bluuebunny/crossref_metadata_2025_split huggingface dataset:
The effects of the fibre environment on the paths taken by regenerating optic nerve fibres in Xenopus
Authors: Taylor J. S. H. , Gaze R. M. 
Published: April 2021

ABSTRACT
The paths taken by fibres regenerating to the tectum from various parts of the Xenopus retina were investigated in whole-mount preparations, after localized retinal labelling with HRP. The effects of different environments on the fibres were studied by comparing contralateral with ipsilateral regeneration, in the presence of the other eye or after it had been removed in embryonic life.
Under all conditions fibres from the various parts of the retina regenerated to the corresponding appropriate parts of the tectum, but they took a variety of pathways, some grossly abnormal, to get there. Contralaterally regenerating fibres tended to behave less abnormally than ipsilateral fibres; and regeneration in the absence of the other eye tended to be more abnormal than in its presence. In any one category of regeneration the most nearly normal pathways were those of fibres from temporal retina, followed by ventral, nasal and dorsal fibres.
Fibres regenerating from all parts of the retina, in the presence of the other eye, tended to become gathered into the medial brachium as they approached the tectum. All regenerating fibres approached their tectal terminations by one or more of three main pathways: round one or both brachia, thus encircling the tectum to get to their terminal zone; directly across the tectum; or by passing on to the tectum before changing course. The changes of direction required to enable fibres wrongly positioned in the tract to reach their correct terminal zones were frequently sudden and considerable, and took place on the tectum or at the tectodiencephalic junction. The results are discussed in relation to the differing substrates over which the fibres regenerate.
Chromosome replication in early development of Xenopus laevis
Authors: Laskey R. A. 
Published: April 2021

ABSTRACT
Eggs of Xenopus laevis contain exceptionally large amounts of materials involved in chromosome replication. This maternal stockpile allows an embryo to produce about 80 000 cells in less than 24 h. The adaptations which achieve this involve the mechanisms of both DNA replication and chromatin assembly.
Cytoplasmic localization and chordamesoderm induction in the frog embryo
Authors: L. Gimlich Robert 
Published: April 2021

ABSTRACT
The experiments described here were designed to reveal the distribution in the frog early embryo of components which are sufficient for specification of the dorsal structures of the embryonic body axis. The approach was to allow cleavage planes to divide the embryo into various well-defined regions and to transplant cells from each region into recipient embryos which would otherwise fail to form axial structures. Partial or complete body axis development could then be scored by the use of external criteria or histological methods. Recipients were embryos which had been irradiated before first cleavage with ultraviolet light on the vegetal surface. Irradiated embryos display a well-characterized set of deficiencies in the dorsal structures of the body axis, but their development can be ‘rescued’ toward normalcy in several ways. In particular, transplantation of certain small groups of blastomeres from the normal 32-to 64-cell embryo into irradiated recipients was sufficient to cause partial or complete axis development. Cell groups which could cause rescue were located in the vegetal and equatorial levels of one quadrant of the normal embryo - the quadrant centered on the future dorsal midline. Clonal marking analysis showed that the vegetal-most cells of this quadrant contribute primarily to endodermal structures in normal development. In rescued recipient embryos, these cells also contributed only to the endoderm; the dorsal mesoderm and central nervous system were formed exclusively by host cells which originated near the transplant. Rescue could also result from transplantation of equatorial cells from the dorsal quadrant of the normal embryo. As in normal development, these cells formed primarily the chordamesoderm of the rescued embryo. Host cells were induced to contribute the somitic mesoderm, central nervous system, and other structures which would have been missing but for the presence of the transplanted cells. The frequency and degree of rescue caused by equatorial and vegetal transplants is variable. This was explained by the discovery that the location of components needed for rescue varies among individual embryos without regard to the positions of cleavage planes. This was true even when donor embryos were selected on the basis of a precisely regular pattern of cleavage. In such selected embryos, particular blastomeres make a predictable contribution of progeny to the body axis. Thus it may be that the positions of components which can cause axis formation vary without exact regard to the fate map of prospective areas. The implications of this for the study of cytoplasmic localization in the early embryo are discussed. In any case, it is likely that regional interactions and a degree of developmental autonomy in the area of the prospective chordamesoderm are both involved in formation of the dorsal structures of the embryonic body axis.
Number of Neoblasts in the Intact Body of Euplanaria torva and Dendrocoelum lacteum
Authors: Agnes  , Brøndsted H. V. 
Published: April 2021

ABSTRACT
The total numbers of neoblasts were counted in a specimen of D. lacteum and in a specimen of E. torva. The numbers were respectively 37,826 and 31,320. The distribution of the neoblasts throughout the planarian body does not coincide either with head-regeneration ability or with cross-sectional area along the cephalo-caudal axis of these animals. Although the distribution curve of the neoblasts shows fair agreement in the two species, the time-graded regeneration field is quite different, in Dendro-coelum stopping short just in front of the pharynx, in Euplanaria distributed throughout the animal (A. & H. V. Brøndsted, 1952). The planarian time-graded regeneration field cannot therefore be ascribed to the available numbers of neoblasts at the level of the cut, as is the case in hydroids.
Localization of specific mRNA sequences in Xenopus laevis embryos by in situ hybridization
Authors: Dworkin-Rastl Eva , Kelley Darcy B. , Dworkin Mark B. 
Published: April 2021

ABSTRACT
In situ hybridization of cloned cDNA probes to frozen sections of Xenopus laevis stage-42 tadpoles has been used to determine the tissue localization of several mRNAs. Nine out of sixteen probes tested hybridized to most or all tadpole tissues; seven probes exhibited tissue-specific hybridization. The non-tissue-specific sequences hybridized to RNA species that are also present in maternal RNA while the tissue-specific sequences hybridized to embryonic RNA species induced after gastrulation and undetectable in maternal RNA. Tissue-specific hybridization was observed with muscle (five clones), epidermis (one clone), and the nervous system (one clone). All muscle-specific sequences hybridized to somites and lateral plate muscles, but they differed in their hybridization to heart muscle.
An examination of the evidence for the existence of preformed pathways in the neural tube of Xenopus laevis
Authors: Scott T. M. , Bunt S. M. 
Published: April 2021

ABSTRACT
We have examined the neural tube in Xenopus laevis tadpoles to investigate the anatomical guidance elements which may be present in the presumptive marginal zone. With appropriate fixation protocols the neuroepithelial cells appeared in contact; electron microscopic observations failed to show any specialized intercellular spaces preceding the growing axons. The first fibres were found in the intercellular clefts between the neuroepithelial cells near the surface of the neural tube. Reconstructions of the neural tube from examination of serial 1 μm sections showed that the intercellular clefts are non-aligned at this stage and branching. Scanning electron microscopy of the surface of the neural tube confirmed that the intercellular spaces are non-aligned and often branch caudal to the growing front of descending axons. Thus to grow in a consistent direction the developing axons may have to make consistent and selective (specific) selections of pathway at numerous branch points if their growth is restricted to these intercellular clefts. As more iixons grow along the neural tube, the intercellular clefts become wider, and the neuroepithelial cells bounding the clefts become indented. At later stages many fibres were observed with both scanning and transmission electron microscopy to grow along the surface of the neural tube. These changes in neuroepithelial cell morphology and fibre pathway allow axons to form bundles which take a fairly straight course in contrast to the winding path which must be taken by the first axons to grow through the intercellular clefts.
Comparison of the effects of vitamin A on limb development and regeneration in the axolotl, Ambystoma mexicanum
Authors: Scadding S. R. , Maden M. 
Published: April 2021

ABSTRACT
The objective of this investigation was to compare the effect of vitamin A on limb development and limb regeneration in the same animal, at the same time, thus eliminating the possibility that species differences or different rates of uptake between animals would influence the results. Axolotl larvae had both right limbs amputated and then were treated with retinol palmitate by immersion at 60 or 300 mg l−1 for 4 or 10 days. Intact left developing limbs at the cone, two-digit, or four-digit stages responded to the treatment by deletion of skeletal elements producing hypomorphic limbs. Severity of the deletions was correlated with higher dose, longer times, and earlier stages of limb development. In contralateral right regenerating limbs, the effect of the same treatment was to cause various degrees of proximodistal duplication as well as occasional hypomorphic regenerates. Thus, there is a marked difference in response to vitamin A between developing and regenerating limbs. The implications of this observation are discussed especially with respect to the underlying morphogenetic mechanisms.
Further studies on the melanophores of periodic albino mutant of Xenopus laevis
Authors: Fukuzawa T. , Ide H. 
Published: April 2021

ABSTRACT
It is still unknown why dermal melanophores disappear during larval development, and why no or very few epidermal melanophores appear during and after metamorphosis, in Xenopus laevis showing periodic albinism (ap). To elucidate these points, we investigated (1) the occurrence of depigmentation in mutant (ap/ap) melanophores during in vitro proliferation and (2) the incidence of melanophore differentiation from mutant melanoblasts in the skin in vitro.
During in vitro proliferation of mutant melanophores, ap-type melanosomes decreased in number gradually and instead the number of premelanosomes increased in the cells, which caused depigmentation at the light microscopic level in the culture. Depigmentation was observed only in mutant melanophores, and not in wild-type (+/+) melanophores. These results suggest that autonomous depigmentation of mutant dermal melanophores is the cause of the disappearance of these cells in vivo.
Dopa-positive melanoblasts were demonstrated in both wild-type and mutant skins. However, the melanoblasts of metamorphosed mutant froglets did not differentiate in vitro, while those of wild-type froglets did. These results suggest that mutant melanoblasts in the skin of froglets lose the potency to differentiate into melanophores, and that this causes the lack of mutant melanophores in the froglets. The site of action of the ap gene is also discussed.
The effect of lethal mutations and deletions within the bithorax complex upon the identity of caudal metameres in the Drosophila embryo
Authors: Whittle J. R. S. , Tiong S. Y. K. , Sunkel C. E. 
Published: April 2021

ABSTRACT
Mutations and deletions of the abdA and AbdB functions in the bithorax complex of Drosophila melanogaster have been examined for their effect upon the hypodermal derivatives of the caudal segments of the embryo, employing light- and scanning electron microscopy. No cuticular structures located posterior to the denticle belt of abdominal segment 8 are affected in abdA- embryos. Embryos of AbdB- genotype no longer have six of the seven pairs of sense organs present in this region, lack posterior spiracles but instead have sclerotized cuticle and sense organs typical of the head region and a rudimentary extra ventral denticle belt. The anal pads, tuft and sense organ 1 do not require BX-C functions for their specification. We discuss the provenance of these cuticular structures and the domain of function of elements within the bi thorax complex in terms of parasegmental metameric units.
Biochemical characterization of the adhesion-related differentiation antigen XT-1
Authors: Bechtol Kathleen B. , Ho Wai Chang , Vaupel Steven 
Published: April 2021

ABSTRACT
The XT-l-molecule, an adhesion-related differentiation antigen of male mouse germ cells, is a 34000 Mr glycoprotein with major charge isomer at pl 5·1 and is an integral component of the cell membrane. On large late pachytene spermatocyte, the molecule is present at a concentration of 2·5×103 moleculesμm-2, which approximates HLA/ABC concentration on lymphocytes. By comparing the reactivity of four anti-XT-1 monoclonal antibodies, three of which elicit germ cell-germ cell adhesion, we have defined two distinct surface regions of the XT-l-molecule. The relationship of the XT-l-molecule with other known adhesion-related molecules and testicular antigens is discussed.
The effect of reducing ATP levels on reorientation of the secondary palate
Authors: Bulleit Robert F. , Zimmerman Ernest F. 
Published: April 2021

ABSTRACT
The force for directing palate shelf reorientation appears to be associated with elements of the presumptive hard palate (Brinkley & Vickerman, 1979; Bulleit & Zimmerman, 1985). The palatal elements that mediate this process do not require palate cells to be metabolically active for expression of the force. This contention was demonstrated using an in vitro system that allows substantial reorientation of the hard palate to occur. ATP levels were reduced by treatment with metabolic inhibitors and the degree of reorientation was measured 1 h following pretreatment with inhibitors. Treatment of cultured embryonic heads under anoxic conditions with 2,4-dinitrophenol or KCN had no effect on the degree of reorientation occurring in vitro. These agents reduced ATP levels by 71 % and 62 %, respectively. Treatment of cultured heads with 2-deoxy-D-glucose under anoxia also had no effect on reorientation. This treatment reduced ATP levels in embryonic heads by 92–94 %. A similar reduction was observed if ATP levels were measured in palate tissue alone. The treatment of cultured heads with 2-deoxy-D-glucose and anoxia not only reduced levels of ATP but also reduced CTP, GTP and UTP. These results indicate that palate shelf reorientation is independent of cellular metabolic activity and supports the hypothesis that reorientation is dependent on a pre-existing infrastructure within the palate shelves.
Modifications of size and pattern of microtubular organelles in overfed cells of a ciliate Dileptus
Authors: Golinska Krystyna 
Published: April 2021

ABSTRACT
The size of mouthparts and their constituent organelles was studied in cells of Dileptus anser, enlarged by overfeeding. The oral structures of large cells were either of normal dimensions and appearance, or they were enlarged and deformed in shape. The increase in size of mouthparts was accompanied by an increase in number of their microtubular organelles. There was, however, no net increase in size of the organelles, as defined by length and number of microtubules they contained.
It is concluded that there exists an upward regulation in the size of the mouthparts as a whole, whilst the size of their constituent organelles is probably not so regulated. It is supposed that the patterning of normal-size organelles into large oral structure may lead to the observed deformations in the shape of the mouthparts.
Neurulation and the cortical tractor model for epithelial folding
Authors: Jacobson Antone G. , Oster George F. , Odell Garrett M. , Cheng Louis Y. 
Published: April 2021

ABSTRACT
We present here a new model for epithelial morphogenesis, which we call the ‘cortical tractor model’. This model assumes that the motile activities of epithelial cells are similar to those of mesenchymal cells, with the added constraint that the cells in an epithelial sheet remain attached at their apical circumference. In particular, we assert that there is a time-averaged motion of cortical cytoplasm which flows from the basal and lateral surfaces to the apical region. This cortical flow carries with it membrane and adhesive structures that are inserted basally and resorbed apically. Thus the apical seal that characterizes epithelial sheets is a dynamic structure: it is continuously created by the cortical flow which piles up components near where they are recycled in the apical region. By use of mechanical analyses and computer simulations we demonstrate that the cortical tractor motion can reproduce a variety of epithelial motions, including columnarization (placode formation), invagination and rolling. It also provides a mechanism for driving active cell rearrangements within an epithelial sheet, while maintaining the integrity of the apical seal. Active repacking of epithelial cells appears to drive a number of morphogenetic processes. Neurulation in amphibians provides an example of a process in which all four of the above morphogenetic movements appear to play a role. Here we reexamine the process of neurulation in amphibians in light of the cortical tractor model, and find that it provides an integrated view of this important morphogenetic process.
Effects of electric field on fusion rate and survival of 2-cell rabbit embryos
Authors: Ozil Jean-Pierre , Modlinski Jacek A. 
Published: April 2021

ABSTRACT
Electric-field-induced blastomere fusion was studied in 2-cell rabbit embryos. Field strengths (1 to 3kVcm−1) and durations (35 to 1000 μs) were chosen so as to provide the right balance between fusion rate, viability and developmental capacity of embryonic cells. Maximum plasma membrane tolerance of 2-cell rabbit embryos was observed at about 3 kV cm−1 for 1000 ps. All surviving ‘fused’ embryos were able to develop in vitro and most of them formed expanded blastocysts. Observation of ‘fused’ embryos immediately after fusion and during the whole cell cycle showed that 27·7 % of the two diploid nuclei remained separated in the hybrid cell. More than one metaphase plate was formed at the onset of mitosis causing direct cleavage into three or four ‘cells’. In the remaining embryos the two diploid nuclei seemed to form a common metaphase plate and cleaved into two equal blastomeres. After transfer to recipient does, 54·4% of these tetraploid embryos developed beyond implantation. Between day 11 and 20, ten live and morphologically fully normal embryos were recovered. Nine embryos were uniformly tetraploid and one recovered on day 18 was a diploid/tetr aploid mosaic. The remaining implantation sites contained either abnormal, very retarded embryos or indefinable embryo remnants. After transfer of ‘nonfused’ embryos treated with 3kVcm−1, 49% gave birth to normal live young. These results suggest that the electric field can be applied successfully in a relatively wide strength and duration range without causing any visible teratogenic effect on treated embryos. Thus, tetraploid embryos can develop normally at least until two-thirds of pregnancy, but the question whether they are able to survive till term remains open.
The postnatal maturation of efferent tubules in the rat: a light and electron microscopy study
Authors: Francavilla Sandro , Moscardelli Sergio , Bruno Bernardino , Barcellona Patrizio Scorza , Martino Cesare De 
Published: April 2021

ABSTRACT
The postnatal maturation of the epithelium and tubule wall of efferent tubules in the rat was investigated by light and transmission electron microscopy, from birth to 50 days of age, when sperms were released from the seminiferous tubules and appeared in the genital duct.
At the end of the first week of life, an endocytotic apparatus is differentiated in the epithelial cells. During the third week of life, efferent tubules developed specializations for the transport of sperms and fluids, namely the appearance of ciliated elements interspersed among the principal cells of the epithelium, and differentiation of myoid elements in the tubule wall.
The appearance of specializations related to endocytosis and fluid transport across the epithelium preceded the canalization of the seminiferous cords which, in fact, is reported to appear at the end of the second week of life in the rat, along with the initial secretion of testicular fluid. This suggested that the maturation of efferent tubules is not triggered by the passage of testicular fluid, as surmised for the postnatal differentiation of caput epididymis.
The postnatal maturation of efferent tubules was almost complete 35 days after birth. The appearance of sperms in the genital duct of 50-day-old animals was not associated with any remarkable structural change.
Scanning electron microscopic observation of mouse embryonic submandibular glands during initial branching: preferential localization of fibrillar structures at the mesenchymal ridges participating in cleft formation
Authors: Nakanishi Yasuo , Sugiura Fukashi , Kishi Jun-Ichi , Hayakawa Taro 
Published: April 2021

ABSTRACT
Branching submandibular glands of 12-day mouse embryos and those cultured in the presence and absence of a collagenase inhibitor from the culture medium of bovine dental pulp or a Clostridial collagenase were examined with the scanning electron microscope. Fracturing of fixed and dried glands with the tip of a fine needle succeeded in exposing the surfaces of the lobules and of their mesenchymal replicas at different stages of branching. At the beginning of branching, corresponding parts of the mesenchyme formed ridges on or in which the fibrillar structures were often found. At the stage forming deeper clefts thicker fibres, 0·5–2·5 μm in diameter, were observed between two adjacent lobules. On the contrary, no apparent differences in the fibrillar structures on the epithelial surfaces were detected between the shallow cleft and noncleft regions at the initial phase of branching. These fibrillar structures were very abundant in glands cultured with collagenase inhibitor and were completely lost in glands cultured with bacterial collagenase, strongly indicating that these materials consisted of collagen. The possible involvement of mesenchyme in epithelial branching is discussed with special reference to mesenchymal traction forces that would be elicited by fibrillar collagens.
Organ distribution of apolipoprotein gene transcripts in 6–12 week postfertilization human embryos
Authors: Hopkins B. , Sharpe C. R. , Baralle F. E. , Graham C. F. 
Published: April 2021

ABSTRACT
In the liver and the yolk sack of 6–12 week postfertilization human embryos, we have detected RNA transcripts from the following apolipoprotein genes: AI, AII, B, CII, CIII and E. The mRNA from the apolipoprotein CIII gene was relatively more abundant in the total RNA from the yolk sack than in that from the liver. The gut and adrenals contained transcripts of all these apolipoprotein genes apart from apolipoprotein AIL The kidneys and heart contained some apolipoprotein transcripts. In conjunction with previous studies, these results suggest that in the human embryo apolipoprotein genes are transcribed in a much larger range of organs than is the case in the adult. Many of these organs lack endoderm tissues.