Showing papers in "Proceedings of The Royal Society B: Biological Sciences in 1978"

Representation and recognition of the spatial organization of three-dimensional shapes.

Abstract: The human visual process can be studied by examining the computational problems associated with deriving useful information from retinal images. In this paper, we apply this approach to the problem of representing three-dimensional shapes for the purpose of recognition. 1. Three criteria, accessibility, scope and uniqueness, and stability and sensitivity, are presented for judging the usefulness of a representation for shape recognition. 2. Three aspects of a representation9s design are considered, (i) the representation9s coordinate system, (ii) its primitives, which are the primary units of shape information used in the representation, and (iii) the organization the representation imposes on the information in its descriptions. 3. In terms of these design issues and the criteria presented, a shape representation for recognition should: (i) use an object-centred coordinate system, (ii) include volumetric primitives of varied sizes, and (iii) have a modular organization. A representation based on a shape9s natural axes (for example the axes identified by a stick figure) follows directly from these choices. 4. The basic process for deriving a shape description in this representation must involve: (i) a means for identifying the natural axes of a shape in its image and (ii) a mechanism for transforming viewer-centred axis specifications to specifications in an object-centred coordinate system. 5. Shape recognition involves: (i) a collection of stored shape descriptions, and (ii) various indexes into the collection that allow a newly derived description to be associated with an appropriate stored description. The most important of these indexes allows shape recognition to proceed conservatively from the general to the specific based on the specificity of the information available from the image. 6. New constraints supplied by a conservative recognition process can be used to extract more information from the image. A relaxation process for carrying out this constraint analysis is described.

A Comparison of the Size Distribution of Collagen Fibrils in Connective Tissues as a Function of Age and a Possible Relation between Fibril Size Distribution and Mechanical Properties

Abstract: Data on the distribution of collagen fibril diameters in various connective tissues have been collected and analysed for common features. The diameter distributions of the collagen fibrils at birth and in the foetal stages of development are unimodal, whereas at maturity the mass-average diameter of the collagen fibrils is generally larger than at birth and the distributions of fibril sizes may be either unimodal or bimodal depending on the tissue. At senescence, few data are available but in most instances both the mean and mass-average diameters of the collagen fibrils are smaller than those at maturity and the fibril distributions are mainly bimodal. The division between tissues showing unimodal or bimodal fibril distributions at maturity does not simply relate to the type I collagen/type II collagen classification, to the distinction between orientated and unorientated material or indeed directly to the levels of stress and strain encountered by the tissue. However, there may prove to be a relation between a bimodal fibril diameter distribution at maturity and the maintenance over long periods of time of either high stress in stretched tissues or low stress in compressed tissues. It has also been noted that the width of the collagen fibril diameter distribution at birth differs between altricious and precocious animals. The ultimate tensile strength of a connective tissue and the mass-average diameter of the constituent collagen fibrils have been shown to have a positive correlation. Further, the form of the collagen fibril diameter distribution can be directly related to the mechanical properties of the tissue. In particular, it is postulated that the size distribution of the collagen fibrils is largely determined by two factors. First, if the tissue is primarily designed to have high tensile strength, then an increase in the diameter of the collagen fibrils will parallel an increase in the potential density of intrafibrillar covalent crosslinks. Consequently large collagen fibrils are predicted to have a greater tensile strength than small fibrils. Secondly, if the tissue is designed to be elastic and hence withstand creep, then a reduction in the diameter of the collagen fibrils will effectively increase the surface area per unit mass of the fibrils thus enhancing the probability of interfibrillar non-covalent crosslinks between the collagen fibrils and the components of the matrix. The idealized description given may indicate how the mechanical properties of a tissue may be interpreted in terms of the collagen fibril diameter distribution.

The mosaic of nerve cells in the mammalian retina.

Abstract: Cells from three different layers of the retina were tested for regular horizontal arrangement or random distribution. Monkey cones, cat cones, cat A-type horizontal cells and cat alpha-ganglion cells are all arranged in a regular mosaic. This was assessed by measuring the distance to the nearest neighbour of each cell. The nearest neighbour distributions differed significantly from those of random dot patterns. The precision of the mosaic decreased in the order monkey cones, cat cones, cat A-type horizontal cells, cat alpha-ganglion cells. All four cell types were - independent of density - more regularly arranged in the centre of the retina than in the periphery. The advantages of a regular arrangement compared with a random distribution and mechanisms which could generate a regular mosaic are discussed.

The Ferrier Lecture, 1977 The neuron network of the cerebral cortex: a functional interpretation

Abstract: An attempt is made at showing that the cerebral cortex has to be envisaged as a mosaic of columnar units of remarkably similar internal structure and surprisingly little variation of diameter (200-300 $\mu $m). A systematic investigation is made of various types of interneurons and also of the local connections of the pyramidal cells by the combined application of four techniques: (i) the classical Golgi procedures, (ii) electron microscope studies of various types of synapses, (iii) light and electron microscope studies in chronically isolated cortical slabs of the local synapses that persist under such circumstances, and (iv) serial reconstruction-under the electron microscope-of clearly identified Golgi-stained interneurons (and pyramidal cells) which has revealed a hitherto unexpected degree of specificity in local connectivity. Most interneurons are not only highly specific with respect to the arborization pattern of their axons and to the size and shape within which they establish synapses with specific sites of certain other neurons, but also with respect to the character, preferential localization, and origin of the synapses that they receive. On the basis of this kind of information the local neuron network of the cerebral cortex can be defined as an intricate system of repetitive but mutually interpenetrating spatial modules of specific interneuron arborizations. As a consequence the synapses of each individual interneuron are distributed in such geometrically defined modules of cortical space. Some of the interneuron types and the corresponding spatial modules can be identified with considerable confidence as being of an excitatory, and others of an inhibitory, nature. By using recent information on the mode of termination of various afferent pathways of the cortex and on the cellular origin of the main efferent pathways, tentative models for the cortical neuron chains can be proposed.

The interplexiform cell system. I. Synapses of the dopaminergic neurons of the goldfish retina.

Abstract: Interplexiform cells are a class of retinal neuron that extends processes widely in both plexiform layers. In goldfish they contain dopamine and readily take up certain biogenic amines. Two of these amines, 6-hydroxyopamine (6-HDA) and 5, 6-dihydroxytryptamine (5,6-DHT), induce fine structural changes in the neurons that accumulate them, allowing the processes of the cells to be recognized by electron microscopy. Typically, the synaptic vesicles within the processes show electron-dense cores. The terminal cytoplasm may also show increased density, as may the cellular and cytoplasmic membranes, presumably an indication of degenerative changes induced by the drugs. 5, 6-DHT gives more readily observable changes than 6-HDA but labels both dopaminergic and indoleamine-accumulating neurons. The terminals of the indoleamine-accumulating terminals were therefore removed by intraocular injections of 5, 7-dihydroxytryptamine (5, 7-DHT) prior to the labelling with 5, 6-DHT. This procedure allowed an analysis of the dopaminergic terminals without interference by the terminals of the indoleamine-accumulating cells. The dopaminergic neurons were found to make synapses of the conventional type. In the outer plexiform layer they contacted both external horizontal cells and bipolar cell dendrites, but not hotoreceptor terminals or intermediate (rod) horizontal cells. No synapses onto the dopaminergic processes were found in the outer plexiform layer despite an extensive search. In the inner plexiform layer the dopaminergic processes were observed to be both pre- and postsynaptic to amacrine cells and their processes. No synaptic contacts between dopaminergic processes and bipolar cell terminals or ganglion cell dendrites were seen. We conclude that the dopaminergic interplexiform cells provide a centri­fugal pathway for information flow in the retina from inner to outer plexiform layer.

A Synaptic Mechanism Possibly Underlying Directional Selectivity to Motion

Abstract: A specific synaptic interaction is proposed as the mechanism underlying the directional selectivity to motion of several nervous cells. It is shown that the hypothesis is consistent with previous behavioural and physiological studies of the motion detection process.

The Croonian Lecture, 1977 - Stretch activation of muscle: function and mechanism

Abstract: Current theories about the mechanism of muscular contraction suppose that the level of enzymic and contractile activity is controlled by the intracellular concentration of calcium ions, the degree of overlap between the myosin and actin filaments and the rate of relative sliding of the filaments. It is now known that in most or all muscles there is a further direct influence of mechanical conditions, usually called stretch activation; changes of length lead to a delayed change of active tension. The effect is large and functionally significant in insect fibrillar flight muscle and in mammalian heart muscle; it is present, but small, in vertebrate skeletal muscle, which probably accounts for its late discovery. In insect fibrillar flight muscle, the delayed tension is responsible for the rhythmic mechanical activity during flight. In mammalian heart muscle it may play a role in Starling9s Law. In insect fibrillar muscle, extension produces a maintained increase in actomyosin ATPase and active tension; in vertebrate skeletal muscle, stretch activation is a transient phenomenon. Mammalian heart muscle shows greater maintenance of stretch activation than skeletal muscle; the duration of higher ATPase activity has not yet been determined. The effective mechanical parameter is not overall strain but is probably the strain on an internal structure related to overall stress. Various lines of evidence point to the myosin filament as the location of the sensor. A considerable degree of molecular synchronization occurs during natural insect flight.

Gametocyte and Gamete Development in Plasmodium falciparum

Abstract: The ultrastructural organization of the mature gametocytes of Plasmodium falciparum isolated from the peripheral circulation of naturally infected Gambians is examined and compared with immature forms obtained from the peripheral circulation of a chloroquine treated patient. The latter are recognized as the stage 2 and 3 developmental forms (Hawking, Wilson & Gammage 1971 Trans. R. Soc. trop. Med. Hyg. 65, 549-559) observed by light microscopy and are distinguished in the electron microscope by three characters; (i) they do not fill the host cell, (ii) they contain few, if any, osmiophilic bodies, (iii) they possess an extensive subpellicular tubule system. Maturation (capacitation) of these immature parasites takes many days and is followed by an extended period of maturity during which the gametocytes will exflagellate. Mature macro- and microgametocytes have numerous characters in common with the gametocytes of avian and reptilian Plasmodiidae, namely a tripartite pellicle, cristate mitochondria, a comparatively high density of osmiophilic bodies in the macrogametocyte, cytostomal feeding, Golgi body, and persistent nucleolus in the female gametocyte. These similarities together with the unexpected nuclear changes detected in macro-gametogenesis suggest that P. falciparum is best considered as pre-dating the 'malariae' and 'vivax' groups and not as having evolved from them. Light microscopy, scanning and transmission electron microscopy and videotape analyses of gamete formation were undertaken. Nuclei in the mature gametocytes are Feulgen negative but upon activation rapidly become Feulgen positive. The gametes also are Feulgen positive. The crescentic parasites swell to become large spherical cells and escape from the host cell by osmotic or enzymic activity. The microgametocyte undergoes three mitotic divisions during which the chromosomes are sequentially reduced in number such that ca. 7 are incorporated into each gametic nucleus. The microtubule organizing centre (m.t.o.c.), which in the mature gametocyte is associated with the intranuclear body, is attached to the centriolar plaque of the first division spindle. There it differentiates into kinetosomes which act as foci for the polymerization of axonemes. The kinetosomes and axonemes remain attached to the centriolar plaques during division and are segregated synchronously with the genome at each division. Subsequently one axoneme enters each haploid gamete at exflagellation. Exflagellation is accompanied by a significant reduction in microgametocyte volume which is associated with an increase in density of the cytoplasm. The female gametocyte does not decrease in volume but undergoes nuclear changes in which a single pole of an intranuclear spindle is detected. Comparisons are made with macrogametogenesis in avian malarial parasites from which it is suggested that this spindle, if not half of a normal mitotic spindle, is an atavistic trait. The possibility of a meiotic gametic division is discussed but discounted. The activity pattern of the microgamete was found to be similar to that of other malarial parasites, with states of high and low activity or immobility. High activity, which results in rapid movement through the medium, is produced by long wavelength (12 $\mu $m), low amplitude (1.1 $\mu $m) waves generated at ca. 12 waves per second; low activity, which results in contorted gyrating on the spot, is produced by long wavelength (14.1), high amplitude (2.3) waves produced at ca. 1 wave per second. Following an initial period of continuous activity the gamete usually alternates between high and low activity states. Subsequent low activity and immobility is in turn followed by death. Microgamete activity was profoundly affected by the plasma of some patients, presumably as a result of the antigametocyte antibodies present. The microgamete contains a single axoneme, at one end of which lies the kinetosome with the juxtakinetosomal sphere and granule. It is this end which emerges first from the parental gametocyte. A single nucleus is centrally located in many microgametes although 23% are anucleate.

The structure of bilirubin

Abstract: Crystals of bilirubin are triclinic, space group P1 with cell dimensions a = 19439, b = 11707, c = 15500 A, †, α = 9719, β =10022, γ = 11820°, Z = 4 The structure was solved by direct methods incorporating information derived from reflexion quartets and by using a set of diffractometer data containing 1408 reflexions with F obs ≥ 3σ( F obs ) The structure was refined with bond-length constraints to a final R of 0125 by using a set of microdensitometer-measured photographic data containing 1323 unique observed reflexions Both data sets were collected from a matured crystal whose diffraction pattern had improved considerably during a period of several weeks Bilirubin is shown to have the Z configuration at both the C5 and C15 bridges The two independent molecules of bilirubin in the crystal structure have very similar ‘ridge tile’ conformations: rings (A + B) and rings (C + D) form two good planes with an interplanar angle of approximately 97° and the molecule is stabilized by six intramolecular hydrogen bonds There is no evidence for any intermolecular hydrogen bonding Observed bond lengths suggest that bilirubin can be regarded as a 2,29 - dipyrrolylmethane with conjugating substituents at the α positions When freshly prepared, most crystals of bilirubin have reflexions hkl ( h odd) diffuse: this observation is discussed in terms of the molecular conformation of bilirubin and the particular packing arrangement which these molecules adopt in the crystal structure

The interplexiform cell system. II. Effects of dopamine on goldfish retinal neurones

Abstract: The effects of atomized solutions of dopamine and certain related compounds have been tested on the intracellularly recorded activity of receptor, horizontal, bipolar and amacrine cells in the goldfish retina. Dopamine depolarizes the cone L-type horizontal cells and reduces the amplitude of light-evoked responses. These effects on L-type horizontal cells are completely abolished by the $\alpha $-adrenergic blocker, phentolamine, but only partially depressed by the $\beta $-blocker, propanolol. L-Dopa, noradrenalin, and serotonin do not have effects on L-type horizontal cells when applied at concentrations similar to those that cause maximal dopamine effects. The results suggest that the effects of dopamine on L-type horizontal cells are specific, and we propose that they mimic the effects of interplexiform cell activity. Dopamine has no effects on rod horizontal cells in goldfish and variable effects on C-type horizontal cells. On bipolar cells, dopamine alters the dark membrane potential, enhances the central response to light, and depresses the surround response. Dopamine also decreases the horizontal cell feedback evident in cone responses. Finally, dopamine strongly depolarizes the transient type of amacrine cells, but it has no significant effect on the sustained type of amacrine cells. Assuming that dopamine is the transmitter of interplexiform cells, we suggest that these neurons regulate lateral inhibitory effects mediated by L-type horizontal cells in the outer plexiform layer and transient amacrine cells in the inner plexiform layer. In addition, it appears as if interplexiform cells have specific effects on bipolar cells and are capable of regulating centresurround antagonism in these cells. The net effect of interplexiform cell activity is to isolate the bipolars from the influence of the surround.

Topography of horizontal cells in the retina of the domestic cat

Abstract: Neurofibrillar methods stain a class of horizontal cells in the cat retina which are shown to be identical with the A-type horizontal cell of Golgi-staining. Thus all of the A-type cells of a single retina can be observed. On this basis the changes in density and dendritic field size of A-type horizontal cells with respect to retinal eccentricity were measured. The decrease in density from centre to periphery is balanced by a corresponding increase in size of the dendritic field. Consequently each retinal point-independent of retinal position - is covered by the dendritic fields of three or four A-type horizontal cells. The nuclei and nucleoli of B-type horizontal cells could also be recognized in neurofibrillar-stained material and thus their distribution was determined. The density ratio B-type: A-type is 2.8 $\pm $ 0.4 and does not vary much from the centre to the periphery of the retina. Each retinal point is also covered by four B-type horizontal cells. Thus a single cone can contact a maximum of eight horizontal cells. The rate of density decrease from centre to periphery is closely similar in cones and horizontal cells but greater in ganglion cells.

A re-examination of curare action at the motor endplate.

Abstract: Recent evidence indicates that curare, in addition to its competitive' interference with endplate receptors, can block open ionic channels by a 'non-competitive' action on the activated acetylcholine-receptor complex. These findings called for further study of the kinetic behaviour of endplate channels and their modification by curare. Examining impulse-evoked endplate currents and acetylcholine-induced current fluctuations, it is found that the lifetime of the open channel is shortened by relatively high concentrations of curare (greater than 5 micrometer), an effect which shows up most strikingly at hyperpolarized levels of membrane potential (-130 mV and above). No shortening of this kind is observed when a neuromuscular block of equal or greater intensity is produced by a dose of alpha-bungarotoxin. Two other neuromuscular blocking agents, gallamine and pancuronium are shown to have an action on channel kinetics which cannot be explained by competitive receptor binding, but conforms to the hypothesis of rapidly repeated blocking and unblocking of individual ion channels, which had been proposed originally to account for the endplate action of local anaesthetics.

The Rapid Synthesis and Destruction of Photoreceptor Membrane by a Dinopid Spider: A Daily Cycle

Abstract: The receptive segments of the posterior median eyes of Dinopis subrufus are hexagonal in transverse profile, and each of the six faces is composed of a rhabdomere. Supportive cell processes are reduced to strands at the junction of the rhabdoms, and each of the six rhabdomeres, therefore, is contiguous with the rhabdomere of a different receptive segment; this arrangement allows the possibility of both optical and electrical coupling between cells. During the day, receptive segments are short, and rhabdomere membrane occupies only a small proportion of their volumes. At nightfall, the segments lengthen, and novel membrane is added in a rapid burst of synthesis almost to fill them. At dawn, the sequence is reversed, and membrane is removed as pinocytotic vesicles which are assembled into multivesicular bodies and lysed in the inter-rhabdomeral cytoplasm and in the swollen receptor axons which underlie the retina. Synthesis and destruction of membrane are shown to be controlled in part by immediate states of retinal illumination, superimposed upon a daily rhythm. It is argued that the evolution of this metabolically extravagant system is unlikely to be primarily concerned with the manipulation of states of adaptation, and some alternative hypotheses are proposed to account for it.

Blue–green algae and freshwater carbonate deposits

Abstract: Twenty-seven contemporary freshwater carbonate deposits were investigated (26 in the British Isles and 1 in S. Australia). The blue-green algae Schizothrix calcicola and Microcoleus vaginatus occurred at 23 of the sites. The remaining sites represented areas where deposition had ceased. About 1% of the dry mass of the deposits consisted of Cyanophyta. The assimilation rates of these algae, measured by $^{14}$CO$\_{2}$ uptake, were low. It was estimated that the equilibrium change caused by CO$\_{2}$ uptake due to photosynthesis could account for only 1-2% of the precipitated carbonate. Cultures of S. calcicola failed to deposit carbonates in solid and liquid media. Deposition rates varied between 0.006 and 8 mm a$^{-1}$ (mean 1.2 mm a$^{-1}$). The laminations encountered at some sites could be correlated to seasonal differences in cyanophyte growth which enabled some deposits to be dated. Thin sections revealed small crystals adhering to the mucilaginous sheaths of the algae. The transmission of light in some deposits was investigated. The algal layer corresponded to the depth to which light was attenuated to about 1% of the surface intensity.

Tendon and Ligament from the Horse: An Ultrastructural Study of Collagen Fibrils and Elastic Fibres as a Function of Age

Abstract: A study has been made of the ultrastructural organization of the collagen fibrils and elastic fibres in tendons and ligaments from horses of ages ranging from 2 months premature to 19 years. Diameter distributions of the collagen fibrils in the common digital extensor tendon, the superficial flexor tendon and the suspensory ligament are unimodal in the foetal tissue and at birth, and at these stages of development the three collagenous tissues are virtually indistinguishable. However, at maturity, the ligament and flexor tendon have bimodal distributions similar to that found for rat-tail tendon. The fibril distribution for extensor tendon remains unimodal until the onset of maturity, beyond which the distribution becomes bimodal. Fibril diameter distributions for ligament, extensor and flexor tendon at old age are, as at birth, virtually identical. An estimate has been made of fibrillar collagen content in the three tissues as a function of age. As with rat-tail tendon, the collagen content increases steadily from birth to maturity, at which stage the content remains fairly constant though it does drop slowly with increasing age. The presence of well developed elastic tissue in foetal and immature tendon and ligament shows that the development of the elastic fibres does not parallel the development of the collagen fibrils. In diseased tissues from a 3 year suspensory ligament and an 8.5 year superficial flexor tendon only immature elastic fibres have been observed. Furthermore, since the collagen fibril diameter distributions in these specimens show a marked change from the norm, it would be expected that the mechanical properties of these tissues would be altered significantly.

Adverse effects of peroxidized lipid on human spermatozoa

Abstract: Peroxidation of the sperm cells' own phospholipids, closely bound up with declining motility, has been demonstrated in human spermatozoa; the rate of peroxidation was determined quantitatively by the reaction with thiobarbituric acid. Immotile or poorly motile spermatozoa from necrospermic or oligospermic semen exhibited a higher rate of peroxidation than highly motile spermatozoa from normal ejaculates. Peroxidized unsaturated fatty acids added to washed sperm suspensions immobilized the spermatozoa rapidly and permanently. Human seminal plasma was found to contain a non-ultrafiltrable factor which effectively prevented, but did not reverse, the toxic effect upon spermatozoa of either endogenous or exogenous lipid peroxides.

Antigenic variation in trypanosomes.

Abstract: The several species of African trypanosomes pathogenic to man and domestic livestock may evade the immune response through a process of antigenic variation. This phenomenon is the major obstacle to immunization. Variation may be recognized in the form of sequential changes in the serological characteristics of the trypanosome surface during the course of an infection. Each serological variant of Trypanosoma brucei possesses a unique glycoprotein covering the entire trypanosome surface. When an immune response occurs against the prevailing variant, the trypanosomes are able to express an alternative and immunologically distinct glycoprotein. The antigenic repertoire of a trypanosome clone or species is so far undetermined and the genetic basis of antigenic variation is unexplored. Structural studies indicate that the immunological uniqueness of each glycoprotein is due primarily to immense variations in amino acid sequence distributed throughout the single polypeptide backbone. The extent of glycosylation is also variable.

Nickel Accumulation by European Species of the Genus Alyssum

Abstract: Herbarium specimens (over 400) of all the 64 species of Alyssum listed in the Flora Europaea were analysed for their nickel and cobalt content in order to identify hyperaccumulators (> 1000 $\mu $g/g dry mass) of either element. In addition to the three previously recorded hyperaccumulators of nickel (A. bertolonii, A. murale and A. serpyllifolium s.sp. lusitanicum), a further 11 were identified. With the exception of A. fischerianum, all hyperaccumulating species are from Section Odontarrhena of the genus and include A. alpestre, A. argenteum, A. corsicum, A. euboeum, A. fallacinum, A. heldreichii, A. markgrafii, A. robertianum, A. smolikanum and A. tenium. Nickel contents exceeding 10000 $\mu $g/g (1%) were recorded in A. bertolonii, A. corsicum, A. heldreichii, A. markgrafii and A. robertianum. The occurrence of 14 hyperaccumulators in a single genus is the highest density so far recorded and poses interesting questions concerning the plant chemistry of these species.

Morphological types of horizontal cell in the retina of the domestic cat

Abstract: Two morphologically distinct types of horizontal cell are described from Golgi-stained whole mounts of the cat retina They are referred to as A-type and B-type cells The two types differ in their dendritic branching pattern, their overall size and the absence or presence of an axon At every retinal position the dendrites of B-type cells branch more densely and overlap each other more frequently than do the dendrites of A-type cells At equivalent retinal positions the dendritic field size of A-type cells is greater than that of B-type cells by a factor of about 15 Only B-type cells have an axon, which branches at the end into a large axon terminal system The axons have no preferred direction of orientation The stain-ability of horizontal cells by different Golgi methods is discussed

Morphology and molecular composition of isolated postsynaptic junctional structures.

Abstract: The solubilization of isolated brain synaptosomal plasma membranes by various detergents was studied and in each case found to depend upon detergent concentration. By using conditions sufficient to extract maximally protein and phospholipid from the membranes, postsynaptic junctional particles were isolated with each of four detergents and their ultrastructural appearances and protein contents compared. Two basic structural forms were identified. One, isolated with Triton X-100, consists of a planar array of dense-staining particles ca. 20 nm in diameter. It resembles the postsynaptic density seen in undigested synaptosomal plasma membranes. The other, isolated with sodium deoxycholate, contains less protein. It has the same overall shape and dimensions as the postsynaptic density, but consists of a branching network of short 5 nm fibres (the postsynaptic junctional lattice) making up an array of contiguous polygons, each ca. 20 nm across. The interior of these polygonal elements seems to be hydrophobic since it cannot be penetrated by metallic salts used for negative staining. It is suggested that the dense-staining 20 nm subunits observed at the postsynaptic junctional site may be composed of hydrophobic proteins inserted into the hollow cores of the lattice polygons. Electrophoretic analysis of the proteins present in the various postsynaptic junctional preparations identified two major common components with molecular masses of 275000 and 47500. The latter is tentatively identified as actin. Components comigrating respectively with $\alpha $-and $\beta $-tubulin are present, and the relation of the lattice structure to subjacent microtubules is discussed.

Random Partitions in Population Genetics

Abstract: One of the most striking results of recent theoretical research in population genetics is the sampling formula associated with the name of W. J. Ewens, who enunciated it in 1972, since which time it has been shown to hold for many different population models. This asserts that, if a sample of n gametes is taken from a population, and classified according to the gene at a particular locus, then the probability that there are a 1 alleles represented once in the sample, a 2 represented twice, a 3 thrice, and so on, is given for some positive value of θ by the expression Pn(a1,a2..., an) = n !/θ(θ+1)...(θ+ n ─1) ∏nj=1 ﴾θaj/jajaj !﴿. Most of the models for which this has been established share three broad features: ( a ) the size of the population is large compared with n , and the expected total number of mutations per generation is moderate (and in fact differs from θ by a constant factor depending on the reproductive mechanism), ( b ) the population is in statistical equilibrium under mutation and genetic drift, with selection at the locus playing a negligible role, and ( c ) mutation is non-recurrent, so that every mutant allele is a completely novel one.

An Abnormal Membrane Glycoprotein Associated with Malignancy in a Wide Range of Different Tumours

Abstract: Malignancy, as measured by the ability of cells to grow progressively in vivo, is intimately linked to the presence of a structural abnormality in the polysaccharide moiety of one particular membrane glycoprotein. This abnormality is present in a wide range of different tumours; it cosegregates with malignancy in all crosses between malignant and non-malignant cells that have so far been tested; and it remains linked to malignancy in a stringent new test in which non-malignant variants are selected from tumour cell populations by the use of a lectin.

Intracellular Calcium and the Regulation of Sodium Transport in the Frog Skin

Abstract: Exposure of frog skins to Na-free solutions on the basal border causes a reduction in transepithelial Na transport. Since Na-free solutions produce an increase in cell Ca concentration in some cell types, we have explored whether such increase plays a role in their inhibitory effects on transepithelial transport. Measurements of 45 Ca uptake in isolated epithelia showed that Na-free solutions caused a threefold stimulation of Ca uptake; nearly all the increased uptake occurred through the basal border. The reduction in transepithelial Na transport caused by Na-free solutions was absent when Ca was also eliminated from the basal solution. These findings suggest that the reduction in transepithelial Na-free solutions is caused by increased cytoplasmic calcium levels. We also found that during the action of metabolic inhibitors, Ca efflux from the epithelium was increased markedly. Since such as increase in efflux is probably due to increased (Ca) in the cytoplasm, we suggest that the reduced epithelial Na permeability observed during metabolic inhibition may be due to increased cytoplasmic calcium.

Some aspects of the H-2 system, the major histocompatibility system in the mouse.

Abstract: The H-2 system is a complex of at least ten loci carried by chromosome 17 of the mouse. The loci can be divided into three or four functional classes concerned with the control and execution of immune response. The different classes are in some specific way involved in: 1. differentiation of bone-marrow derived (B) lymphocytes, leading to the production of humoral antibodies detectable by serological methods; 2. differentiation of thymus-derived (T) lymphocytes, leading to the production of killer cells capable of specifically attacking relevant targets; 3. regulation of immune response, both humoral and cellular (this response can be either enhanced or suppressed); 4. biosynthesis and activation of components of the complement system. On the same chromosome as the H-2 is the T/t complex, which controls embryonic and spermatozoal differentiation. An interesting relation between H-2 and T/t might exist between the complexes of loci.

Gap junctions between dendrites and somata of neurons in the primate sensori-motor cortex.

Abstract: Gap junctions have been found infrequently between two dendrites or a dendrite and a cell soma in the deep layers of both the motor and somatic sensory cortices of the primate. At these junctions the outer leaflets of the plasma membranes of both profiles are intimately apposed with a gap of 2 nm between them which shows a structure of hexagonal subunits in tangential sections. These gap junctions occur mainly between the dendrites or dendrites and somata of large stellate cells but are also associated in some examples with a dendro-dendritic synapse and thus occur between large stellate dendrites and presynaptic dendrites; a desmosome may also occur in association with a gap junction and dendro-dendritic synapse. Gap junctions have been identified as sites of electrical transmission between cells in a number of sites and it is therefore suggested that some neurons in the sensori-motor cortex are electrotonically coupled.

Observations on the vegetation of northeastern Mato Grosso II. Forests and soils of the Rio Suiá--Missu area.

Abstract: The vegetation of the well drained soils along the Suia─Missu road in the Serra do Roncador region of NE Mato Grosso is Evergreen Seasonal forest of Amazonian type. The area lies close to the meeting place of the Amazonian forest (the hylaea) and the cerrado (savanna) formation of Central Brazil. The structure of the forest is simple: the canopy is at about 18−23 m, and is exceeded by a few scattered emergents; no recognizable strata can be distinguished among the understorey trees and the shrub and herb layers are sparse. Table 1 lists the most important species and gives information on stratification and general distribution. Most of the species appear to have a hylaean centre of distribution but extend into other vegetation types. The forest differs from related communities which lie closer to the cerrado/forest boundary in its greater height and luxuriance, the presence of additional tall tree species, and the great reduction in abundance of a cerrado floristic element. A survey on the Xavantina─Sao Felix road allowed us to extend previous observations on the distance to which the cerrado tree Pterodon pubescens extends into the forest. The results obtained indicate a considerable extension of forest into cerrado during the life of an individual tree. A characteristic low forest occurs in the flood plain of the Rio Suia─Missu while Swampy Gallery forests occur on permanently waterlogged soils around the headwaters of streams. The well drained soils of the Suia─Missu forest are very uniform, deep latosols (oxisols) of very dystrophic nature with pH (in water) between 4.0 and 5.0 (see table 2, p. 203).

Growth and cell replacement in the new-born pig intestine

Abstract: Both macroscopical and microscopical aspects of growth in the pig small intestine have been measured and related to the kinetics of cell turnover and changing physiological function in the period immediately following birth. The first 10 days of post-natal life was associated with an 80% increase in length and a 30% increase in diameter of the small intestine. The absorptive area doubled during this period of post-natal growth. Villi formed at birth were 29, 50 and 75% longer by day 10 (proximal, mid and distal regions respectively). Maximal elongation occurred on day 6 of post-natal life. Thymidine labelling of cells showed mitosis to be confined to the crypts. Newly formed cells migrated onto the villi. The mean rate of cell appearance in a section was 0.4 cell/h. This rate remained unchanged and independent of the area of the small intestine chosen for study during the first 4 days of post-natal life. Examination of villus sections at different times following injection of radioactive thymidine showed considerable cell mixing to occur. Labelled cells, which never formed protein-filled vacuoles, became mixed with vacuolated and non-vacuolated, unlabelled cells. There was also a redistribution of labelled cells upon the villus. The level of radioactivity within cells did not always correspond to that predicted from their respective positions on the villus. Two alternative theories are put forward to account for these findings. The time taken for complete cell replacement is approximately 19 days. The mucosal uptake of macromolecules finally ceases and the resistance of piglets to enteric viral infections increases at about this time. It is suggested that both events are dependent on the final disappearance of cells originally present in the pig intestinal mucosa at birth.

The Photoreceptors and Pigment Epithelium of the Larval Xenopus Retina: Morphogenesis and Outer Segment Renewal

Abstract: Light microscopic autoradiography and electron microscopy were used to examine outer segment renewal and the development of photoreceptors and pigment epithelium in the larval Xenopus retina. Following the injection of [ 3 H]-leucine at stages 37/38–40 (when outer segments first develop) or 53–54 (when rod outer segments (r. o. s.) attain adult length), a band of label accumulated at the base of r. o. s. and was displaced sclerally with time, whereas label was diffusely distributed in cone outer segments (c. o. s.). By taking into account the change in shape of r. o. s. from conical to cylindrical around stage 46, and calculating outer segment growth (determined from the rate of band displacement) as volume of material added with time, we found a constant rate of membrane addition (1.59 μm/day) from the time of initial outer segment formation. The changes observed in r. o. s. length therefore indicate variations in the rate of disk shedding and phagocytosis, which is minimal before stage 46 and rises to 1.19 μm/day after stages 53–54. Ultrastructural observation showed that although all photoreceptor outer segments form by the repeated evagination of the plasma membrane of the connecting cilium, r. o. s. and c. o. s. are distinguishable by differences in membrane appearance even before they develop divergent membrane topologies. Fibrous granules near the basal body of young receptors may be precursors to the elongating ciliary microtubules. Clusters of cisternae observed near the ciliary base in photoreceptor inner segments may represent a stage in the transport of newly-synthesized opsin to the outer segment base.

Intracellular recording from visually identified motoneurons in rat spinal cord slices.

Abstract: Motoneurons were directly visualized with Nomarski optics in slices prepared from new born rat spinal cord. Intracellular recordings from these neurons showed spontaneous potentials, probably triggered by interneuronal activity. Action potentials could also be evoked by direct intracellular stimulation of the motoneurons. Iontophoretically applied L-glutamate caused a fast depolarization of the motoneuronal membrane. Considerable differences in local sensitivity to L-glutamate were found on the surface of the motoneuron.

Receptor contacts of horizontal cells in the retina of the domestic cat

Abstract: The terminal aggregations of A- and B-type horizontal cells, stained by the Golgi-Colonnier method, have been analysed. The pattern of the aggregations is regular and is shown to be in register with the cone mosaic. Both types of horizontal cell are in contact with at least 80% of the cones above their dendritic fields. Therefore, the different horizontal cell classes cannot be selective for a special kind of cone but must have at least 60% of the cone input in common. Each A-type horizontal cell makes contacts with between 120 and 170 cones, and each B-type horizontal cell with 60-90 cones. An individual A-type horizontal cell occupies an average of 20% of the lateral elements of the triads in a cone pedicle, but an individual B-type cell fills only some 13%. Each and every cone is connected with several of both types of horizontal cell. An estimation of the number of rods converging onto a single axon terminal system showed that it could be as many as 3000.