Presence Of Contacts Research Articles

New data on the formation of local variations in the Upper Paleolithic of the Caucasus

As of today, very few Upper Paleolithic sites are known on both the southern and the northern slopes of the Caucasus. Their materials allow tracing settlement dynamics in the region from 40/39 to 20 cal ka BP. The change of the research methodology, which today is focused on thorough excavations involving a range of natural science disciplines for complex investigation of the materials, including complete water sieving of the cultural de-posits, enables obtaining many new, often unique data on human occupation, life-support strategies, and adapta-tions of humans in different periods of the Upper Paleolithic in the Caucasus. In our paper, a wide range of issues is discussed, concerned with the cultural affinity, diversity, and interaction of the Upper Paleolithic population of the Caucasus. The earliest Upper Paleolithic industries (40–35 cal ka BP) of the southern as well as the northern slopes of the Caucasus show the closest affinity. Studies of the raw-material strategies indicate the presence of contacts between populations of the northwestern and South Caucasus throughout the entire Upper Paleolithic and the development of extensive social networks. During the Late Upper Paleolithic period, the materials of the North and South Caucasus acquire certain features of distinctiveness, as in the stone industry and in bone tools and decorations. The appearance of geometric microliths in the South Caucasus contemporaneously with the Near East attests to the contacts between these regions. The delivery of seashells from the Caspian Sea coast to the Lesser Caucasus also indicates the southeastern direction of the contacts. The import of seashells from the Black Sea coast to the northwestern Caucasus indicates the southwestern direction of contacts. The unique finds that have analogies in the Upper Paleolithic of the Russian Plain indicate contacts between populations of the northwestern Caucasus and the Russian Plain that belong to different cultural areas. The current data demon-strate formation of local differences in the culture of the Upper Paleolithic populations in both the South and the North Caucasus, including under the influence of the neighboring regions.

Humoral immunity to SARS-CoV-2 among the population of Murmansk region amid COVID-19 epidemic

Purpose of the study. Determination of the level and structure of population immunity to SARS-CoV-2 among the population of the Murmansk region against the background of the incidence of COVID-19.Materials and methods. The study was carried out according to a unified method for determining the seroprevalence of the population, developed by Rospotrebnadzor with the participation of the St. Petersburg Pasteur Institute. A total of 3117 volunteers were examined, distributed into 7 age groups. The content of antibodies to SARS-CoV-2 was determined by ELISA using a set of reagents for analyzing human serum or plasma for the presence of specific immunoglobulins of class G to the nucleocapsid of the SARS-CoV-2 virus produced by the FBUN GNCPMiB Rospotrebnadzor (Obolensk) in accordance with the instructions for application.Results. The level of population immunity in the Murmansk region as a whole was 31,2%. The highest seroprevalence was found among children aged 1–6 and 7–13 years (35.6% and 44,1%, respectively), the lowest among the elderly population aged 60–69, as well as 70 and more years (20,4% and 20,9%, respectively). In the cities of the Murmansk region. the herd level of immunity varied from 19,6% (Kola city) to 46,1% (Kandalaksha city). It was not possible to find a reliable relationship between the incidence of the population of the cities of the Murmansk region and seroprevalence. The highest level of seroprevalence was noted among office workers (38,0%), the lowest among transport workers (19,5%). In the presence of contacts with patients with COVID-19, seropositivity increased 1,4 times compared to the average population value. The level of specific humoral immunity in convalescents after COVID-19 is 64,1%, which is 2,2 times higher than the average for half-day. The proportion of asymptomatic forms among seroprevalence volunteers was 89%.

Impact of the fracture contact area on macro-dispersion in single rough fractures

In the scientific literature, the study of the impact of the fracture contact area on macro-dispersion in single rough fractures is still an open question. In this work, we study numerically the combined effects of the fracture roughness and the fracture contact area on the non-Fickian transport in single rough fractures. In particular, we quantify the contribution of the fracture contact area on macro-dispersion. These objectives are achieved by estimating the macro-dispersion coefficient from Monte Carlo parallel numerical simulations in pure advection and advection–diffusion cases. When the fractional void S O is equal to 1 (i.e., for σ lnb <0.25), the Monte Carlo simulations show that macro-dispersion results of two contributions, dispersion caused by the heterogeneity of fracture apertures that induces a channelization of flow paths and molecular diffusion, as shown by the analytical solution proposed by Gelhar in 1993. When the fraction void S O is different from 1 (i.e., for σ lnb >0.25), a third mechanism plays in macro-dispersion. The presence of contacts or obstacles causes a disruption of flow paths. This mechanism is identical to that induced by the fracture roughness with a lower amplitude. Its amplitude is the function of the fractional void S O .

Структура серопревалентности к вирусу SARS-CoV-2 среди жителей Московской области в период эпидемической заболеваемости COVID-19

A pandemic of the disease caused by the SARS-CoV-2 virus (hereinafter referred to as COVID-19) was announced by WHO in February 2020. In the Moscow Region (MR), the first case of the disease was detected on March 7, 2020 in a woman who arrived from Switzerland. Four weeks later, a rapid rise began, the peak of which fell on the 20th week (May 11, 2020 – May 17, 2020), which was replaced by a gradual steady decline that lasted for 13 weeks. Objective. To determine the level and structure of population immunity to the SARS-CoV-2 virus among the population of the Moscow Region between June 22, 2020 and July 11, 2020. Patients and methods. A serological study of seroprevalence among residents of MR to COVID-19 was carried out from June 22, 2020 till July 11, 2020 during the period of a steady decline in the incidence. The work was conducted as part of the first stage of the large-scale Rospotrebnadzor program to assess population immunity to the COVID-19 virus among the population of the Russian Federation. Volunteers for the study were selected by the method of questioning and randomization. The analysis included the results of a survey of 2688 people. The number of volunteers in all age groups was equal. The content of specific IgG to the COVID-19 nucleocapsid was determined by the enzyme-linked immunosorbent assay using a test system manufactured by the FBIS SRCAMB according to the manufacturer's instructions. Results. The results of the study showed that the collective immunity of the total population of the Moscow Region was 21.0%. The maximum level of population immunity was established in children aged 14–17 years (30.7%) and 1–6 years old (25.2%). There were no statistically significant differences in the level of seroprevalence between men and women. As a result of seroepidemiological research, it was shown that in the medical center, in the presence of contacts with patients with COVID-19, the risk of infection increases by 2.5 times. After infection with COVID-19, antibodies were detected in 78.7% of cases. In persons with a positive result of the PCR analysis obtained earlier, antibodies were detected in 82.8% of cases. A high proportion of asymptomatic infection among seropositive volunteers was found, which amounted to 83.4%. Key words: coronaviruses, SARS-CoV-2, incidence, seroprevalence, the Moscow Region, population, COVID-19

Коллективный иммунитет к SARS-CoV-2 жителей Москвы в эпидемический период COVID-19

The coronavirus disease pandemic (COVID-19) was announced by WHO in February 2020. In Moscow, the first case of the disease was detected on March 2, 2020 in a man who arrived from Italy. Two weeks after the first case, there was an exponential increase in the number of cases. The incidence peaked in the 19th week of the year, followed by a steady decline, lasting 16 weeks. Objective. To investigate the level and structure of population immunity to the SARS-CoV-2 virus among the population of Moscow against the background of the incidence of COVID-19. Patients and methods. The study of population immunity of Moscow residents to SARS-CoV-2 was carried out in the period from July 1, 2020 to July 30, 2020 against the background of stabilization of the incidence rate. The work was conducted as part of the first stage of the Rospotrebnadzor project to assess population immunity to the SARS-CoV-2 virus among the population of the Russian Federation, taking into account the protocol recommended by WHO. Volunteers for the study were selected by the method of questioning and randomization. The results of the survey of 2688 people were included in the analysis. The number of volunteers in all age groups was 384–385 people. The content of specific IgG to the SARS-CoV-2 nucleocapsid was determined by the enzyme-linked immunosorbent assya using a kit produced by FBIS SRCAMB according to the manufacturer's instructions. Results. During the survey of volunteers it was found that the share of seroprevalent residents of Moscow was 22.1%. By age groups, the highest proportion of seroprevalent was found among children aged 14–17 years (44.6%), the lowest (15.9%) – among people aged 18–29 years. Seroprevalence had no gender differences. The lowest level of herd immunity was revealed in the North-Western Administrative District (16.8%), the highest – in the South-Eastern Administrative District, as well as in the Troitsky and Novomoskovsky Administrative Districts of Moscow (in both, 24.1%). The smallest number of seropositive persons was among cultural workers (5.8%), the largest – among medical workers (27.0%). In the presence of contacts with patients with COVID-19, the probability of seroconversion increases by 1.5 times, and among convalescing persons COVID-19 antibodies were detected in 60.0% of cases. The proportion of people with asymptomatic infection among seropositive residents was 82.4%. Conclusion. The results obtained make it possible to characterize the population immunity of Moscow residents and are essential for the planning and implementation of anti-epidemic measures during the incidence of COVID-19. Key words: coronaviruses, SARS-CoV-2, seroprevalence, population, COVID-19

A Two-Stage Trajectory Optimization Strategy for Articulated Bodies With Unscheduled Contact Sequences

In this letter, we propose a two-stage strategy for optimal control problems of robotic mechanical systems that proves to be more robust, and yet more efficient, than straightforward solution strategies. Specifically, we focus on a simplified humanoid model, represented as a two-dimensional articulated serial chain of rigid bodies, in the tasks of getting up (sitting down) from (to) the supine and prone postures. Interactions with the environment are integral parts of these motions, and a priori unscheduled contact sequences are discovered by the solver itself, opportunistically making or breaking contacts with the ground through feet, knees, hips, elbows, and hands. The present investigation analyzes the effects on the computational performance of: 1) the explicit introduction of contact forces among the optimization variables, 2) the substitution of undesired contact forces with geometric constraints that prevent interpenetrations, and 3) the splitting of the planning problem into two consecutive phases of increasing complexity. To the best of our knowledge, these tests represent the only quantitative analysis of the performances achievable with different solution strategies for optimization-based, whole-body dynamic motion planning in the presence of contacts.

Dynamics of mechanical systems with two sliding contacts: new facets of Painlevé’s paradox

We investigate the dynamics of finite degree-of-freedom, planar mechanical systems with multiple sliding, unilateral frictional point contacts. A complete classification of systems with 2 sliding contacts is given. The contact-mode based approach of rigid body mechanics is combined with linear stability analysis using a compliant contact model to determine the feasibility and the stability of every possible contact mode in each class. Special forms of non-stationary contact dynamics including "impact without collision" and "reverse chattering" are also investigated. Many types of solution inconsistency and the indeterminacy are identified and new phenomena related to Painlev\'e"s non-existence and non-uniqueness paradoxes are discovered. Among others, we show that the non-existence paradox is not fully resolvable by considering impulsive contact forces. These results contribute to a growing body of evidence that rigid body mechanics cannot be developed into a complete and self-consistent theory in the presence of contacts and friction.

Comparative analysis of DNA sequences of regions of X-chromosome attachment to the nuclear envelope of nurse cells Anopheles messeae Fall.

Polytene chromosomes of ovarian nurse cells of Anopheles mosquitoes form strong contacts with the nuclear envelope. The presence of contacts, their position at nurse cell chromosomes, and their morphological features are species-specific in malaria mosquitoes. It is important to determine the nature of these interspecies differences in the nuclear architecture, both to understand the function of the nucleus and to assess the role of the spatial organization of chromosomes in evolution. Using dot-blot hybridization, we compared DNA sequences of the clone library from the X-chromosome attachment region to the nuclear envelope of ovarian nurse cells of Anopheles messeae with DNA-probes: (1) of the X-chromosome attachment region of An. atroparvus, (2) of the 3R chromosome attachment region ofAn. messeae, and (3) of the chromosome 2 pericentromeric region of An. messeae, without expressed contacts with the nuclear envelope. It has been shown that the chromosome attachment regions have a significantly higher number of homologous DNA sequences as compared with the pericentromeric region of chromosome 2. Sequences that are common for attachment regions are largely potentially able to participate in the formation of chromatin loop domains and to interact with some nucleus frameworks, according to the analysis in the ChrClass program. The obtained results support the important role of DNA in the formation of strong chromosomal attachments to the nuclear envelope in nurse cells of Anopheles mosquitoes.

On the importance of optical contacts in gold hybrid structures for enhancement of localized surface plasmon resonance sensing

Gold/polymer hybrid structures consisting of gold-coated dielectric particles resting on a thin gold mirror layer are investigated in view of their optical sensitivity toward molecular adsorption. Three different methods, i.e., metal evaporation, sputtering, and wet chemistry, are used to prepare metal coatings of different roughness and different extent to which the dielectric cores are engulfed by the metal. Further, the presence of contacts between the dielectric cores is varied. As a well-understood organic monolayer system, the sensitivity of the different hybrid structures toward adsorption of octadecanethiol is studied from ethanolic solution. Thereby, it is found that besides surface roughness, core–core contacts are important for the layers’ optical response, which is governed by surface plasmon effects. The system prepared by means of wet chemistry that allows for core–core contacts achieves an about 4 fold higher sensitivity toward molecular adsorption as compared with that prepared by evaporation.

MEDICAL AND SOCIAL CHARACTERISTICS OF PENSIONERS LIVING AT HOME AND IN THE BOARDING HOUSE

The article presents a comparative medico-social characteristics of pensioners living at home and in the boarding house. The analysis of age structure, education level, place of work and specialty until retirement, the presence of contacts with occupational hazards was carried out. We studied the living conditions, financial status, the presence of disability, the reasons for staying at a boarding house, an assessment of relations with relatives.

Fundamental Rules to Construct Highly Integrated Organic Nanowires as Nanodevices

Recent advances in inorganic nanowires techniques have led to more efficient and reliable electronic nanodevices as bipolar transistors, field effect transistors, junctions, logic-gate structures and applications from barcodes to LEDs. Otherwise, a true organic nanowire (ONW) device has not been realized. In this paper we describe few rules that will be useful to construct ONWs as well as the influence of mettalic contacts. Based on an equilibrium technique, we present electronic transport simulation for a well-known one-dimensional polymer prototype attached with donor/acceptor groups. Our results are consistent with: (i) asymmetric electronic transport (Charge Accumulation/Depletion-Voltage) curve for the ONW with and without the presence of contacts; (ii) for donor-polyacetylene-acceptor we find out assymetric nonresonance tunneling type conduction in the I–V curve; (iii) the aluminum atoms show to be a good candidate as mettalic contact when bonded with Sulfur atoms provoking rectification for lower values when compared with the system without Aluminum; (iv) For forward and reverse bias the resonance takes place suggesting in operational limit working as non-symetric bi-directional molecular field effect transistor (FET).

Influence of the contacts on the conductance of interacting quantum wires

We investigate how the conductance G through a clean interacting quantum wire is affected by the presence of contacts and noninteracting leads. The contacts are defined by a vanishing two-particle interaction to the left and a finite repulsive interaction to the right or vice versa. No additional single-particle scattering terms (impurities) are added. We first use bosonization and the local Luttinger liquid picture and show that within this approach G is determined by the properties of the leads regardless of the details of the spatial variation of the Luttinger liquid parameters. This generalizes earlier results obtained for step-like variations. In particular, no single-particle backscattering is generated at the contacts. We then study a microscopic model applying the functional renormalization group and show that the spatial variation of the interaction produces single-particle backscattering, which in turn leads to a reduced conductance. We investigate how the smoothness of the contacts affects G and show that for decreasing energy scale its deviation from the unitary limit follows a power law with the same exponent as obtained for a system with a weak single-particle impurity placed in the contact region of the interacting wire and the leads.

Role of wide contacts in the ac response of a double-barrier structure

The role of wide contacts is investigated in the ac response of a double-barrier structure. Several physical quantities, such as the distributions of internal potential and charge density, capacitance, and low-frequency ac conductance, are calculated. The results show that when the system is far away from resonance, the charge distribution exists only around the barrier regions as a response to the applied voltage, and hence the contacts have almost no effect on the results. In the resonant case or near resonance, the charge distribution displays large peaks in the contact regions, but almost no charge distribution within the conductor. In the case of small transmission probability, we find a considerable amount of charge distribution both in the contact regions and surrounding the double-barrier conductor. Moreover, we find that qualitatively the presence of contacts does not change the main features of the emittance obtained from a calculation that neglects the widening of contacts. But the contact effect on the capacitance is significant when the chemical potential is very close to the resonant energy: There is a sharp capacitance peak at the resonance, which corresponds to the charge peaks in the contact regions.

Cross-Linking of Escherichia coli Formamidopyrymidine-DNA Glycosylase to DNA Duplexes Containing Photoactivatable Phenyl(Trifluoromethyl)diazirine Groups

New reactive analogs of substrates for DNA repair enzyme E. coli Fpg protein containing the residues of 8-oxoguanine and photoactivatable phenyl(trifluoromethyl)diazirine groups were synthesized. Their substrate properties were investigated. Using photocross-linking technique, we established the presence of contacts of two nucleosides located near the oxoG with amino acids from the Fpg protein. The cross-linking efficiency achieved 10%.

Tracing tools to resolve neural circuits

In this paper we summarize neuroanatomical tracing methods, in particular combinations of methods designed to achieve the combined goals of tracing connectivity and extracting extra information from the projection or the target neurons. These combinations include techniques that identify projection fibres together with the neurons from which they originate, methods which establish the morphological or chemical identity of the target neurons and techniques to verify the presence of contacts between the terminal boutons on fibres and prospective target neurons. In the second part of this paper we describe a method recently developed in our laboratory, which consists of fluorescence tracing (introducing marker #1) combined with immunofluorescence with different fluorochromes (markers #2 and #3). This method enables us to observe the distribution of terminals of a particular set of projection fibres in contact with neurons belonging to two chemically different populations of interneuron. Multifluorescence confocal laser scanning is used for image acquisition of fibres and processes of presumed target neurons, and follow-up is by three-dimensional computer reconstruction. These reconstructions are used to determine that the contacts between the differentially labelled structures are real, i.e. with no optically empty space in between regardless of the angle of inspection. The specific experiments reported in this paper comprise the tracing with biotinylated dextran amine of fibres in the rat brain running from the presubiculum to layer III of the medial division of the entorhinal cortex. We studied the possibility of contacts in the latter area between the terminal boutons of these projecting fibres in association with presumed target neurons, i.e. parvalbumin and calretinin expressing interneurons.

Separation of Fibres from the Capsule Enhances Mitotic Activity of Human Lens Epithelium

Proliferation and migration of lens epithelial cells is at the origin of after-cataract, currently the main side effect of cataract surgery. Extracapsular cataract surgery necessitates opening of the anterior lens capsule and removal of the lens fibres before an intraocular lens can be placed in the cleaned capsular bag. The present study was aimed at testing the potential of the lens capsule and the fibres to affect the mitotic activity of the lens epithelial cells in human donor eyes. Pairs of human lenses used in this study were subjected to two different experimental conditions. In the first, only a small anterior capsulorhexis was made. In the other, fibres were separated from the epithelium by gentle hydrodissection, the nucleus was rotated and, either left in the capsular bag or removed by hydroexpression. The specimens were cultured for 48 hr in MEM with 2% serum. Mitotic activity was assessed by immunohistochemistry using the bromodeoxyuridine (BrdU) incorporation technique. Fellow lenses of the same donors were cultured intact as controls. Anterior capsulorhexis showed a small but significant (P<0.02) increase in the number of BrdU positive nuclei in the equatorial region but not in the wound area. Lenses where fibres were separated from the epithelium showed a large increase in BrdU incorporation in the germinative region, as compared to the intact control lenses (P<0.01). BrdU incorporation was highest when fibres were removed. Lens capsule integrity and the presence of contacts between fibres and epithelial cells may have a role in the control of the mitotic activity of the lens epithelium. The data recorded here could be useful for understanding lens cell biology and prevention of after-cataract.

Opioid regulation of proopiomelanocortin (POMC) gene expression in the rat brain as studied by in situ hybridization

Proopiomelanocortin (POMC) is the precursor of the potent opioid peptide β-endorphin as well as a number of other active peptides. On the basis of neuroanatomical data indicating the presence of contacts between POMC neurons in the rat arcuate nucleus, 3,7 it has been proposed that POMC neurons could be autoregulated. In order to investigate the role of opiates in the regulation of POMC gene expression in the rat arcuate nucleus, we studied the effects of chronic administration of the opioid drug morphine and an opiate receptor antagonist naloxone on POMC mRNA levels as measured by in situ hybridization. 4-day treatment with naloxone (4 mg/kg/day) produced a 60% increase in the number of silver grains overlying POMC neurons. Conversely, morphine (40 mg/kg/day) also administered during 4 days decreased the hybridization signal by 30%. The concomitant administration of morphine and naloxone completely prevented the effect of morphine on POMC gene expression indicating that the inhibitory influence of morphine is likely to be mediated by opioid receptors. The data obtained clearly indicate that activation of opioid receptors decreased the biosynthetic activity of POMC neurons and that conversely opiate receptor blockade caused an increase in the activity of these neurons. They are consistent with the hypothesis of an autoregulation of the POMC neuronal system by endogenous opiate peptides(s).

Ultrastructural single‐ and double‐label immunohistochemical studies of substance P‐containing terminals and dopaminergic neurons in the substantia nigra in pigeons

The vast majority of striatonigral projection neurons in pigeons contain substance P (SP), and the vast majority of SP-containing fibers terminating in the substantia nigra arise from neurons in the striatum. To help clarify the role of striatonigral projection neurons, we conducted electron microscopic single- and double-label immunohistochemical studies of SP+ terminals and/or dopaminergic neurons (labeled with either anti-dopamine, DA, or anti-tyrosine hydroxylase, TH) in pigeons to determine: (1) the synaptic organization of SP+ terminals, (2) the synaptic organization of TH+ perikarya and/or dendrites, and (3) the synaptic relationship between SP+ terminals and TH+ neurons in the substantia nigra. Tissue single-labeled for SP revealed numerous SP+ terminals contacting thin unlabeled dendrites in the substantia nigra, but few SP+ terminals were observed contacting perikarya or large-diameter dendrites. SP+ terminals contained round, densely packed, clear vesicles, and often contained one or more dense-core vesicles. Synaptic junctions between SP+ terminals and their targets were more often symmetric (86%) than asymmetric. In tissue single-labeled for DA, we observed few terminals contacting DA+ perikarya, whereas terminals contacting DA+ dendrites were more abundant. Terminals contacting DA+ structures comprised at least four different morphologically distinct types based on the morphology of the clear synaptic vesicles and the type of synaptic junction. One type of terminal contained round clear vesicles and made symmetric synapses, and thus resembled the predominant type of SP+ terminal. The second type contained round clear vesicles and made asymmetric synapses, the third type contained medium-size pleomorphic clear vesicles and made symmetric synapses, and the fourth type contained small pleomorphic clear vesicles and made symmetric synapses. The presence of contacts between SP+ terminals and dopaminergic dendrites in the substantia nigra was directly demonstrated in tissue double-labeled for SP (by the peroxidase-antiperoxidase procedure, or PAP, with diaminobenzidine) and TH (by either the silver-intensified immunogold procedure or the PAP procedure with benzidine dihydrochloride). SP+ terminals commonly contacted thin TH+ dendrites in the substantia nigra, but few SP+ terminals contacted large-diameter TH+ dendrites or perikarya. Synapses between SP+ terminals and TH+ neurons were always symmetric. TH+ dendrites also were contacted by terminals not labeled for SP, which were more abundant than were SP+ terminals. Non-TH+ neurons were also contacted by both SP+ terminals and non-SP+ terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron microscopic study of cardiac ganglia in human fetuses

The early events in the development of the heart ganglia and nerves in human fetuses ranging from 5 to 12 weeks of gestation age were studied by transmission electron microscopy. The first neuroblasts in the atrial mesenchyme differ from surrounding cells in the presence of short cisternae of rough endoplasmic reticulum and in the absence of glycogen particles. The most valuable criterion for identification of neuroblasts is the presence of contacts with preganglionic nerve terminals. Only ganglia composed of compact aggregations of neuronal cells and nerve terminals have a complete glial sheath. The first signs of synapse formation were seen in 5-week-old fetuses; well developed synapses with many synaptic vesicles were found from the 8th week of gestation onwards. These were predominantly axodendritic synapses. It is proposed that synaptogenesis begins with the appearance of osmiophilic zones at the sites of interneuronal contacts, then synaptic vesicles move by axonal transport to reach the preformed specialized junctions and stop in axonal presynaptic varicosities.

Fine structure of gels prepared from an actin-binding protein and actin: comparison to cytoplasmic extracts and cortical cytoplasm in amoeboid cells of cortical cytoplasm in amoeboid cells of Dictyostelium discoideum.

We have identified the three-dimensional ultrastructure of actin gels that are formed in well-characterized cell extracts and mixtures of purified actin and the 120K actin-binding protein and compared these to the ultrastructure of the cytoplasmic matrix in regions of nonextracted Dictyostelium amoebae that are rich in actin and 120K. This ultrastructural characterization was achieved by using critical-point-dried whole-mount preparations. All three preparations--gelled extracts, purified proteins, and cortical cytoplasm--are composed of filament networks. The basic morphological feature of these networks is the presence of contacts between convergent filaments resulting in "T" or "X" shaped contacts. The finding that actin-containing gels are composed of filament networks, where the primary interaction occurs between convergent filaments, reconciles the known requirement of F actin for gelation with the amorphous appearance of these gels in thin sections. Increasing the molar ratio of 120K dimer to actin monomer increases the number of contacts between filaments per unit volume and decreases the lengths of filaments between contacts. This indicates that 120K stabilizes interactions between filaments and is consistent with biochemical evidence that 120K crosslinks actin filaments. The cortical network in situ resembles more closely networks formed in 120K-rich extracts than networks assembled in mixtures of purified 120K and actin. The heterogeneity of filament diameters and variation of network density are properties shared by extracts and the cytomatrix in situ while networks found in purified 120K-actin gels have filament diameters and densities that are more uniform. These differences are certainly due to the more complex composition of cell extracts and cortical cytoplasm as compared to that of purified 120K-actin gels.