bluuebunny/crossref_metadata_2025 huggingface dataset:
Disruption of the cytokeratin filament network in the preimplantation mouse embryo
Authors: Emerson Julia Andrew
Published: April 2021

The distribution of the cytokeratin network in the intact preimplantation mouse embryo and the role of cytokeratin filaments in trophectoderm differentiation were investigated by means of whole-mount indirect immunofluorescence microscopy and microinjection of anti-cytokeratin antibody. Assembled cytokeratin filaments were detected in some blastomeres as early as the compacted 8-cell stage. The incidence and organization of cytokeratin filaments increased during the morula stage, although individual blastomeres varied in their content of assembled filaments. At the blastocyst stage, each trophectoderm cell contained an intricate network of cytokeratin filaments, and examination of sectioned blastocysts confirmed that extensive arrays of cytokeratin filaments were restricted to cells of the trophectoderm. Microinjection of anticytokeratin antibody into individual mural trophectoderm cells of expanded blastocysts resulted in a dramatic rearrangement of the cytokeratin network in these cells. Moreover, antibody injection into 2-cell embryos inhibited assembly of the cytokeratin network during the next two days of development. Despite this disruption of cytokeratin assembly, the injected embryos compacted and developed into blastocysts with normal morphology and nuclear numbers. These results suggest that formation of an elaborate cytokeratin network in preimplantation mouse embryos is unnecessary for the initial stages of trophectoderm differentiation resulting in blastocyst formation.
Retinotopic refinement of the regenerating goldfish optic tract is not linked to activity-dependent refinement of the retinotectal map
Authors: Cook Jeremy E. , Becker David L.
Published: April 2021

The cut optic nerve of a goldfish can regenerate, restoring an orderly projection from the retina to the optic tectum. At first, regenerating axons make transient connections, many of them in inappropriate tectal locations. Later, their arrangement is gradually refined into an accurate retinotectal map by a process that depends on afferent activity.

On their way to the tectum, many regenerating axons make erroneous choices between the two arms (brachia) of the optic tract. However, since they commonly possess divergent collateral branches, a secondary refinement of the brachial pattern can occur by selective collateral elimination. How or why a particular collateral is lost is not known, but we have previously suggested that sibling branches might compete to form stable tectal synapses, implying that there might be a causal link between refinement of the brachial pattern and refinement of the retinotectal map.

In this paper, we have tested directly for such a link, blocking map refinement with tetrodotoxin (TTX) or stroboscopic light, verifying the effectiveness of the block and measuring the extent of brachial refinement by standard methods in experimental and control fish. Both TTX and stroboscopic light reliably prevented map refinement, their results being indistinguishable. However, neither had even the slightest detrimental effect on brachial refinement, either 42 days or 70 days after nerve cut.

Evidently, neither activity nor a sharp retinotectal projection is necessary for brachial refinement. Theory and experiment both dictate that the basic projection pattern be controlled by a mechanism (such as chemoaffinity) that is independent of activity, and it would seem that selective collateral loss must depend on the same mechanism.
Control of cell cycle length in amphibian eggs: evidence for a temporal relationship between the nucleus and egg cytoplasm
Authors: Aimar C.
Published: April 2021

The role of the cytoplasm and nucleus in the control of the length of the division cycle was investigated in Pleurodeles waltl eggs. Injection of spermine into enucleated eggs showed that the ability to cleave was not restricted just to the period of normal cytokinesis (T = 1·0) but was possible throughout most of the first egg cycle. The cytoplasmic components required for cytokinesis seem to increase progressively during the first division cycle.

Nuclear transfer experiments indicated that the timing of cleavage was normal only when the nucleus and egg cytoplasm were reassociated between T = 0 ·0 (activation time) and T = 0·50. Delayed associations, after T = 0·50, provoked an alteration in the chronology of first cleavage and led to abnormal embryonic development. In the absence of a nucleus, the egg cycle seemed to stop at T = 0·50. These different observations suggest that the normal timing of cleavage not only depends on a ‘cytoplasmic clock’ but is also determined by an isochronous nucleocytoplasmic relationship during the early phase of egg development.
The albino–deletion complex in the mouse defines genes necessary for development of embryonic and extraembryonic ectoderm*
Authors: Niswander Lee , Yee Della , Rinchik Eugene M. , Russell Liane B. , Magnuson Terry
Published: April 2021

A detailed embryological analysis has been undertaken on embryos carrying the c4FR60Hd-, c5FR60Hg- or c2YPSj- albino deletions of mouse chromosome 7. Embryos homozygous for the c4FR60Hd deletion are abnormal at day 7 · 5 of gestation. The extraembryonic ectoderm does not develop, and primitive-streak formation and mesoderm production do not occur. In contrast, extensive development of the extraembryonic ectoderm, as well as mesoderm production, are observed in the c5FR60Hg-and c2YPSj- homozygous embryos. The mesoderm does not, however, organize into somites and the neural axis does not form. The embryos are grossly abnormal by day 8-5 of development. There are two other albino deletions (c6H and c11DSD) that are known to affect the embryo around the time of gastrulation (Niswander et al. 1988), and the lethal phenotype observed for the c4FR60Hd - homozygous embryos is similar to that described for c6H- homozygous embryos, whereas the c5FR60Hg- and c2YPSj- homozygous embryos display a phenotype that is similar to c11DSD -homozygous embryos. A detailed complementation analysis using these five deletions revealed that the c5FR60Hg, c2YPSj- and C11DSD deletions could partially complement the phenotype produced by the c4FR60Hd and c6H deletions in any combination. Extensive development of the extraembryonic structures and production of mesoderm occurs in the compound heterozygotes. These results suggest that the distal breakpoints of the c5FR60Hg, c2YPSj and c11DSD deletions lie more proximal than the distal breakpoints of the c4FR60Hd and c6H deletions. This arrangement defines new functional units of chromosome 7 such that a gene(s) important for normal development of the extraembryonic ectoderm would be located in the distal region of non-overlap between the two groups of deletions, and a gene(s) important for the development of the embryonic ectoderm would be located in the region deleted by both groups of chromosomes. The c4FR60Hd and c6H deletions appear to be missing both genes and belong to one complementation group; whereas the c5FR60Hg, c2YPSj and c11DSD deletions are missing only the gene(s) affecting the development of the embryonic ectoderm and, therefore, belong to another complementation group.
A new approach to the development of the cerebellum provided by the quail–chick marker system
Authors: Hallonet Marc E. R. , Teillet Marie-Aimée , Le Douarin Nicole M.
Published: April 2021

We have used the quail-chick chimera system to reveal the cell migrations and settling pattern involved in the construction of the cerebellum. Three types of orthotopic transplantations were carried out, between quail and chick embryos, at the 12-somite stage: exchanges of (i) the whole metencephalic vesicle, (ii) the lateral half of this vesicle and (iii) the diencephalic plus the mesencephalic vesicles.

Histological study of chimeric embryos and young chicks provided the following results: longitudinal morphogenetic movements distort the embryonic neural tube as early as the fifth embryonic day, so that the dorsal limit of the mes-, met-and myelencephalic vesicles are displaced caudad and their ventral limits rostrad. This leads to a participation of mesencephalic vesicular material in the construction of the cerebellum. Cells originating in the mesencephalic vesicle are found in a rostromedial V-shaped region, in all the cerebellar cellular layers, except the external granular layer, the presumptive territory of which is entirely located in the metencephalic vesicle.

The chimerism of the rostromedial part of the cerebellum allows the analysis of the origin of the various cerebellar cell types. We find (i) that the Purkinje cells always have the same cellular marker as the ventricular epithelium radially beneath them. This strongly suggests that these cells reach their final localization following strictly radial migrations, (ii) Most of the small cells surrounding the Purkinje neurons and most of the neurons and glial cells of the molecular layer are also of the same type as the ventricular epithelium they surmount, i.e. different from the type of the external granular layer cells. Therefore, they are not derived from the external granular layer and are not of the same origin as the granule cells as previously believed.

Unilateral substitutions of the metencephalic vesicle revealed that transverse cell migrations occur across the sagittal plane. They have been observed mainly in the inner and external granular layers, but also, though to a lesser extent, in the molecular layer and in the cell layer located at the level of the Purkinje neurons.

These observations show that the position of cerebellar cells is determined by both morphogenetic movements and cell type-specific active radial and tangential migrations. The quail-chick chimera system is thus able to provide new information both on the origin of cerebellar cells and how each cell type assumes its final position.
Retinoic acid treatment alters the distribution of retinoic acid receptor-β transcripts in the embryonic chick face
Authors: Rowe Annie , Richman Joy M. , Brickell Paul M.
Published: April 2021

Retinoic acid is a metabolite of vitamin A that can act as a signalling molecule in a number of developmental systems. Retinoic acid is also known to be teratogenic in mammals, causing a range of defects including abnormalities in craniofacial development. Exposure of the developing chick face to retinoic acid released slowly from a bead implanted in the wing bud results in a specific facial defect, in which outgrowth of the frontonasal mass is inhibited. This results in clefting of the primary palate and absence of the upper beak. To investigate the role of nuclear retinoic acid receptors in normal and abnormal chick face morphogenesis, we isolated chick retinoic acid receptor-β (RAR-β) cDNA clones and probed northern blots of RNA isolated from chick embryos at stages 22, 24 and 25 and from adults. RAR-β transcripts of 2.8 and 3.5 kb were present in several regions of the embryo, including the facial primordia, and were also present at much lower levels in adult tissues. In situ hybridisation showed that RAR-β transcripts were present in all of the facial primordia at embryonic stages 20, 24 and 28, but that their distribution was not uniform. Transcripts were abundant in the lateral nasal processes, at the edges and corners of the frontonasal mass and in the anterior part of the maxillary primordia. Lower levels were present elsewhere. Treatment of stage 20 embryos with retinoic acid altered the distribution of RAR-β transcripts in the maxillary primordia, such that high levels of transcripts were present throughout, rather than being confined to the anterior part. This change was detectable at stage 24, before any alterations in the morphology of the facial primordia were apparent. By stage 28, when the morphology of the facial primordia was clearly abnormal, there were more widespread changes in the distribution of RAR-/1 transcripts. These results show that RAR-β transcripts are particularly concentrated in regions of the primordia that give rise to the upper beak, the development of which is specifically affected by retinoic acid. In addition, they demonstrate that retinoic acid can induce changes in the pattern of expression of RAR-β tr in vivo.
Timing of paternal Pgk-1 expression in embryos of transgenic mice
Authors: Pravtcheva Dimitrina D. , Adra Chaker N. , Ruddle Frank H.
Published: April 2021

In mouse development, the paternal allele of the X-linked gene Pgk-1 initiates expression on day 6, two days later than the maternal allele, which is activated on day 4. The different timing of expression of the maternal and paternal alleles may be determined by (i) imprinting of the chromosome region in which the gene resides, but not aimed specifically at the Pgk-1 gene; (ii) gene specific imprinting, acting on Pgk-1 irrespective of the chromosomal localization of the gene; (iii) an interplay between embryo cell differentiation, timing of X-inactivation and Pgk-1 expression, without the involvement of imprinting at the Pgk-1 locus itself (Fundele R., Illmensee, K., Jagerbauer, E. M., Fehlau, M. and Krietsch, W. K. (1987) Differentiation 35, 31–36). Our findings in transgenic mouse lines, carrying Pgk-1 on autosomes, indicate the importance of the X chromosomal location for the delayed expression of the paternal Pgk-1 allele, and are in agreement with the first of the explanations listed above. We propose that the late activation of the paternal Pgk-1 locus is a consequence of imprinting targeted at, and centered around, the X chromosome controlling element (Xce).
Deep cytoplasmic rearrangements during early development in Xenopus laevis
Authors: Danilchik M. V. , Denegre J. M.
Published: April 2021

The egg of the frog Xenopus is cylindrically symmetrical about its animal-vegetal axis before fertilization. Midway through the first cell cycle, the yolky subcortical cytoplasm rotates 30° relative to the cortex and plasma membrane, usually toward the side of the sperm entry point. Dorsal embryonic structures always develop on the side away from which the cytoplasm moves. Details of the deep cytoplasmic movements associated with the cortical rotation were studied in eggs vitally stained during oogenesis with a yolk platelet-specific fluorescent dye. During the first cell cycle, eggs labelled in this way develop a complicated swirl of cytoplasm in the animal hemisphere. This pattern is most prominent on the side away from which the vegetal yolk moves, and thus correlates in position with the prospective dorsal side of the embryo. Although the pattern is initially most evident near the egg’s equator or marginal zone, extensive rearrangements associated with cleavage furrowing (cytoplasmic ingression) relocate portions of the swirl to vegetal blastomeres on the prospective dorsal side.
Vital dye labelling demonstrates a sacral neural crest contribution to the enteric nervous system of chick and mouse embryos
Authors: Serbedzija George N. , Burgan Scott , Fraser Scott E. , Bronner-Fraser Marianne
Published: April 2021

We have used the vital dye, Dil, to analyze the contribution of sacral neural crest cells to the enteric nervous system in chick and mouse embryos. In order to label premigratory sacral neural crest cells selectively, Dil was injected into the lumen of the neural tube at the level of the hindlimb. In chick embryos, Dil injections made prior to stage 19 resulted in labelled cells in the gut, which had emerged from the neural tube adjacent to somites 29–37. In mouse embryos, neural crest cells emigrated from the sacral neural tube between E9 and E9.5. In both chick and mouse embryos, Dil-labelled cells were observed in the rostral half of the somitic sclerotome, around the dorsal aorta, in the mesentery surrounding the gut, as well as within the epithelium of the gut. Mouse embryos, however, contained consistently fewer labelled cells than chick embryos. Dil-labelled cells first were observed in the rostral and dorsal portion of the gut. Paralleling the maturation of the embryo, there was a rostral-to-caudal sequence in which neural crest cells populated the gut at the sacral level. In addition, neural crest cells appeared within the gut in a dorsal-to-ventral sequence, suggesting that the cells entered the gut dorsally and moved progressively ventrally. The present results resolve a long-standing discrepancy in the literature by demonstrating that sacral neural crest cells in both the chick and mouse contribute to the enteric nervous system in the postumbilical gut.
Expression of the mouse α1 (II) collagen gene is not restricted to cartilage during development
Authors: Cheah Kathryn S. E. , Lau Elizabeth T. , Au Patrick K. C. , Tam Patrick P. L.
Published: April 2021

The mouse αl(II) collagen gene has been isolated and a 5′ portion of the gene which has low homology to other collagen genes was used to study the pattern of expression during mouse embryogenesis. In situ hybridization studies show that in the mouse, like the chick, α1(11) collagen is expressed in chondrogenic tissues in advance of chondrocyte differentiation. The gene is expressed early in embryogenesis at 9.5 days both in the cranial mesenchyme destined for the chondrocranium, and the sclerotome of the somites, and at 12.5 days in the primordia of the hyoid and the laryngeal cartilage. Type II collagen gene transcripts were found in all the chondrogenic tissues of the axial and appendicular skeleton until the onset of endochondral ossification. Expression of α1(11) collagen mRNA was also observed in non-chondrogenic tissues such as the notochord which may be responsible for inducing chondrogenesis in somitic mesoderm, neural retina, the corneal and conjunctival epithelia and sclera of the developing eye. Expression in the tail tendon was late, at 16.5–18.5 days.

Transient expression was also found in the heart at 9.5–12.5 days, the epidermis at 10.5–14.5 days, the calvarial mesenchyme at 12.5–16.5 days, the inner ear at 14.5 days and the fetal brain from 9.5-14.5 days. Within the neural tube, α1(11) collagen mRNA was localized in the proliferative ventricular cells of the forebrain and midbrain of 9.5-to 10.5-day embryos. Subsequently, transcription of the αl(II) collagen gene was confined to restricted areas of the rhombencephalic basal plate, the ventricular layer of the hindbrain and the cervical spinal cord. These examples of expression of the type II collagen gene in the developing nervous system seem to suggest that active transcription of this gene might be associated with early stages of neuroblast differentiation. Type II collagen may therefore have additional roles in development unrelated to chondrogenesis.
Glucose transporter gene expression in early mouse embryos
Authors: Hogan Aileen , Heyner Susan , Charron Maureen J. , Copeland Neal G. , Gilbert Debra J. , Jenkins Nancy A. , Thorens Bernard , Schultz Gilbert A.
Published: April 2021

The glucose transporter (GLUT) isoforms responsible for glucose uptake in early mouse embryos have been identified. GLUT 1, the isoform present in nearly every tissue examined including adult brain and erythrocytes, is expressed throughout preimplantation development. GLUT 2, which is normally present in adult liver, kidney, intestine and pancreatic β cells is expressed from the 8-cell stage onward. GLUT 4, an insulin-recruitable isoform, which is expressed in adult fat and muscle, is not expressed at any stage of preimplantation develop- ment or in early postimplantation stage embryos. Genetic mapping studies of glucose transporters in the mouse show that Glut-1 is located on chromosome 4, Glut-2 on chromosome 3, Glut-3 on chromosome 6, and Glut-4 on chromosome 11.
Cells with persistent twist expression are the embryonic precursors of adult muscles in Drosophila
Authors: Bate Michael , Rushton Emma , Currie Douglas A.
Published: April 2021

twist expression in the embryonic mesoderm of Drosphila declines during germ band retraction to leave a residual population of twisr-expressing cells in the late embryo. In the abdomen, the pattern of twist expression is a simple one: a single cell ventrally, pairs of cells laterally and three cells dorsally in each hemisegment. In the thorax, there are patches of cells associated with the imaginai discs and there are additional clusters in A8 and A9. During larval life, the twist-expressing cells proliferate and, in the abdomen, they form ventral, lateral and dorsal clusters, which are the precursors of the adult abdominal muscles, while in the thorax, they form populations of cells in the imaginai discs that correspond to the adepithelial cells described by previous authors. While most thoracic twist-expressing cells are associated with the discs, the abdominal cells are separate from the precursors of the adult abdominal epidermis, the abdominal histoblasts, and lie on branches of peripheral nerves. The distribution of these cells is tightly linked to the pattern of peripheral nerves, but they segregate normally in da/da embryos despite the absence of the peripheral nervous system.
The development of adult abdominal muscles in Drosophila: myoblasts express twist and are associated with nerves
Authors: Currie Douglas A. , Bate Michael
Published: April 2021

During metamorphosis, the adult muscles of the Drosophila abdomen develop from pools of myoblasts that are present in the larva. The adult myoblasts express twist in the third larval instar and the early pupa and are closely associated with nerves. Growing adult nerves and the twist-expressing cells migrate out across the developing abdominal epidermis, and as twist expression declines, the myoblasts begin to synthesize β3 tubulin. There follows a process involving cell fusion and segregation into cell groups to form multinucleate muscle precursors. These bipolar precursors migrate at both ends to find their correct attachment points, β3 tubulin expression continues at least until 51 h APF by which time the adult muscle pattern has been established.
Activation of p34cdc2 protein kinase activity in meiotic and mitotic cell cycles in mouse oocytes and embryos
Authors: Choi Taesaeng , Aoki Fugaku , Mori Makoto , Yamashita Masakane , Nagahama Yoshitaka , Kohmoto Kaoru
Published: April 2021

p34cdc2 protein kinase is a universal regulator of M-phase in eukaryotic cell cycle. To investigate the regulation of meiotic and mitotic cell cycle in mammals, we examined the changes in phosphorylation states of p34cdc2 and its histone Hl kinase activity in mouse oocytes and embryos.

We showed that p34cde2 has three different migrating bands (referred to as upper, middle and lower bands) on SDS-PAGE followed by immunoblotting with anti- PSTAIR antibody, and that the upper and middle bands are phosphorylated forms since these two bands shifted to the lower one by alkaline phosphatase treatment.

In meiotic cell cycle, only germinal vesicle (GV) stage oocytes had the three forms. The phosphorylated forms decreased gradually in oocytes up to 2 h after isolation from follicles, and thereafter the phosphorylation states did not change significantly until metaphase H. How ever, the histone Hl kinase activity oscillated, being activated at the first and second metaphase in meiosis and inactivated at the time of the first polar body extrusion. These results suggest that changes in phosphorylation states of p34cd” triggered its activation at the first metaphase, but not inactivation and reactivation at the first and second metaphase, respectively.

In mitotic cell cycle, phosphorylated forms appeared at 4h after insemination, increased greatly just before metaphase, and were dephosphorylated in metaphase. Histone Hl kinase activity was high only at metaphase. This kinase activation is probably triggered by déphosphorylation of p34cdc2.
Specific expression of a retinoic acid-regulated, zinc-finger gene, Rex-1, in preimplantation embryos, trophoblast and spermatocytes
Authors: Rogers Melissa B. , Hosler Betsy A. , Gudas Lorraine J.
Published: April 2021

We have previously isolated a cDNA clone for a gene whose expression is reduced by retinoic acid (RA) treatment of F9 embryonal carcinoma cells. The nucleotide sequence indicated that this gene, Rex-1, encodes a zinc-finger protein and thus may be a transcriptional regulator. The Rex-1 message level is high in two lines of embryonic stem cells (CCE and D3) and is reduced when D3 cells are induced to differentiate using four different growth conditions. As expected for a stem-cell-specific message, Rex-1 mRNA is present in the inner cell mass (ICM) of the day 4.5 mouse blastocyst. It is also present in the polar trophoblast of the blastocyst. One and two days later, Rex-1 message is found in the ectoplacental cone and extraembryonic ectoderm of the egg cylinder (trophoblast-derived tissues), but its abundance is much reduced in the embryonic ectoderm which is directly descended from the ICM. Rex-1 is expressed in the day 18 placenta (murine gestation is 18 days), a tissue which is largely derived from trophoblast. The only tested adult tissue that contains detectable amounts of Rex-1 mRNA is the testis. In situ hybridization and northern analyses of RNA from germ-cell-deficient mouse testis and stage-specific germ cell preparations suggest that Rex-1 expression is limited to spermatocytes (germ cells undergoing meiosis). These results suggest that Rex-1 is involved in trophoblast development and spermatogenesis, and is a useful marker for studies of early cell fate determination in the ICM.
Expression pattern of the Brachyury gene in whole-mount T
WIS/T
WIS mutant embryos
Authors: Herrmann Bernhard G.
Published: April 2021

The murine Brachyury (T) gene is required in mesoderm formation. Mutants carrying different T alleles show a graded severity of defects correlated with gene dosage along the body axis. The phenotypes range from shortening of the tail to the malformation of sacral vertebrae in heterozygotes, and to disruption of trunk development and embryonic death in homozygotes. Defects include a severe disturbance of the primitive streak, an early cessation of mesoderm formation and absence of the allantois and notochord, the latter resulting in an abnormality of the neural tube and somites. The T gene is expressed in nascent mesoderm and in the notochord of wild-type embryos. Here the expression of T in whole-mount mutant embryos homozygous for the T allele Twis is described. The Twisgene product is altered, but the Twis/Twis phenotype is very similar to that of T/T embryos which lack T. In early Twis/Twis embryos T expression is normal, but ceases prematurely during early organogenesis coincident with a cessation of mesoderm formation. The archenteron/node region is disrupted and the extension of the notochord precursor comes to a halt, followed by a decrease and finally a complete loss of T gene expression in the primitive streak and the head process/notochord precursor. It appears that the primary defect of the mutant embryo is the disruption of the notochord precursor in the node region which is required for axis elongation. Thus the T gene product is directly or indirectly involved in the organization of axial development.
The transcription factor TTF-1 is expressed at the onset of thyroid and lung morphogenesis and in restricted regions of the foetal brain
Authors: Lazzaro Domenico , Price Melanie , Felice Mario De , Lauro Roberto Di
Published: April 2021

TTF-1, a homeodomain-containing transcription factor, which is required for the specific expression of the thyroglobulin and thyroperoxidase gene promoters in differentiated thyroid cell lines, is expressed at the very beginning of rat thyroid differentiation. TTF-1 mRNA is detected in the endodermal cells of the thyroid rudiment in the rat embryo and precedes the expression of the two known target genes by 5 days. No delay is observed between the appearance of TTF-1 mRNA and protein, which shows a clear nuclear localization. In the adult thyroid, TTF-1 is present only in the endoderm-derived follicular cells.

Two additional domains of expression of TTF-1 have been observed, the lung and restricted areas of the brain. In the lung, TTF-1 mRNA and protein are also present at the earliest stages of differentiation and are later confined to the bronchial epithelium. In the brain, TTF-1 appears to be restricted to structures of diencephalic origin, including the developing neurohypophysis. The early detection of TTF-1 in the endodermal cells of the thyroid and lung anlage and in restricted neuroblast populations indicates that TTF-1 may have a role in cell determination in these three systems and that additional mechanisms may be involved in the activation of thyroid-specific gene expression.
Retinoic acid modifies development of the midbrain-hindbrain border and affects cranial ganglion formation in zebrafish embryos
Authors: Holder Nigel , Hill James
Published: April 2021

Considerable evidence now suggests that retinoic acid (RA) is an important modulator of patterning events in early neuronal development in vertebrates. In this paper, we describe the effects of exogenously applied RA on early neural development in the zebrafish embryo. Neural anatomy is assessed by immunocytochemical and histochemical analysis of the developing embryo in whole mounts at 24 h post-fertilization. RA was applied for one hour at concentrations ranging from 10−9 to 10−6M to embryos at 50% epiboly, the midgastrula stage, and at 10−7M to embryos at early and late gastrula stages.

The neuroanatomical analysis shows that 10−7M RA causes a defined lesion to the developing central nervous system which corresponds to a loss of a region of the brain in the caudal midbrain-rostral hindbrain area, the precursor of the cerebellum and associated neural structures. The region that fails to develop corresponds to the cranial expression domain of the engrailed protein as assessed by the monoclonal antibody 4D9 (Patel et al. 1989: Expression of engrailed proteins in arthropods, annelids and chordates. Cell 58, 955-968). Structures caudal to rhombomere 4 are unaffected by 10 7 M RA, as are the cranial midbrain and forebrain: 10−7M RA also affects the development of cranial ganglia, principally the Vth, anterior lateral line and VUIth ganglia, suggesting that RA affects normal development of the cranial neural crest. Effects of RA at stages immediately prior to and after gastrulation show some similar and some distinct features.

Results are discussed in terms of the possible role of RA as an endogenous moderator of normal head development.
Control of dorsoventral pattern in vertebrate neural development: induction and polarizing properties of the floor plate
Authors: Placzek Marysia , Yamada Toshiya , Tessier-Lavigne Marc , Jessell Thomas , Dodd Jane
Published: April 2021

Distinct classes of neural cells differentiate at specific locations within the embryonic vertebrate nervous system. To define the cellular mechanisms that control the identity and pattern of neural cells we have used a combination of functional assays and antigenic markers to examine the differentiation of cells in the developing spinal cord and hindbrain in vivo and in vitro. Our results suggest that a critical step in the dorsoventral patterning of the embryonic CNS is the differentiation of a specialized group of midline neural cells, termed the floor plate, in response to local inductive signals from the underlying notochord. The floor plate and notochord appear to control the pattern of cell types that appear along the dorsoventral axis of the neural tube. The fate of neuroepithelial cells in the ventral neural tube may be defined by cell position with respect to the ventral midline and controlled by polarizing signals that originate from the floor plate and notochord.
Conserved segmental expression of Krox-20 in the vertebrate hindbrain and its relationship to lineage restriction
Authors: Nieto M. Angela , Bradley Leila C. , Wilkinson David G.
Published: April 2021

The zinc-finger gene Krox-20 is expressed in two alternating segments, rhombomeres (r) 3 and 5, in the developing mouse hindbrain. This expression pattern is established prior to rhombomere formation in the mouse, but it is not known how the timing of expression relates to cellular events of segmentation, such as lineage restriction. We have cloned Krox-20 sequences from Xenopus and the chick and shown that its alternating expression pattern is conserved in these systems, suggesting that its role in hindbrain development is conserved. Analysis of the early stages of Krox-20 expression in the chick show that both domains of expression precede the restriction of cell lineage to specific rhombomeres, consistent with a role of this gene in early events of hindbrain segmentation. The finding that expression is not coincident with lineage restriction indicates that early expression may not reflect an irreversible commitment of cells to r3 and r5 and/or may be mosaic.
Purine utilisation, de novo synthesis and degradation in mouse preimplantation embryos
Authors: Alexiou Maria , Leese Henry J.
Published: April 2021

The importance of de novo purine synthesis as opposed to the réutilisation of metabolites by salvage pathways, and the nature of the excretory product(s) of purine degradation, have been examined in cultured pre-implantation mouse embryos. In the presence of azaser-ine and mycophenolic acid, which inhibit de novo purine synthesis, embryo cleavage was blocked prior to com-paction, the precise stages at which this occurred depended on whether the cultures were established on day 1 or day 2 after fertilisation, and indicated that salvage pathways were insufficient to fulfil the demand for nucleotides during early preimplantation develop-ment. The end-product of purine degradation appeared to be xanthine, which was excreted in very small amounts on days 1, 2 and 3, with a pronounced rise from the early to late blastocyst. Uric acid formation or excretion could not be detected. Exogenous hypoxan-thine and adenine, which partially inhibited develop-ment, were taken up by the embryos and converted to xanthine, most probably by salvage pathways, since the enzyme xanthine oxidase, which converts hypoxanthine directly to xanthine and then to uric acid, could not be detected. Exogenous guanine bad little effect on develop-ment and was also converted to xanthine, but in this case, the conversion was probably in a single step, via the enzyme guan ase.
Spatial and temporal patterns of distribution of the gap junction protein connexin43 during mouse gastrulation and organogenesis
Authors: Yancey S. Barbara , Biswal Sandip , Revel Jean-Paul
Published: April 2021

Connexin43 (Cx43) is a member of the family of channel-forming proteins that make up the gap junction and are believed to provide pathways for cell-cell exchange of developmental signals. We have used immunofluor-escence and confocal microscopy to characterize the patterns of distribution of Cx43 in postimplantation mouse embryos representing stages of development extending through gastrulation and the major period of organogenesis [through 13.5 days post coitum (dpc)]. We find that Cx43 is expressed early after implantation by the undifferentiated, pluripotent cells of the primitive embryonic ectoderm from which all tissues of the fetus are believed to be derived. As cells become committed to particular developmental pathways, there is a progressive restriction of Cx43 to specific areas and organ systems. The patterns are complex and not limited by germ layer of origin, although there is a clear preference for expression in ectodermal and, to a lesser extent, mesodermal derivatives. Expression in lens, retina, kidney, brain, pineal and pituitary glands is initiated early in organogenesis. In heart, the first clear signal for Cx43 appears in the ventricle at about 10 dpc and is only subsequently detected in the atrium at about 13–13.5 dpc. Particularly intriguing with regard to functional implications is the high level expression observed at sites of inductive interaction; the eye lens and optic cup, the infundibulum and the apical ectodermal ridge of the limb bud.
The Drosophila pumilio gene: an unusually long transcription unit and an unusual protein
Authors: Macdonald Paul M.
Published: April 2021

Specification of the posterior body plan in Drosophila requires the action of a determinant prelocalized to the posterior pole of the embryo. During embryogenesis this determinant appears to move anteriorly in a process dependent on the pumilio (pum) gene. This report describes the cloning and molecular characterization of a cDNA derived from the pum gene, and the analysis of pum mRNA and protein expression during early Drosophila development. The pum gene is unusually large; comparison of genomic and cDNA sequences reveals that the pum transcription unit is at least 160 kb in length.

The pum cDNA encodes a 157 × 103Mr protein which consists mainly of regions enriched in a single amino acid, usually glycine, alanine, glutamine or serine/threonine. Six tandem repeats of a 36 amino acid repeat unit are also present. Pum protein is cytoplasmic and is concentrated in a subcortical region of the embryo. The distribution of pum protein exhibits no asymmetry along the anteroposterior axis of the embryo. .
Lineage of radial glia in the chicken optic tectum
Authors: Gray Grace E. , Sanes Joshua R.
Published: April 2021

In many parts of the central nervous system, the elongated processes of radial glial cells are believed to guide immature neurons from the ventricular zone to their sites of differentiation. To study the clonal relationships of radial glia to other neural cell types, we used a recombinant retrovirus to label precursor cells in the chick optic tectum with a heritable marker, the E. coli lacZ gene. The progeny of the infected cells were detected at later stages of development with a histochemical stain for the lacZ gene product. Radial glia were identified in a substantial fraction of clones, and these were studied further. Our main results are the following, (a) Clones containing radial glia frequently contained neurons and/or astrocytes, but usually not other radial glia. Thus, radial glia derive from a multipotential progenitor rather than from a committed radial glial precursor, (b) Production of radial glia continues until at least embryonic day (E) 8, after the peak of neuronal birth is over (∽E5) and after radial migration of immature neurons has begun (E6–7).

Radial glial and neuronal lineages do not appear to diverge during this interval, and radial glia are among the last cells that their progenitors produce, (c) As they migrate, many cells are closely apposed to the apical process of their sibling radial glia. Thus, radial glia may frequently guide the migration of their clonal relatives, (d) The population of labelled radial glia declines between E15 and E19–20 (just before hatching), concurrent with a sharp increase in the number of labelled astrocytes. This result suggests that some tectal radial glia transform into astrocytes, as occurs in mammalian cerebral cortex, although others persist after hatching. To reconcile the observations that many radial glia are present early, that radial glia are among the last offspring of a multipotential stem cell, and that most clones contain only a single radial glial cell, we suggest that the stem cell is, or becomes, a radial glial cell.
Thyroid hormone receptors in chick retinal development: differential expression of mRNAs for α and N-terminal variant β receptors
Authors: Sjöberg Maria , Vennström Björn , Forrest Douglas
Published: April 2021

Thyroid-hormone-dependent development of the neuroretina has principally been described in amphibia. Here, we show by in situ hybridisation that mRNAs coding for three distinct thyroid hormone receptors (TRs), TRα and two TRβ variants, are differentially expressed during chick retinal development. We isolated a cDNA for a novel N-terminal variant of chick TRβ (cTRβ) that is predominantly expressed in retinal development. Interestingly, in its N-terminal A/B domain cTRβ2 is 70% homologous to the rat pituitaryspecific TRβ. Expression of cTRβ mRNA was high at embryonic day 6 (Eli) in the retinal outer nuclear layer (ONL) and decreased to low levels at hatching. Mrna for the previously described chick β receptor, cTRβ0, was expressed at low levels in both the ONL and the inner nuclear layer (INL) after E10. In contrast, cTRa expression occurred in the ONL, INL and ganglion cell layer at intermediate and later stages. Finally, cTRβ2 confers a stronger irans-activation of reporter gene transcription than cTRβ0. The distinctive kinetics and localisation of TRa and β gene expression suggest cell- and stage-specific functions for TRs, both individually and in combinations, in chick neuroretinal development.
Probing spermatogenesis in Drosophila with P-element enhancer detectors
Authors: Gönczy Pierre , Viswanathan Sridhar , Dinardo Stephen
Published: April 2021

Formation of motile sperm in Drosophila melanogaster requires the coordination of processes such as stem cell division, mitotic and meiotic control and structural reorganization of a cell. Proper execution of spermatogenesis entails the differentiation of cells derived from two distinct embryonic lineages, the germ line and the somatic mesoderm. Through an analysis of homozygous viable and fertile enhancer detector lines, we have identified molecular markers for the different cell types present in testes. Some lines label germ cells or somatic cyst cells in a stage-specific manner during then-differentiation program. These expression patterns reveal transient identities for the cyst cells that had not been previously recognized by morphological criteria. A marker line labels early stages of male but not female germ cell differentiation and proves useful in the analysis of germ line sex-determination. Other lines label the hub of somatic cells around which germ line stem cells are anchored. By analyzing the fate of the somatic hub in an agametic background, we show that the germ line plays some role in directing its size and its position in the testis. We also describe how marker lines enable us to identify presumptive cells in the embryonic gonadal mesoderm before they give rise to morphologically distinct cell types. Finally, this collection of marker lines will allow the characterization of genes expressed either in the germ line or in the soma during spermatogenesis.
Expression of vascular endothelial growth factor during embryonic angiogenesis and endothelial cell differentiation
Authors: Breier Georg , Albrecht Ursula , Sterrer Sylvia , Risau Werner
Published: April 2021

Vascular endothelial growth factor (VEGF) is a secreted angiogenic mitogen whose target cell specificity appears to be restricted to vascular endothelial cells. Such factors are likely candidates for regulatory molecules involved in endothelial growth control. We have characterized the murine VEGF gene and have analysed its expression pattern in embryogenesis, particularly during brain angiogenesis. Analysis of cDNA clones predicted the existence of three molecular forms of VEGF which differ in size due to heterogeneity at the carboxy terminus of the protein. The predicted mature proteins consist of 120,164 or 188 amino acid residues. Homodimers of the two lower molecular weight forms, but not of the higher molecular weight form, were secreted by COS cells transfected with the corresponding cDNAs and were equally potent in stimulating the growth of endothelial cells. During brain development, VEGF transcript levels were abundant in the ventricular neuroectoderm of embryonic and postnatal brain when endothelial cells proliferate rapidly but were reduced in the adult when endothelial cell proliferation has ceased. The temporal and spatial expression of VEGF is consistent with the hypothesis that VEGF is synthesized and released by the ventricular neuroectoderm and may induce the ingrowth of capillaries from the perineural vascular plexus. In addition to the transient expression during brain development, a persistent expression of VEGF was observed in epithelial cells adjacent to fenestrated endothelium, e.g. in choroid plexus and in kidney glomeruli. The data are consistent with a role of VEGF as a multifunctional regulator of endothelial cell growth and differentiation.
Organization and regulation of cortical microtubules during the first cell cycle ofXenopuseggs
Authors: Schroeder Marianne M. , Gard David L.
Published: April 2021

Anti-tubulin antibodies and confocal immunofluorescence microscopy were used to examine the organization and regulation of cytoplasmic and cortical microtubules during the first cell cycle of fertilized Xenopus eggs. Appearance of microtubules in the egg cortex temporally coincided with the outgrowth of the sperm aster. Microtubules of the sperm aster first reached the animal cortex at 0.25, (times normalized to first cleavage), forming a radially organized array of cortical microtubules. A disordered network of microtubules was apparent in the vegetal cortex as early as 0.35. Cortical microtubule networks of both animal and vegetal hemispheres were reorganized at times corresponding to the cortical rotation responsible for specification of the dorsal-ventral (D-V) axis. Optical sections suggest that the cortical microtubules are continuous with the microtubules of the sperm aster in fertilized eggs, or an extensive activation aster in activated eggs.

Neither assembly and organization, nor disassembly of the cortical microtubules coincided with MPF activation during mitosis. However, cycloheximide or 6-dimethyl-aminopurine, which arrest fertilized eggs at interphase, blocked cortical microtubule disassembly. Injection of pl3, a protein that specifically inhibits MPF activation, delayed or inhibited cortical microtubule breakdown. In contrast, eggs injected with cycΔ90, a truncated cyclin that arrests eggs in M-phase, showed normal microtubule disassembly. Finally, injection of partially purified MPF into cycloheximide-arrested eggs induced cortical microtubule breakdown. These results suggest that, despite a lack of temporal coincidence, breakdown of the cortical microtubules is dependent on the activation of MPF.
Retinal fate and ganglion cell differentiation are potentiated by acidic FGF in an in vitro assay of early retinal development
Authors: Guillemot François , Cepko Constance L.
Published: April 2021

One of the earliest events in vertebrate eye development is the establishment of the pigmented epithelium and neural retina. These fundamentally different tissues derive from the invaginated optic vesicle, or optic cup. Even after achieving a fairly advanced state of differentiation, the pigmented epithelium exhibits the same potential as the optic cup in that it can “transdifferentiate” into neural retina. C. M. Park and M. J. Hollenberg (Dev. Biol. 134, 201-205, 1989) discovered that administration of basic fibroblast growth factor, coupled with retinal removal, could trigger this transformation in vivo. We have developed a quantitative in vitro assay to study the role(s) of the fibroblast growth factor (FGF) family in this phenomenon and more generally in early retinal development. We found that several aspects of the process, including inhibition of pigmented epithelium differentiation, proliferation, and conversion to a retinal fate, were not strictly correlated. Both acidic and basic FGFs were found to potentiate all aspects of the process, with acidic FGF being 4 to 20 times more potent than basic FGF for inhibition of pigmentation and induction of retinal antigens. Depending upon its concentration, acidic FGF induced from 40% to 80% of the cells in the explants to produce antigens normally expressed by retinal ganglion cells, the first cell type to be generated in retinal development. Expression of such a ganglion cell marker could be directly stimulated in non-dividing cells as well as in dividing cells, indicating that conversion from the pigmented epithelial to retinal fate did not require cell division. These data suggest that acidic FGF, or a related molecule, may function in establishment of retinal fate from the optic cup. This effect may be directly or indirectly mediated by induction of retinal ganglion cell fate among multipotent progenitor cells.