bluuebunny/crossref_metadata_2025 huggingface dataset:
The trigeminal system: an advantageous experimental model for studying neuronal development
Authors: Davies Alun M.
Published: April 2021

The trigeminal system is a well-characterized sensory system that has been studied extensively in mammals and birds. The clear definition and easy accessibility of the components of this system throughout development have facilitated investigation of several fundamental aspects of neural development. This has led to important advances in our understanding of the mechanism of axonal guidance, the physiology of neurotrophic factors and the establishment and refinement of neural connections. The first convincing evidence for axonal guidance by chemotropism has been obtained. The site and timing of nerve growth factor (NGF) synthesis and NGF receptor expression have been elucidated, thereby clarifying the role of this neurotrophic factor in development. A novel concept in neurotrophic factor physiology has emerged: the survival of neurones that innervate two separate target fields is regulated by two different neurotrophic factors derived from the respective target fields. The development of somatotopic maps of the periphery in the central nervous system (CNS) is dependent on spatial information provided by the periphery. The transfer of this information from the periphery to the CNS is not simply achieved by the ordered growth and arrangement of the intervening sensory nerve fibres.
Histochemical localization of IGF-I and IGF-II mRNA in the rat between birth and adulthood
Authors: Beck Felix , Samani N. J. , Byrne S. , Morgan K. , Gebhard R. , Brammar W. J.
Published: April 2021

We describe the postnatal ontogeny and localization of insulin-like growth factors I and II (IGF-I and -II) in the rat. We have used oligodeoxyribonucleotide probes for in situ hybridization (hybridization histochemistry) and for Northern blotting. IGF-II mRNA is strongly expressed in liver, skeletal muscle, perichondrium, leptomeninges and choroid plexus of the newborn. Demonstrable levels fall dramatically in the liver at 18-20 days postnatally but persist for longer periods in muscle and remain undiminished throughout life in the pia/choroid plexus, indicating that different control mechanisms operate in these tissues. IGF-I mRNA is predominantly found in the liver. Its level in this organ rises well before levels of IGF-II fall. This suggests that distinct factors govern the expression of IGF-I and -II genes.
Anterior determinants in embryos of Chironomus samoensis: characterization by rescue bioassay
Authors: Elbetieha Ahmed , Kalthoff Klaus
Published: April 2021

Embryos of Chironomus samoensis are programmed, by anterior u.v. irradiation, to form the abnormal body pattern ‘double abdomen’. Most double abdomen embryos show a mirror-image duplication of abdominal segments in the absence of cephalic or thoracic segments. Such embryos can be ‘rescued’, i.e. restored to normal development, by microinjection of cytoplasm or RNA from unirradiated donor embryos. Most of the rescued embryos look completely normal and many of them hatch spontaneously. The rescuing activity decreases from the anterior to the posterior pole in the donor cytoplasm and must be delivered near the anterior pole of the recipient for maximum efficiency. Rescuing activity is present in total RNA extracted from whole, unirradiated embryos. Upon fractionation, the activity is associated with poly(A)+RNA, with LiCl precipitate depleted of RNA smaller than 250 nucleotides (nt) and with a sucrose gradient fraction depleted of RNA larger than 500 nt. Corresponding fractions of RNA from Xenopus oocytes have no rescuing activity. The activity of Chironomus RNA is sensitive to u.v. irradiation with low fluence affecting less than 2 % of the pyrimidine bases. Rescuing activity is present in cytoplasm until the blastoderm stage but disappears earlier from poly(A)+RNA. Rescuing activity is also present, and localized, in cytoplasm of embryos from two related dipterans, Smittia sp. and Drosophila melanogaster, although the extent of rescue observed in Chironomus decreases with the phylogenetic distance between donor and recipient. The results of these and previous experiments indicate that dipteran embryos contain localized RNP particles acting as anterior determinants. In Chironomus, the activity of these particles seems to depend on the integrity of polyadenylated RNA of about 250 to 500 nt length.
Regulation of transcript encoding the 43K subsynaptic protein during development and after denervation
Authors: Baldwin Timothy J. , Theriot Julie A. , Yoshihara Corinne M. , Burden Steven J.
Published: April 2021

The postsynaptic membrane of vertebrate neuromuscular synapses is enriched in the four subunits of the acetylcholine receptor (AChR) and in a peripheral membrane protein of Mr = 43×103 (43K). Although AChRs are virtually restricted to the postsynaptic membrane of innervated adult muscle, developing and denervated adult muscle contain AChRs at nonsynaptic regions. These nonsynaptic AChRs accumulate because the level of mRNA encoding AChR subunits increases in response to a loss of muscle cell electrical activity. We have determined the level of mRNA encoding the 43K subsynaptic protein in developing muscle and in innervated and denervated adult muscle. We isolated a cDNA that encodes the entire protein-coding region of the 43K subsynaptic protein from Torpedo electric organ and used this cDNA to isolate a cDNA that encodes the 43K subsynaptic protein from Xenopus laevis. We used the Xenopus cDNA to measure the level of transcript encoding the 43K protein in embryonic muscle and in innervated and denervated adult muscle by RNase protection. The level of transcript encoding the 43K protein is low in innervated adult muscle and increases 25- to 30-fold after denervation. The level of transcript encoding the alpha subunit of the AChR increases to a similar extent after denervation. Moreover, during development, transcripts encoding the 43K protein and the alpha subunit are expressed initially at late gastrula and are present in similar quantities in embryonic muscle. These results demonstrate that transcripts encoding the 43K protein and AChR subunits appear coordinately during embryonic development and that the level of mRNA encoding the 43K protein is regulated by denervation.
Implants of quail thymic epithelium generate permanent tolerance in embryonically constructed quail/chick chimeras
Authors: Ohki Hiroko , Martin Claude , Coltey Monique , Douarin Nicole M. LE
Published: April 2021

In situ implantation of a quail wing bud into a chick embryo at 4 days of incubation (E4) regularly results in the normal development of the implant followed by its acute rejection starting within two weeks post-hatching. If the epithelial thymic rudiments of the quail donor are implanted into the branchial arch area of the chick recipient after partial removal of its own thymic primordia, a chimeric thymus develops in the chick host and this induces tolerance to the quail wing by the chick recipient. The species identity of cells in chimeric thymuses was mapped using Feulgen-Ros-senbeck’ staining and immunolabelling with monoclonal antibodies directed against quail or chick B-L antigens. Certain lobes contained only chick cells both at the stromal and hemopoietic cell levels. Others had a quail epithelial stroma containing host hemopoietically derived cells. Only chimeras in which at least one third of the thymic lobes were chimeric showed permanent tolerance to the grafted wing.

Since the two species exhibit distinct developmental rates, we decided to study the kinetics of thymic involution after birth. Although the changes in thymus weight and histological structure are fundamentally similar in quail and chick, those in the quail start about 7-8 weeks earlier. In the chimeric thymuses, the lobes whose epithelial cells were quail involuted at the rate of control quail showing no influence of the hemopoietic thymic compartment in this process. Tolerance induced by the thymic epithelium during embryogenesis and in early postnatal life was maintained after a profound involution of the quail thymic graft had occurred.
The interaction of proteins encoded by Drosophila homeotic and segmentation genes with specific DNA sequences
Authors: Laughon Allen , Howell William , Scott Matthew P.
Published: April 2021

The ANT-C gene cluster is part of a network of genes that govern pattern formation in the development of Drosophila. The ANT-C genes encode proteins that contain a conserved 60 amino acid sequence, the homeodomain. Here we show that the homeodomains encoded by two of the ANT-C loci confer sequencespecific DNA-binding activity. The DNA sequence specificities of the Dfd and ftz homeodomains appear to overlap completely in vitro, indicating that differences in regulatory specificity among ANT-C and BX-C proteins (assuming that differences exist) must be a consequence of the nonconserved protein sequences found outside of the homeodomains. Deletions that remove sequences from either end of the ftz homeodomain abolish DNA-binding activity, consistent with the commonly held assumption that the homeodomain is a structural domain. The relevance of in vitro DNA-binding experiments to the regulatory function of ftz is supported by our finding that a temperature-sensitive ftz mutation that causes a pairwise fusion of embryonic segments also reduces the affinity of the ftz homeodomain for DNA. Restriction fragments containing ftz homeodomain binding sites were identified within a 90 kb stretch of DNA extending the Antp Pl and P2 promoters. Binding sites appear to be clustered near the Pl promoter but also occur near P2 and in the region between the two. The task remains of determining which of these sequences mediate regulation of Antp by ftz or by other genes that encode closely related homeodomains.
Expression of intermediate filament proteins during development of Xenopus laevis: II. Identification and molecular characterization of desmin
Authors: Herrmann Harald , Fouquet Bernadette , Franke Werner W.
Published: April 2021

During embryogenesis of avian and mammalian species the formation of intermediate filaments (IFs) containing desmin is characteristic for myogenesis. In view of important differences of patterns of IF protein expression in embryogenic pathways of amphibia on the one hand and birds and mammals on the other, we have decided to study the expression of desmin during early embryogenesis of Xenopus laevis by cDNA hybridization and antibody reactions. Here we describe the isolation of a cDNA clone encoding Xenopus desmin and the deduced amino acid sequence (458 residues; Mr52800) which displays a very high degree of conservation during vertebrate evolution from Xenopus to chicken and hamster, with a similar degree of sequence divergence between all three species compared. In addition, we have noted, by both cDNA-hybrid-selection-translation and immunoblotting of cytoskeletal proteins a second desmin-related polypeptide of Mr∼49000. RNA (Northern) blot analyses show the occurrence of three different desmin mRNAs (1 · 9, 2 · 6 and 3 · 0kb) which seem to represent different polyadenylation sites, displaying quantitative differences in different kinds of muscle tissues. During embryogenesis, desmin mRNA has first been detected in stage-14 embryos and then increases drastically to high levels at stage 18 and thereafter. Immunofluorescence microscopy using desmin-specific antibodies shows that this synthesis of desmin is restricted to somite tissue. The embryonic time course of synthesis of desmin and desmin mRNA is discussed in relation to those of other muscle proteins.
Polyspermic eggs in the anuran Discoglossus pictus develop normally
Authors: Talevi Riccardo
Published: April 2021

Fertilization and development in 400 eggs of the anuran Discoglossus pictus has been followed. In these eggs successful sperm interaction is restricted to a small area of the animal dimple called DI and causes a rapid depolarization. A high incidence of polyspermy (36 %) was detected by in vivo observations of fertilization cone formation. Polyspermie eggs gave rise to fertilization potentials comparable to monospermic eggs and developed normally. By using current-injection technique it is shown that sperm penetration is independent of mem-brane potential. The role of the egg envelopes in regulating sperm–egg interaction is discussed.
Three Strongylocentrotus purpuratus actin genes show correct cell-specific expression in hybrid embryos of S. purpuratus and Lytechinus pictus
Authors: Nisson Paul E. , Dike Laura E. , Crain William R.
Published: April 2021

The cell-specific expression of three actin genes from the sea urchin species Strongylocentrotus purpuratus was examined in hybrid embryos of S. purpuratus and another species, Lytechinus pictus, by in situ hybridization. The mRNAs from each of these genes displayed distinct spatial patterns of expression in late-stage hybrid embryos (constructed in either direction), being detected only in the cell lineages where they are normally found in S. purpuratus embryos (i.e. Cyllla, only in the aboral ectoderm lineage; Cyl, in the gut, oral ectoderm and some mesenchyme cells of plutei, and preferentially in the archenteron of gastrulae; M, only in two small clusters of cells near the esophagus in plutei). These results, together with our previous observation that expression of each of these genes is activated at the same stage in these hybrid embryos as in normal S. purpuratus embryos, demonstrate that the trans-acting factors which are necessary to regulate both the temporal and spatial expression of these genes are present in the hybrid embryos. Previous experiments have shown that the expression of a chimeric gene containing the Cyllla promotor fused to a bacterial chloramphenicol actetyl-transferase (CAT) gene is not confined to the correct cell lineage (aboral ectoderm) when injected into Lytechinus embryos. The conclusion from these sets of data is that the factor(s) that regulate the spatial expression of at least one of the actin genes must derive from transcription of the zygotic genome.
Ascidian eggs release glycosidase activity which aids in the block against polyspermy
Authors: Lambert Charles C.
Published: April 2021

To ensure normal development, most animals have evolved a number of mechanisms to block polyspermy including prevention of binding to surface coats as well as sperm-egg fusion. Ascidian sperm bind to vitelline coat (VC) glycosides. In the genus Ascidia, N-acetylglu-cosamine (GlcNAc) is the ligand to which sperm bind. The number of sperm bound to the VC is biphasic following fertilization; sperm binding increases through the first minute or so, then abruptly declines. At fertilization, the eggs of Ascidia callosa, A. ceratodes, A. méntula, A. nigra and Phallusia mammillata release N- acetylglucosaminidase into the sea water (SW). This has been shown to inactivate VC GlcNAc groups, blocking the binding of supernumerary sperm and polyspermy in A. nigra. This block to polyspermy is inactivated by GlcNAc (2mm) or 150mm-Na+ (choline substituted) SW. These treatments are not additive and therefore probably affect the same process. In A. callosa, fertilization in low Na+ SW causes a 60 % decline in enzyme release and a similar increase in the number of sperm remaining on the VC at 4 min as well as a great increase in polyspermy. Thus the principal block to polyspermy in ascidian eggs involves the release of N-acetylglucosa-minidase which appears to be Na+ dependent. Enzyme activity is found in the supernatant SW by 15 s after fertilization, suggesting that it is stored very near the egg surface. Histochemical staining of whole eggs and embryos shows loss of surface-associated enzyme activity following fertilization. Like other lysosomal enzymes this N-acetylglucosaniinidase is mannosylated and has an acidic pH optimum.
Non-genic inheritance of cellular handedness
Authors: Nelsen E. Marlo , Frankel Joseph , Jenkins Leslie M.
Published: April 2021

Ciliates exhibit an asymmetry in arrangement of surface structures around the cell which could be termed handedness. If the usual order of placement of structures defines a ‘right-handed’ (RH) cell, then a cell with this order reversed would be ‘left-handed’ (LH). Such LH forms appear to be produced in Tetrahymena thermophila through aberrant reorganization of homopolar doublets back to the singlet condition. Four clones of LH forms were selected and subjected to genetic analysis to test whether this drastic phenotypic alteration resulted from a nuclear genetic change. The results of this analysis indicate that the change in handedness is not due to a genetic change in either the micronucleus or macronucleus. The LH form can, under certain circumstances, revert to the RH form, but typically it propagates itself across both vegetative and sexual generations with similar fidelity.

While this analysis does not formally rule out certain possibilities of nuclear genic control involving regulatory elements transmitted through the cytoplasm, when the circumstances of origin and propagation of the LH condition are taken into account direct cortical perpetuation seems far more likely. Here we outline a conceptual framework centred on the idea of longitudinally propagated positional information; the positive evidence supporting this idea as well as further application of the idea itself are presented in the accompanying paper.
Re-expression of 117 antigen, a cell surface glycoprotein of aggregating cells, during terminal differentiation of Dictyostelium discoideum prespore cells
Authors: Browne Lois H. , Sadeghi Homa , Blumberg Daphne , Williams Keith L. , Klein Claudette
Published: April 2021

117 antigen is a glycoprotein expressed on the surface of D. discoideum cells at aggregation. It then disappears and is later re-expressed on the surface of a subpopulation of cells at culmination, the terminal differentiation stage (Sadeghi et al. 1987). A cDNA clone was used to show that the appearance of cell surface 117 antigen accurately reflects the expression of the 117 gene as measured by mRNA levels. It was also shown that during multicellular development there is a reciprocal relationship between the levels of 117 mRNA and the mRNA which codes for prespore surface glycoprotein, PsA. Dual parameter flow cytometry was used to demonstrate that the 117 antigen is found on the surface of maturing prespore cells after the PsA glycoprotein disappears, but that it is not found on mature spores. Using three monoclonal antibodies which identify respectively 117 antigen, PsA, and MUD3 antigen (a spore coat glycoprotein - probably Sp96), two new stages of final spore maturation were defined. These results indicate that there is a recapitulation of at least one aggregative cell surface glycoprotein in the prespore subpopulation of cells as they rise up the stalk during final spore development. This raises the possibility that culmination, which involves complex three dimensional morphogenetic movements not unlike those observed during animal embryogenesis, involves components of the two-dimensional pattern seen during aggregation.
XIHbox 8: a novel Xenopus homeo protein restricted to a narrow band of endoderm
Authors: Wright Christopher V. E. , Schnegelsberg Patrick , Robertis Eddy M. De
Published: April 2021

We report the isolation of a new homeobox gene from Xenopus laevis genomic DNA. The homeodomain sequence is highly diverged from the prototype Antennapedia sequence, and contains a unique histidine residue in the helix that binds to DNA. The homeo-domain is followed by a 65 amino acid carboxy-terminal domain, the longest found to date in any vertebrate homeobox gene. We have raised specific antibodies against an X1Hbox 8-β-gal fusion protein to determine the spatial and temporal expression of this gene. The nuclear protein first appears in a narrow band of the endoderm at stage 33 and develops into expression within the epithelial cells of the pancreatic anlagen and duodenum. Expression within the pan-creatic epithelium persists into the adult frog. This unprecedented restriction to an anteroposterior band of the endoderm suggests that vertebrate homeobox genes might be involved in specifying positional information not only in the neuroectoderm and mesoderm, but also in the endoderm. Our data suggest that X1Hbox 8 may therefore represent the first member of a new class of position-dependent transcription factors affecting endodermal differentiation.
IGF I induces differentiation in a transformed human keratinocyte line
Authors: Kamalati T. , Howard M. , Brooks R. F.
Published: April 2021

A comparison of normal epithelial cells with their transformed counterparts could lead to the definition of parameters related to growth and differentiation which are altered by viral transformation and which may be relevant to malignant changes in vivo. Using the SV40-transformed human keratinocyte line, SVK14, which exhibits characteristics of simple, nonkeratinizing epithelia, we have shown that IGF I stimulation of these cells results in extensive multilayering, increased cell size, accumulation of involucrin, modulation of keratin 18 and expression of keratins 14 and 10, whilst T-antigen expression is maintained in the multilayered cells. Since T-antigen expression is correlated directly with impairment of stratification and differentiation, it is interesting that treatment of SVK14 with a single growth factor, IGF I, results in molecular events characteristic of differentiating normal keratinocytes.
Cross-species recognition of tectal cues by retinal fibers in vitro
Authors: Godement Pierre , Bonhoeffer Friedrich
Published: April 2021

The retinae of vertebrates project in a topographic manner to several visual centers of the brain. The formation of these projections could depend on the existence of position-specific properties of retinal and target cells. In this study, we have tested the in vitro growth of mouse retinal fibers on membranes derived from various regions of the embryonic superior colliculus, a main target of the retina in this species. Fibers had the choice of elongating on membranes taken from either the anterior or the posterior half of the superior colliculus. Fibers from temporal areas of the retina prefer to elongate on anterior collicular membranes, while fibers from nasal areas do not show a preference. These phenomena are observed with membranes from embryonic (E15–E18) or young postnatal mice. In interspecies cultures where mouse retinal fibers had to grow on chick tectal membranes, or vice versa, the same preference for anterior tectal or collicular membranes in growth of temporal retinal fibers is observed, suggesting some similarities in the cues used in both species.
Cutaneous nerves of the embryonic chick wing do not develop in regions denuded of ectoderm
Authors: Martin Paul , Khan Akbar , Lewis Julian
Published: April 2021

Peripheral nerves travel to their targets along precise routes, and it is likely that different cues provide guidance at different stages of the journey. In a develop­ing chick limb, the cutaneous nerve fibres follow at first deep mixed nerve trunks, in company with motor axons; they branch from these trunks at predictable points and approach the skin; they then ramify profusely to form a plexus at a precisely defined depth beneath the ecto­derm, at exactly the same level as the blood vascular plexus. To analyse the role of signals from the target patch of skin in regulating cutaneous nerve develop­ment, we have ablated patches of dorsal wing ectoderm using short-wave ultraviolet irradiation at E4 (embry­onic day 4), approximately one day before nerves grow into the limb bud. The irradiated patches remain denuded of ectoderm for more than a week, by which time the cutaneous nerve plexus on the contralateral control side is well developed and can be revealed by whole-mount silver staining. Where the ectoderm has been ablated, no cutaneous nerve plexus forms, and the nerve branches that normally would have diverged from the neighbouring mixed nerve trunk to innervate the missing patch of skin are absent - ab initio, apparently. The routes of the mixed nerve trunks are not affected. Partial ablation of the territory of a cutaneous nerve branch often leads to loss of the whole nerve branch; the intact skin territory thus left vacant is invaded by ramifications from the remaining cutaneous branches, as expected if the normal extent of a cutaneous nerve’s territory is regulated by competition. Where there is an ectodermal lesion, cutaneous innervation stops precisely at its boundary, even though the vascular plexus extends for some distance beyond this margin, beneath the denuded surface. The data suggest that the embryonic skin is required firstly to trigger divergence of cutaneous nerve branches from the mixed nerve trunks, and secondly, once the nerve fibres have reached the skin, to supply a trophic cue (probably NGF) encouraging growth of a plexus; at the same time, the embryonic skin generates a signal inhibiting nerves from approaching closer than about 70 μm to the surface.
Transcriptional regulation of a Xenopus embryonic epidermal keratin gene
Authors: Jonas Erzsebet A. , Snape Alison M. , Sargent Thomas D.
Published: April 2021

XK81A1 is a type I epidermal keratin gene expressed in early developmental stages of Xenopus (Jonas et al. 1985). Fusion of the keratin promoter (-5900 to +26) to a human β globin gene led to fully epidermis-specific accumulation of human globin mRNA and protein when this DNA was injected into fertilized eggs. Further localization of regulatory sequences was accomplished by injecting marked, 5’-deleted keratin gene DNA into fertilized eggs and evaluating tissue specificity of expression. All 5’ flanking DNA upstream from -487 could be removed without interfering with keratin gene expression or regulation. These results suggest that the primary mode of regulation of epidermis-specific keratin gene expression is at the level of transcription, and that sequence elements in the 5’ flanking region of the keratin gene, between -487 and +26, are responsible for this regulation.
Fibronectin gene expression during limb cartilage differentiation
Authors: Kulyk William M. , Upholt William B. , Kosher Robert A.
Published: April 2021

A critical event in limb cartilage differentiation is a transient cellular condensation process in which pre-chondrogenic mesenchymal cells become closely juxtaposed and interact with one another prior to initiating cartilage matrix deposition. Fibronectin (FN) has been suggested to be involved in regulating the onset of condensation and chondrogenesis by actively promoting prechondrogenic aggregate formation during the process. We have performed a systematic quantitative study of the expression of the FN gene during the progression of chondrogenesis in vitro and in vivo. In high-density micromass cultures of limb mesenchymal cells, FN mRNA levels increase about 5-fold coincident with the crucial condensation process, and remain relatively high during the initial deposition of cartilage matrix by the cells. Thereafter, FN mRNA levels progressively decline to relatively low levels as the cultures form a virtually uniform mass of cartilage. The changes in FN mRNA levels in vitro are paralleled closely by changes in the relative rate of FN synthesis as determined by pulselabeling and immunoprecipitation analysis. The relative rate of FN synthesis increases 4- to 5-fold at condensation and the onset of chondrogenesis, after which it progressively declines to low levels as cartilage matrix accumulates. High levels of FN gene expression also occur at the onset of chondrogenesis in vivo. In the proximal central core regions of the limb bud in which condensation and cartilage matrix deposition are being initiated, FN mRNA levels and the relative rates of FN synthesis become progressively about 4-fold higher than in the distal subridge region, which consists of undifferentiated mesenchymal cells that have not yet initiated condensation. The striking increase in FN gene expression that occurs during condensation and the onset of chondrogenesis is consistent with the suggestion that this molecule may play a role in the crucial cell-cell or cell-matrix interactions involved in regulating the onset of cartilage differentiation.
Mammalian neuronal differentiation: early expression of a neuronal phenotype from mouse neural crest cells in a chemically defined culture medium
Authors: Boisseau Sylvie , Simonneau Michel
Published: April 2021

We show that mouse neural crest cells cultured in a serum-deprived chemically defined medium on appropriate culture substrata can be induced to express a neuronal phenotype. The uncommitted neural crest cells express a mesenchymal intermediate filament protein such as vimentin, but not the usual neuronal markers such as receptor sites for tetanus toxin or neurofilaments. In the chemically defined medium, receptor sites for tetanus toxin or neurofilaments can be characterized after a few hours in culture. Furthermore, these cells acquire tetrodotoxin-sensitive voltage-dependent Na+ channels and can generate action potentials. Such an in vitro system should allow us to analyze and manipulate early stages of neuronal differentiation in a mammalian embryo, at a level so far restricted to lower vertebrate embryos.
Melanogenesis of avian neural crest cells in vitro is influenced by external cues in the periorbital mesenchyme
Authors: Campbell Steven
Published: April 2021

Avian melanoblast differentiation was studied by explantation of the neural tube and periorbital mesenchyme. Outgrowths from the mesenchymal expiants consisted of a mixed population of melanocytes, melano-blasts and fibroblasts, whilst typical neural crest populations migrated from the neural tube expiants. Cells that differentiated within expiants of mesenchyme, produced elongate black eumelanosomes of normal ultrastructure which were identical to those found in the ocular connective tissues. However, melanoblasts that differentiated within outgrowths of mesenchyme or neural tube produced round brown melanosomes of highly abnormal ultrastructure. Some of these melanosomes contained a few disorganised melanosomal filaments whilst others had granular melanin with complete absence of filaments. This abnormality of phenotype was invariant over a range of culture conditions that modified cell behaviour, the timing of differentiation and the abundance of the pigmented cells. These experiments suggest that local factors in the mesenchyme are essential for the induction of melanogenesis in the presumptive connective tissue melanocyte.
The appearance of neural and glial cell markers during early development of the nervous system in the amphibian embryo
Authors: Messenger N. J. , Warner Anne E.
Published: April 2021

Cell-type-specific antibodies have been used to follow the appearance of neurones and glia in the developing nervous system of the amphibian embryo. Differentiated neurones were recognized with antibodies against neurofilament protein while glial cells were identified with antibodies against glial fibrillary acidic protein (GFAP). The appearance of neurones containing the neurotransmitters 5-hydroxytryptamine and dopamine has been charted also.

In Xenopus, neurofilament protein in developing neurones was observed occasionally at NF stage 21 and was present reliably in the neural tube and in caudal regions of the brain at stage 23. Antibodies to the low molecular weight fragment of the neurofilament triplet recognized early neurones most reliably. Radial glial cells, identified with GFAP antibody, were identified from stage 23 onwards in the neural tube and caudal regions of the brain. In the developing spinal cord, GFAP staining was apparent throughout the cytoplasm of each radial glial cell. In the brain, the peripheral region only of each glial cell contained GFAP. By stage 36, immunohistochemically recognizable neurones and glia were present throughout the nervous system.

In the axolotl, by stage 36 the pattern of neural and glial staining was identical to that observed in Xenopus. GFAP staining of glial cells was obvious at stage 23, although neuronal staining was clearly absent. This implies that glial cells differentiate before neurones.

5-HT-containing cell bodies were first observed in caudal regions of the developing brain on either side of the midline at stage 26. An extensive network of 5-HT neurones appeared gradually, with a substantial subset crossing to the opposite side of the brain through the developing optic chiasma. 5,7-dihydroxytryptamine prevented the appearance of 5-HT. Depletion of 5-HT had little effect on development or swimming behaviour. Dopamine-containing neurones in the brain first differentiated at stage 35-36 and gradually increased in number up to stage 45-47, the latest stage examined. The functional role of 5-HT- or dopamine-containing neurones remains to be elucidated.

We conclude that cell-type-specific antibodies can be used to identify neurones and glial cells at early times during neural development and may be useful tools in circumstances where functional identification is difficult.
Regeneration and pattern formation in planarians: III. Evidence that neoblasts are totipotent stem cells and the source of blastema cells
Authors: Baguñà Jaume , Saló Emili , Auladell Carme
Published: April 2021

In most regenerating systems, blastema cells arise by dedifferentiation of functional tissue cells. In planarians, though, it is still debatable whether dedifferentiated cells or a population of undifferentiated cells, the neoblasts, are the main source of blastema cells. Moreover, it is unclear whether in the intact organisms neoblasts are quiescent cells ‘reserved’ for regeneration or if they serve as functional stem cells of all differentiated cell types. Both uncertainties partly stem from the failure to distinguish by conventional labelling methods neoblasts from differentiated cells.

Here we describe a new approach to these problems based on testing the regenerative and stem cell capabilities of purified neoblasts and differentiated cells when introduced, separately, into irradiated hosts. Introduc tion of neoblasts led to resumed mitotic activity, blastema formation, and extended or complete survival of the host; differentiated cells, in contrast, never did so.

Therefore, planarian neoblasts can be qualified as totipotent stem cells and the main source of blastema cells, while dedifferentiation does not seem to operate either in intact or regenerating organisms. In addition, these results strengthen the idea that different types of regeneration and blastema formation, linked to the tissular complexity of the organisms, are present in the animal kingdom.
Changes in cell surface glycoproteins during Dictyostelium development analysed using monoclonal antibodies
Authors: Crandall Ian E. , Newell Peter C.
Published: April 2021

We have produced a series of monoclonal antibodies that recognize carbohydrate epitopes on cell surface glycoproteins of developing amoebae of Dictyostelium discoideum. The antibodies were found to have differential specificity for amoebae at different stages of development and were classified into types A to E on the basis of their temporal pattern of reactivity with the developing amoeba! cell surface. Evidence from Western Blots and digestion of the glycoproteins with alkaline phosphatase were consistent with previous reports that the cell surface glycoproteins are extensively processed during development, leading at 16 h of development to the exposure of a highly antigenic core recognized by antibodies in group E. The nature of this core structure is indicated by the finding that antibodies in group E were found also to bind with high avidity to the plant glycoprotein horse radish peroxidase.
Antisera to basal lamina and glial endfeet disturb the normal extension of axons on retina and pigment epithelium basal laminae
Authors: Halfter Willi
Published: April 2021

In order to determine the role of the extracellular matrix in regulating the directed growth of embryonic neurites, antisera to retina (a-RBL I and II), to pigment epithelium (a-PBL) and to glomerular (a-GBL) basal lamina were probed for an effect on the ordered extension of neurites. In the assays, retina explants from chick and quail were cultured on basal lamina from embryonic chick retina and pigment epithelium either in the presence of anti-basal lamina antisera or in the presence of the corresponding preimmune sera. In the presence of all anti-basal lamina antisera, normal extension of axons was greatly inhibited both on retina and on pigment epithelium basal lamina. The antisera affected the growth pattern and the morphology of the individual axons in two ways: in the presence of a-RBL I the short axons were less directed, developed more and longer side branches, and the lamellipodia of the growth cones were reduced in size compared to axons from control cultures. In the presence of a-RBL II and a-GBL, axons grew slowly out from the explants as very thick bundles, strikingly different from axons in control cultures. The antiserum to pigment epithelium basal lamina induced both strong fasciculation and disorganization of the linear fiber extension, being intermediate between the two types of effects observed after antiserum addition. The results suggest that adhesive matrix molecules in basal laminae have important functions in elongation, fasciculation and in the morphology of growing axons.
Developmental loss of functional laminin receptors on retinal ganglion cells is regulated by their target tissue, the optic tectum
Authors: Cohen James , Nurcombe Victor , Jeffrey Peter , Edgar David
Published: April 2021

The ability of chick retinal ganglion cells (RGCs) to extend neurites on tissue culture substrata of the extracellular matrix protein laminin is lost during embryonic development. In order to establish the mechanism responsible for the loss of response, the number of high affinity (KD 10−9M) laminin receptors on both the cell bodies and neurites of RGCs were determined throughout this period by a ligand binding assay using radiolabelled laminin. It was found that the loss of response paralleled a decrease in receptor numbers on both the cell bodies and the neurites of the RGCs. Bilateral tectalablation at embryonic day 6 resulted in the subsequent maintenance of laminin-stimulated neurite outgrowth, together with a partial inhibition of the loss of laminin receptors. Thus, the loss of response of the RGCs to laminin reflects a decrease in the numbers of laminin receptors on these neurons, and furthermore, this downregulation is in turn dependent on innervation of the target tissue.
Another role for melanocytes: their importance for normal stria vascularis development in the mammalian inner ear
Authors: Steel Karen P. , Barkway Chris
Published: April 2021

The stria vascularis of the mammalian cochlea is composed primarily of three types of cells. Marginal cells line the lumen of the cochlear duct and are of epithelial origin. Basal cells also form a continuous layer and they may be mesodermal or derived from the neural crest. Intermediate cells are melanocyte-like cells, presumably derived from the neural crest, and are scattered between the marginal and basal cell layers. The marginal cells form extensive interdigitations with the basal and intermediate cells in the normal adult stria. The stria also contains a rich supply of blood vessels.

We investigated the role of melanocytes in the stria vascularis by studying its development in a mouse mutant, viable dominant spotting, which is known to have a primary neural crest defect leading to an absence of recognisable melanocytes in the skin. Melanocytes were not found in the stria of most of the mutants examined, and from about 6 days of age onwards a reduced amount of interdigitation amongst the cells of the stria was observed. These ultrastructural anomalies were associated with strial dysfunction. In the normal adult mammal, the stria produces an endocochlear potential (EP), a resting dc potential in the endolymph in the cochlear duct, which in mice is normally about + 100 mV. In our control mice, EP rose to adult levels between 6 and 16 days after birth. In most of the mutants we studied, EP was close to zero at all ages from 6 to 20 days.

Melanocyte-like cells appear to be vital for normal stria vascularis development and function. They may be necessary to facilitate the normal process of interdigitation between marginal and basal cell processes at a particular stage during development, and the lack of adequate interdigitation in the mutants may be the cause of their strial dysfunction. Alternatively, melanocytes may have some direct, essential role in the production of an EP by the stria. Melanocytes may be important both for normal strial development and for the production of the EP. We believe this is the clearest demonstration yet of a role for migratory melanocytes other than their role in pigmentation.
Genetic structure of the abd-A gene of Drosophila
Authors: Busturia Ana , Casanova Jordi , Sánchez-Herrero Ernesto , González Rosa , Morata Ginés
Published: April 2021

We report the embryonic and adult phenotypes of a number of mutations of the abd-A gene of the bithorax complex. Some of them result in loss of abd-A function in the whole abd-A domain and are usually lethal. These probably eliminate or inactivate abd-A protein products. Other mutations affect only part of the abd-A domain. These are viable, appear to map outside the abd-A transcription unit, and presumably alter the normal spatial regulation of abd-A products.

We propose a model of abd-A structure based on a protein-coding region and two cis-regulatory regions. Regulatory region 1, 3′ to the transcription unit, contains positive and negative regulatory elements. Regulatory region 2, 5′ to the transcription unit, establishes the correct level of abd-A activity in the abdominal metameres.
Myosin light chain gene expression in developing and denervated fetal muscle in the mouse
Authors: Barton Paul J. R. , Harris A. John , Buckingham Margaret E.
Published: April 2021

We have investigated the accumulation of mRNA transcripts of the atrial (or embryonic) myosin light chain MLC1A (MLCLmb), and the two adult fast muscle myosin light chains (MLC1F and MLC3F) during fetal skeletal muscle development in the mouse. In 15-day fetal muscle, MLC1A is the predominant mRNA detectable, by 18 days MLC1F has become the major transcript and MLC3F mRNA is detectable for the first time. By 12 days after birth, MLC1A transcripts are undetectable and MLC1F and MLC3F are similar in abundance. In fetuses treated with β-bungarotoxin and which therefore develop in the absence of functional nerve, MLC1A and MLC1F undergo normal transitions but MLC3F mRNA accumulation is significantly retarded. This demonstrates that these myosin light chain mRNAs accumulate with differing kinetics, and that MLC3F mRNA accumulation is nerve-dependent during fetal development. The results are discussed in terms of secondary muscle fibre formation, and in relation to the independent regulation of MLCIF and MLC3F mRNAs which are transcribed from the same gene.
Expression of the SMP antigen by oligodendrocytes in the developing avian central nervous system
Authors: Cameron-Curry P. , Dulac C. , Le Douarin N. M.
Published: April 2021

Expression of the avian antigen SMP (Schwann cell Myelin Protein, Mr 75–80 000), first characterized in the PNS with a monoclonal antibody as an early and strictly specific Schwann cell marker, was further studied in the CNS.

Comparing SMP immunoreactive areas in the different parts of the CNS with those expressing the Myelin Basic Protein (MBP), we showed a strict colocalisation of both phenotypes. In vitro, MBP+ oligodendrocytes express the surface antigen SMP as well.

SMP cellular expression was followed in situ and in culture using nervous tissues from embryos at different stages. We were thus able to detect an early expression of this marker by oligodendroblasts before the first appearance of MBP immunoreactivity.

We have also identified a subpopulation of SMP+/ MBP– and SMP+/GC– cells, which persists under our culture conditions as precursors remaining in an immature state.
Temporal and tissue-specific expression of the proto-oncogene c-fos during development in Xenopus laevis
Authors: Mohun T. J. , Garrett N. , Taylor M. V.
Published: April 2021

We describe the isolation and complete sequence of the Xenopus c-fos proto-oncogene, c-fos expression throughout Xenopus development was analysed using a homologous probe derived from the cloned gene, c-fos RNA is accumulated during oogenesis to reach a plateau of 2 x105 transcripts per stage VI oocyte, suggesting an unusual stability of the c-fos message. The amount of RNA per embryo decreases substantially after fertilisation to reach a level corresponding to less than 0.1 molecule per cell at the tailbud stage. Subsequently, at the swimming tadpole stage, the amount of c-fos mRNA increases; an increase that is correlated with the start of skeleton formation. In the newly metamorphosed frog-let, c-fos mRNA shows a marked tissue-specific distribution, with the highest level in intestine and lowest in gall bladder, lung and spleen. We also demonstrate that the Xenopus c-fos gene is serum-inducible in Xenopus cultured cells, a property attributable to a promoter sequence known as the Serum Response Element (SRE). A protein activity (indistinguishable from Serum Response Factor) in both whole cell and nuclear Xenopus embryo extracts binds specifically to the SRE and is present at an approximately constant level throughout early development. Our results suggest roles for c-fos in aspects of both the rapid cell proliferation and cell differentiation characteristic of early Xenopus development.
The contribution made by cells from a single somite to tissues within a body segment and assessment of their integration with similar cells from adjacent segments
Authors: Bagnall K. M. , Higgins S. J. , Sanders E. J.
Published: April 2021

Somites represent the first visual evidence of segmentation in the developing vertebrate embryo and it is becoming clear that this segmental pattern of the somites is used in the initial stages of development of other segmented systems such as the peripheral nervous system. However, it is not known whether the somites continue to contribute to the maintenance of the segmental pattern after the dispersal of the somitic cells. In particular, the extent to which cells from a single somite contribute to all of the tissues of a single body segment and the extent to which they mix with cells from adjacent segments during their migration is not known. In this study, we have replaced single somites in the future cervical region of 2-day-old chick embryos with equivalent, similarly staged quail somites. The chimerae were then allowed to develop for a further 6 days when they were killed. The cervical region was dissected and serially sectioned. The sections were stained with the Feulgen reaction for DNA to differentiate between the chick and quail cells. The results showed that the cells from a single somite remained as a clearly delimited group throughout their migration. Furthermore, the sclerotome, dermatome and myotome portions from the single somites could always be recognised as being separate from similar cells from other somites. The somitic cells formed all of the tissues within a body segment excluding the epidermis, notochord and neural tissue. There was very little mixing of the somitic cells between adjacent segments. The segmental pattern of the somites is therefore maintained during the migration of the somitic cells and this might be fundamental to a mechanism whereby the segmentation of structures, such as the peripheral nervous system, is also maintained during development.