Gastrulation and the evolution of development

Development ◽  
1992 ◽  
Vol 116 (Supplement) ◽  
pp. 7-13 ◽  
Author(s):  
L. Wolpert

The original eukaryotic cell may have possessed the key processes necessary for metazoan development - cell differentiation, patterning and motility - and these are present in the cell cycle. Protozoa also possess key patterning processes. It remains a problem as to why there should be two main modes of development - one based on asymmetric cell division and the other on cellular interactions. The latter may be related to asexual reproduction. The morphogenetic movements of gastrulation - as distinct from specifying the body plan - are highly conserved in a wide variety of organisms. This may reflect the requirement for patterning being specified in two dimensions, sheets of cells, and a third dimension being created by cell infolding. The origin of the gastrula can be accounted for in terms of Haeckel's gastrea theory - an early metazoan resembling the gastrula. Gastrulation in Cnidaria may resemble the primitive condition but there is nevertheless considerable diversity. While this may reflect, for example, yolkiness, it seems that there is little selection on developmental processes other than for reliability. Thus it is possible that the embryo is privileged with respect to selection and this may help account for the evolution of novel processes like the origin of the neural crest. Reliability is the key demand made on development This may be provided by apparent redundancy. Since many developmental processes involve switches and spatial patterning reliability is provided by parallel buffering mechanisms and not by negative feedback.

Development ◽  
1994 ◽  
Vol 1994 (Supplement) ◽  
pp. 79-84
Author(s):  
L. Wolpert

A scenario for the evolution of a simple spherical multicellular organism from a single eukaryotic cell is proposed. Its evolution is based on environmentally induced alterations in the cell cycle, which then, by the Baldwin effect, become autonomous. Further patterning of this primitive organism - a Blastaea, could again involve environmentally induced signals like contact with the substratum, which could then become autonomous, by, perhaps, cytoplasmic localization and asymmetric cell division. Generating differences between cells based on positional information is probably very primitive, and is well conserved; its relation to asymmetric cell division is still unclear. Differentiation of new cell types can arise from non equivalence and gene duplication. Periodicity also evolved very early on. The origin of gastrulation may be related to mechanisms of feeding. The embryo may be evolutionarily privileged and this may facilitate the evolution of novel forms. Larvae are secondarily derived and direct development is the primitive condition as required by the continuity principle.


1967 ◽  
Vol 22 (4) ◽  
pp. 422-431 ◽  
Author(s):  
Kyozaburo Kambe

A general theory of electron diffraction by crystals is developed. The crystals are assumed to be infinitely extended in two dimensions and finite in the third dimension. For the scattering problem by this structure two-dimensionally expanded forms of GREEN’S function and integral equation are at first derived, and combined in single three-dimensional forms. EWALD’S method is applied to sum up the series for GREEN’S function.


Parasitology ◽  
1953 ◽  
Vol 42 (3-4) ◽  
pp. 161-186 ◽  
Author(s):  
D. R. Arthur

The palps of all stages of Ixodes trianguliceps are provided with a ventral plate below the basal article; formerly this plate was thought to be the first palpal article. This article in the larva and nymph is produced forward into a spur, but in the female tick this spur is incorporated into the basis capituli as the sella. The hypostomes of I. trianguliceps and I. canisuga are redescribed to clear up existing inaccuracies.Ticks in which the rostrum does not extend beyond the apex of the first palpal article are found on birds, and this probably represents a primitive condition. Those with palpal spurs, which may or may not be fused with the basis capituli, are found on birds (not in Britain) or on mammals of the mouse size group, and those where the rostrum is produced beyond the first palpal article occur on a wide range of large and small animals. The longer and more heavily toothed digits of ticks appear to be associated with a wide host range and vice versa. The structure of the digit may also influence the choice of attachment sites by ticks on their hosts because the microstructure of the skin varies in different parts.Variations in size and form of the scuta of some British ticks are described, and the mean growth rate is ascertained from this data. The information shows that the material of I. ricinus and I. hexagonus is homogeneous, and that specific differences occur in size, shape, the position and type of dermal ducts and in the relation between scutal and alloscutal bristles.The morphology of Gené's organ in I. hexagonus is described. It consists of a basal sac-like portion surmounted by four horns and lined with a cuticle beneath an epithelium. The gland is a proliferation of the epithelium and located near the bifurcation of the base into the horns. A watery refractile fluid, secreted by the gland, accumulates between the epidermis and the cuticle in the horn-like extensions. Proximally the thick endocuticle and epidermis lie close together, and two cuticularized rods penetrate the endocuticle for about half-way along the stalk. The rods arise from the postero-dorsal margin of the basis capituli. Muscles, arising from the free ends of the rods, pass back to near the hind-margin of the scutum; they retract the basis capituli after egg laying and indirectly assist in the retraction of Gené's organ. A suggested mechanism for everting Gené's organ in I. hexagonus is outlined.During feeding the opisthosomatic cuticle of all developmental stages of the tick is much stretched. In the larva, where the cuticle is thin, this is effected by the flattening of the epicuticular pleats, but nymphs and females have, in addition, two longitudinal folds alongside the body which stretch to a far greater extent than do the epicuticular pleats. Similar folds are present between the hard ventral plates and the scutum and between the epimeral and median plates in the male. Coupled with the large size of the male digit, this suggests that morphologically, at least, males can imbibe blood.The growth of the leg segments of I. hexagonus is not constant from stage to stage, and as a result the shortest ‘leg-length’ of the female exceeds the longest ‘leg-length’ of the male.


2002 ◽  
Vol 16 (08) ◽  
pp. 1217-1223 ◽  
Author(s):  
K. V. MCCLOUD ◽  
M. L. KURNAZ

The roughness exponent of surfaces obtained by dispersing silica spheres into a quasi-two-dimensional cell is examined. The cell consists of two glass plates separated by a gap, which is comparable in size to the diameter of the beads. Previous work has shown that the quasi-one-dimensional surfaces formed have two roughness exponents in two length scales, which have a crossover length about 1 cm. We have studied the effect of changing the gap between the plates to a limit of about twice the diameter of the beads. If the conventional scaling analysis is performed, the roughness exponent is found to be robust against changes in the gap between the plates; however, the possibility that scaling does not hold should be taken seriously.


Author(s):  
Babak Ommani ◽  
Nuno Fonseca ◽  
Trygve Kristiansen ◽  
Christopher Hutchison ◽  
Hanne Bakksjø

The bilge keel induced roll damping of an FPSO with sponsons is investigated numerically and experimentally. The influence of the bilge keel size, on the roll damping is studied. Free decay tests of a three-dimensional ship model, for three different bilge keel sizes are used to determine roll damping coefficients. The dependency of the quadratic roll damping coefficient to the bilge keel height and the vertical location of the rotation center is studied using CFD. A Navier-Stokes solver based on the Finite Volume Method is adopted for solving the laminar flow of incompressible water around a section of the FPSO undergoing forced roll oscillations in two-dimensions. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. An averaged center of rotation is estimated by comparing the results of the numerical calculations and the free decay tests. The obtained two-dimensional damping coefficients are extrapolated to 3D by use of strip theory argumentations and compared with the experimental results. It is shown that this simplified approach can be used for evaluating the bilge keel induced roll damping with efficiency, considering unconventional ship shapes and free-surface proximity effects.


1964 ◽  
Vol s3-105 (71) ◽  
pp. 281-299
Author(s):  
ELAINE A. ROBSON

The integument of Peripatopsis moseleyi has been examined by light and electron microscopy with particular reference to the structure and formation of the cuticle. The evidence supports the idea that Peripatus is a true arthropod but not that it has direct affinities with the annelids. The characteristics of arthropod cuticle are present in their simplest form and pore canals and dermal glands are lacking. The cuticle is 1 or 2 µ, thick except in the hardened claws and spines. Above the procuticle (chitinprotein) is a thin 4-layered epicuticle. It is possible that the innermost of the 4 layers (prosclerotin) may correspond to cuticulin of other arthropods. In the claws and spines tanning in this layer extends to the procuticle. Hydrofuge properties of the cuticle probably depend on the outer layers of epicuticle, and it is suggested that the lamina concerned might consist of oriented lipid associated with lipoprotein (Dr. J. W. L. Beament). Wax and cement are absent. Non-wettability of the cuticle is probably ensured by the contours of micropapillae which cover the surface. Similar structures arise in Collembola and other terrestrial arthropods by convergence. The formation of new cuticle before ecdysis is described. After the epicuticular layers are complete, the bulk of the procuticle is laid down in a manner probably common to all arthropods. Secreted materials originate in small vesicles derived from rough endoplasmic reticulum and from scattered Golgi regions. The latter contribute to larger vacuoles which rise to the surface of the cell and liberate material in a fluid state. This later consolidates to form procuticle. Vesicles may also open to the surface directly, and ribosomes probably occur free in the cytoplasm. At this stage the cell surface is reticulate, especially under micropapillae. The ordinary epidermis has only one kind of cell, attached to the cuticle by tonofibrils disposed like the ribs of a shuttlecock, and to the fibrous sheaths of underlying muscle-fibres by special fibres of connective tissue. These features and the presence of numerous sensory papillae are associated with the characteristic mobility of the body wall. The appearance of epidermal pigment granules, mitochondria, the nuclear membrane, and a centriole are noted. No other cells immediately concerned in the formation of cuticle have been found. By contrast myriapods, which do not have wax either, possess dermal glands secreting far more lipid than is found in the Onychophora. The wax layer found in insects and some arachnids constitutes an advance of high selective value which emphasizes the primitive condition of the Onychophora. It is noted that the thick layer of collagen separating the haemocoel from the epidermis probably restricts the transfer of materials. It is suggested that since some features of cuticular structure and formation appear to be common to all arthropods, it is possible that some of the endocrine mechanisms associated with ecdysis may also be similar throughout the phylum.


1984 ◽  
Vol 28 (01) ◽  
pp. 48-54 ◽  
Author(s):  
A. Nestegard ◽  
P. D. Sclavounos

A numerical technique is presented for the solution of deep water linear and time-harmonic wave-body-interaction problems in two dimensions. A mathematical boundary of circular shape surrounding the body is introduced in the fluid domain, thus defining two flow regions. A multipole expansion valid in the outer region is then matched to an integral representation of the solution in the inner region which is obtained by applying Green's theorem and by using the fundamental logarithmic singularity as the Green function. The method applies both to surface-piercing and submerged bodies. Numerical results are presented for the forced oscillations of three surface-piercing ship-like sections of regular shape. No irregular frequencies are encountered. Comparisons with existing numerical methods are made to test the accuracy and emphasize the computational efficiency of the present approach.


2009 ◽  
pp. 45-73
Author(s):  
Giovanni Sacchini

- According to a common conception - among researchers as well - there is no apparent relation between crime perception (or insecurity) indicated in surveys and the crime trend (indicated by the police force), since the perception seems to be influenced above all by what mass-media conveys. Literature indicates two dimensions of crime perception: social concern about criminality and personal fear of the phenomenon. However resorting to a useful empirical indicator, a third dimension may be contemplated between these two: the evaluation of criminality in the residence area. This article considers data that refers to the twenty Italian regions, collected by the National Institution for Statistics - Istat, regarding the Multipurpose Survey on Families. It describes the strong correlation existing between criminality perception in the residence area and the diffusion of certain kinds of predatory crimes, in particular those in which there is a strong author-victim interaction (robbery and bag-snatching) and those in which crime authors are interested in citizens' relevant economical and symbolic goods (houses or cars).


Development ◽  
1993 ◽  
Vol 117 (1) ◽  
pp. 149-162 ◽  
Author(s):  
U. Mayer ◽  
G. Buttner ◽  
G. Jurgens

gnom is one of several genes that make substantial contributions to pattern formation along the apical-basal axis of polarity in the Arabidopsis embryo as indicated by the mutant seedling phenotype. The apical and basal end regions of the body pattern, which include the meristems of the shoot and the root, fail to form, and a minority of mutant embryos lack morphological features of apical-basal polarity. We have investigated the developmental basis of the gnom mutant phenotype, taking advantage of a large number of EMS-induced mutant alleles. The seedling phenotype has been traced back to the early embryo in which the asymmetric division of the zygote is altered, now producing two nearly equal-sized cells. The apical daughter cell then undergoes abnormal divisions, resulting in an octant embryo with about twice the normal number of cells while the uppermost derivative of the basal cell fails to become the hypophysis, which normally contributes to root development. Consistent with this early effect, gnom appears to be epistatic to monopteros in doubly mutant embryos, suggesting that, without prior gnom activity, the monopteros gene cannot promote root and hypocotyl development. On the other hand, when root formation was induced in bisected seedlings, wild-type responded whereas gnom mutants failed to produce a root but formed callus instead. These results suggest that gnom activity promotes asymmetric cell division which we believe is necessary both for apical-basal pattern formation in the early embryo and for root formation in tissue culture.


2007 ◽  
Vol 56 (5) ◽  
Author(s):  
Jean Laffitte

A partire dalla seconda metà del secolo scorso, la Chiesa si è trovata a dover ripensare i rapporti tra fede, teologia e antropologia in problematiche nuove come, ad esempio, la sessualità umana. Interprete privilegiato di questa rielaborazione è stato, senza dubbio, Giovanni Paolo II che in più occasioni ha avuto modo di riflettere e illustrare la teologia, la antropologia e l’etica che sostengono la visione cristiana della sessualità umana. Di questa vasta produzione, l’articolo prende in esame soprattutto le Catechesi di Giovanni Paolo II con frequenti richiami e illustrazioni del pensiero del filosofo Karol Wojty´la. L’analisi dell’autore prende le mosse dall’esposizione di Giovanni Paolo II dei dati creaturali dei tre primi capitoli del libro della Genesi, esaminando, in particolar modo, i significati fondamentali della solitudine originaria dell’uomo verso la creazione e poi il rapporto maschio-femmina. Vengono illustrati quindi l’esperienza dell’amore e l’ethos del dono: l’esperienza cristiana è presentata dal Pontefice come evento e saggezza e legata all’esperienza di amore che l’uomo sperimenta nel rapporto di filiazione che lo unisce a Dio; l’esperienza dell’amore coniugale ruota attorno alla corporeità umana e ai suoi valori/significati. Il corpo assume dunque un significato sponsale che conserva anche dopo la caduta, testimonianza dell’innocenza originaria e della libertà del dono. In tale contesto l’esperienza dell’amore è vissuta come mediazione di una conoscenza che va al di là della persona dell’amato aprendo l’orizzonte al dono divino anteriore. Nella seconda parte del contributo si prendono in esame i significati dell’amore e l’esperienza etica della sessualità così come sviluppati da Giovanni Paolo II: nella corporeità umana, in cui è impressa la complementarietà biologica, vi è una chiamata alla comunione che non è solo comunione tra i due sessi, ma che rimanda a una divina comunione di Persone. L’autore esamina anche l’esercizio della sessualità in rapporto alla legge naturale intesa come conformità alla ragione umana protesa verso la verità. Tale conformità conduce alla retta comprensione dell’intima struttura dell’atto coniugale, la cui “verità ontologica” si manifesta nell'inscindibilità delle due dimensioni unitiva e procreativa. In questa ampia visione della sessualità è compreso anche il mistero dell’amore nuziale tra Cristo e la Chiesa: la comunione di vita e d’amore tra l’uomo e la donna ha come missione propria di significare e rendere attuale l’unione tra Cristo e la sua Chiesa. L’articolo termina con l’analisi del legame tra corpo e sacramento e della dimensione sacrificale e nuziale del dono eucaristico. ---------- Since the second half of the last century, the Church has found herself having to rethink the relationship between faith, theology, and anthropology within new problems concerning, for example, human sexuality. Without any doubt, a privileged interpreter of this reprocessing was John Paul II, who on more occasions had a way of reflecting upon and illustrating the theology, anthropology, and ethics that support the Christian vision of human sexuality. Out of the vast work produced, the article examines especially the Catecheses of John Paul II with frequent appeals to and illustrations of the thought of Karol Wojty´la. The author’s analysis begins its quest with John Paul II’s exposition of creatural data in the first three chapters of the Book of Genesis, examining in particular the fundamental meanings of the original solitude of man toward creation and then the relationship between male and female. The experience of love and the ethos of gift thus come to be illustrated: Christian experience is presented by the Pontiff as event and wisdom and is connected to the experience of love that man experiences in the relationship of filiation that unites Him to God. The experience of conjugal love revolves around human corporeity and its values/meanings. The body thus assumes a spousal meaning that remains even after the Fall, serving as testimony of original innocence and the freedom of gift. Within such a context, the experience of love is lived out as the mediation of knowledge that goes beyond the person of the loved, opening up the horizon to the earlier divine gift. In the second part of this contribution, the meanings of love and the ethical experience of sexuality as such are examined as developments by John Paul II: In human corporeity, upon which biological complementarity is impressed, there is a call to communion that is not only communion between the two sexes, but which refers back to a divine communion of Persons. The author also examines the exercise of sexuality in relation to a natural law intended as conformity to a human reason reaching toward truth. Such conformity leads to the proper understanding of the intimate structure of the conjugal act, whose “ontological truth” manifests itself through the inseparability of the two dimensions: unitive and the procreative. Within this comprehensive vision of sexuality also resonates the mystery of nuptial love between Christ and the Church: The communion of life and love between man and woman that has as its own mission to signify and render present the union between Christ and His Church. The article ends with an analysis of the connection between body and sacrament and of the sacrificial and nuptial dimension of the Eucharistic gift.


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