lymphatic system have been studied by means of electron microscopic methods in combination with the trasser technique. Light optic identification of the vascular segments and selection of samples for electron microscopic analysis make it possible to reveal certain differences in the pathways of protein transport via the walls of the blood capillaries and venules. The vesicular transport is the only means for peroxidase to be transferred via the walls of the mesenteric blood capillaries. The time for transendothelial transfer of the marker is more than 10 min. In the venules the vesicular transport of protein does not differ from that in the capillaries, however, the predominant leakage of peroxidase from blood into the interstitium is performed through open interendothelial contacts. The hemato-interstitial transport via the intercellular clefts takes less than 3 min. For transferring protein from the interstitium into the lumen of the lymphatic capillaries and postcapillaries, the vesicular mechanism is used, and to a less extent--the open intercellular contacts. A suggestion is made that the term "open contact" should be understood in functional meaning and this means should be considered as an intercellular pathway for transporting molecules of a definite size.

Morphology of peritumoral proteoglycan alterations in the rabbit mesentery invaded by V2 carcinoma cells

After intraperitoneal implantation into Swiss Silver rabbits, V2 rabbit carcinoma cells invade the mesentery where they form nodules of different size and texture: compact (less than 120 microns in diameter), loose (120-250 microns) and mixed (above 200 microns). Together with tumor development, certain changes take place in the loose connective tissue of the mesentery. Application of TEM, together with use of safranin O, has shown that, in areas free of tumor growth, collagen bundles become thick and heavy and proteoglycan density is increased. Concurrently, the number of fibrocytes, now transformed to fibroblasts, increases. Small, compact nodules are surrounded by a concentrically arranged extracellular matrix. Its overall density is similar to that of nodule-free areas. In the immediate vicinity of large, loose nodules, all constituents of the extracellular matrix disappear. Adjacent connective tissue is partly destroyed but still contains collagen fibers and proteoglycans. These findings suggest the following: The presence of V2 carcinoma cells induces marked alterations in the structured and non-structured components of the extracellular matrix. These changes are, at the same time, progressive and regressive and the occurrence of one or the other depends on local tumor progression. Progressive alterations may result from an increased activity of fibroblasts. Since degradative effects, on the other hand, are only seen in the immediate vicinity of larger tumor aggregates, it is assumed that a minimal number of tumor cells is essential for destruction of extracellular matrix.

Blood-nerve barrier in the Vater-Pacini corpuscle of cat mesentery

Correlated thin-section, freeze-fracture and tracer examinations were used to examine the blood-nerve barrier of the Vater-Pacini corpuscles in cat mesentery. A laminar inner core and a multilayered outer core enfolded the terminal nerve fiber of the corpuscle. The lamellar cells of both cores were characterized by numerous vesicular membrane invaginations. Freeze-fracture images and tracer experiments employing lanthanum nitrate proved that these invaginations are static structures mediating in neither active pinocytosis nor the transcellular transport of metabolites. In both inner and outer cores, lamellar cells were connected to one another by tight junctions of either the zonula or the fascia type, that occurred between lamellar-cell processes within the lamella and between the cells of adjacent lamellae. Intravascularly applied lanthanum lay at the outermost regions of the corpuscles without entering their internal zones, apparently because lamellar-cell tight junctions hindered further penetration. The results of our investigations suggest strongly that the Vater-Pacini corpuscle lamellae enfolding the nerve terminal form an effective diffusion barrier against the permeation of tissue fluids, thus preserving the corpuscle internal circumference.

Cellular and extracellular changes following mast-cell secretion in avascular rat mesentery. An electron-microscopic study

Mast cell (MC) secretion induces local cell proliferation lasting 48-72 h in fibroblasts and mesothelial cells in the almost avascular true mesentery of the rat. We studied this membranous tissue by transmission electron microscopy with regard to cellular and extracellular features occurring during the first 72 h following MC secretion. After MC secretion elicited by compound 48/80, apparently all individual tissue-bound cells (i.e. fibroblasts, mesothelial cells, and macrophages) show signs of accelerated metabolic activity. In fibroblasts, conspicuous increases in the volume of Golgi apparatus and rough endoplasmic reticulum and in the amount of plasmalemmal indentations suggest an increased production and secretion of the extracellular matrix. Released MC granules lying close to projections of nearby phagocytosing cells cause areas free from electron-dense material in the extracellular matrix. MC secretion therefore appears to produce a remodelling of extracellular matrix. Most of the activities initiated by MC secretion start to subside within (48-)72 h. The findings indicate a close functional relationship between the tissue MC and all its neighbouring cells and the surrounding extracellular matrix. The striking chain of events that it induces emphasizes strongly that the secreting MC plays a prominent although as yet in many respects enigmatic role in normal tissue.

Morphological studies on V2 carcinoma invasion and tumor-associated connective tissue changes in the rabbit mesentery

The mesentery is a duplicature of the peritoneum consisting of loose connective tissue and covered on both sides by mesothelium. Rabbit V2 carcinoma cells implanted i.p. adhere to the mesenteric surface between contracted mesothelial cells. While invasion from these sites sets in, progressive changes of the connective tissue, reflecting fibroblast stimulation, become apparent and comprise multiplication of connective tissue cells, transformation of fibrocytes into fibroblasts, and enhanced production of fibrillar and nonfibrillar constituents of the extracellular matrix. Tumor invasion into this increasingly dense tissue proceeds in 2 ways. (a) Single cells penetrate into and locomote within the interior, where they divide and give rise to nodules which become surrounded by zones of tissue destruction. (b) Proliferation of surface-attached tumor cells results in the formation of nodules which, preceded by zones of tissue damage, extend into the interior. While evidence for lytic effects in the microenvironment of single tumor cells is lacking, degradation of the fibrillar extracellular matrix is regularly found around tumor nodules and indicates a collective lytic action achieved by tumor cells and, possibly, host cells. These morphological findings are discussed in relation to published bio- and histochemical data on spread of the V2 carcinoma.