Nerve-independent DNA synthesis and mitosis in regenerating hindlimbs of larval Xenopus laevis

Summary Xenopus laevis larvae at stage 52–53 (according to Nieuwkoop and Faber 1956) were subjected to amputation of both limbs at the thigh level as well as to repeated denervations of the right limb. Results obtained in larvae sacrificed during wound healing (1 after amputation), blastema formation (3 days) and blastema growth (5 and 7 days) showed that denervated right limbs have undergone the same histological modifications observed in innervated left limbs and have formed a regeneration blastema consisting of mesenchymal cells with a pattern of DNA synthesis and mitosis very similar to that in presence of nerves. Also, the patterns of cellular density in regenerating right and left limbs were very similar. On the whole, the data here reported show a highly remarkable degree of nerve-independence for regeneration in hindlimbs of larval Xenopus laevis at stage 52–53 and lend some substance to the hypothesis that, in early limbs, there would exist trophic factors capable of replacing those released by nerves, promoting DNA synthesis and mitosis in blastemal cells. Offprint requests to: S. Filoni     

Transforming Growth Factor: β Signaling Is Essential for Limb Regeneration in Axolotls

Axolotls (urodele amphibians) have the unique ability, among vertebrates, to perfectly regenerate many parts of their body including limbs, tail, jaw and spinal cord following injury or amputation. The axolotl limb is the most widely used structure as an experimental model to study tissue regeneration. The process is well characterized, requiring multiple cellular and molecular mechanisms. The preparation phase represents the first part of the regeneration process which includes wound healing, cellular migration, dedifferentiation and proliferation. The redevelopment phase represents the second part when dedifferentiated cells stop proliferating and redifferentiate to give rise to all missing structures. In the axolotl, when a limb is amputated, the missing or wounded part is regenerated perfectly without scar formation between the stump and the regenerated structure. Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele limb regeneration. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-β). In the present study, the full length sequence of the axolotl TGF-β1 cDNA was isolated. The spatio-temporal expression pattern of TGF-β1 in regenerating limbs shows that this gene is up-regulated during the preparation phase of regeneration. Our results also demonstrate the presence of multiple components of the TGF-β signaling machinery in axolotl cells. By using a specific pharmacological inhibitor of TGF-β type I receptor, SB-431542, we show that TGF-β signaling is required for axolotl limb regeneration. Treatment of regenerating limbs with SB-431542 reveals that cellular proliferation during limb regeneration as well as the expression of genes directly dependent on TGF-β signaling are down-regulated. These data directly implicate TGF-β signaling in the initiation and control of the regeneration process in axolotls.     

Cellular contribution to symmetrical forelimbs from triploid-marked "polarizing region" in the embryo of axolotl, Ambystoma mexicanum

Grafts of posterior tissue placed anterior to the limb bud in the salamander embryo exert a polarizing influence. To explain this result, the idea that the anteroposterior axis of the developing forelimb is polarized by a diffusible morphogen has been proposed. An alternative hypothesis, and the working hypothesis of the present study, is that the polarization of the developing salamander forelimb is accomplished by short-range cellular interactions resulting in intercalation rather than by the more global influence of a diffusible morphogen. One prediction of this intercalation hypothesis is that cells will be contributed to the limb from the "polarizing tissue." To test this idea, grafts of triploid marked polarizing tissue were implanted anterior to the limb bud in 82 diploid axolotl embryos at stages 32-34 of development. A total of 27 (33%) of the limbs that resulted were symmetrical and ranged in complexity from one to seven digits. Histological analysis of a subgroup of the original symmetrical limbs revealed that mesodermally derived tissues in the anterior side of these limbs (the side which formed as a duplication in response to the influence of the graft) contained high percentages of trinucleolate cells (muscle, 12.1%; connective tissue tissue, 12.5%; and cartilage, 13.4%) when compared to similar tissues in the posterior side of the same symmetrical limbs (muscle, 1.8%; connective tissue , 0.7%; and cartilage, 0.6%). When symmetrical limbs were amputated, 73% regenerated symmetrical limbs. When these regenerated limbs were again amputated, 63% formed symmetrical secondary regenerates. Histological analysis of the first generation of regenerated limbs revealed that the pattern of distribution of trinucleolate cells in each regenerate was similar to the pattern seen in the original symmetrical limb. These results indicate that there is considerable cellular contribution to the anterior side of the symmetrical forelimb from the mesoderm of grafted "polarizing tissue." This result supports the idea that short-range cellular interaction are sufficient for formation of symmetrical forelimbs in salamander embryos.     

Anatomical Network Comparison of Human Upper and Lower,Newborn and Adult,and Normal and Abnormal Limbs,with Notes on Development,Pathology and Limb Serial Homology vs. Homoplasy

How do the various anatomical parts (modules) of the animal body evolve into very different integrated forms (integration) yet still function properly without decreasing the individual’s survival? This long-standing question remains unanswered for multiple reasons, including lack of consensus about conceptual definitions and approaches, as well as a reasonable bias toward the study of hard tissues over soft tissues. A major difficulty concerns the non-trivial technical hurdles of addressing this problem, specifically the lack of quantitative tools to quantify and compare variation across multiple disparate anatomical parts and tissue types. In this paper we apply for the first time a powerful new quantitative tool, Anatomical Network Analysis (AnNA), to examine and compare in detail the musculoskeletal modularity and integration of normal and abnormal human upper and lower limbs. In contrast to other morphological methods, the strength of AnNA is that it allows efficient and direct empirical comparisons among body parts with even vastly different architectures (e.g. upper and lower limbs) and diverse or complex tissue composition (e.g. bones, cartilages and muscles), by quantifying the spatial organization of these parts—their topological patterns relative to each other—using tools borrowed from network theory. Our results reveal similarities between the skeletal networks of the normal newborn/adult upper limb vs. lower limb, with exception to the shoulder vs. pelvis. However, when muscles are included, the overall musculoskeletal network organization of the upper limb is strikingly different from that of the lower limb, particularly that of the more proximal structures of each limb. Importantly, the obtained data provide further evidence to be added to the vast amount of paleontological, gross anatomical, developmental, molecular and embryological data recently obtained that contradicts the long-standing dogma that the upper and lower limbs are serial homologues. In addition, the AnNA of the limbs of a trisomy 18 human fetus strongly supports Pere Alberch''s ill-named "logic of monsters" hypothesis, and contradicts the commonly accepted idea that birth defects often lead to lower integration (i.e. more parcellation) of anatomical structures.     

Muscle development in dendrobranchiate shrimp, with comparison with Artemia

The muscle pattern of malacostracan and entomostracan crustacean nauplius larvae was compared using fluorescent phallotoxins. In the dendrobranchiate malacostracan Sicyonia ingentis, F-actin staining was first detected in limb setae at 12 h, likely within sensory nerves. Staining of F-actin was detected in the trunk at 15 h and grew into the naupliar limbs. Sarcomeres were detected at 19 h, identifying the structures as extrinsic limb muscles. The extrinsic limb muscles enlarged but retained their general pattern during the later nauplius stages. Longitudinal trunk muscles and circumferential visceral muscle (VM) developed in the post-naupliar region during nauplius instars 4 and 5, at the time when the gut also formed. In the anostracan branchiopod Artemia salina, the newly hatched nauplius contained an extensive system of extrinsic and intrinsic limb muscles. The gut was almost complete at hatching, along with its associated circumferential VM. Muscles similar in position and structure could be identified in nauplii from the two taxa, but different anatomical origins of extrinsic muscles were evident. Whether the naupliar limb muscles are homologous in malacostracans and branchiopods remains an open question. The strong musculature of the dendrobranchiate naupliar limbs correlates with the use of all three pairs of limbs for swimming.     

Central program of hind limb interaction during locomotion in cats

The central program for interaction between the hind limbs, expressed as the time structure of motor discharges in the nerves to the various muscles, was studied in immobilized decerebrate spinal cats during fictitious locomotion. The program of hind limb interaction (alternating or inphase) in the decerebrate cats was shown to be determined by the relations between the flexor hemicenters. The activity of the latter is either antiphased or cophased. The character of activity of the extensor hemicenters is determined secondarily on account of alternating interaction of each of them with the ipsilateral flexor hemicenter. After injection of dopa into the animals the cophased program of hind limb interaction may be determined by the cophased working of the extensor center.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 1, pp. 65–73, January–February, 1979.     

Comparative morphology among trunk limbs of Caenestheriella gifuensis and Leptestheria kawachiensis (Crustacea: Branchiopoda: Spinicaudata)

Morphological differences among groups of the 24 trunk limbs of Caenestheriella gifuensis (Ishikawa, 1895) and differences between males and females are described and illustrated. A setose attenuate lobe located proximally near enditic lobe 1 and a discoid lobe covered with small setae proximal to enditic lobe 1 are newly described. The five ventral enditic lobes, endopod, exopod, and dorsal exite of traditional spinicaudatan morphology are redescribed. Trunk limbs 1–4 of females bear a palp on enditic lobe 5 and trunk limbs 1–15 of males bear a similar palp. A second, articulating palp is associated with the base of the endopod of trunk limbs 1–2 of males. The proximal part of trunk limbs 19–24, bearing enditic lobe 1, articulates by an arthrodial membrane with the remainder of the limb, and the exite is distal to this arthrodial membrane. Development of trunk limbs, ascertained through an examination of early juvenile instars of Leptestheria kawachiensis Uéno, 1927, includes an asetose limb followed in time by a series of setose limbs that increase in morphological complexity with age. The number of lobes on the asetose limb varies from seven (corresponding to five enditic lobes, an endopod, and an exopod) on anterior limbs to five on trunk limb 24, which lacks the lobes corresponding to enditic lobe 4 and the endopod; these two structures are added later to setose limbs. The attenuate lobe, the discoid lobe, the exite of all trunk limbs, and the palps of the anterior trunk limbs are added to the setose limbs. Development of anterior limbs is accelerated relative to that of posterior limbs, and development of the more posterior limbs is truncated relative to that of limbs immediately anterior to them. Enditic lobe 4 and the endopod of limbs like trunk limb 24 develop from, or are patterned by, enditic lobe 5; the articulating palp of male trunk limbs 1–2 also may be added in this way. A comparison of these observations with development of the copepod maxilliped suggests that the spinicaudatan trunk limb is composed of a praecoxa with three lobes, a coxa and a basis each with one lobe, and an endopod of three segments in females and four in males. This is similar to the homology scheme previously proposed by Hansen in 1925. A critique is given of attempts to homologize parts of arthropod limbs based on developmental gene expression patterns. Stenopodal to phyllopodal transformations of maxillipeds in copepods provide a model at least partly applicable to spinicaudatans, and a ‘multibranched’ interpretation of spinicaudatan (and by extension branchiopodan) limb morphology is rejected. There is nothing intrinsic to the structure of the adult trunk limbs suggesting that they are similar to the adult limbs of the ancestral branchiopod or the ancestral crustacean, but early developmental steps of more posterior limbs are good matches for the morphology of an ancestral crustacean biramal limb predicted by a hypothesis of duplication of the proximo‐distal axis. © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 139 , 547–564. No claim to original US government works.     

The ontogeny of the cypridid ostracod Eucypris virens (Jurine, 1820) (Crustacea,Ostracoda)

The chaetotaxy (shape, structure and distribution of setae) of appendages and valve allometry during the post embryonic ontogeny of the cyprididine ostracod Eucypris virens are described. It is shown that the basic ontogenetic development of E. virens is very similar to that of other species of the family Cyprididae. During ontogeny, the chaetotaxy shows continual development on all podomeres of the limbs with the exception of the last podomere on the antennulae. The long setae on the exopodite and protopodite of the antennae have a natatory function until the actual natatory setae develop in later instars. Aesthetascs (presumed chemoreceptors) ya and y3 are the first to develop and may have an important function in the first instars. Cyprididae require a pediform limb in the posterior of the body presumably to help them to attach to substrates and this is reflected by the pediform nature of one limb at all times throughout all instars. This study has also shown that the fifth limb is most probably of thoracic origin and hence ostracods have only one pair of maxillae.     

The feeding behaviour of Leptodora kindti and its impact on the zooplankton community of Neusiedler See (Austria)

Leptodora kindti is a very efficient invertebrate predator. Its searching mode of preying is tactile. The setae of the first thoracic limb act as mechanoreceptors, the other thoracic limbs, thorax and head together form the shape of an open basket in which after encounter the prey is pushed in by the aid of the first thoracic limbs and the furca. In Neusiedler See, small individuals of Diaphanosoma brachyurum (0.6–0.9 mm) are the preferred prey, rarely copepods are taken. The predation rate is influenced by temperature, prey density and predator size and varies between less than one and 12 prey items per predator per day. At high predator densities, Leptodora will have a substantial effect on the Diaphanosoma population of Neusiedler See.     

The evolutionary transformation of phyllopodous to stenopodous limbs in the Branchiopoda (Crustacea)--is there a common mechanism for early limb development in arthropods?

Arthropods and in particular crustaceans show a great diversity concerning their limb morphology. This makes the homologization of limbs and their parts and our understanding of evolutionary transformations of these limb types problematical. To address these problems we undertook a comparative study of the limb development of two representatives of branchiopod crustaceans, one with phyllopodous the other with stenopodous trunk limbs. The trunk limb ontogeny of a 'larger branchiopod', Cyclestheria hislopi ('Conchostraca') and the raptorial cladoceran Leptodora kindtii (Haplopoda) has been examined by various methods such as SEM, Hoechst fluorescent stain and expression of the Distal-less gene. The early ontogeny of the trunk limbs in C. hislopi and L. kindtii is similar. In both species the limbs are formed as ventrally placed, elongate, subdivided limb buds. However, in C. hislopi, the portions of the early limb bud end up constituting the endites and the endopod of the phyllopodous filtratory limb in the adult, whereas in L. kindtii, similar limb bud portions end up constituting the actual segments in the segmented, stenopodous, and raptorial trunk limbs of the adults. Hence, the portions of the limbs corresponding to the endites of the phyllopodous trunk limbs in C. hislopi (and other 'larger branchiopods') are homologous to the segments of the stenopodous trunk limbs in L. kindtii. It is most parsimonious to assume that the segmented trunk limbs in L. kindtii have developed from phyllopodous limbs with endites and not vice versa. This study has demonstrated at least one way in which segmented limbs have been derived from phyllopodous, multi-lobate limbs during evolution. Similar pathways can be assumed for the evolution of stenopodous, segmented and uniramous limbs in other crustaceans. Irrespective of the differences in the adult limb morphology, the early patterning of arthropod limbs seems to follow a similar principle.     

Larval development of Japanese 'conchostracans': part 2, larval development of Caenestheriella gifuensis (Crustacea, Branchiopoda, Spinicaudata, Cyzicidae), with notes on homologies and evolution of certain naupliar appendages within the Branchiopoda

As part of a larger project examining and comparing the ontogeny of all major taxa of the Branchiopoda in a phylogenetic context, the larval development of Caenestheriella gifuensis (Ishikawa, 1895), a Japanese spinicaudatan ‘conchostracan’, is described by scanning electron microscopy. Seven different larval stages are recognised, in most cases based on significant morphological differences. They range in length from about 200 to 850 μm. Nauplius 1 has a plumb and lecithotrophic appearance with a rounded hind body and a labrum with an incipient medial spine. Limb segmentation is mostly unclear but the second antennae have more putative segments delineated than are expressed in the later stages. Feeding structures such as the mandibular coxal process and antennal coxal spine are only weakly developed. Nauplius 2 is very different from nauplius 1 and has three large spines on the labral margin and two long caudal spines. Feeding structures such as the mandibular coxal process and various spines and setae are developed, but whether feeding begins at this stage was not determined. The mandible has developed an ‘extra’ seta on endopod segment 1, absent in Nauplius 1. The segmentation of the second antenna has changed significantly due to fusions of various early segments. Nauplius 3 is like nauplius 2 in morphological detail, but larger and more elongate. Nauplius 4 has developed a pair of small anlagen of the carapace and rudiments of the first five pairs of trunk limbs, and the coxal spine of the antenna has become distally bifid. Nauplius 5 has a larger carapace anlage, externally visible enditic portions of the elongate trunk limbs, and a pair of primordial dorsal telson setae. Nauplius 6 has a larger and partly free carapace and better-developed, partly free trunk limbs with incipient enditic, endopodal, and exopodal setation. A pair of caudal spines, dorsal to the large caudal spines, has appeared. Nauplius 7 is quite similar to nauplius 6 but is larger and has slightly longer caudal and labral spines; also, the setation of the most anterior trunks limbs is better developed. The larval development is largely similar to that of other spinicaudatans. The larval mandible, which is evolutionarily conservative within the Branchiopoda, reveals a setation pattern similar to that of the Anostraca and Notostraca (two setae on mandibular endopod segment 1). Most other spinicaudatans and all examined laevicaudatans share another setal pattern (one seta on mandibular endopod segment 1), which could indicate a close relationship among these taxa. The second antenna undergoes a special development, which provides an insight into the evolution of this limb within the Branchiopoda. In nauplius 1 the basipod, endopod, and exopod are all superficially divided into a relatively high number of segments. In later nauplii some of these have fused, forming fewer but larger segments. We suggest that this ontogeny reflects the evolution of antennae in the conchostracans. Various aspects of the morphology of the antennae are discussed as possible synapormorphies for either the Diplostraca or subgroups of the Conchostraca.     

Alona brandorffi sp. n. (Crustacea: Anomopoda: Chydoridae) – a new species from Brazil, related to A. verrucosa Sars, 1901

Alona brandorffi sp. n, related to A. verrucosa Sars, 1901 is described from Boa Vista, Brazil. Parthenogenetic females and males of A. brandorffi were studied. Examination of trunk limbs of A. brandorffi reveals several unusual modifications in structure, unique for the genus, such as very short setae on endites 2 and 3 of limb I, peculiar IDL setae, limb II with scrapers 7–8 with reduced distal part and only six, instead of seven setae in the filter plate, limb IV with only three, instead of four, setae on the inner lobe. The relationships and place of A. brandorffi within the genus Alona are discussed.     

Role of afferent activity in the generation of stepping movements

Efferent fibers of the hind limbs were divided and electrical activity of a filament from the ventral root of S1 was recorded in experiments on mesencephalic cats capable of locomotion in response to stimulation of the "locomotor region" of the midbrain. In response to weak stimulation of the locomotor region, when the forelimbs were not performing stepping movements, regular waves of activity appeared in the filament with a period close to the duration of the step during walking (0.5–1.0 sec). This periodic process was largely dependent on the tonic afferent inflow: various extero- and interoceptive stimuli applied to the hind limb could change the period of generation or abolish it. Active stepping movements of the forelimbs as well as passive movements of the hind limbs led to synchronization of activity in the filament with these movements. After division of the afferent fibers to the hind limbs the animals performed one or two steps in response to stimulation of the dorsal root of S1 by a short series of pulses. They could also perform independent stepping movements of the hind limb if 15–30% of the fibers in the dorsal root of L7 remained intact.M. V. Lomonosov Moscow State University. Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 401–409, 1972.     

External morphology of limb development in the amphipod <Emphasis Type="Italic">Orchestia cavimana</Emphasis> (Crustacea,Malacostraca, Peracarida)

The external morphology of limb development in Orchestia cavimana is examined by scanning electron microscopy and fluorescence staining from the appearance of the first limb buds until hatching. As other amphipods, O. cavimana undergoes direct development and the degree of segmental differentiation shows a more or less continual decrease in anteroposterior direction. Limbs form ventrally as small buds, which elongate and divide into podomeres early in development. This early subdivision largely corresponds with the limb segmentation of the hatchling. When the post-naupliar limbs start to develop, the germ band begins to split into two halves along the midline, so that the trunk limbs transiently occupy a very dorsolateral position. After the germ band has closed again, the differentiation into the characteristic amphipodan tagmata (cephalothorax, pereon, pleon) takes place and the limb podomeres lose their round-shape. The late embryo is covered by a so-called intermediate cuticle, which is formed after an embryonic moult and shed after hatching. The early development of O. cavimana reveals the Anlage of a vestigial seventh pleonic segment that is assumed to belong to the ground pattern of malacostracans, but is retained as a free, limbless segment only in adult Leptostraca. A transient subdivision of the proximal segment of the pleopods suggests the occurrence of a coxa and a basis in these limbs. The mandible attains its upright, adult position via a characteristic bending process that is strikingly similar to that in Archaeognatha (Insecta).     

Comparative analysis of the kinematics of hind limb movements in rats during different kinds of locomotion

A comparative analysis of phases of the locomotor cycle and the dynamics of changes in hind limb joint angles during swimming and stepping movements (on a treadmill), involving the fore- and hind limbs to different degrees, were undertaken in rats. Differences in the sequence and degree of changes in joint angles during locomotion of the types investigated were participation of the forelimbs in locomotion was found to be accompanied by more marked forward carrying of the hind limb. Dependence of the swing phase on duration of the cycle was observed and differences were found in the period of protraction of the limb (F period) during swimming and stepping. The role of central spinal processes and influences of peripheral afferents in the formation of different types of locomotion is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 189–198, March–April, 1985.     

Effects of phosphoramide mustard and acrolein, cytotoxic metabolites of cyclophosphamide, on mouse limb development in vitro

Phosphoramide mustard and acrolein are toxic and reactive metabolites of the widely used anticancer drug and known teratogen cyclophosphamide. To study the mechanism(s) involved and to determine which of the active metabolites of cyclophosphamide is responsible for the production of limb malformations, the effects of exposure of cultured limb buds to phosphoramide mustard and acrolein were investigated. Fore- and hindlimbs were excised from ICR mice on day 12 of gestation and cultured in roller bottles for 6 days. Limbs were exposed to either phosphoramide mustard or acrolein (10 or 50 micrograms/ml) for the first 20 hours of the culture period. Exposure to phosphoramide mustard produced limb reduction malformations in both the fore- and hindlimbs; total limb bone area was greatly reduced, while the relative contribution of the paw to this area in forelimbs was increased. There was a fourfold reduction in both DNA and RNA; protein content was reduced only by one-half. Alkaline phosphatase activity was significantly decreased in fore- and hindlimbs exposed to phosphoramide mustard, whereas creatine phosphokinase activity was only reduced in hindlimbs in the limbs exposed to the higher concentration of phosphoramide mustard. Exposure to acrolein also produced malformed limbs with a mangled appearance; however, total limb bone area and the relative contribution of the long bones versus paw structures were not altered. Acrolein exposure had little effect on growth parameters such as DNA (decreased only in hindlimbs exposed to 50 micrograms/ml), RNA (increased in hindlimbs exposed to 50 micrograms/ml), or protein content. Alkaline phosphatase and creatine phosphokinase activities were not altered in acrolein-exposed fore- or hindlimbs. Thus, phosphoramide mustard and acrolein have dramatically different effects on developing limbs in vitro; this observation may indicate that they have different targets and/or mechanisms of action as teratogens in the limb. The effects of phosphoramide mustard are very similar to those of "activated" cyclophosphamide (4-hydroperoxycyclophosphamide).     

Effect of local cooling of the cortex on evoked potentials in the reticular formation in response to somatosensory stimulation

Potentials evoked in nuclei of the reticular formation by electrodermal stimulation of the limbs were investigated in acute experiments on unanesthetized, immobilized rats during cooling of the somatosensory cortex in the area of representation of one forelimb. Evoked potentials in the reticular formation were found to depend on the degree of cold inhibition of the cortical primary response to the same stimulation. The peak time of the main negative wave increased from 40–50 to 60–80 msec with a simultaneous decrease in its amplitude or its total disappearance in the case of deep cooling of the cortex. Cooling of the cortex had a similar although weaker effect on the earlier wave of the evoked potential with a peak time of 14 msec, recorded in the ventral reticular nucleus. In parallel recordings of potentials evoked by stimulation of other limbs they remained unchanged at these same points of the reticular formation or were reduced in amplitude while preserving the same temporal parameters. Cooling of the cortex thus selectively delays the development and reduces the amplitude of the response to stimulation of the limb in whose area of representation transformation of the afferent signal into a corticofugal volley is blocked. Consequently the normal development of both late and early components of the potential evoked in the reticular formation by somatic stimulation requires an additional volley, descending from the cortex, and formed as a result of transformation of the same afferent signal in the corresponding point of the somatosensory cortex.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 32–38, January–February, 1981.     

Stimulation-induced changes at crayfish (Procambarus clarkii) neuromuscular terminals

Summary The fine structure of neuromuscular terminals of the single excitor axon was examined in the limb stretcher muscle of the crayfish Procambarus clarkii. A morphometric comparsion of the neuromuscular terminals of the left and right limbs of a control crayfish showed them to be similiar in qualitative as well as quantitative features. The excitor axon to the stretcher muscle of the right side was stimulated, by backfiring its branches in the adjacent opener muscle, at 20 Hz for 4–5 h per day over 4–5 days. The stretcher muscle on the left side was not stimulated and served as a control. Morphometric analysis of stimulated terminals revealed an increase in the number of dense bars and synaptic vesicles compared to their non-stimulated, contralateral counterparts. Since dense bars are regarded as active sites of transmitter release, changes in their number provide a morphological basis for synaptic plasticity.     

Dependence of efferent activity parameters on limb position during fictitious scratching in decerebrate cats

The influence of tonic afferent inflow as conditioned by ipsilateral hindlimb position on the efferent activity parameters of the spinal generator governing scratching motion was investigated in immobilized decerebrate cats. A significant correlation was observed between motor activity parameters and ensuing bouts of scratching in the absence of afferent flow (after deafferentation of the limbs). This correlation was less pronounced when afferentation remained intact and declined when the limb was shifted from the "aimed" to either the "overaimed" or "deflecting backwards" placing of the limb. The statistically significant correlations found between the parameters of different stages of motor activity and their dependence on hindlimb positions during actual scratching could be responsible for the stability of intended placing of the limbs during the performance of oscillatory movements. Hindlimb deafferentation would appear closest to "aimed" position judging by the parameters of efferent activity and the nature of correlations between them.A. A. Bogomolets Institute of Physiology. Academy of Sciences of the Ukrainian SSR. Kiev. Translated from Neirofiziologiya, Vol. 15, No. 5, pp. 636–645, September–October, 1986.     

Limb Regeneration in the Fiddler Crab, Uca pugilator: Histological, Physiological and Molecular Considerations

This paper summarizes our recent work on the histological, physiologicaland molecular aspects of limb regeneration in the fiddler crabUca pugilator and new information is presented on mitotic activityin the blastema during the first days of blastemal organization.We also report for the first time the localization of vertebrategrowth factor immunoreactivity (FGF 2 and FGF 4) in the regeneratingblastema. In the first part of this paper we review recent histologicalfindings concerning the physical events that accompany autotomyof limbs and propose a new function for the autotomy membrane—thetethering of the regenerating pedal nerve to the walls of thecoxa. In the second part of the paper we review our recent findingson the identification and characterization of the Uca ecdysteroidreceptor (UpEcR, and its potential dimer partner, the retinoid-X-receptor,UpRXR). Using Uca-specific antibody probes raised in our lab,we have been able to identify specific cells in the early blastemathat express receptor proteins. The regenerating limb of thefiddler crab is responsive to both steroids and retinoids andmRNA for steroid and retinoid receptors are expressed in theregenerating limb buds during all stages of regeneration. TheDNA and deduced amino acid sequences of the ecdysteroid receptoris very similar to the sequences of insect EcRs, while the retinoidreceptor is similar to insect protein (ultraspiracle) in theDNA-binding domain, but closer to vertebrate RXRs in the ligandbinding domain