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Biomedical subjects

V French

Publications and source records attributed to V French.

32 records · Page 2Linked to original sources
PubMed1986

Regeneration in the anterior-posterior axis of the insect thoracic segment.

After removal of a transverse strip of ventral thorax from the beetle, Tenebrio molitor, interaction occurred between epidermis posterior to the mesothoracic leg and that anterior to the metathoracic leg. Depending on the size and position of the excision, this interaction resulted in either the regeneration of the extirpated tissue or its replacement by an A/P reversed pattern of sclerites and supernumerary leg. By either route, local pattern continuity was restored between the normal meso- and metathoracic legs. Similarly, when a leg plus adjacent tissue was extirpated, continuity was restored by leg regeneration or by formation of an A/P reversed duplication of sclerites. The results of these strip excisions can be understood in terms of two current models of the ventral thorax (the Boundary Model and the Polar Coordinate Model), each of which postulates a distinct compartment or region intervening between the epidermis surrounding the bases of successive legs. However, the models do not explain the large differences in the frequency of formation of the duplication/deletion pattern after excisions of different widths. The results are also compatible with a different model, involving an A-P sequence of positional values similar to that proposed for the abdominal segment. Regeneration would restore continuity within the sequence by the shortest route, forming either the midsegment (including the leg) or the intersegmental region. The meso- and metathorax differ in the structure of the ventral sclerites and in the segmentation of the tarsus of the leg. The structures regenerated after the various excisions show that the segment border is not crossed during regeneration and indicate that an A/P compartment border running through the leg is usually also respected. There is no sign, however, of a third line of lineage restriction that would indicate a subdivision of the segment into three compartments (as proposed in the Boundary Model).

Animals↗
PubMed1986

Interaction between the leg and surrounding thorax in the beetle.

Interactions between the insect leg and surrounding thoracic epidermis were studied in the beetle, Tenebrio, by grafting the entire larval prothoracic leg into the metathoracic leg site in various orientations. Control grafts simply heal, but A/P reversed grafts lead to regeneration of supernumerary legs of host orientation in A and P positions. M/L reversed grafts also give supernumeraries, again with host orientation but in M (or MP) and L (or LA) positions. The differences in structure between adult prothoracic and metathoracic legs allow the origin of these supernumeraries to be analysed at coxa and tarsus level. The A/P supernumeraries are consistent and complementary in structure, of host origin on the host side and graft origin on the graft side, and with the borders in apparently constant midmedial and midlateral positions. The M/L supernumeraries, however, are variable and often non-complementary in structure. The results of the A/P and M/L reversals are similar to those found at a more distal level in Tenebrio legs and the legs of several hemimetabolous insects, suggesting that the arrangement of positional values and A and P compartments extends from the epidermis of the leg onto the surrounding thorax. The results of a 180 degrees rotation of the entire leg, however, differ from those found at a more distal level in that the grafted leg rarely derotated and two (or occasionally one or three) supernumeraries are formed in a wide variety of positions, some with constant and others with variable orientation. These results are not readily explained by current models of insect leg formation and regeneration.

Animals↗
PubMed1986

Disruption of segmentation in a short germ insect embryo. I. The location of abnormalities induced by heat shock.

The effect of heat shock (15 min at 48 degrees C) on segmentation has been investigated in the short germ embryo of the locust (Schistocerca gregaria). Prior to formation of the germ anlage and at the disc stage heat shock considerably reduced the survival of eggs but appeared to have little effect upon segmentation. At later stages heat shock had no effect on survival but resulted in disruptions of the segmental pattern. The location of abnormal segments depended upon the stage at heat shock and the number affected depended on its severity. A constant number of normal segments developed between the last segment visible at the time of heat shock and the first abnormal segment. These results are similar to the disruptions observed in amphibian somites following heat shock. However, different parts of the segment pattern varied in their response; the head segments were very rarely affected, and disrupted regions rarely started in the middle abdomen (segments A5 and A6). The results are discussed in relation to two models (the clock and wavefront and progress zone models) that have been proposed as an explanation for the specification of the somite pattern in amphibians.

Animals↗
PubMed1986

Disruption of segmentation in a short germ insect embryo. II. The structure of segmental abnormalities induced by heat shock.

A heat shock (of 15 min at 48 degrees C) given to early embryos of the locust, Schistocerca gregaria, results in localized abnormalities in the segment pattern subsequently formed. Most defects involve two consecutive segments of the thorax or abdomen, and these are analysed in detail. The abdominal defects fall into three main classes each of which involves the absence of a particular region of the segment pair and, in one class, duplication of the region which remains. The thoracic defects similarly involve absence of parts of the segments and the formation of a single limb base from which one, two, or three limbs develop. Heat shock may result in the absence of parts of segments in two distinct ways. It may interfere with the process of segmentation or it may delete parts of already formed segment primordia. These possibilities are discussed although, at present, neither can be excluded. The duplication observed in some abdominal disruptions and the formation of triple limbs indicates that the absence of parts of embryonic segments is followed by pattern regulation similar to that occurring in regeneration studies on larval segments and appendages of other insects. Two out of the three classes of abnormality can be explained in terms of intercalary regeneration restoring pattern continuity, but it is possible that discontinuities persist in the remaining class.

Abdomen↗
PubMed1985

Cell division during intercalary regeneration in the cockroach leg.

In a series of grafting operations on cockroach legs, epidermal cells from different positions or from the same position on the circumference of the femur were placed together. Where cells from different positions were confronted, new cuticular structures corresponding to the positions which would normally have lain between them were formed during the following moults. At the control junctions, where cells from the same positions were placed together, no new structures were formed. Grafted legs were examined histologically at various times after the operation. The events following grafting fell into four phases: wound healing--when epidermal cells migrated over the wound to re-establish epidermal continuity and cells adjacent to the wound divided to compensate for cell emigration; intercalation--when cell divisions took place at the host-graft borders where there was a positional discrepancy; proliferation--when the general growth of the epidermis occurred by widespread cell division; cuticle secretion--when apolysis occurred, cell division ceased, and cuticle secretion began. The results show that intercalary regeneration is associated with local cell division at the graft-host borders, and that these divisions are not confined to the normal proliferative phase of the moult cycle, but begin much earlier in the cycle, as soon as wound healing is complete. These results support epimorphic models (such as the Polar Coordinate Model) of pattern regulation, where change of positional value is tied to cell division, but they do not discount the possibility of a limited initial morphallactic phase.

Animals↗
PubMed1984

The structure of supernumerary leg regenerates in the cricket.

In many insects, grafting a larval leg onto the contralateral leg stump reverses one transverse axis and leads to the regeneration of supernumerary legs at each of the two points of maximum discrepancy on the graft/host junction. These operations were performed on the cricket Acheta domesticus, grafting between tibiae of pro- and metathoracic legs, and between tibia and tarsus, in order to deduce the mode of origin of the supernumeraries from their graft type, host type or composite structure. Supernumerary legs formed after A/P axis reversal are always half-and-half in structure, being of host type on the host side and graft type on the graft side, while supernumeraries formed after M/L axis reversal are variable in structure. These results are generally consistent with the recent Polar Co-ordinate Model (whereby supernumeraries form because the pattern of intercalation between graft and host generates two complete circumferences at the junction), provided that intercalation is restricted by the borders between anterior and posterior leg compartments. However there are features of the structure of the M/L supernumeraries which the Polar Co-ordinate Model does not explain. Medial supernumeraries are often of pure graft type (and lateral ones of host type) or half-and-half with anterior graft type and posterior of host type (while lateral supernumeraries often have the converse structure).

Animals↗
PubMed1984

Butterfly wing patterns: how good a determining mechanism is the simple diffusion of a single morphogen?

The formation of the wing pigmentation patterns of three species of butterflies has been modelled using a mechanism based on a tripod of assumptions. First, that there may be morphogen sources in the foci of eyespots and morphogen sinks at some parts of the wing margin, all other cells being passive. Second, that the morphogen has a finite half life and diffuses simply and freely away from the sources throughout a wing of hexagonally packed cells. Third, that the overt pattern derives from cells interpreting the local morphogen concentration with respect to thresholds which determine scale colours. The final pattern thus follows lines of constant morphogen concentration and may, depending on the distribution of sources, comprise rings, curves or bands. With such a model, we have been able to compute stable patterns having the essential topology of the compound spots of Tenaris domitilla, the large rings of Diaethria marchalii and the pattern of eyespots, rings and asymmetric bands of Ragadia minoa. Quantitative analysis of the pattern-forming process shows that, with a biologically realistic diffusion constant (approximately 5.10(-7) cm2 sec-1) and a morphogen half life less than 6h, the patterns form within approximately 12h over a wing of approximately 1000 cells in length. The limitations of the model are that the exact morphology of the eyespots and bands do not match precisely those of the original wings, that there are edge distortions and that optimal patterns may be critically dependent on the exact positions of sources and sinks. An explanation for part of the discrepancy is that we have assumed an adult wing shape and foci coordinates in modelling a process that took place earlier in development. Nevertheless, the limitations of the model argue against a mechanism based on a single morphogen operating in vivo. However, as the model can generate many features of butterfly wing patterns, it may be considered as a degenerate case of that mechanism.

Animals↗
PubMed1981

Pattern regulation and regeneration.

Insect legs develop from small regions of the embryonic thorax. In most insects they differentiate in the embryo, forming functional larval legs, which grow and moult through larval life. In Drosophila the presumptive legs invaginate to form imaginal discs, which grow through larval life but only differentiate in the pupal stage. Analysis of the structures formed after amputation, grafting and wounding experiments on larval legs and on mature and immature imaginal discs suggests that the same organization of positional information and cellular behaviour is involved in the response of tahe developing leg to disturbance at early stages (termed 'regulation') and at later stages (termed 'regeneration'). The results suggest that developing legs form pattern in accordance with positional information specified in two dimensions within the epidermis, along polar coordinates. A continuous sequence of positional values runs around the circumference and an independent sequence runs down the leg. Two rules govern cellular behaviour after a disturbance. The shortest intercalation rule: interaction between cells with different positional values provokes local growth, producing cells with intermediate values (by the shortest route in the case of the circumferential values). The distalization rule: if intercalated cells have positional values identical to those of adjacent pre-existing cells then the new cells adopt a more distal value. These rules will produce a complete distal regenerate from a complete circumference and may produce a symmetrical regenerate from a symmetrical wound surface. This regenerate may taper (converge) or widen (diverge) and branch into two distal tips, depending on the extent of the original wound and the way in which it heals. The polar coordinate model provides a simple and unified interpretation, in terms of only local interactions, of a wide range of experimentally produced and naturally occurring insect (and crustacean and amphibian) limbs showing regeneration of missing structures, duplication of structures, and the formation of complete, tapering or branching supernumeraries. It is not yet clear what molecular mechanisms could underlie a polar map of positional information, nor how such a map could be initially established at a particular site in the early embryo.

Animals↗
PubMed1980

Positional information around the segments of the cockroach leg.

Epidermal cells from different circumferential positions around the femur of Blabera craniifer can interact to form an intercalary regenerate consisting of that section of the circumference normally separating graft and host positions, by the shorter route. This results is extended to other leg segments; the tibia and coxa (TT and CC Grafts). Grafting strips of integument from the tibia (TF Grafts) or the coxa (CF Grafts) to a corresponding position on the host femur results in simple healing. Grafting to a non-corresponding position leads to intercalation of the shorter intermediate arc of circumference, composed partly of graft-segment and partly of host-segment structures. These results show that that same continuous sequence of positional values is distributed around the circumferences of the coxa, femur and tibia. Cellular interactions along the edges of strip-grafts obey the Shortest Intercalation Rule. At the ends of strip-grafts intercalation usually restores continuity of positional values where possible but, when a complete circumference is generated, a supernumerary distal regenerate is usually formed. This is in general agreement with the Complete Circle Rule and the exceptions are discussed. In intercalary regeneration following the intersegmental strip-grafts, the host femur cells seem unable to intercalate beyond two positions (posterior/internal and posterior/external). These lineage restrictions operating during regeneration indicate that the cockroach leg, like the Drosophila leg disc, may consist of an anterior and a (smaller) posterior 'compartment'.

Animals↗
PubMed1978

Intercalary regeneration around the circumference of the cockroach leg.

Epidermal cells from different circumferential positions around the femur of Blabera craniifer can interact to form an intercalary regenerate. Removal of a longitudinal strip of integument (cuticle plus epidermis) from any position around the circumference leads to the cut edges healing, localized growth and intercalary regeneration of the missing section of the circumference, so that the resulting femur is approximately normal in size and pattern of cuticular structures. Grafting a longitudinal strip of femur integument into a different circumferential position on the host femur confronts epidermal cells from different positions along both the inner and outer longitudinal graft/host junctions. In numerous different situations this results in local growth and intercalary regeneration of that section of the circumference normally separating graft and host positions, by the shorter route around the circumference. Confrontation of opposite positions results in the intercalation of either of the intervening half circumferences. In one opposite confrontation, between mid-anterior and mid-posterior, there was also a third result where graft and host healed together, provoking no intercalary regeneration. Grafts made with reversed proximal/distal polarity show that a confrontation between different circumferential positions gives the same result, regardless of the proximal/distal levels involved, hence circumferential position is an independent aspect of position on the femur. These results strongly suggest that epidermal position is not specified with respect to two transverse axes running through the epidermis and internal tissue of the leg, but that there is a continuous circular sequence of positional values running around the circumference, in the epidermis. This is analogous to but independent of the sequence previously shown by Bohn (1967) and Bullière (1971) to run proximal/distal along a leg segment. Hence epidermal position on the femur is specified in two dimensions and can be represented in terms of the French, Bryant & Bryant (1976) polar co-ordinate model. Interactions along the edges of the strip-grafts conform to the Shortest Intercalation Rule (French et al. 1976). At the proximal and distal ends of strip-grafts intercalation restores normal sequences of positional values where possible. However, where the graft, together with the intercalary regenerates formed at the longitudinal graft/host junctions and the adjacent host tissue formed a complete sequence of circular values, then a supernumerary distal regenerate was formed, in agreement with the Complete Circle Rule of French et al. (1976). The problem of generating a continuous circular sequence of positional values by one or more circumferential gradients, is briefly discussed.

Animals↗
PubMed1976

Pattern regulation in epimorphic fields.

We have described a formal model for pattern regulation in epimorphic fields in which positional information is specified in terms of polar coordinates in two dimensions. We propose that cells within epimorphic fields behave according to two simple rules, the shortest intercalation rule and the complete circle rule, for both of which there is direct experimental evidence. It is possible to understand a large number of different behaviors of epimorphic fields as a straight-forward consequence of these two rules, and the model therefore provides a context in which to view many of the results of experimental embryology. Although we have confined our discussion to cockroach legs, the imaginal disks of Drosophila, and regenerating and developing amphibian limbs, the fact that the model can explain regulative behavior in such evolutionarily diverse animals suggests that it may have general applicability to epimorphic fields. The predictions which the model makes should make it possible to assess its applicability to other developing systems, and to investigate the cellular mechanisms involved.

Amphibians↗
PubMed1976

Leg regeneration in the cockroach, Blatella germanica. II. Regeneration from a non-congruent tibial graft/host junction.

The interactions occurring between host and graft leg epidermis at a non-congruent junction were studied in the cockroach, Blatella germanica. Graft and host tibia were cut perpendicular to the proximal-distal axis and two heteropleural combinations were used to reverse separately the two transverse axes of the graft relative to the host. Use of dark and light cuticle colour mutants gave a good indication of the graft or host origin of regenerated structures. Graft/host junctions regenerated segmented structures in various spatial arrangements, always comprising two copies of all structures distal to the level of the junction. It is concluded that the categories--two separate laterals, double lateral, completely and partially autonomous regeneration--reflect two processes. (i) If the graft tarsus is removed, graft and host may not heal together and interact, but form autonomous regenerates lying in mirror-image symmetry separating original graft and host levels. (ii) If interaction occurs between graft and host (or their developing autonomous regenerates) two laterals of dual origin are produced, one from each point of transverse axis incongruity. These laterals may secondarily fuse together to form a double structure originating from a point of congruity. The orientation and composition of the component tarsi of the double structure depend on the site of origin and the extent to which the two laterals fuse. It is argued that the four 'faces' and two 'transverse axes' of the leg are merely descriptive terms. A new model is developed whereby lateral regeneration arises directly from the circumferential organisation of the leg epidermis. Previous work has shown that position is specified continuously around the circumference, and that intercalary regeneration occurs by the shortest route between confronted positions. After reversal of one 'transverse axis' the shortest route between confronted graft and host positions is different on the two sides of each of the two points of 'axis' incongruity, and at these points the two halves of a complete circumference are formed. These lateral circumferences, like the terminal circumference exposed by amputation, cannot heal over by intercalary regeneration, and this leads to regeneration of distal structures. The model accounts for lateral regeneration after reversal of both 'transverse axes' by 180 degrees rotation of a homopleural graft. The possibility is discussed that there may be clonal restrictions on the circumferential positions which the progeny of a cell may occupy.

Animals↗