bluuebunny/crossref_metadata_2025 huggingface dataset:
Small eye (Sey): a mouse model for the genetic analysis of craniofacial abnormalities
Authors: Hogan Brigid L. M. , Hirst Elizabeth M. A. , Horsburgh Gwynn , Hetherington Colin M.
Published: April 2021

Small eye (Sey) is a dominant mutation in the mouse affecting the embryonic development of the eyes and nose. In homozygous Sey/Sey embryos, the optic vesicles grow out but there is no lens induction and the nasal pits fail to develop. Scanning electron microscope studies of Sey/ Sey embryos show that the maxillary processes develop normally and fuse with ridges of ectoderm in the frontonasal position. In Sey/ + heterozygotes, the vacuolated lens is smaller than normal, and there is folding of the margins of the optic cup and ingrowth of mesodermal cells. Evidence is presented that Sey is not allelic with Coloboma (Cm), another mutation affecting eye development on chromosome 2.
XY follicle cells in ovaries of XX↔XY female mouse chimaeras
Authors: Burgoyne Paul S. , Buehr Mia , Mclaren Anne
Published: April 2021

Oocytes with adhering follicle cells were sampled from ovaries obtained from 11 GPI-1A↔GPI-1B chimaeras, comprising 10 females and 1 hermaphrodite. GPI analysis of individual oocytes revealed a marked bias towards the GPI-1B component in the germ line of this chimaeric combination. GPI-IB XY oocytes were identified in the ovary from the hermaphrodite, the bias towards the GPI-1B germ line perhaps helping to counterbalance the normally severe selection against XY oocytes. GPI analysis of follicle cells revealed a much more balanced contribution of the two components to this ovarian cell type. Importantly, GPI-1A follicle cells were identified in more than half the follicles from an XX↔XY female in which the GPI-1A component was XY, supporting an earlier conclusion of Ford et al. (1974) that XY cells can contribute to the follicles of XX↔XY female mice.

It is suggested that XY cells can be recruited to form follicle cells in XX↔XY chimaeras when there is a developmental mismatch between the two components, such that an ovary-determining signal produced by the XX component pre-empts the testisdetermining action of the Y.
Segmentation in frogs
Authors: Davidson Duncan
Published: April 2021

This paper reviews evidence relating to the question, at what stage in the development of the frog embryo are segment boundaries specified? Current evidence leads to the hypothesis that a spatiotemporal series of cell states leading to segmentation is continuously initiated at a position 200 to 300 μm from the posterior end of the presomitic mesoderm, about nine somite intervals before the formation of a definitive somite. The evidence suggests, though by no means proves, that segment boundaries are specified close to this time. This hypothesis relies critically on evidence concerning the effects of disruptive agents, the extent of cell mixing prior to the early gastrula stage, fate map data, and a comparison with development in the mouse where a similar fate map can be related to morphological evidence of somitomeric segmentation.

Evidence regarding the organization of the posterior, undifferentiated zone of the mesoderm in the frog embryo indicates that the cells are not proliferating rapidly, but are undergoing cell movements and rearrangements associated with caudal extension. The speculation that the segment pattern derives from inductive interactions in this region is discussed.
Involvement of oocyte-coded message in cell differentiation control of early human embryos
Authors: Tesařík Jan
Published: April 2021

Considerable evidence indicates that the first phenotypical diversification of embryonic cells during mammalian preimplantation development is achieved in two successive steps: (i) generation of cell asymmetry and (ii) unequal cell division. This paper shows that ultrastructural signs of blastomere surface regionalization in human preimplantation embryos are evident as early as the 2-cell stage when modifications of the plasma membrane (loss of microvilli and endocytotic activity, formation of cell junctions) are induced in places of blastomere contact. The capacity of the plasma membrane to undergo these cell-contact-dependent changes precedes any detectable activity of the embryonic genome. The area of the modified plasma membrane shows a continuous increase during the first three cleavage stages. The progression of these membrane modifications is the same in embryos that have properly enhanced their transcriptional activity at the 8-cell stage and in those that have not. In spite of the failure of this early-cleavage-progressed-cleavage transition of gene activity, the formation of zonula adherens and gap junctions goes on apparently normally in the respective embryos and morphologically distinct inner cell mass and trophectoderm cell lineages are subsequently segregated in 16-cell morulae. However, tight junctions do not develop under these conditions. The occurrence of the progressed-cleavage pattern of gene activity in the majority of embryonic cells is a necessary prerequisite for the appearance of the blastocyst cavity. Thus, oocyte-coded message is apparently involved in the control of relatively late stages of human preimplantation development including the differentiation of the first two embryonic tissues, but the embryonic genome is required for the full achievement of this early differentiative event.
Maintenance and regulation of cellular handedness in Tetrahymena
Authors: Nelsen E. Marlo , Frankel Joseph
Published: April 2021

The left-handed phenotype of Tetrahymena thermophila (LH) is a global mirror image of its right-handed counterpart (RH). LH cells are ‘wound’ in the opposite direction from that of RH cells with respect to the placement of all structures that are asymmetrically disposed on the cell circumference. However, the local geometry of ciliary rows, including the asymmetrically placed microtubule bands and other accessory structures, is identical in RH and LH cells. Populations of LH cells grow more slowly than those of RH cells, probably because of nutritional problems due to faulty construction of the cell mouth. LH cells, like RH cells, conjugate in a homopolar configuration, while LH cells mate with RH cells in a heteropolar union which suffices to initiate the conjugal nuclear events but is insufficient to allow survival of progeny. Subclonal analyses indicate that reversion of the LH to the RH form is relatively rare. However, the frequency of reversion is greatly increased by conditions that promote the formation of doublets by fission arrest. An analysis of intermediate doublet forms in such cultures strongly suggests that reversion takes place through a specific pathway, with LH-LH doublets regulating to LH-RH forms that then may give rise to RH singlets. The origin and fate of the LH-RH intermediate forms can be explained by applying a modified polar coordinate model of positional information with the proviso that there is a preferred direction for the intercalation of new positional values.
Relationship between vasculogenesis, angiogenesis and haemopoiesis during avian ontogeny
Authors: Pardanaud Luc , Yassine Fouzia , Dieterlen-Lievre FranÇoise
Published: April 2021

Quail-chick intracoelomic grafts of organ rudiments were used to study the origin of endothelia and haemopoietic cells during avian organogenesis in conjunction with the monoclonal antibody QH1 which recognizes the quail haemangioblastic lineage. Results differed according to the germ-layer constitution of the grafted rudiments. In the case of the limb buds, endothelial cells from the host invaded the graft through an angiogenic process. Haemopoietic progenitors from the host also colonized the grafted bone marrow.

In contrast, rudiments of internal organs provided their own contingent of endothelial precursors, a process termed vasculogenesis. Nevertheless, haemopoietic cells in these organs were all derived from the host. In the lung, this extrinsic cell population appeared regularly scattered around the parabronchi and had a macrophage-like phenotype. In the pancreas, the granulocytes which differentiate as dense aggregates located in the wall of the largest vessels were extrinsic. Similarly in the spleen, a mesodermal primordium that develops in close association with the pancreatic endoderm, endothelial cells were intrinsic and haemopoietic cells host-derived.

This study demonstrates that, in ontogeny, vascularization obeys different rules depending on which germ layer the mesoderm is associated with: in mesodermal/ectodermal rudiments angiogenesis is the rule; in mesodermal/endodermal rudiments, vasculogenesis occurs. However, in these internal organs undergoing vasculogenesis, endothelial and haemopoietic cells have separate origins.

We put forward the hypothesis that the endoderm induces the emergence of endothelial cells in the associated mesoderm. Formation of blood stem cells may also involve interactions between endoderm and mesoderm, but in this case the responding capacity of the mesoderm appears restricted to the paraaortic region.
Pattern formation in the Drosophila embryo: allocation of cells to parasegments by even-skipped and fushi tarazu
Authors: Lawrence Peter A. , Johnston Paul
Published: April 2021

The first sign of metamerization in the Drosophila embryo is the striped expression of pair-rule genes such as fushi tarazu (ftz) and even-skipped (eve). Here we describe, at cellular resolution, the development of ftz and eve protein stripes in staged Drosophila embryos. They appear gradually, during the syncytial blastoderm stage and soon become asymmetric, the anterior margins of the stripes being sharply demarcated while the posterior borders are undefined. By the beginning of germ band elongation, the eve and ftz stripes have narrowed and become very intense at their anterior margins. The development of these stripes in hairy−, runt−, eve−,ftz− and engrailecT embryos is illustrated. In eve− embryos, the ftz stripes remain symmetric and lack sharp borders. Our results support the hypothesis (Lawrence et al. Nature 328, 440-442, 1987) that individual cells are allocated to parasegments with respect to the anterior margins of the eve and ftz stripes.
Differential gene expression in the anterior neural plate during gastrulation of Xenopus laevis
Authors: Jamrich Milan , Sato Sheryl
Published: April 2021

We have isolated three cDNA clones that are preferentially expressed in the cement gland of early Xenopus laevis embryos. These clones were used to study processes involved in the induction of this secretory organ. Results obtained show that the induction of this gland coincides with the process of neural induction. Genes specific for the cement gland are expressed very early in the anterior neural plate of stage-12 embryos. This suggests that the anteroposterior polarity of the neural plate is already established during gastrulation. At later stages of development, two of the three genes have secondary sites of expression. The expression of these genes can be induced in isolated animal caps by incubation in 10mm-NH4Cl, a treatment that is known to induce cement glands.
Spatial and temporal variation in the structure of the basal lamina in embryonic grasshopper limbs during pioneer neurone outgrowth
Authors: Anderson Hilary , Tucker Richard P.
Published: April 2021

The pioneer neurones of the embryonic grasshopper limb use the basal lamina underlying the limb ectoderm as a substratum over which to grow from the periphery to the CNS (Anderson & Tucker, 1988). In this paper we use transmission electron microscopy to describe the structure of this substratum before, during, and after the time of axon navigation.

The organization of the basal lamina varies considerably in different regions and at different times of development of the embryonic limbs, and is unlike that of the fully developed limb at the time of hatching. We suggest that this spatial and temporal variation could play a role in regulating the direction of outgrowth of pioneer neurones.
Anin situtransgenic enzyme marker for the midgestation mouse embryo and the visualization of inner cell mass clones during early organogenesis
Authors: Beddington Rosa S. P. , Morgernstern Jay , Land Hartmut , Hogan Aileen
Published: April 2021

In order to study the deployment of cells during gastrulation and early organogenesis, it is necessary to have an in situ cell marker which can be used to follow cell fate. To create such a marker a transgenic mouse strain, designated Tg(Act-lac Z)-1, which carries 6 copies of the Escherichia coli lac Z gene under the control of the rat β-actin promoter, was made by pronuclear injection of DNA. Staining early postimplantation hemizygous mouse conceptuses, during gastrulation and early organogenesis, for β-galactosidase activity shows that lac Z expression is ubiquitous and constitutive in all epiblast derivatives of the 10th day conceptus. No activity is seen in trophectoderm and primitive endoderm derivatives.

Postimplantation grafts of [3H]thymidine-labelled transgenic cells establish the cell autonomy of this transgenic marker. Preliminary observations on the distribution of inner cell mass (ICM) descendant clones, identified in situ in midgestation conceptuses, confirm the pluripotency of individual ICM cells. The implications regarding patterns of cell growth in nascent fetal primordia are discussed.
A set of cell surface glycoproteins forms an early marker of cell position, but not cell type, in the root apical meristem of Daucus carota L.
Authors: Knox J. Paul , Day Susan , Roberts Keith
Published: April 2021

A monoclonal antibody (JIM4) has been derived that recognizes a series of glycoproteins associated with the plasma membrane of a suspension-cultured carrot cell line and also an arabinogalactan proteoglycan secreted by the cultured cells. Immunocytochemistry indicated that the plasma membrane antigen(s) recognized by JIM4 are specific to certain cells of Daucus carota L. seedlings. In the root apex JIM4 labelled two segments of the stele. These were centred upon the poles of the protoxylem. An axis of unlabelled cells connected the two phloem regions. Two sections of the pericycle with characteristic oblique longitudinal divisions were particularly reactive with JIM4. This pattern of reactive cells, reflecting cell position rather than a specific future cell type, would appear to be a unique observation in plants. The association of JIM4 antigens with these vascular tissues is maintained through the transition from root to the shoot tissue of the cotyledons and the mature plant. Examination of JIM4 labelling upon ultrathin frozen sections of the carrot seedling root apical meristem indicated that the expression of the antigen is a very early event in root development. Cells express the surface epitope, within one or two cells of the dome of apical initials, before the pattern of future vascular tissue can be discerned and well before its actual differentiation.
Mesoderm-inducing properties of INT-2 and kFGF: two oncogene-encoded growth factors related to FGF
Authors: Paterno G. D. , Gillespie L. L. , Dixon M. S. , Slack J. M. W. , Heath J. K.
Published: April 2021

Many theories of neoplasia suggest that oncogenic transformations result from aberrations in the control mechanisms which normally regulate growth and differentiation during embryonic development. It has recently become clear that many proto-oncogenes are differentially expressed during embryonic development and may thus be important embryonic regulatory molecules. We report here that the products of two transforming oncogenes int-2 and hst / ks (now called kfgf) can, with different potencies, induce mesoderm formation in isolated Xenopus laevis animal pole explants and stimulate DNA synthesis in mammalian fibroblasts. The results suggest that these proteins may function as mesoderm inducers in mammalian embryogenesis and that similar receptor/signalling pathways may be utilized for developmental and oncogenic processes. Finally, we have shown that the Xenopus assay system used in this study provides a powerful screen for protein factors that are active in development.
Gene expression during mammalian oogenesis and early embryogenesis: quantification of three messenger RNAs abundant in fully grown mouse oocytes
Authors: Roller R. J. , Kinloch R. A. , Hiraoka B. Y. , Li S. S.-L. , Wassarman P. M.
Published: April 2021

Ribonuclease protection assays have been used to quantitatively assess changes in steady-state levels of specific mRNAs during oogenesis and early embryogenesis in mice. The mRNAs encode ZP3 (a glycoprotein that serves as a sperm receptor), LDH-B (heart-type lactate dehydrogenase), and MOM-1 (a protein of unknown function). MOM-1 and LDH-B are expressed in a variety of adult mouse tissues and midgestation embryos, whereas ZP3 expression is restricted completely to oocytes. All three mRNAs are expressed by growing mouse oocytes and accumulate to unusually high levels in fully grown oocytes as compared to somatic cells; 240000, 200000 and 74000 copies mRNA per fully grown oocyte for ZP3, LDH-B and MOM-1, respectively. Steady-state levels of LDH-B and MOM-1 mRNA undergo a modest decline (∼20–40 %) during ovulation when fully grown oocytes become unfertilized eggs and, in general, mirror the reported change in poly(A)+RNA levels during this period of development. On the other hand, the level of ZP3 mRNA declines dramatically (∼98%) during ovulation, from ∼240000 copies per oocyte to ∼5000 copies per unfertilized egg, and ZP3 mRNA is undetectable in fertilized eggs (<1000 copies per fertilized egg). MOM-1 mRNA is expressed at relatively low levels in morulae ∼2000 copies per embryo) and blastocysts ∼5000 copies per embryo), whereas ZP3 mRNA remains undetectable (<1000 copies per embryo) at these stages of preimplantation development. These findings are discussed in the context of overall gene expression during oocyte growth, meiotic maturation and early embryogenesis in mice.
Expression of N-CAM precedes neural induction in Pleurodeles waltl (Urodele, amphibian)
Authors: Saint-Jeannet Jean-Pierre , Foulquier Françoise , Goridis Christo , Duprat Anne-Marie
Published: April 2021

The appearance and localization of N-CAM during neural induction were studied in Pleurodeles waltl embryos and compared with recent contradictory results reported in Xenopus laevis.

A monoclonal antibody raised against mouse N-CAM was used. In the nervous system of Pleurodeles, it recognized two glycoproteins of 180 and 140 xlO3 which are the Pleurodeles equivalent of N-CAM-180 and -140.

Using this probe for immunohistochemistry and immunocytochemistry, we showed that N-CAM was already expressed in presumptive ectoderm at the early gastrula stage. In late gastrula embryos, a slight increase in staining was observed in the neurectoderm, whereas the labelling persisted in the noninduced ectoderm.

When induced ectodermal cells were isolated at the late gastrula stage and cultured in vitro up to 14 days, a faint polarized labelling of cells was observed initially. During differentiation, the staining increased and became progressively restricted to differentiating neurons.
Development and phenotypic variability of genetically identical half mouse embryos
Authors: Papaioannou Virginia E. , Mkandawire John , Biggers John D.
Published: April 2021

The growth and development of three groups of genetically identical F1 C57BL/6J♀ × SJL/J♂ mice were compared to examine whether embryo manipulation affects subsequent postnatal growth and development of mammalian embryos: (1) controls - the natural offspring of timed matings, (2) transferred controls - offspring from 2-cell embryos transferred to recipients 1 day asynchronous, and (3) transferred half embryos - offspring developing from one blastomere from the 2-cell stage transferred to recipients 1 day asynchronous. The recipients were C57BL/6J females. No differences were found in the age at eye opening and vaginal opening. At 5 days after birth the median body weights of the controls were lower than the weights of the transferred groups. This result could be explained by the larger litter size in the control group. The overall variances of the body weights did not differ between the groups. By the second week after birth a marked increase in overall variances of body weights of the transferred groups, compared with the control group, was observed. At 5 days after birth, the median tail lengths did not differ between groups, and overall variances were the same. By the second week, the overall variances of the tail lengths of the transferred groups were significantly greater than that of the control group. Possibly the increased overall variances of the body weight and the tail length of the transferred groups are related to the smaller litter size in these groups which affects competition for food and the ambient temperature in the nest. The overall results suggest newborn mice that have developed from half embryos have compensated for their initial deficiency. The intraclass correlation coefficients for body weight and tail length are approximately the same in all groups. Thus, producing artificial identical twins by embryo bisection may not affect their potential usefulness in the design of experiments.
Gene regulation during cartilage differentiation: temporal and spatial expression of link protein and cartilage matrix protein in the developing limb
Authors: Stirpe Norine S. , Goetinck Paul F.
Published: April 2021

The temporal and spatial expression of link protein and cartilage matrix protein genes was defined during chondrogenesis in the developing chick embryonic wing bud, using RNA in situ hybridization. For comparison, the expression of genes encoding type II collagen and cartilage proteoglycan core protein was also examined. Link protein transcripts are first detected at stage 25 of Hamburger and Hamilton, together with proteoglycan core protein transcripts. Type II collagen transcripts were first detected as early as stage 23 whereas cartilage matrix protein transcripts could not be detected before stage 26. The results of the study indicate that the temporal expression of the genes for cartilage matrix protein and type II collagen are independent of each other and also independent of that for link protein and proteoglycan core protein.
Complementary patterns of expression of c-ets 1, c-myb and c-myc in the blood-forming system of the chick embryo
Authors: Vandenbunder B. , Pardanaud L. , Jaffredo T. , Mirabel M. A. , Stehelin D.
Published: April 2021

We have used in situ hybridization to study the spatial and temporal distribution of the transcription of three cellular oncogenes encoding DNA-binding proteins, c-ets 1, c-myb and c-myc during the development of the chick embryo.

c-ets 1 mRNA expression appears linked to the mesodermal lineage and is strongly expressed in early endothelia; it subsequently becomes restricted to small vessel endothelia. Hemopoietic cells in extraembryonic blood islands at E2 express c-ets 1, while intraembryonic hemopoietic cells in aortic clusters (E3) and paraaortic foci (E6) express c-myb. c-myc transcripts are detected in cells undergoing hemopoiesis in both these extraembryonic and intraembryonic sites. Outside the blood forming system, c-myc is transcribed in a large variety of cells; c-ets 1 displays tissue-specific expression in groups of mesodermal cells engaged in morphogenetic processes and appears excluded from all epithelia; finally the expression of c-myb is the most tightly linked to hemopoietic cells. In any case, it is clear that these three oncogenes display complementary expression in endothelial and hemopoietic cells where their patterns are modulated in relationship to multiplication and differentiation.
Retinoic acid inhibits growth in agarose of early chick embryonic cells and may be involved in regulation of axis formation
Authors: Mitrani Eduardo , Shimoni Yael
Published: April 2021

The mechanisms involved in the generation of axial structures in the chick are well documented, yet, little is known about the actual factors that generate such a complex pattern. The recent demonstrations that all-trans-retinoic acid (RA) acts as a morphogen during limb development (Thaller and Eichele, 1987) lead us to examine whether during axis formation in the developing chick, RA could be one of the factors involved. We now show that retinoic acid can block a very unusual property of normal early chick embryonic cells, mainly their capacity to grow in semisolid medium. We also present experiments that suggest that RA may play a direct role during axis formation in the developing chick.
Spatially ordered transcription of regulatory DNA in the bithorax complex of Drosophila
Authors: Sánchez-herrero Ernesto , Akam Michael
Published: April 2021

The identities of the second through ninth abdominal segments of Drosophila are specified by two genes of the bithorax complex (BX-C), abdominal-A (abd-A) and Abdominal B (Abd-B). The correct deployment of these two genes requires an extensive region (the iab region) located between the two protein-coding transcription units. We show here that one iab mutation affects the pattern of expression of Abd-B. We also show that most or all of the DNA in this regulatory iab region is transcribed. In blastoderm stage embryos we can define three distinct domains within the iab DNA, each transcribed in a region that extends from a characteristic anterior limit to the posterior end of the segmented part of the embryo. The anterior limits of expression for the three regions are colinear with the sequence of the domains on the chromosome, and lie at about two-segment intervals. We suggest that these early transcription patterns reflect the initial activation of the BX-C.
Mitogenic effect of muscle on the neuroepithelium of the developing spinal cord
Authors: Fontaine-Pérus Josiane C. , Chanconie Marcelle , Le Douarin Nicole M. , Gershon Michael D. , Rothman Taube P.
Published: April 2021

A previous study revealed that segments of bowel grafted between the neural tube and somites of a younger chick host embryo would induce a unilateral increase in cellularity of the host’s neural tube. The current experiments were done to test the hypotheses that muscle tissue in the wall of the gut is responsible for this growth-promoting effect and that the spinal cord enlargement is the result of a mitogenic action on the neuroepithelium. Fragments of skeletal (E8–15) or cardiac muscle (E4–14) were removed from quail embryos and grafted between the neural tube and somites of chick host embryos (E2). Both skeletal and cardiac muscle grafts mimicked the effect of bowel and induced an increase in cell number as well as a unilateral enlargement of the region of the host’s neural tube immediately adjacent to the grafts. The growth-promoting effect of muscle-containing grafts was restricted to the neural tube itself and was not seen in proximate dorsal root or sympathetic ganglia. The action of the grafts of muscle was neither species-nor class-specific, since enlargement of the neural tube was observed following implantation of fetal mouse skeletal muscle into quail hosts. Grafts of skeletal muscle or gut increased the number of cells taking up [3H]thymidine in the host’s neuroepithelium as early as 9h following implantation of a graft. The increase in the number of cells entering the S phase of the cell cycle preceded the increase in cell number. These observations demonstrate that muscle-containing tissues can increase the rate of proliferation of neuroepithelial cells when these tissues are experimentally placed together.
Presence of cdc2+-like proteins in the preimplantation mouse embryo
Authors: McConnell Josie , Lee Melanie
Published: April 2021

An antibody raised against a portion of the human equivalent of the yeast cdc2+ protein reacts with a 34K protein in mouse cell lines and early embryonic cells. Western blot analysis coupled with phosphatase treatment of material collected from the early preimplantation embryo has shown that the murine cdc2+ homologue does not correspond to the previously described newly synthesised proteins that are phosphorylated in a cell-cycle-dependent fashion [Howlett, 1986]. The cdc2+-like protein is converted into a slower migrating form on entry into S-phase and is further modified during G2 prior to mitosis. Studies of embryos that are held in extended periods of M-phase, i.e. unfertilised eggs or 1-cell embryos treated with nocodazole, demonstrate that the cdc2+-like protein becomes demodified in these cells.
Sex determination in the germ line of Drosophila depends on genetic signals and inductive somatic factors*
Authors: Nöthiger Rolf , Jonglez Martine , Leuthold Margrit , Meier-Gerschwiler Pia , Weber Thomas
Published: April 2021

We have analyzed the mechanism of sex determination in the germ line of Drosophila by manipulating three parameters: (1) the ratio of X-chromosomes to sets of autosomes (X: A); (2) the state of activity of the gene Sexlethal (Sxl), and (3) the sex of the gonadal soma. To this end, animals with a ratio of 2X:2A and 2X:3A were sexually transformed into pseudomales by mutations at the sex-determining genes Sxl (Sex-lethal), tra (transformer), tra-2 (transformer-2), or dsx (double-sex). Animals with the karyotype 2X;3A were also transformed into pseudofemales by the constitutive mutation SxlM1. The sexual phenotype of the gonads and of the germ cells was assessed by phase-contrast microscopy.

Confirming the conclusions of Steinmann-Zwicky et al. (Cell 57, 157, 1989), we found that all three parameters affect sex determination in germ cells. In contrast to the soma in which sex determination is completely cell-autonomous, sex determination in the germ line has a non-autonomous component inasmuch as the sex of the soma can influence the sexual pathway of the germ cells. Somatic induction has a clear effect on 2X; 2A germ cells that carry a Sxl+ allele. These cells, which form eggs in an ovary, can enter spermatogenesis in testes. Mutations that cause partial loss of function or gain of function of Sxl thwart somatic induction and, independently of the sex of the soma, dictate spermatogenesis or oogenesis, respectively. Somatic induction has a much weaker effect on 2X;3A germ cells. This ratio is essentially a male signal for germ cells which consistently enter spermatogenesis in testes, even when they carry SxlMl. In a female soma, however, Sxl™1 enables the 2X;3A germ cells to form almost normal eggs.

Our results show that sex determination in the germ line is more complex than in the soma. They provide further evidence that the state of Sxl, the key gene for sex determination and dosage compensation in the soma, also determines the sex of the germ cells, and that, in the germ line, the state of activity of Sxl is regulated not only by the X: A ratio, but also by somatic inductive stimuli.
The zygotic control of Drosophila pair-rule gene expression: I. A search for new pair-rule regulatory loci
Authors: Vavra Stephanie H. , Carroll Sean B.
Published: April 2021

The examination of pair-rule gene expression in wildtype and segmentation mutant embryos has identified many, but not necessarily all, of the elements of the regulatory system that establish their periodic patterns. Here we have conducted a new type of search for previously unknown regulators of these genes by examining pair-rule gene expression in blastoderm embryos lacking parts of or entire chromosomes. This method has the advantage of direct inspection of abnormal pair-rule gene patterns without relying upon mutagenesis or interpretation of larval phenotypes for the identification of segmentation genes. From these experiments we conclude that: (i) most zygotically required regulators of the fushi tarazu (ftz), even-skipped (eve) and hairy (h) pair-rule genes have been identified, except for one or more loci we have uncovered on chromosome arm 2L; (ii) the repression of the ftz and eve genes in the anterior third of the embryo is under maternal, not zygotic control; and (iii) there are no general zygotically required activators of pair-rule gene expression. The results suggest that the molecular basis of pair-rule gene regulation can be pursued with greater confidence now that most key trans-acting factors are already in hand.
The origin of secondary myotubes in mammalian skeletal muscles: ultrastructural studies
Authors: Duxson M. J. , Usson Y. , Harris A. J.
Published: April 2021

The distribution of secondary myotubes and undifferentiated mononucleated cells (presumed to be myoblasts) within foetal IVth lumbrical muscles of the rat was analyzed with serial section electron microscopy. In all myotube clusters for which the innervation zone was located, every secondary myotube overlapped the endplate region of the primary myotube. No secondary myotubes were ever demonstrated to occur at a distance from the primary myotube innervation zone. This indicates that new secondary myotubes begin to form only in the innervation zone of the muscle. Some young secondary myotubes made direct contact with a nerve terminal, but we cannot say if this is true for all developing secondary myotubes. Myoblasts were not clustered near the innervation zone, but were uniformly distributed throughout the muscle. Myoblasts were frequently interposed between a primary and a secondary myotube, in equally close proximity to both cell membranes. We conclude that specificity in myoblast-myotube fusion does not depend on restrictions in the physical distribution of myoblasts within the muscle, and therefore must reflect more subtle mechanisms for intercellular recognition.
Positional information revisited
Authors: Wolpert Lewis
Published: April 2021

Positional information has been suggested to play a central role in pattern formation during development. The strong version of positional information states that there is a cell parameter, positional value, which is related to position as in a coordinate system and which determines cell differentiation. A weaker version merely emphasises position as a key determinant in cell development and differentiation. There is evidence for boundaries and orthogonal axes playing an important role in positional systems. A positional signal is distinguished from an inductive interaction because the former specifies multiple states, confers polarity, and can act over a long range. A gradient in a diffusible morphogen is just one way of specifying position. There is now good evidence in several systems for substances which may be the morphogen for positional signalling. The product of the bicoid gene in early Drosophila development is the best prospect. Retinoic acid is unique in its ability to alter positional value and may also be a morphogen. The best evidence for positional value, a concept fundamental to positional information, remains a biological assay based on grafting. The idea of positional value uncouples differentiation and position, and allows considerable freedom for patterning. It is not clear whether positional value or differentiation involves a combinatorial mechanism.

Interpretation of positional information remains a central problem. There is good evidence that cells can respond differentially to less than a two-fold change in concentration of a chemical signal. It may be that interpretation involves listing the sites at which a particular class of cell differentiation will occur. The problem is made less severe when blocks of cells are specified together as in mechanisms based on an isomorphic prepattern. Isomorphic prepatterns could establish repeated structures which are equivalent and which are then made non-equivalent by positional information. This would enable local differences to develop. The combination of these two mechanisms may be widespread.

There is evidence that positional signals within a single animal and in related animals are conserved. It is not clear just how wide this conservation is, but it is at phylotypic stages, rather than in eggs, that similarity might be expected. It is nevertheless impressive that the polar coordinate model can be applied to regulation in systems as diverse as insects, vertebrates and protozoa. The molecular basis of positional signalling is just becoming accessible; the molecular basis of positional value is still awaited.

A brief personal history of positional information is provided in an appendix.
The development of a simple scaffold of axon tracts in the brain of the embryonic zebrafish, Brachydanio rerio
Authors: Wilson Stephen W. , Ross Linda S. , Parrett Timothy , Easter Stephen S.
Published: April 2021

We have examined neuronal differentiation and the formation of axon tracts in the embryonic forebrain and midbrain of the zebrafish, between 1 and 2 days postfertilisation. Axons were visualised with three techniques; immunocytochemistry (using HNK-1 and antiacetylated tubulin antibodies) and horseradish peroxidase (HRP) labelling in whole-mounted brains, and transmission electron microscopy. Differentiation was monitored by histochemical staining for acetylcholinesterase (AChE).

These independent methods demonstrated that a simple grid of tracts and commissures forms the initial axon scaffold of the brain. At 1 day, the olfactory nerve, four commissures, their associated tracts and three other non-commissural tracts are present. By 2 days, these tracts and commissures have all greatly enlarged and, in addition, the optic nerve and tract, and three new commissures and their associated tracts have been added.

Small applications of HRP at various sites revealed the origins and projections of some of these earliest axons. Retrogradely labelled cell bodies originated from regions that were also positive for AChE activity. At 1 day, HRP-labelled axons were traced: (1) from the olfactory placode through the olfactory nerve to the dorsal telencephalon; (2) from the telencephalon into the tract of the anterior commissure and also to the postoptic region of the diencephalon; (3) from the hindbrain through the ventral midbrain and diencephalon to the postoptic commissure; (4) from the dorsal diencephalon (in or near the epiphysis) to the tract of the postoptic commissure; (5) from ventral and rostral midbrain through the posterior commissure. Three new projections were demonstrated at 2 days: (1) from the retina through the tract of the postoptic commissure to the tectum; (2) from the telencephalon to the contralateral diencephalon; and (3) from the telencephalon to the ventral flexure.

These results show that at 1 day, the zebrafish brain is impressively simple, with a few small, well-separated tracts but by 2 days the brain is already considerably more complex. Most of the additional axons added onto pre-existent tracts rather than pioneered new ones supporting the notion that other axons play a crucial role in the guidance of early central nervous system (CNS) axons.
Abnormal meiotic spindles cause a cascade of defects during spermatogenesis in asp males of Drosophila
Authors: Casal Jose , Gonzalez Cayetano , Wandosell Francisco , Avila Jesus , Ripoll Pedro
Published: April 2021

Since spermatogenesis in Drosophila is a series of interconnected and interdependent steps and most of the spermatogenic events take place in the absence of transcription, failures in a given stage can give rise to a cascade of defects later on. The asp locus of Drosophila melanogaster codes for a non-tubulin component implicated in proper spindle structure and/or function (Ripoll et al. 1985). Homozygous asp males exhibit abnormal meiotic spindles giving rise to altered segregation of chromosomes and mitochondria and failures in cytokinesis. Postmeiotic spermatogenic stages of asp males show a series of alterations that we interpret as due to the previously occurring defective meiosis because meiotic spindles are the only microtubular structure altered in mutant testes. The most conspicuous alterations are: (i) variable size of nuclei and nebenkerns of early spermatids, which are also multinucleate instead of having single and uniformly sized nuclei; (ii) elongating spermatids in which abnormal-sized mitochondrial derivatives elongate alongside more than one axoneme; (iii) failures in the individualization process, where abnormal spermatids remain syncytial, and seem to be eliminated during the coiling stage.
The three most downstream genes of the Hox-3 cluster are expressed in human extraembryonic tissues including trophoblast of androgenetic origin
Authors: Oudejans Cees B. M. , Pannese Maria , Simeone Antonio , Meijer Chris J. L. M. , Bonc1Nelli Edoardo
Published: April 2021

Human first trimester extraembryonic tissues of normal and androgenetic origin (molar pregnancies) were investigated for the expression of 6 homeobox genes from the chromosome 12-encoded Hox-3 cluster by non-autoradiographic in situ hybridization with biotinylated RNA probes. By comparative in situ hybridization involving the use of exon- or region-specific RNA probes, analysis included the cellular distribution of alternative Hox-3 transcripts in chorionic villous tissues. A bias in extraembryonic distribution was seen between transcripts of the three most upstream Hox-3 genes (Hox-3.7, -3.6, and -3.1) versus transcripts of the 3 most downstream genes (Hox-3.3, 3.4, and 3.5). Only genes from the latter group are transcribed in human extra-embryonic tissues including extraembryonic tissues of androgenetic origin. Moreover, comparative in situ hybridization showed that distinct alternative transcripts of Hox-3.3, Hox 3.4 and Hox-3.5 are exclusively found in trophoblast cells while others are present in chorionic villous stromal cells as well. These data demonstrate the existence of tissue- and cell-specific use of transcriptional (alternative gene promoters) or post-transcriptional (alternative splicing) regulation of homeobox genes in extraembryonic tissues.
Developmental expression of the creatine kinase isozyme system of Xenopus: maternally derived CK-IV isoform persists far beyond the degradation of its maternal mRNA and into the zygotic expression period
Authors: Robert Jacques , Wolff Jacques , Juakli Hassan , Graf Jean-Daniel , Karch François , Kobel Hans Rudolf
Published: April 2021

The differential expression of the multilocus CK isozyme system throughout development of the two Xenopus species X. laevis and X. borealis was investigated. A cDNA containing the nearly complete coding sequence of the CK-IV subunit of X. laevis was isolated and sequenced. Early development of X. laevis proceeds with a stock of maternally derived CK-IV/IV isozyme. While the mRNA declines rapidly after fertilization and disappears before neurulation, maternal CK-IV/IV isozyme is active far beyond the onset of zygotic expression and is still detectable when tadpoles start feeding. Zygotic expression of CK-IV begins after neurulation, at stage 22/24, and seems to start simultaneously with that of another gene, CK-IIL Modulation in the expression of these two genes and the appearance of two other isoforms, the CK-I and CK-II/III isozymes, take place during development in a tissue-specific manner. During metamorphosis, the CK phenotypes of eyes and skeletal musculature undergo additional changes. The final adult pattern only appears several weeks after metamorphosis. The presumed orthologous CK isozymes of X. borealis show a developmental profile similar to that of X. laevis, except that CK-Il/n is equally present in oocytes and during early development, in addition to CK-IV/IV isozyme. These results show that the expression of each of the four CK genes of Xenopus is under differential developmental control.
Transcription of a mutant collagen I gene is a cell type and stage-specific marker for odontoblast and osteoblast differentiation
Authors: Schwarz Michaela , Harbers Klaus , Kratochwil Klaus
Published: April 2021

The Movl3 allele of the mouse al(I) collagen gene carries a retroviral insert in its first intron and had been reported to be transcriptionally silent. We have recently shown, however, that this mutant gene is expressed in odontoblasts of transplanted teeth derived from homozygous and heterozygous carrier embryos. The expression of the Movl3 allele has now been followed throughout in vivo development of mandibular teeth and bone in heterozygous animals, by in situ hybridization with a probe that specifically recognizes transcripts of the mutant gene. We show that the onset of its transcription precisely coincides with the final differentiation of odontoblasts and the onset of dentinogenesis, i.e. on day E16 for the incisor and at birth for the first molar. The mutant allele is also transcribed in osteoblasts of mandibular bone, again starting precisely with the onset of osteogenesis (day E13/14). No other cells were seen to transcribe the mutant gene. By these criteria, transcription of the Movl3 allele constitutes a true differentiation marker for odontoblasts and osteoblasts. Expression of the mutant allele in these two specialized cell types, in contrast to its transcriptional block in all other mesodermal cells (‘fibroblasts’), suggests tissue-specific differences in the regulation of the al(I) collagen gene.
The parthenogenetic development of rabbit oocytes after repetitive pulsatile electrical stimulation
Authors: Ozil Jean Pierre
Published: April 2021

Freshly ovulated rabbit oocytes were activated parthenogenetically by periodically repeated calcium stimuli generated by electric field pulses applied onto the plasma membrane. Electric field pulses of 1.8 kV cm-1 were delivered every 4 min for 1h 30 min (22 double pulses) in a specially designed chamber. Before each pulse, the culture medium was replaced by an isotonic glucose solution containing 10 μM Ca2+.

The effects of modulating the ionic stimuli (by changing the duration of EF pulse) on a postactivation reaction, and/or on the pre- and postimplantation development, were studied.

The rate of activation increased progressively as the pulse duration lengthened. For 22 pulses of 200 μs, 13 % of oocytes were activated versus 100 % for 1200 μs. The uniformity of the parthenogenetic response was obtained when oocytes were exposed to a series of pulses within which the reduction of pulse duration followed a negative exponential law. The influence of such activating treatment on the preimplantation development was tested using two treatments of 22 pulses with a total pulse duration equal to 14868 and 11228 μs, respectively. For the weaker treatment, a lower proportion of embryos underwent compaction and those that compacted were irregular. In contrast, the majority of embryos resulting from the stronger treatment compacted and developed into blastocysts. The most significant result that emerges from this study is that the level of stimulation affects in vitro developmental potency after the third cleavage division.

The postimplantation viability of parthenogenetic eggs was tested and the results showed that parthenogenetic rabbit embryos died at a similar stage of development to the parthenogenetic mouse embryos. But, in the present series, high implantation rates and embryonic development (66 %) till day 10−11 of pregnancy were obtained after the appropriate pulsatile EF treatment of oocytes. The parthenogenetic fetuses were of smaller size than the controls, but the developement of the trophoblast tissue was proportional to the development of the fetuses. Anomalies of fetuses were also observed. This study reveals that activation is not a time-limited event and that the type of activating treatment has a marked effect on the ability of the resulting parthenogenetic embryos to develop to the early postimplantation stages. The sustained alteration of the cytoplasmic activity provides a useful tool to study the function of embryonic or somatic nuclei introduced during the earliest stages of activation.