bluuebunny/crossref_metadata_2025 huggingface dataset:
Retinoids reprogramme pre-bud mesenchyme to give changes in limb pattern
Authors: Wilde Susan M. , Wedden S. E. , Tickle C.
Published: April 2021

Retinoic acid was locally applied to presumptive limb regions of chick embryos to find out the earliest time at which the limb pattern can be reprogrammed. When beads soaked in retinoic acid were placed in the appropriate positions in embryos at stage 10 or older, duplicated or reduced leg patterns resulted. To pin point the time at which the cells in the limb rudiment respond to the retinoid, beads were removed at various times and the lengths of exposure required to reprogramme limb development found. The early limb rudiments require longer exposures to give duplications than late rudiments. The effective treatment periods last at least until stage 17 when the limb bud and apical ectodermal ridge develop. In contrast, the length of exposure to reduce the limb is constant at early stages. Retinoids first start acting to produce duplicated structures between stages 10 and 13. Therefore, retinoids appear to begin to reprogramme the cells as soon as they are determined to give rise to a limb.
Two distinct classes of prestalk-enriched mRNA sequences inDictyostelium discoideum
Authors: Jermyn K. A. , Berks M. , Kay R. R. , Williams J. G.
Published: April 2021

We have isolated cDNA clones derived from three mRNA sequences which are inducible by DIF, the putative stalk-specific morphogen of Dictyostelium. The three mRNA sequences are selectively expressed in cells on the stalk cell pathway of differentiation and we have compared them with previously characterized prestalk-enriched mRNA sequences. We find these latter sequences are expressed without a dependence on DIF, are much less highly enriched in prestalk over prespore cells and are expressed earlier during development than the DIF-inducible mRNA sequences. We propose two distinct mechanisms whereby a mRNA may become enriched in prestalk cells. An apparently small number of genes, represented by those we have isolated, is inducible by DIF and accumulates only in prestalk cells. We suggest that a second class of prestalk-enriched mRNA sequences are induced by cAMP to accumulate in all cells during aggregation and then become enriched in prestalk cells by selective loss from prespore cells.
The molecular basis for metameric pattern in the Drosophila embryo
Authors: Akam Michael
Published: April 2021

The metameric organization of the Drosophila embryo is generated in the first 5 h after fertilization. An initially rather simple pattern provides the foundation for subsequent development and diversification of the segmented part of the body. Many of the genes that control the formation of this pattern have been identified and at least twenty have been cloned. By combining the techniques of genetics, molecular biology and experimental embryology, it is becoming possible to unravel the role played by each of these genes.

The repeating segment pattern is defined by the persistent expression of engrailed and of other genes of the ‘segment polarity’ class. The establishment of this pattern is directed by a transient molecular prepattern that is generated in the blastoderm by the activity of the ‘pair-rule’ genes.

Maternal determinants at the poles of the egg coordinate this prepattern and define the anteroposterior sequence of pattern elements. The primary effect of these determinants is not known, but genes required for their production have been identified and the product of one of these, bicoid is known to be localized at the anterior of the egg. One early consequence of their activity is to define domains along the A–P axis within which a series of ‘cardinal’ genes are transcribed.

The activity of the cardinal genes is required both to coordinate the process of segmentation and to define the early domains of horneotic gene expression. Further interactions between the homeotic genes and other classes of segmentation genes refine the initial establishment of segment identities.
Histochemical localization of IGF-I and -II mRNA in the developing rat embryo
Authors: Beck Felix , Samani N. J. , Penschow Jennifer D. , Thorley Bruce , Tregear Geoffrey W. , Coghlan John P.
Published: April 2021

We describe the histological localization of embryonic and fetal tissues whose cells express the genes coding for insulin-like growth factors I and II (IGF-I and IGF-II) in the developing rat. Our studies span the period between early somite stages and full term. We have used oligodeoxyribonucleotide probes and obtained results which are both topographically precise and highly reproducible. The gene coding for IGF-II is predominant throughout development. It is strongly expressed in the liver and yolk sac. A variety of other tissues also expresses the IGF-II gene, especially many mesodermally derived structures in the process of differentiation. Many tissues do not express IGF genes. Thus no IGF mRNA was demonstrable in ectodermally derived structures, including the central and peripheral nervous systems as well as the skin and its derivatives.
Somatomedins/insulin-like growth factors, their receptors and binding proteins are present during mouse embryogenesis
Authors: Smith Eric P. , Sadler Thomas W. , D’Ercole A. J.
Published: April 2021

Somatomedins/insulin-like growth factors (Sm/IGFs) are considered to have important roles in regulating fetal growth; however, because of limited quantities of tissue, few studies have been performed on their effects on embryonic growth. To assess a potential role for these factors, we evaluated mouse embryonic tissues for the presence of Sm/IGF and insulin receptors and Sm/IGF-binding proteins by chemical affinity labelling. In addition, we measured extractable Sm-C/IGF-I radioimmunoactivity in mouse embryonic tissues. Finally, we compared these data with those from the embryonal carcinoma cell line, PC13. All embryos from day 9 (3–4 somites) to day 12 (45 somites) possessed both Sm-C/IGF-I and IGF-II receptors in apparent greater abundance than insulin receptors. The visceral yolk sac appeared to have proportionally more insulin receptors than the corresponding embryonic tissue. Extracts from the embryos contained immunoreactive Sm-C/IGF-I and binding proteins of 30–45×103MT. PC13 cells possessed all three receptors and the apparent abundance of the insulin and IGF-II receptors was reduced after differentiation was induced with retinoic acid. PC13 cells released both immunoreactive Sm-C/IGF-I- and Sm-C/IGF-I-binding proteins into their medium. When differentiated, the binding proteins resembled the native ones extracted from the intact embryos. The presence of Sm/IGF activity, receptors and binding proteins in early embryogenesis suggests a role for these factors in embryonic growth. The PC13 cell line appears to only partially reflect normal development.
Posterior expression of a homeobox gene in early Xenopus embryos
Authors: Condie Brian G. , Harland Richard M.
Published: April 2021

The homeobox containing transcript Xhox-36 is expressed exclusively in the posterior mesoderm and ectoderm of early Xenopus embryos. Therefore, the transcript shows region-specific rather than tissuespecific expression in the gastrula and neurula, a time when cells are becoming committed to defined fates. Exposure of early embryos to LiCl, which shifts posterior cells to more anterior fates, reduces the abundance of this posterior-specific transcript. In contrast, embryos ventralized by u.v. treatment express normal levels of the transcript, implying that expression of the gene is not absolutely linked to dorsal cell identity. The sequence of a full-length cDNA corresponding to this transcript predicts a homeodomain-containing protein of 209 amino acids.
A monoclonal antibody stains myogenic cells in regenerating newt muscle
Authors: Griffin Kevin J. P. , Fekete Donna M. , Carlson Bruce M.
Published: April 2021

Monoclonal antibodies have been used to study minced muscle regeneration in the adult newt, Notophthalmus viridescens. The contralateral limb was amputated and the immunostaining patterns in the regenerating blastema were compared with the minced tissue in sectioned material. Staining with a myofibre-specific antibody, called 12/101 (Kintner & Brockes, 1984), showed that myofibre degeneration was complete by 8– 10 days after mincing, with myogenesis commencing 2 days later. Another monoclonal antibody, called 22/18, previously shown to label a subset of cells in the regeneration blastema of the newt (Kintner & Brockes, 1984, 1985), was found also to recognize a population of cells in regenerating minced muscle. At 6 days after mincing, the number of 22/18-positive (22/18+) cells was low but by days 12– 16, during the period of myogenesis, their number had increased to become a major population within the minced tissue. A small number of the 22/18+ cells could be double labelled with 12/101 at this time.

Prior to this, there was a phase in which 12/101 staining had disappeared from the mince. Cells immunoreactive with both antibodies after this phase confirm that at least some of the 22/18+ cells are myogenic. The number of 22/18+ cells decreased as muscle repair and maturation progressed. These results show that 22/18 is not specifically associated with blastemal cells but is a more general marker for regenerating systems in the newt. They further suggest an alternative interpretation of the doublelabelled cells used by Kintner & Brockes (1984) as evidence for myofibre dedifterentiation in limb regeneration. Instead, we propose that such cells represent new myogenesis occurring by tissue repair of locally damaged muscle fibres.
Initial disorder and secondary retinotopic refinement of regenerating axons in the optic tract of the goldfish: signs of a new role for axon collateral loss
Authors: Becker David L. , Cook Jeremy E.
Published: April 2021

The optic tract of the goldfish splits into two brachia just before it reaches the tectum, normal optic axons being distributed systematically between the two according to their retinal origins. The orderliness of this division, like that of the retinotectal projection itself, is conventionally attributed to a system of specific axonal guidance cues. However, the brachial distribution of regenerated axons is much less orderly; and, since there is evidence that these axons have many collateral branches in the nerve and tract, the gross order that remains after regeneration could potentially arise secondarily, in parallel with refinement of the retinotectal map, by a preferential loss of collaterals from the inappropriate brachium.

The brachial paths of normal axons, and axons regenerated after optic nerve cut for periods ranging from 19 days to 5 years, were therefore studied by anterograde labelling with horseradish peroxidase from discrete retinal lesions or retrograde labelling of ganglion cells from a cut brachium.

From 19 to 28 days, regenerating axons showed little or no preference for their normal brachium. During this period (which includes the first week of tectal synaptogenesis) an average of 46·3 % of cells retrogradely labelled from a cut medial brachium were in dorsal retina, compared with only 1· 45% in normal fish. Some preference for the normal brachium was evident at 35 days and significant order had returned by 42–70 days, when the average proportion of labelled cells in dorsal retina had fallen to 25·4 % though the average number in the whole retina was unchanged. Thus a brachial refinement had occurred in parallel with refinement of the retinotectal map.

These results support the idea of a selective loss of axon collaterals from the inappropriate brachium, though they do not exclude the possibility of some concurrent gain in the appropriate one. We suggest that refinement may depend on a process we term ‘sibling rivalry’: competition between different collaterals of the same axon to form a critical number of stable tectal synapses, in which the most-normally-routed branches have the best chance of succeeding and surviving. Developing normal axons might also make use of collateral formation and ‘sibling rivalry’ to generate and refine the complex interwoven patterns of the normal optic tract.
Comparative development of the extensor and flexor tibiae muscles in the legs of the locust, Locusta migratoria
Authors: Myers Camilla M. , Ball Eldon E.
Published: April 2021

The embryonic development of the extensor and flexor tibiae muscles in the pro- and mesothoracic legs of the locust, Locusta migratoria, is described, and compared to the previously described development of these muscles in the metathoracic legs. The basic pattern of development of each muscle is the same in all three pairs of legs. The extensor tibiae (ETi) forms a giant syncytium, or supramuscle pioneer, which then breaks up into a series of muscle pioneers. The flexor tibiae muscle (FITi) is formed directly by sequential addition of individual muscle pioneers. Thus, there are at least two fundamentally different patterns of muscle development in the embryonic locust, as exemplified by these two muscles, and supramuscle pioneer formation is not a unique feature of the metathoracic ETi associated with its evolutionary hypertrophy.

In spite of a basically similar pattern of development of homologous muscles in all three pairs of legs, there are significant developmental differences between the metathoracic ETi and FITi and their homologues in the anterior legs. First, during development of the ETi muscles, the ETi MP forms a double row of attachment sites along both walls of the metathoracic leg, while in the anterior legs there is only a single row. Second, during development of the FITi muscles, the proximal MP, which lies at the tip of the apodeme, dies and breaks down in the metathoracic limbs. In the pro- and mesothoracic limbs it remains intact and eventually forms a large proximal muscle bundle. Third, the accessory ETi and FITi muscles, which develop in the metathoracic legs, are not formed in the anterior legs.
Distribution of laminin and collagens during avian neural crest development
Authors: Duband Jean-Loup , Thiery Jean Paul
Published: April 2021

The distribution of type I, III and IV collagens and laminin during neural crest development was studied by immunofluorescence labelling of early avian embryos. These components, except type III collagen, were present prior to both cephalic and trunk neural crest appearance. Type I collagen was widely distributed throughout the embryo in the basement membranes of epithelia as well as in the extracellular spaces associated with mesenchymes. Type IV collagen and laminin shared a common distribution primarily in the basal surfaces of epithelia and in close association with developing nerves and muscles. In striking contrast with the other collagens and laminin, type III collagen appeared secondarily during embryogenesis in a restricted pattern in connective tissues.

The distribution and fate of laminin and type I and IV collagens could be correlated spatially and temporally with morphogenetic events during neural crest development. Type IV collagen and laminin disappeared from the basal surface of the neural tube at sites where neural crest cells were emerging. During the course of neural crest cell migration, type I collagen was particularly abundant along migratory pathways whereas type IV collagen and laminin were distributed in the basal surfaces of the epithelia lining these pathways but were rarely seen in large amounts among neural crest cells. In contrast, termination of neural crest cell migration and aggregation into ganglia were correlated in many cases with the loss of type I collagen and with the appearance of type IV collagen and laminin among the neural crest population. Type III collagen was not observed associated with neural crest cells during their development.

These observations suggest that laminin and both type I and IV collagens may be involved with different functional specificities during neural crest ontogeny, (i) Type I collagen associated with fibronectins is a major component of the extracellular spaces of the young embryo. Together with other components, it may contribute to the three-dimensional organization and functions of the matrix during neural crest cell migration, (ii) Type III collagen is apparently not required for tissue remodelling and cell migration during early embryogenesis, (iii) Type IV collagen and laminin are important components of the basal surface of epithelia and their distribution is consistent with tissue remodelling that occurs during neural crest cell emigration and aggregation into ganglia.
The development of animal cap cells in Xenopus: a measure of the start of animal cap competence to form mesoderm
Authors: Jones E. A. , Woodland H. R.
Published: April 2021

Grafting, together with tissue identification by monoclonal antibodies, has been used to study the allocation of Xenopus animal cap cells to the ectodermal or mesodermal lineages. Animal cap cells become responsive at stage and lose responsiveness to mesodermal induction at, or just after, stage (depending on the batch of embryos). The ability of the vegetal yolky cells to induce mesoderm disappears between stages and 11. It is present at stage 6– and may exist before this. The emergence of competence to respond at stage , coupled with the fact that the endoderm is already capable of induction at this stage, suggests that mesodermal induction begins at this point in the intact embryo.
Role of the neural crest in development of the trabeculae and branchial arches in embryonic sea lamprey, Petromyzon marinus (L)
Authors: Langille Robert M. , Hall Brian K.
Published: April 2021

Lamprey embryos were obtained by artificial fertilization to ascertain the contributions made by the neural crest to the head skeleton. Early-neurula-stage embryos of Petromyzon marinus were subjected to neural crest extirpation along the anterior half from one of seven zones, raised to a larval stage at which control larvae exhibit well-developed skeletons and analysed by light microscopy for any abnormalities to the cranial and visceral skeleton. The removal of premigratory neural crest at the level of the anterior prosencephalon (zone I) and at the level of somites 6 to 8 (zone VII) had no effect on skeletal development. However, the extirpation of neural crest from the intervening regions was positively correlated with deletions/reductions to the trabeculae (basicranial elements) and to the branchial arches (viscerocranial elements). Alterations to the trabeculae (16/27 cases, or 59 %) occurred only after extirpation of zones II-V (corresponding to the posterior prosencephalon to midrhombencephalon) while alterations to the branchial arches (21/28 cases, or 75 %) occurred only after removal of neural crest from zones IH-VI (corresponding to the mesencephalon to the level of the fifth somite). Furthermore, the first three branchial arches were correlated in a majority of cases with neural crest from zone III, the next two arches with zones IV, V and VI and the last two arches with zone VI. Organs that develop within or adjacent to the area of neural crest extirpation such as the brain, notochord and lateral mesodermal derivatives were not affected. Parachordals were never altered by the operations nor were there any discernible changes to developing mucocartilage or to the prechondrogenic otic capsule. The contributions of the neural crest to the petromyzo-nid head skeleton described herein are compared with the roles of neural crest in the development of cranial and visceral skeletal elements in other vertebrates. The importance of these findings to the current hypothesis of the phylogeny of the vertebrate skeleton and the central role of the neural crest in vertebrate cephalization is discussed.
Development of macroglial cells in rat cerebellum II. An in situ immunohistochemical study of oligodendroglial lineage from precursor to mature myelinating cell
Authors: Reynolds Richard , Wilkin Graham P.
Published: April 2021

Using immunofluorescence with a panel of antibodies that recognize antigens expressed by oligodendroglia, the myelin-producing cells of the CNS, at different stages of differentiation from precursor to mature cell, we have investigated the development of cells of this lineage in cryostat sections of rat cerebellum. Our results are consistent with the view that glial precursors, identified by their expression of the ganglioside GD3, arise in the subependymal layers of the 4th ventricle and migrate to their final position in the cerebellum via the superior medullary velum, and to some extent the peduncles. As the cells reach their final destination they make the transition to recognizable galactocerebroside (GC)-expressing oligodendroglia, via a GD3+/GC+ intermediate. The myelin-associated protein 2′,3′-cyclic nucleotide 3′-phospho-hydrolase (CNP) appears at the same time as GC, whereas myelin basic protein (MBP) is expressed 2–3 days after GC and CNP, immediately prior to myelin formation. A very clear progression of oligodendroglial differentiation was observed from the SMV into the base of the cerebellum, up into the white matter (WM) tracts of the folia, and then away from this central white matter into the granule cell and Purkinje cell layers, and finally the molecular layer. The time delay between the expression of GC, CNP and MBP was the same for oligodendroglia in all of these layers, suggesting the presence of an intrinsic clock controlling the initial expression of these myelin components. The early appearance of CNP in oligodendroglia suggests a role for this protein in the early stages of myelinogenesis.
Vasculogenesis and angiogenesis in embryonic-stem-cell-derived embryoid bodies
Authors: Risau Werner , Sariola Hannu , Zerwes Hans-Günter , Sasse Joachim , Ekblom Peter , Kemler Rolf , Doetschman Thomas
Published: April 2021

Embryonic stem cells (ESC) have been established previously from the inner cell mass cells of mouse blastocysts. In suspension culture, they spontaneously differentiate to blood-island-containing cystic embryoid bodies (CEB). The development of blood vessels from in situ differentiating endothelial cells of blood islands, a process which we call vasculogenesis, was induced by injecting ESC into the peritoneal cavity of syngeneic mice. In the peritoneum, fusion of blood islands and formation of an in vivo-like primary capillary plexus occurred. Transplantation of ESC and ESC-derived complex and cystic embryoid bodies (ESC-CEB) onto the quail chorioallantoic membrane (CAM) induced an angiogenic response, which was directed by nonyolk sac endoderm structures. Neither yolk sac endoderm from ESC-CEB nor normal mouse yolk sac tissue induced angiogenesis on the quail CAM. Extracts from ESC-CEB stimulated the proliferation of capillary endothelial cells in vitro. Mitogenic activity increased during in vitro culture and differentiation of ESC. Almost all growth factor activity was associated with the cells. The ESC-CEB-derived endothelial cell growth factor bound to heparin-sepharose. The identification of acidic fibroblast growth factor (FGF) in heparin-sepharose-puri-fied material was accomplished by immunoblot experiments involving antibodies against acidic and basic FGF. We conclude that vasculogenesis, the development of blood vessels from in situ differentiating endothelial cells, and angiogenesis, the sprouting of capillaries from preexisting vessels, are very early events during embryogenesis which can be studied using ESC differentiating in vitro. Our results suggest that vasculogenesis and angiogenesis are differently regulated.
Transcription of the segment-polarity gene wingless in the imaginal discs of Drosophila, and the phenotype of a pupal-lethal wg mutation
Authors: Baker Nicholas E.
Published: April 2021

wingless (wg) is a segment-polarity gene in Drosophila which is related to the murine proto-oncogene inl-1. In Drosophila embryos, wg transcription defines part of each parasegment. In situ hybridization shows that wg is also expressed in the imaginal discs which give rise to the adult during metamorphosis. Transcripts are localized in the apical cytoplasm of disc cells, and accumulate in different patterns in dorsal and ventral discs. The wgCX3 mutation produces morphological defects in the adult structures derived from these imaginal discs. The results show that wg is involved in the development of the adult, as well as the embryo, but that the imaginal discs do not express this segment-polarity gene in an identical pattern to the embryonic segments.
Developmental expression of a neurofilament-M and two vimentin-like genes in Xenopus laevis
Authors: Sharpe C. R.
Published: April 2021

A hamster vimentin cDNA probe has been used to isolate and characterize three Xenopus laevis intermediate filament genes, named XIF1, XIF3 and XIF6. Of these, XIF6 shows 89 % homology at the amino acid level to a portion of porcine neurofilament-M. XIF6 is transcribed solely in nervous tissue of embryos, commencing at the late neural tube stage. Expression is totally dependent on an interaction between mesoderm and ectoderm during gastrulation and can be used as a marker of neural induction.

XIF1 shows 94 % homology and XIF3 83 % homology to hamster vimentin at the amino acid level over a region of the protein. Although XIF1 and XIF3 show more homology to vimentin than to any other intermediate filament gene, they have distinct temporal and spatial patterns of expression. XIF1 expression most resembles that of vimentin in higher vertebrates, being expressed in embryonic myotome and nerve cord, whilst XIF3 is unusual in that its expression is restricted predominantly to the head in tailbud embryos.
Stripes of positional homologies across the wing blade of Drosophila melanogaster
Authors: Simpson P. , Messal M. El , Prado J. Moscoso Del , Ripoll P.
Published: April 2021

Clones of cells mutant for shaggy transform all hairs into bristles on the wing blade of Drosophila. Different types of bristles are formed at different locations. It is shown that, although shaggy cells are unable to make a correct decision between an epidermal cell pathway and that of a sensory bristle, they are nevertheless able to respond correctly to positional cues. A compilation of many clones led to the construction of a map of positional homologies in which all of the cells in any one area will produce the same kind of bristle. The result is a series of stripes oriented perpendicular to the anteroposterior axis of the wing and parallel to the dorsoventral axis. The significance of these stripes in relation to mechanisms of pattern formation is discussed.
The application of molecular genetics to detection of craniofacial abnormality
Authors: Moore Gudrun , Ivens Alasdair , Chambers Joanna , Bjornsson Arni , Arnason Alfred , Jensson Olafur , Williamson Robert
Published: April 2021

Congenital malformations such as secondary cleft palate can be exclusively monogenic or polygenic, but most cases have a multifactorial origin involving both environmental and genetic factors, making genetic analysis difficult. The new techniques of molecular genetics have allowed the successful chromosomal localization of mutant genes in disorders that show a simple Mendelian segregation, whether autosomal dominant (e.g. Huntington’s disease), autosomal recessive (cystic fibrosis) or X-Iinked (Duchenne muscular dystrophy). Recently, a large Icelandic family (over 280 members) with X-linked secondary cleft palate and ankyloglossia (tongue-tied) has been used as a model to localize the mutant gene associated with this craniofacial clefting. The gene has been sub-chromosomally localized to Xql3–q21.1, using anonymous probe DXYS1; a LOD score of 3· 07 was obtained.

We are preparing cosmid libraries from DNA from mouse cell lines containing only the relevant part of the human X chromosome, introduced by chromosome-mediated gene transfer. Cosmids that contain human X-chromosome sequences will be isolated and analysed for overlapping sequences and RFLPs (restriction fragment length polymorphisms) and the regions further defined by pulsed-field gel electrophoresis and the identification of coding sequences. This should give data on the location and structure of a gene involved in the craniofacial development of the human palatine shelves. This gene, and its protein product, will identify one component of the pathway that causes nonfusion of the palate. In the long term, the understanding of the expression of this sex-linked gene for secondary cleft palate and ankyloglossia will provide a model for the molecular identification of other genes regulating processes in craniofacial development whose expression is hidden in phenotypic, polygenic complexity.
Pattern formation in the facial primordia
Authors: Wedden S. E. , Ralphs J. R. , Tickle C.
Published: April 2021

Pattern formation is the developmental process that leads to the spatial ordering of cell differentiation. We have explored the problem of pattern formation in the development of the face of chick embryos. At early stages, the developing face consists of a series of small buds of tissue, the facial primordia that encircle the primitive mouth. The concepts of positional information provide a framework for considering how the patterns of differentiated cells are generated in the face. We suggest that the cranial neural crest cells must first be informed to which facial primordium they belong and then of their position within that primordium.

The cells of the early primordia appear indistinguishable. However, when the mesenchyme cells are placed in high-density culture, cartilage differentiates. The extent and pattern of cartilage differentiation is characteristic for the cell population of each facial primordium. Myogenic cells also differentiate in the cultures, but the proportion of myogenic cells is independent of the extent of chondrogenesis. Within the facial primordia, a set of epithelial-mesenchymal interactions appears to be required for outgrowth and pattern formation along the proximodistal axis of the chick beaks. In culture, face epithelium locally inhibits cartilage differentiation and suggests that another set of epithelial-mesenchymal interactions may be involved in cell patterning. The mechanisms involved in specifying the mediolateral axis of the face, for example, the midpoint of the upper beak, are not known.

Vitamin A derivatives, collectively known as retinoids, affect the development of the face of chick embryos and lead to a specific facial defect. Upper beak development is inhibited but the lower beak develops normally. The response to retinoids could be related to the specification of cells to belong to the facial primordium that will form the upper beak. Alternatively, retinoids may interfere with positional cues that operate to inform cells of their position within that primordium.
Control of pathfinding by the avian trunk neural crest
Authors: Erickson Carol A.
Published: April 2021

We have determined the pathways taken by the trunk neural crest of quail and examined the parameters that control these patterns of dispersion. Using antibodies that recognize migratory neural crest cells (HNK-1), we have found that the crest cells take three primary pathways: (1) between the ectoderm and somites, (2) within the intersomitic space and (3) through the anterior somite along the basal surface of the myotome.

The parameters controlling dispersion patterns of neural crest cells are several. The pathways are filled with at least two adhesive molecules, laminin and fibronectin, to which neural crest cells adhere tenaciously in culture. The pattern of migration through the somite may be accounted for in part by the precocious development of the basal lamina of the dermamyotome in the anterior half of the somite; this basal lamina contains both fibronectin and laminin and the neural crest cells prefer to migrate on it. In contrast, the regions into which the crest cells do not invade are filled with relatively nonadhesive molecules such as chondroitin sulphate. Some of the pathways are filled with hyaluronic acid, which stimulates the migration of neural crest cells when they are cultured in three-dimensional gels, presumably by opening spaces. Neural crest cells are also constrained to stay within the pathways by basal laminae, which act as barriers and through which crest cells do not go. Therefore, crest pathways are probably defined by several redundant factors.

The directionality of crest cell migration is probably due to contact inhibition, which can be demonstrated in tissue culture. Various grafting experiments have suggested that chemotaxis and haptotaxis do not play a role in controlling the dispersion of the crest cells away from the neural tube.

We have documented the extraordinary ability of neural crest cells to disperse in the embryo, even when they are grafted into sites in which they would normally not migrate. We have evidence that the cells’ production of plasminogen activator, a proteolytic enzyme, and also the minimal tractional force that crest cells exert on the substratum as they migrate, contribute to this migratory ability.
Accumulation of collagen III at the cleft points of developing mouse submandibular epithelium
Authors: Nakanishi Yasuo , Nogawa Hiroyuki , Hashimoto Yoko , Kishi Jun-Ichi , Hayakawa Taro
Published: April 2021

The distribution of collagens I, III, IV and V was studied by immunoperoxidase staining of early developing mouse submandibular glands. Collagen I was always present in the extracellular matrices of the mesenchyme and at the epithelial-mesenchymal interfaces of the 12-day gland with no clefts and of the 13-day gland with a few definite clefts. Collagen III was found in a similar fashion to that of collagen I in the mesenchyme, but the distribution at the epithelial-mesenchymal interfaces was very different. In the mid 12-day gland with a round lobule, collagen III was distributed at every slightly indented site of basal epithelial surfaces. At the late 12-day stage, a few initial signs of cleft appeared on the surface, at which accumulation of collagen III became evident. Intense immunoreaction of collagen III in the early 13-day gland was seen at the bottom of every narrow cleft. No specific accumulation of collagens IV and V was observed in clefts of the late 12-day and early 13-day glands.

Staining of collagen III in the 12-day gland cultured for 10 h in the presence of bovine dental pulp collagenase inhibitor, which has been shown to stimulate cleft initiation, was very prominent at the bottom of every narrow cleft. These observations suggest that collagen III works as a key substance for either in vitro or in vivo cleft initiation of the mouse embryonic submandibular epithelium.
Spatial- and temporal-restricted pattern for amelogenin gene expression during mouse molar tooth organogenesis
Authors: Snead Malcolm L. , Luo Wen , Lau Eduardo C. , Slavkin Harold C.
Published: April 2021

Position- and time-restricted amelogenin gene transcription was analysed in developing tooth organs using in situ hybridization with asymmetric complementary RNA probes produced from a cDNA specific to the mouse 26×103Afr amelogenin. In situ analysis was performed on developmentally staged fetal and neonatal mouse mandibular first (MO and maxillary first (M1) molar tooth organs using serial sections and three-dimensional reconstruction. Amelogenin mRNA was first detected in a cluster of ameloblasts along one cusp of the M1 molar at the newborn stage of development. In subsequent developmental stages, amelogenin transcripts were detected within foci of ameloblasts lining each of the five cusps comprising the molar crown form. The number of amelogenin transcripts appeared to be position-dependent, being more abundant on one cusp surface while reduced along the opposite surface. Amelogenin gene transcription was found to be bilaterally symmetric between the developing right and left M1 molars, and complementary between the M1 and M1 developing molars; indicating position-restricted gene expression resulting in organ stereoisomerism. The application of in situ hybridization to forming tooth organ geometry provides a novel strategy to define epithelial-mesenchymal signal(s) which are believed to be responsible for organ morphogenesis, as well as for temporal- and spatial-restricted tissue-specific expression of enamel extracellular matrix.
Early innervation of the metanephric kidney
Authors: Sariola H. , Holm K. , Henke-Fahle S.
Published: April 2021

During kidney differentiation, the nephrogenic mesenchyme converts into renal tubules and the ureter bud branches to form the collecting system. Here we show that in the early undifferentiated kidney rudiment there is a third cell type present. In whole-mount preparations of cultured undifferentiated metanephric kidneys, neurones can be detected by immunohistochemical means with antibodies against the neurofilament triplet, 13AA8, and against neuronal cell surface gangliosides, Q211. Clusters of neuronal cell bodies can be seen in the mesenchyme close to the ureter bud. The terminal endings of neurites are found around the mesenchymal condensates that later become kidney tubules. A similar distribution of neurites can be revealed in tissue sections of kidney grafts growing in the chicken chorioallantoic membranes. In primary cultures of the ureter bud cells, neurones are con-stantly present. In another report, we have shown that, in experimental conditions, neurones are in-volved in regulation of kidney morphogenesis. The present results raise the possibility that neurones of the metanephric kidney may have this function in vivo as well.
Induction by acid load of the maturation of prestalk cells in Dictyostelium discoideum
Authors: Inouye Kei
Published: April 2021

During the process of fruiting body construction in the cellular slime mould Dictyostelium discoideum, prestalk cells become mature stalk cells in a well-controlled manner. To identify the natural inducer of stalk cell maturation, substances known to induce stalk cell differentiation under in vitro conditions, and some other related compounds, were examined for their effects in vivo on migrating slugs, the precursor structures of the fruiting bodies.

Among these substances, addition of weak acids such as CO2, and addition followed by removal of weak bases such as NH3, strikingly induced the maturation of prestalk cells in situ in slugs. On the other hand, inhibitors of the plasma membrane proton pump did not efficiently induce the maturation of prestalk cells in intact slugs. Differentiation inducing factor (DEF), an endogenous inducer of prestalk differentiation, seemed to be an even poorer inducer of stalk cell maturation when applied to intact slugs.

The activities of these substances in inducing stalk cell maturation showed a good correlation with their effects on the cytoplasmic pH (pH|) of prestalk cells; the larger the pH| drop, the stronger the induction of stalk cell maturation, suggesting a requirement for a pHi decrease for the maturation of prestalk cells. Based on these results, it was proposed that stalk cell differentiation, which is induced by DIF, is blocked halfway during normal development by (an) agent(s) that prevent(s) the decrease in pH,.
A phylogenetic interpretation of the patterns of gene expression in Drosophila embryos
Authors: Tear G. , Bate C. M. , Arias A. Martinez
Published: April 2021

Two fundamental processes of the development of insect embryos are the generation and the morphological diversification of metameric units. In Drosophila, these processes are under the control of the products of the segmentation (generation) and the homeotic (morphological diversification) genes. Molecular studies of the activity of these genes has revealed spatial and temporal patterns of expression consistent with the requirements inferred from the mutant phenotypes but, in addition, these studies have revealed transient patterns which are difficult to reconcile with those phenotypes. It is possible that these patterns reflect ancestral regulatory elements which are still operational in more primitive insects. The validity of this interpretation can be tested by comparing the embryonic development of long germ band insects like Drosophila melanogaster with that of the more primitive short germ band insects like the locust Schistocerca gregaria and by obtaining and studying locust homologues of Drosophila segmentation and homeotic genes.
A cell lineage analysis of segmentation in the chick embryo
Authors: Stern Claudio D. , Fraser Scott E. , Keynes Roger J. , Primmett Dennis R. N.
Published: April 2021

We have studied the lineage history of the progenitors of the somite mesoderm and of the neural tube in the chick embryo by injecting single cells with the fluorescent tracer, rhodamine-lysine-dextran. We find that, although single cells within the segmental plate give rise to discrete clones in the somites to which they contribute, neither the somites nor their component parts (sclerotome, dermatome, myotome or their rostral and caudal halves) are ‘compartments’ in the sense defined in insects. Cells in the rostral two thirds or so of the segmental plate contribute only to somite tissue and divide about every 10 h, while those in the caudal portions of this structure contribute both to the somites and to intermediate and lateral plate mesoderm derivatives. In the neural tube, the descendants of individual prospective ventral horn cells remain together within the horn, with a cycle time of 10 h.

We have also investigated the role of the cell division cycle in the formation and subsequent development of somites. A single treatment of 2-day chick embryos with heat shock or a variety of drugs that affect the cell cycle all produce repeated anomalies in the pattern of somites and vertebrae that develop subsequent to the treatment. The interval between anomalies is 6 –7 somites (or a multiple of this distance), which corresponds to 10 h. This interval is identical to that measured for the cell division cycle. Given that cell division synchrony is seen in the presomitic mesoderm, we suggest that the cell division cycle plays a role in somite formation.

Finally, we consider the mechanisms responsible for regionalization of derivatives of the somite, and conclude that it is likely that both cell interactions and cell lineage history are important in the determination of cell fates.
A note on segmentation and the scale of pattern formation in insects and in vertebrates
Authors: Cooke Jonathan
Published: April 2021

Little or nothing was said, by workers on vertebrate somite formation at this symposium, about theories for the overall control of segment size, and thus the number of segments in the pattern. The current mood may well be that further modelling of the machinery of segmental patterning should await knowledge of the spatial patterns of synthesis of all relevant gene products, because understanding of the dynamics of segmentation in insects is only just beginning to emerge even though such knowledge for Drosophila genes is well advanced. What can be said, nevertheless, about the ‘system properties’ of the segmentation process in the two groups?
Interactions between the pair-rule genes runt, hairy, even-skipped and fushi tarazu and the establishment of periodic pattern in the Drosophila embryo
Authors: Ingham Philip , Gergen Peter
Published: April 2021

The pair-rule genes of Drosophila play a fundamental role in the generation of periodicity in the early embryo. We have analysed the transcript distributions of runt, hairy, even-skipped and fushi tarazu in single and double mutant embryos. The results indicate a complex set of interactions between the genes during the blastoderm stage of embryogenesis.
Development of surface pattern during division in Paramecium; I. Mapping of duplication and reorganization of cortical cytoskeletal structures in the wild type
Authors: Iftode Francine , Cohen Jean , Ruiz Françoise , Rueda Antonio Torres , Chen-Shan Lily , Adoutte André , Beisson Janine
Published: April 2021

The shape of a Paramecium is determined by the organization of its cortex which constitutes most of the cell cytoskeleton. These structures and networks are organized in relation to the approx. 4000 ciliary basal bodies present at the surface. Each basal body is the centre of a polarized and asymmetrical cortical unit. At the whole-cell level, all units are tandemly arranged in parallel rows and form a defined asymmetrical pattern with dorsoventral and anteroposterior polarities. During division, the cortex is the site of the major morphogenetic processes.

In order to analyse how the surface pattern and the shape of the cell are reconstructed at each division, we have used specific immunological and cytological probes to map, in space and time, the reorganization of each of the major cytoskeletal cortical components: basal bodies and microtubules, kinetodesmal fibres, epiplasm and outer lattice. This cytological dissection demonstrates that the surface of the dividing cell is progressively invaded by morphogenetic waves which successively and individually trigger the duplication, assembly or reorganization of each structure and which all spread from the same epicentre (oral apparatus and fission furrow) with the same shape. Furthermore, the response of units to the morphogenetic waves depends on their position on the cell. It thus appears that despite the structural local constraints within units, the development of surface pattern is controlled in an integrated manner by transcellular signals.

This paper is dedicated to the memory of T. M. Sonneborn
Distribution and possible interactions of actin-associated proteins and cell adhesion molecules of nerve growth cones
Authors: Letourneau Paul C. , Shattuck Terri A.
Published: April 2021

Actin filaments and their interactions with cell surface molecules have key roles in tissue cell behaviour. Axonal pathfinding during embryogenesis, an especially complex cell behaviour, is based on the migration of nerve growth cones. We have used fluorescence immunocytochemistry to examine the distribution in growth cones, their filopodia and lamellipodia of several actin-associated proteins and nerve cell adhesion molecules. The leading margins of chick dorsal root ganglion nerve growth cones and their protrusions stain strongly for f-actin, filamin, a-actinin, myosin, tropomyosin, talin and vinculin. MAP2 is absent from DRG growth cones, and staining for spectrin/fodrin extends into growth cones, but not along filopodia. Thus, organization of the leading margins of growth cones may strongly resemble the leading lamella of migrating fibroblasts. The adhesion-mediating molecules integrin, LI, N-CAM and A-CAM are all found on DRG neurites and growth cones. However, filopodia stain relatively more strongly for integrin and LI than for A-CAM or N-CAM. In fact, the 180×103Mr,. form of N-CAM may be absent from most of the length of filopodia. DRG neurones cultured in cytochalasin B display differences in immunofluorescence staining which further emphasize that these adhesion molecules interact differentially with the actin filament system of migrating growth cones. Several models for neuronal morphogenesis emphasize the importance of regulation of the expression of adhesion molecules. Our results support hypotheses that cellular distribution and transmembrane interactions are key elements in the functions of these adhesion molecules during axonal pathfinding.