bluuebunny/crossref_metadata_2025 huggingface dataset:
Cell to substratum contacts of chick fibroblasts and their relation to the microfilament system. a correlated interference-reflexion and high-voltage electron-microscope study
Authors: Heath J. P. , Dunn G. A.
Published: April 2021

The interference reflexion microscope may be used to examine the pattern of cell-substratum separation of cultured cells. Earlier studies on chick fibroblasts using this technique identified 3 types of separation: (1) focal contacts, representing approx. 10—14 nm separation and which are sites of cell-substratum adhesion and form at the leading edge of a cell during locomotion; (2) close contacts, representing approx. 30 nm separation; and (3) regions of greater separation, generally 100 – 140 nm. Using light microscopy these studies described the association of the focal contacts with the distal ends of cytoplasmic fibres and noted their similarity to the adhesion plaques described in EM studies of thin vertical sections of fibroblasts.

In this investigation we have directly confirmed these earlier observations by using a correlated interference-reflexion and high-voltage stereo electron-microscopic study of cultured chick heart fibroblasts. The results show that focal contacts are always associated with the distal ends of linear bundles of microfilaments which pass centripetally and often obliquely through the cytoplasm. At their proximal ends the bundles insert into a loose matrix of microfilaments and organelles surrounding the nucleus. The close contacts are often associated with a meshwork of microfilaments.

The evidence supports the hypothesis that the microfilament bundles of a fibroblast form synchronously with the focal adhesions to the substratum. Our results are discussed with reference to the current evidence for the role of contractile proteins in cell locomotion.
The effects of tumour cells on embryonic cell aggregation, adhesion and detachment
Authors: Maslow D.E. , Weiss L.
Published: April 2021

The presence of small numbers of tumour cells inhibits the aggregation of embryonic chicken neural retina cells grown in gyratory shaker culture. The aggregation of neural retina cells was also inhibited by ascites cell medium. We investigated whether the inhibitory effect of the tumour cells on aggregate size is effected by inhibition of the initial adhesion or by enhancement of their separation. The number of neural retina cells adherent to microtest plate surfaces was significantly reduced after incubation with either Ehrlich ascites cells or cell-free, conditioned medium, while the percentage of cells removed from glass by shearing was unchanged under those conditions. These results suggest that the reduction in neural retina cell aggregate size produced by Ehrlich ascites cells and their products is due to partial inhibition of neural retina cell adhesion processes, as distinct from enhancement of separation.
A morphological study of the cholinergic receptor protein from Torpedo marmorata in its membrane environment and in its detergent-extracted purified form
Authors: Cartaud Jean , Benedetti E. Lucio , Sobel André , Changeux Jean-Pierre
Published: April 2021

The highly specialized structural features of the postsynaptic membrane of Torpedo mar-morata electric organ have been studied by freeze-fracturing, freeze-etching and negative staining applied both to intact electroplaques and to isolated purified excitable membrane fragments.

The external surface of the postsynaptic membrane, revealed by deep-etching and negative staining, is characterized by the presence of geometrically packed arrays of repeating particles having a subunit structure and a central pit. The isolated receptor protein molecules (molecular weight : 300 000) show structural features identical to those characteristic of the repeating particles visualized on the external surface of the postsynaptic membrane.

Freeze-fracture cleaves the postsynaptic membrane into 2 asymmetric halves, showing that numerous identical particulate entities are more strongly anchored to the cytoplasmic side of the bilayer. On the other hand, geometrically packed arrays of pits or depressions found on the external fracture face suggest that the receptor protein extends across the outer leaflet and is exposed to the external surface facing the synaptic cleft.

The homogeneous size-distribution of the particulate entities associated with the postsynaptic membrane element is consistent with the finding that the receptor protein accounts for at least 40% of the total protein of purified excitable membrane fractions. These results strongly favour the assumption that the particulate entities represent the receptor protein, which forms oligomeric complexes either of identical polypeptides or quasi-equivalently related proteins.

Thus, the cholinergic receptor oligomers forming a bidimensional lattice of repeating particles are exposed both to the synaptic cleft and to the cytoplasmic environment. It is proposed that the regulating properties of the excitable membrane depend upon oligomeric or polymeric associations of receptor proteins spanning the bilayer.
Duration of availability of tritiated thymidine following intraperitoneal injection
Authors: Blenkinsopp W. K.
Published: April 2021

Much indirect evidence supports the assumption that tritiated thymidine does not label cells which enter the deoxyribonucleic acid synthesis phase (S(more than 1 h after injection. Direct evidence confirming this assumption was obtained by counting labelled epithelial nuclei in mice killed 1, 4 or 6 h after a single intraperitoneal injection of [3H]thymidine; colchicine was used to prevent the increase in number of labelled nuclei which would otherwise have occurred because of cell division. The proportion of cells labelled was the same at 1 h as at 4 or 6 h after injection of [3H]thymidine. Nuclei were regarded as labelled if they were overlaid by 4 grains or more; comparison of nuclear and background labelling indicated that nuclei overlaid by 3 grains or less represented background labelling.
Contractility and the fibre systems of Stentor Coeruleus
Authors: Bannister L. H. , Tatchell E. C.
Published: April 2021

Observations of the microtubular ‘km’ system and filamentous ‘M’ systems in living specimens of Stentor coeruleus show that on contraction of the body both systems shorten, thicken and remain straight, but that on initial relaxation they behave differently: the km fibres begin to lengthen later than the M fibres, the latter being thrown into sinuous folds. Electron microscopy of specimens cooled before fixation appears to confirm this difference in behaviour in relaxing specimens.

The M fibres of the stalk region are discrete and prominent bundles, but in the adoral half of the body they are extensively linked together by side branches. There are diffuse filamentous attachments between the M fibres and the kinetosomes.

The vertical microtubular stacks which make up the km fibres are each linked with a pair of kinetosomes. Each stack contains about 21 microtubules grouped in a 2 + 19 pattern. Crossbridges are present between the microtubules of adjacent stacks. The number of stacks in each km fibre cross-section is smaller in extended specimens than in contracted ones, indicating that the stacks slide upon one another as the body changes its length.
On the effect of motor nerve degeneration on the fine-structural localization of esterases in the mammalian motor end-plate
Authors: Csillik B. , Knyihár Elizabeth
Published: April 2021

Electron-microscopic localizations of acetylcholinesterase and non-specific esterases have been studied in the motor end-plates of the rat diaphragm, using a modified Koelle acetylthiocholine technique for acetylcholinesterase, the Holt technique for non-specific esterases (substrates: indoxyl acetate and indoxyl butyrate) and the Crevier-Bélanger technique for thiolacetic acid esterases. Acetylcholinesterase is located in the pre- and post-synaptic membranes and in the junctional folds built up by the latter. Indoxyl acetate esterase shows a homogeneous localization in primary and secondary synaptic clefts. Indoxyl butyrate esterase is located in the ‘middle dense layer’ of the synaptolemma. Thiolacetic acid esterase is predominantly post-synaptic, accompanied by a slighter pre-synaptic activity.

Eleven days after motor nerve transection, the synaptic gutter is occupied by a large number of collagenous fibres, and only small granular fragments of the degenerated nerve fibres remain. These fragments are devoid of acetyl- and non-specific esterases, but exert a thiolacetic acid esterase activity, probably demonstrating proteolytic enzymes. The post-synaptic membrane exhibits a slightly reduced enzyme activity without any major alteration in the fine-structural localization of acetyl- and non-specific esterases.
Plant protoplast agglutination by artificial carbohydrate antigens
Authors: Larkin P. J.
Published: April 2021

The existence of β-lectins on protoplast surfaces is confirmed by the agglutination of proto-plasts by those Yariv antigens that have sugar specificities which also interact with isolated fi-lectins. Agglutination by β-mal tosyl but not by β-D-mannosyl Yariv antigens is used to identify some of the structural features required of the ligand for β-lectin binding. Inhibition of agglutination by phenolic glycosides and the effect of protoplast fixation are also investigated.
Induction of cleavage in nucleated and enucleated frog eggs by injection of isolated sea-urchin mitotic apparatus
Authors: Masui Y. , Forer A. , Zimmerman A. M.
Published: April 2021

Mitotic apparatus (MA) were isolated in glycerol-dimethylsulphoxide solution (MTME) from zygotes of sea urchins (Strongylocentrotus purpuratus). Freshly isolated MA were stored in 1/10 strength MTME for varying periods of time and were then injected into unfertilized frog (Rana pipiens) eggs. These injections induced 40–60 % of the recipient frog eggs to initiate cleavage, resulting in the formation of blastula cell clusters. The cleavage-inducing activity of MA stored in 1/10 MTME at room temperature decreased with time of storage in 1/10 strength MTME, and disappeared by about 6 h. There was no change in the ultrastructure of MA during storage.

MA isolated and stored in MTME at room temperature had a constant level of cleavageinducing activity during the first 48 h of storage, but this activity slowly declined upon further storage; almost no activity was left after 3 weeks.

MA isolated in hexylene glycol (HG) and immediately transferred into MTME were compared with MA isolated in MTME; both MA had the same cleavage-inducing activity on the day of isolation, after which the MA isolated in HG quickly lost activity. On the other hand, MA isolated and stored in HG had little cleavage-inducing activity when tested 3 h following isolation.

Cleavage-inducing agent (CIA) isolated from frog brains induced cleavage and blastula formation when injected into nucleated frog eggs, but had no such activity when injected into enucleated frog eggs. MA isolated in MTME induced cleavage and blastula formation in enucleated frog eggs as well as in nucleated frog eggs. Cytological examination revealed that blastula cells which developed from MA-injected enucleated eggs contained Feulgen-negative nuclei, whereas cells which developed from CIA-injected nucleated eggs contained Feulgen-positive nuclei. These results suggest that sea-urchin nuclear materials participate in mitosis in frog eggs.

Isolated MA which had been stored in MTME for 3 weeks and which exhibited little cleavage-inducing activity were injected together with frog brain CIA into either normal or enucleated eggs; normal recipient eggs cleaved with significantly higher frequencies (70%) than those injected with CIA alone (40 %). Furthermore, enucleated eggs injected with CIA alone failed to cleave, while those injected with MA and CIA together cleaved with significant frequencies (overall 29 %). This result suggests a cooperative interaction between CIA and the inactivated MA to restore the cleavage-inducing activity of MA.
The role of phosphodiesterase in aggregation of Dictyostelium discoideum
Authors: Darmon Michel , Barra Jacqueline , Brachet Philippe
Published: April 2021

The role of cAMP phosphodiesterase in the cAMP-mediated aggregation of the cellular slime mould Dictyostelium discoidettm was investigated with a morphogenetic mutant defective in phosphodiesterase production. Mutant cells become capable of aggregating normally when incubated in the presence of exogenous phosphodiesterase isolated from Dictyostelium or rat brain. Direct contact between enzyme and the cell membrane is not required for this phenotypic suppression. The aggregateless character of this strain presumably results from an over-accumulation of cAMP in the extracellular medium since aggregation can be induced in the absence of added phosphodiesterase under conditions facilitating diffusion of the nucleotide. This suggests that phosphodiesterase is not involved in the generation or recognition of cAMP signals, but that the enzyme is essential in the control of the cAMP signal-to-noise ratio.
Intracellular fibres in cultured cells: analysis by scanning and transmission electron microscopy and by sds-polyacrylamide gel electrophoresis
Authors: Trotter John A. , Foerder Beverly A. , Kellerj John M.
Published: April 2021

The 3-dimensi0nal structure of the fibrous cytoskeleton of 3T3cells was examined by scanning electron microscopy of cells extracted with the non-ionic detergent Triton X-100. Detergent-extracted cells consist of the nucleus and an extensive system of fibres, the largest of which correspond to stress fibres visible by phase-contrast microscopy. The system of fibres, which is coterminous with the borders of the native cell, remains firmly adherent to the substratum. The major fibres branch into smaller fibrils which appear to end by ravelling out into fine filaments that constitute a matted network in a plane very close to that of the substratum. In the nuclear region all the major fibres pass over the top of the nucleus, where they may also branch into a system of fine fibrils. Thin-section transmission electron microscopy in conjunction with heavy meromyosin treatment of extracted cells shows the fibres to be composed of native F-actin. Intermediate filaments are also present, and are prominent in the matted network, together with actin filaments. The major proteins of the residue are identified by SDS-polyacrylamide gel electrophoresis as actin, a 56000 Dalton peptide, and histones. Also present are myosin heavy chain, peptides of 225000 and 250000, and minor bands at 60000 and 94000 Daltons. The non-ionic detergent extracts 70 % of the cellular protein, including 50 % of the actin and 75 % of the myosin. The Triton-insoluble fraction of 3T3cells appears to constitute, in addition to the nucleus, a stable cytoskeletal system, composed largely of contractile proteins and 10-nm filaments, which functions in maintenance of cell shape, in substratum adhesion, and in positioning the nucleus within the cell.
The arrangement of microtubules in serially sectioned spindles of the alga cryptomonas
Authors: Oakley Berl R. , Heath I. Brent
Published: April 2021

Serial sections of mitotic spindles of the marine cryptophycean alga, Cryptomonas, were analysed to determine what types of microtubules they contained and which of these microtubules came close enough to each other (50 nm or less) for the commonly described crossbridging to be possible. Interpolar microtubules were rare (≤ 1 %) but from prometaphase through anaphase there was a substantial interpolar framework of free and polar microtubules which came close enough to one another to cross-bridge and generate anaphase spindle elongation by intermicrotubule sliding. However, such elongation would also require some concomitant polar microtubule polymerization. In contrast, only about 12% of the chromosomal microtubules came within bridging distance of interpolar framework microtubules. Thus, assuming that microtubules were accurately fixed in their in vivo positions, crossbridging between chromosomal and interpolar framework microtubules is unlikely to function in chromosome-to-pole movement. In all stages the great majority of chromosomal microtubules were found to extend all the way from the chromatin to the pole. Although the identification of intermicrotubular bridges in section is often ambiguous, the best and most frequent examples of bridges in the present material were between polar microtubules and between chromosomal microtubules but not between the 2 different categories. The spindles also contained 5-nm- diameter microfilaments associated mainly with chromosomal microtubules and occasionally with polar microtubules. A specific model for the possible involvement of these filaments in mitosis is presented.
Chromatin influence on the function and formation of the nuclear envelope shown by laser-induced psoralen photoreaction
Authors: Peterson Scott P. , Berns Michael W.
Published: April 2021

Potorous tridactylis (PTK2) cells growing in culture were treated with psoralen derivatives and dividing cells were located by phase-contrast microscopy. Psoralens, light-sensitive DNA-photoadducting drugs, were reacted with mitotic chromosomes through exposure to 365-nm light from an argon laser microbeam system. It was found that following mitosis and photoreaction, cells without nuclear envelopes were produced when psoralen-treated cells received 60 light pulses over their entire chromosome complement. These ‘non-nuclear membrane* cells were found to incorporate PHjuridine and, to a lesser extent, [3H]thymidine by autoradiography. Reduction of the light exposure by half (30 near-u.v. pulses) over the entire chromosome complement in the presence of psoralen also produced non-nuclear-membrane cells as seen by light microscopy. Further examination of these cells (30 light pulses) by singlecell electron microscopy revealed that unlike the high light exposure (60 near-u.v. pulses), the low light dosage resulted in cells with membrane patches associated with their chrcmatin. Since neither actinomycin D nor cycloheximide impeded nuclear envelope reformation, the psoralen-DNA reaction is concluded to produce non-nuclear-membrane cells by a mechanism other than transcription or translation inhibition. The association of Golgi with areas of nuclear membrane patches gives indirect evidence of a possible Golgi contribution to the reformation of the nuclear envelope after mitosis. It is concluded that DNA plays a role in envelope reformation.
Genetic analysis of developmental mechanisms in hydra: V. cell lineage and development of chimera hydra
Authors: Sugiyama Tsutomu , Fujisawa Toshitaka
Published: April 2021

Chimeric hydra were produced by making use of a strain (nf-1) which lacks interstitial cells, nerve cells and nematocytes. This strain arises by spontaneous loss of interstitial cells from its parental strain (sf-1) (Sugiyama & Fujisawa, 1978). Réintroduction of interstitial cells from other strains intonf-1 leads to the creation of chimeric strains that consisted of epithelial cells derived from strain sf-i and interstitial cells and their derivatives (nerves and nematocytes) from other strains.

In chimeras, interstitial or epithelial cells apparently maintain very stable cell lineages; no indication was obtained that suggested interstitial cell differentiation into epithelial cells or dedifferentiation in the opposite direction during the long courses of chimera cultures (up to one year).

Developmental characters of chimeras were examined and compared to those of the epithelial cell (sf-1) and the interstitial cell donors. Almost all of the chimera’s characters examined (growth rate, budding rate, tentacle numbers, polyp size, regenerative capacity, etc.) closely resembled those of the epithelial cell donor, but not of the interstitial cell donors. This suggests that epithelial cells, rather than interstitial or nerve cells, are the primary determinant of most, if not all, of hydra developmental characters.
Respiratory tract epithelium in primary culture: concurrent growth and differentiation during establishment
Authors: Heckman C. A. , Marchok Ann C. , Nettesheim Paul
Published: April 2021

Studies of cellular function in the respiratory tract lining have traditionally been limited by the small tissue mass and functional diversity of the epithelium. Recent improvements in culture conditions have permitted long-term maintenance of epithelial cells derived from the upper respiratory tract of rats. The present study determined the extent to which proliferation and differentiation took place in such epithelial cell cultures. The labelling index obtained after ‘H-dThd administration was approximately 100-fold higher than that of the quiescent epithelium in vivo; therefore, a large proportion of the cells were probably in the cycling population.

Ultrastructural studies showed that features unique to the specialized mucous secretory cells and ciliated cells were lost rapidly with entry of these cells into the in vitro environment. With long-term maintenance, the cultures were reorganized into a stratified epithelium containing large, squamous, apical cells and small basal cells. The (Ultrastructural appearance of basal cells in vitro was nearly identical to that of basal cells in vivo. Squamous cells were frequently joined by tight junctions. Because hemicysts originated by detachment of squamous cells from the basal layers but not from adjacent squamous cells, they were considered to indicate stratification in the cultures. The proliferative and differentiative status of the mucociliary epithelium was altered by in vitro conditions, and came to resemble that of early squamous metaplasia in the respiratory tract epithelium.
Cell wall differentiation and stages involved with intercellular gas space opening
Authors: Roland J. C.
Published: April 2021

The development of the intercellular gas system has been followed during the growth of Pisttm sativum root and Phaseolus aureus hypocotyl by means of ultrastructural cytochemistry.

The extension of the system is sequential and takes place according to a defined programme of cell wall reconstruction. Contrary to current views, the actual pectic middle lamella does not seem to be directly engaged in the initiation of the aerating system, which conversely appears subordinate to the presence of specialized substructures within the wall. The process is characterized by the early differentiation of a particular layer of wall called the ‘splitting layer’. The splitting layer differs from the pectic middle lamella particularly in its insolubility in an incubating medium which removes the wall subunits (EDTA, DMSO, pectinases, cellulases) and its non-reactivity to polysaccharide test involving periodic oxidation (PATAg staining). With ultracryotomy, it displays a distinctive β-glycerophosphatase activity.

The layer gradually splits apart from lateral sites in a manner which somewhat evokes the opening of a zip fastener. The primordial opening, and later the intercellular space, keeps a thin (10–20 nm) extramural coat which is apparently non-glucidic and derives from the splitting layer. Ultimately, local shifts and resorptions of the wall lead to fusion of the early intercellular channels.

One of the peculiarities of the opening of the air-space is that because of the mechanism involved the polysaccharides of the wall are not left naked. The processes observed are compared with other cases of cell wall separation.
A scanning electron-microscopic study of the local degeneration of cilia during sexual reproduction in Paramecium
Authors: Watanabe Tsuyoshi
Published: April 2021

The location and extent of local degeneration of cilia during sexual reproduction of Paramecium was studied using scanning electron microscopy to examine cells undergoing conjugation and autogamy. At some time during the mating reaction, but prior to conjugant pair formation, ciliary degeneration begins at the antero-ventral tip of cells and proceeds posteriorly along the suture. In the anterior part of the cell, degeneration occurs on both sides of the suture, but in the posterior part it is restricted to the right side of the suture. In 5 species of Paramecium examined, degeneration occurred in nearly the same region. No degeneration of cilia is observed in natural autogamy of P. tetraurelia, whereas in chemically induced autogamy of P. caudatum degeneration occurs as in ordinary conjugation. Conjugant pairs never expose any deciliated cell surface except at the poetero-ventral tip. The maximum extent of ciliary degeneration is best seen in the chemically induced autogamous cells: 7 kineties (rows of unit teritories) at the anterior-left, 4 kineties at the anterior-right, 10 or more kineties at the posterior-right and the right wall of the vestibule of the mouth. Before complete disappearance of the cilia, many short cilia are observed. This suggests that ciliary degeneration is due to resorption. Degeneration extends more rapidly in cells with stronger mating reactivity. The relations between mating reactivity, ciliary degeneration and nuclear activation are discussed.
Adenosinetriphosphate, calcium and temperature requirements for the final steps of exocytosis in Paramecium cells
Authors: Matt H. , Bilinski M. , Plattner H.
Published: April 2021

In Paramecium cells a synchronized discharge of trichocysts (which involves only the final exocytosis steps of membrane fusion, content discharge and membrane resealing) was achieved with ATPase-blockers, Ca3+-ionophores, lipid solvents (including lysolecithin), polyethyleneglycol, anaesthetics (Dibucain) and cationic detergents (cetyltrimethylammonium bromide (CTMAB) and cetylpyridinium chloride (CPC)). Only Dibucain - and to some extent cationic detergents -can trigger exocytosis independently of extracellular Ca2+, possibly by mobilizing intracellular Ca2+. The internal free [Ca2+] necessary for exocytosis can be estimated to be > 10−6 to 10−4 M. Membrane-free trichocyst contents were isolated by density gradient centrifugation; they are converted from the contracted to the expanded state by Dibucain, CTMAB and CPC, and also by exogenous ATPase (Apyrase). Thus, it is possible to de-couple the discharge (stretching) process from membrane-related phenomena. Since only the latter are inhibited by low temperature (0°C), membrane lipids probably have to be in a fluid state for exocytosis to occur. At least 2 steps appear to be involved: when membrane fusion is initiated, an independent matrix-bound system is activated for the synchronized stretching process. The energy requirement for one discharge event is estimated to be about 14 × 106 ATP molecules.
Microtubules in protozoan cells: III. ultrastructural changes during disintegration and reformation of heliozoan microtubules
Authors: Toyohara A. , Shigenaka Y. , Mohri H. H.
Published: April 2021

In the heliozoan, Echinosphaerium nucleofilum strain MA, cold temperature (2 °C) induced axopodial retraction to about 36% of the initial length after 3 h. By electron microscopy, it was found that such axopodial shortening is accompanied by degradation of axonemal microtubules (25 nm in diameter), followed by the appearance of macrotubules (37 nm in diameter) and filamentous structures (14 nm in diameter) of a tubular and twisted appearance. All of these structures (microtubules, macrotubules and filamentous structures) were found to be depolymerized completely by applying 10 MM colchicine for 1–2 h, and to be replaced by regions with low electron density. Axopodial re-extension was induced rapidly by returning the cold-treated organisms to room temperature (20 °C). At a very early stage of axopodial re-extension, the filamentous structures were often observed to be continuous with the macrotubules. At a late stage of axopodial re-extension, the reforming axoneme was composed only of normal microtubules, while the macrotubules and filamentous structures had disappeared. On the basis of these results, the processes of disintegration and reformation of microtubules are discussed.
A collision analysis of lymphoid cell aggregation
Authors: Evans Clive W. , Proctor John
Published: April 2021

We have obtained data on the frequency of aggregates of different size classes and on the maximum aggregate diameter of lymphoid cells subjected to aggregation in the laminar shear field of a Couette viscometer. Maximum aggregate diameter reached a plateau level during aggregation. This plateau is considered to be the result of a balance between the hydrodynamic shear forces tending to resist the formation of aggregates, and the adhesive forces of the aggregated cells tending to resist dissociation. Stepwise increases in the shear rate produced aggregates of progressively smaller maximum diameter until the limiting (i.e. control equivalent) diameter was reached. Equations governing these stepwise changes in aggregate diameter were obtained by regression analysis, and an estimate of the force of dissociation (FD,) was made from derived values of the critical shear rate. Thymocytes (FD= 2 ·54 × 10−6 N m−2 /cell) were found to be more adhesive than lymphocytes (FD = 2 ·05 × 10−6 N m−2 /cell), in agreement with current concepts.

The observed data on aggregate frequency were seen to be of poor fit with a model of aggregation derived by collision analysis of the aggregation process. This led us to consider the possibility that all cells may not share the same probability of forming an adhesion. We thus derived further models of aggregation in which some fraction of the total cells was considered to have enhanced possibilities of a collision producing an adhesion. Of the models we considered, a 15% preferred fraction offered best agreement with the experimental observations. We therefore conclude that the populations of cells studied in this report are not ‘homogenous’ in that some cells are more adhesive than others. Alterations in the percentage of the preferred fraction of cells will lead to different aggregate-frequency indices. Such changes might be expected to occur during the initial stages of carcinoma development.
Contact behaviour and pattern formation of bhk and polyoma virus-transformed bhk fibroblasts in culture
Authors: Erickson C. A.
Published: April 2021

Certain behavioural and morphological aspects of cellular transformation have been studied, using baby hamster kidney cells (BHK21/13) and polyoma virus-transformed BHK cells. BHK cells grow to monolayers arranged in parallel arrays, whereas the transformed cells show a much greater incidence of crisscrossing and multilayering. Time-lapse cinemicrography and scanning electron microscopy were used to examine the behaviour producing these striking differences in cellular pattern.

It was found, contrary to previous thought, that in both cell lines, when contact is made ruffle to ruffle, ruffling is inhibited. When BHK cells contact each other ruffle to side, strong adhesions always occur, as evidenced by a large deformation of the contacted cell margin, with accompanying paralysis of ruffling. Then, the contacting cell either changes direction, usually spreading along the side of the contacted cell, or occasionally continues to protrude and underlap the other cell, although the original adhesions are seen to remain. Transformed cells never form strong rufHe-to-side adhesions, and usually underlap each other totally. When the cells were filmed, fixed and the same cells relocated in the scanning electron microscope, neither cell type was seen to move over the surface of another (overlap). Rather, all cells crisscross by underlapping (moving under the other cell). SEM also reveals PyBHK cells to have many fewer side-to-substratum adhesions than BHK cells. The smaller number of these attachments could explain the ease with which PyBHK cells underlap.
Regulation of interstitial cell differentiation in Hydra Attenuata : III. Effects of I-Cell and nerve cell densities
Authors: Yaross Marcia S. , Bode Hans R.
Published: April 2021

The interstitial cell (i-cell) of hydra, a multipotent stem cell, produces two classes of differentiated cell types, nerve cells and nematocytes, throughout asexual growth. Using a new assay, the regulation of i-cell commitment to either nerve cell or nematocyte differentiation was investigated. This assay was used to determine the fractions of i-cells differentiating into nerve cells and nematocyte precursors in a variety of in vivo cellular milieus produced by hydroxyurea treatment, differential feeding, and reaggregation of dissociated cells.

Nematocyte commitment was found to be positively correlated with the size of the i-cell population and independent of the axial position of the i-cells along the body column. This indicates that i-cell commitment to nematocyte differentiation may be regulated by feedback from the i-cell population. Nerve cell commitment was found to be correlated with regions of high nerve cell density. This suggests that nerve cell commitment is regulated by feedback from the nerve cell population or is dependent on axial position. Implications of such mechanisms for the regulation of i-cell population size and distribution are discussed.
An analysis of con A-mediated agglutination in a chinese hamster ovary subclone which responds morphologically to growth in dibutyryl cyclic amp: III. The role of microvilli in the agglutination process
Authors: Noonan Kenneth D. , Ukena Thomas
Published: April 2021

We have used the H-7 subclone of a Chinese hamster ovary cell line (K1) to investigate the role of cell surface architecture (specifically microvilli, blebs, and sheets) in determining the relative agglutinability of a cell line with Con A. Our evidence clearly demonstrates that no specific, immediately recognizable surface architecture is associated with the agglutinable or non-agglutinable phenotype. Our data suggest that the expression of microvilli on the cell surface is neither necessary to nor sufficient for the phenotype described by enhanced agglutinability with Con A. Furthermore our work demonstrates that cells covered with blebs are as agglutinable as cells covered with microvilli thereby suggesting that the intertwining of microvilli may not be an essential facet of the agglutination phenomenon.
Change of the nuclear pore frequency during the nuclear cycle of Physarum polycephalum
Authors: Schel Jan H. N. , Steenbergen Leo C. A. , Bekers A. G. M. , Wanka Friedrich
Published: April 2021

The change of the nuclear pore frequency during the nuclear cycle of synchronous plasmodia of Physarum polycephalum was investigated. Counts were made on platinum-carbon replicas of isolated nuclei. Pore numbers varied markedly at any given time of the nuclear cycle, possibly due to the variable DNA contents of the nuclei. The average pore frequency per nucleus increased from 336 at 1 h after mitosis to 770 at 50 min before the subsequent mitosis. The results suggest a quantitative relation between the nuclear DNA content and pore frequency. This is compatible with the hypothesis that pore complexes serve as attachment sites for nuclear DNA.
RNA transcription on the giant lateral loops of the lampbrush chromosomes of the american newt Notophthalmus viridescens
Authors: Hartley S. E. , Callan H. G.
Published: April 2021

By counting silver grains in autoradiographs of lampbrush chromosomes the rates of incorporation of [3H]adenine, [3H]cytidine, [3H]guanosine and [3H]uridine, administered separately, into RNA on the giant loops of chromosome II of Notophthalmus viridescens, were compared with the rates of incorporation of these same precursors into RNA on other, unidentified loops. The overall rate of RNA transcription on the giant loops is only about half that on the generality of other loops, and the RNA transcribed on the giant loops is computed to have a base ratio of approximately 25 A:39 C:9 G:27 U, implying that there must be about 4 times as many guanine residues on the transcribed as on the non-transcribed strand of the giant loops’ DNA.
Evidence for centriolar region RNA functioning in spindle formation in dividing PTK2 Cells
Authors: Peterson Scott P. , Berns Michael W.
Published: April 2021

The light-activated, nucleic acid-binding drugs, psoralens, were used in conjunction with a 365-nm laser microbeam to selectively bind to any nucleic acids in the centriolar region. 4′-aminomethyl-4,5′,8-trimethyl-psoralen (AMT) has a high affinity for both RNA and DNA and can be shown to cause mitotic abortion when centriolar regions of prophase PTK, cells are reacted with AMT and 365-nm laser light. Other psoralen derivatives which have a high affinity for DNA and a low affinity for RNA are not effective in blocking mitosis in dividing PTK, cells. Examination of psoralen-bound centriolar regions by single-cell electron microscopy shows that at various times after treatment, the number of microtubules associated with the irradiated poles is much lower than in normal, dividing cells. Light-activated psoralen binding of the centriolar regions does not seem to affect the condensation or structure of mitotic chromosomes. It is concluded that there is an RNA in the centriolar region that is responsible for the formation of the spindle in dividing cells.
Associated changes in the size and number of mitochondria present in the midgut of the larvae of the housefly, Musca domestica and phospholipid composition of the larvae
Authors: Sohal R. S. , Bridges R. G.
Published: April 2021

A possible relationship between the phospholipid base composition of larvae of the housefly and the size, number and total volume of midgut mitochondria was examined. The phospholipid composition of the larvae was altered by restricting the amount of choline, substituting choline by N-dimethylethyl-(2-hydroxyethyl)-ammonium salt or 2-N-dimcthylaminoethanol and by the addition of 2-aminobutan-1-01 to the larval diet. Mitochondrial measurements were made on electron micrographs of the epithelial cells in the midgut of third instar larvae. Reduction in the normal proportion of phosphatidylcholine in the phospholipids of the larvae was associated with an increase in the average size of the mitochondria and a decrease in their number, probably due to enhanced mitochondrial fusion. The opposite effect was noted when phosphatidyl-2-aminobutan-1-01 was present as a part replacement of the phosphatidylethanolamine normally present in the larvae. Larvae which contained both phosphatidyl-2-aminobutan-1-01 as well as reduced amounts of phosphatidylcholine, contained mitochondria which were intermediate in both size and number. The effects associated with the changes in phosphatidylcholine and phosphatidyl-2-aminobutan-1-01 were independent of each other. The presence of the analogue of phosphatidylcholine and phosphatidyldimethylaminoethanol had no significant effect on the mitochondrial distribution. Total mitochondrial volume remained unaffected by alterations in the phospholipid composition of the larvae.
The distribution of spectrin along the membranes of normal and echinocytic human erythrocytes
Authors: Ziparo Elio , lemay André , Marchesi Vincent T.
Published: April 2021

Spectrin molecules are distributed uniformly throughout the submembranous regions of intact human erythrocytes. Spectrin does not appear to extend into the red blood cell cytoplasm to any significant extent. Thus, it does not form a recognizable internal scaffolding nor does it seem to connect distant segments of the cell membrane. Spectrin retains its submembranous location in the spiny processes of echinocytes produced by ATP depletion. Thus, these processes do not seem to form by a simple extrusion mechanism powered by contraction of the spectrin network. Spectrin seems to be important for the stability of the lipid bilayer of the red cell membrane, and it probably also plays a role in regulating red cell shape. How it performs either function is still unknown.
Do human spermatozoa reactivate in the cytoplasm of somatic cells?
Authors: Van Meel F. C. M. , Pearson P. L.
Published: April 2021

The morphological changes following Sendai virus-enhanced interaction of purified fractions of human spermatozoa with various types of fibroblast lines were examined over a period of several days. The incorporation of spermatozoa was monitored by a variety of immunofluorescent staining techniques, including one for protamin. The initially high incorporation frequency (30–40 %) was accompanied by swelling of the acrosome and nucleus, which, in most instances, was followed by gradual lysis of the sperm components. However, a small proportion of all incorporated spermatozoa (2 %) exhibited chromatin decondensation, accompanied by a shift from a protamin to histone content and induction of RNA and DNA synthesis. These latter cells appear to fit the criteria for true reactivation and electron-microscope studies show that they do not undergo phagocytosis as do the majority of incorporated spermatozoa.
The route of entry of cytoplasmic ally synthesized proteins into: Chloroplasts of algae possessing chloroplast ER
Authors: Gibbs Sarah P.
Published: April 2021

In 8 classes of algae, namely the Cryptophyceae, Raphidophyceae, Haptophyceae, Chryso-phyceae, Bacillariophyceae, Xanthophyceae, Eustigmatophyceae and Phaeophyceae, the chloroplasts, in addition to being surrounded by a double-membraned chloroplast envelope, are also enclosed by a cisterna of endoplasmic reticulum called the chloroplast ER. Often this ER cisterna is continuous with the outer membrane of the nuclear envelope in such a manner that the nuclear envelope forms a part of the ER sac enclosing the chloroplast. In all these classes of algae except the Cryptophyceae, a regular network of tubules and vesicles, named the periplastidal reticulum, is present at a specific location between the chloroplast envelope and the chloroplast ER. In the Cryptophyceae, scattered vesicles are found between the chloroplast envelope and the chloroplast ER.

Ribosomes which have been shown to be arranged in polysomes are found on the outer membrane of the chloroplast ER. It is proposed that nuclear-coded proteins which are destined for the chloroplast are synthesized on these polysomes, passing during synthesis into the lumen of the ER cisterna. Vesicles containing these proteins then pinch off the chloroplast ER and form the periplastidal reticulum. Vesicles then fuse with the outer membrane of the chloroplast envelope thereby delivering their contents to the lumen of the chloroplast envelope. Proteins then cross the inner membrane of the chloroplast envelope in an as yet unknown manner.

Experimental evidence for this hypothesis comes from studies on Ochromonas danica using chloramphenicol and spectinomycin, which inhibit protein synthesis on plastid ribosomes, and cycloheximide, which inhibits protein synthesis on cytoplasmic ribosomes. In cells of Ochromonas exposed to chloramphenicol or spectinomycin, the periplastidal reticulum proliferates markedly becoming several layers thick. Presumably this build up of periplastidal reticulum occurs because the transport of cytoplasmically synthesized plastid proteins is slowed down when protein synthesis in the chloroplast is inhibited. Conversely, when cells of Ochromonas are treated with cycloheximide, there is a reduction in the amount of periplastidal reticulum presumably because there are no cytoplasmically synthesized proteins to be transported into the chloroplast.
On the reduced intercellular adhesiveness of virally transformed BHK21 cells
Authors: Edwards J. G. , Dysart J. Mck. , Edgar D. H. , Son R. T. Rob
Published: April 2021

Baby hamster kidney fibroblasts (BHK21 cells) transformed by polyoma or Rous sarcoma viruses aggregate less than the untransformed parental cells when incubated in growth medium in a gyratory shaker for 18–24 h. This difference can be measured by electronic particle counting, or by filtering aggregated suspensions of 32 P-labelled cells through bolting fabric. The aggregation of transformed derivatives is not enhanced by the presence, during aggregation of ∈-amino caproic acid, an inhibitor of plasmin activation. Some lines of transformed BHK21 cells do not appear less adhesive than untransformed cells in a short-term aggregation assay, and none adheres markedly less well when seeded onto homotypic cell sheets. The decreased aggregation of transformed cells is consistent with suggestions that LETS protein is involved in intercellular adhesion of fibroblasts as well as in attachment of cells to non-cellular substrates. If so, the short-term aggregation of freshly trypsinized cells may depend on secretion of LETS from an intracellular pool.
Growth of human diploid cells (Strain MRC-5) in defined medium; replacement of serum by a fraction of serum ultrafiltrate
Authors: Lambert Kathleen , Pirt S.J.
Published: April 2021

A calf serum ultrafiltrate fraction permitted growth for at least 3 · 5 generations, including one subculture, of MRC-5 cells in defined medium in the absence of whole serum. The active material has a molecular weight of 10000 Daltons or less. This suggests that there may be no requirement for a large macromolecular component of serum. The ultrafiltrate was assayed by maximum cell yield from a serum-limited inoculum in a defined medium containing nonlimiting amounts of vitamins, amino acids, glucose, a 68-component supplement, iron and methylcellulose. The levels of vitamins, amino acids and glucose were based on quantitative measurements of uptake and the levels of the other components by minimum amount required for maximum yield in defined medium without ultrafiltrate or serum. With excess ultrafiltrate maximum cell yield was limited by the defined part of the medium, probably the supplement. The cell doubling time in defined medium with ultrafiltrate fractions was 70 h compared with 27 h in the medium with serum. Excess ultrafiltrate did not inhibit growth. The lowered growth rate is attributed to a nutritional deficiency in the supplement.
Experimentally induced differentiation of slow tonic and fast twitch muscles in the chick
Authors: Ashhurst Doreen E. , Vrbová Gerta
Published: April 2021

The anterior latissimus dorsi (ALD) muscle of chickens is a slow tonic muscle, while the posterior latissimus dorsi (PLD) is a fast twitch muscle. These muscles on opposite sides of a 3-week-old chick were removed, minced and replaced in the site of the other muscle and left to regenerate. The regenerating muscles were examined at various periods from 4 days onwards and their contractile properties were found to resemble those typical of the muscle they replaced and not the original muscle. The regenerating muscles from 8 days onwards displayed the morphological features of the control muscles in the contralateral site. By 14 days, differentiation was almost complete and neuromuscular junctions were seen.

It is suggested that the physiological and morphological characteristics of a muscle are determined by its position and possibly also by its innervation.
Functional characterization of an adhesive component from the embryonic chick neural retina
Authors: Rutz Richard , Lilien Jack
Published: April 2021

We have developed a quantitative assay for tissue-specific adhesive components which is based on the agglutination of glutaraldehyde-fixed cells. At least 2 components are required for fixed-cell agglutination: a cell-surface ligand which is obtained from tissue culture-conditioned medium, and a soluble ‘agglutinin’ which accumulates in conditioned medium from monolayer cultures. Our results suggest that the surface-binding ligand and the agglutinin interact directly, resulting in tissue-specific agglutination of cells. The agglutination reaction exhibits divalent cation, temperature, and pH dependence. Several models of cell adhesion are described; the simplest of these which can account for the data is a multicomponent model in which the 2 adhesive components have structural roles.