bluuebunny/crossref_metadata_2025 huggingface dataset:
Differentiation of the Yolk Sac of the Chick Studied by Electron Microscopy
Authors: Bellairs Ruth
Published: April 2021

The marginal cells of the area opaca and early yolk sac are extended distally into long processes which may measure up to 500μ in length by as little as μ in depth. In whole mounts of blastoderms, examined by fight microscopy, these processes can be seen forming a clear rim to the blastoderm. They possess cytoplasmic granules, but no mitochondria or endoplasmic reticulum. It is suggested that these processes enable the blastoderm to migrate under the vitelline membrane. The endoderm of the region lying immediately proximal to the edge has been found to be composed of cells and not of a syncytium as is generally supposed. These cells are frequently separated by patches of modified yolk, consisting of a meshwork of membranes. These membranes possess a fine structure similar to that of myelin figures and it is suggested that they are formed by hydration of yolk lipids by water drawn through the vitelline membrane from the egg white. This meshwork corresponds with the region known at this stage as the periblast by some light microscopists. No evidence about the nature of the periblast in early cleavage is presented. The incidence of desmosomes in the area opaca appears to increase between 1 and 2 days of incubation. It is shown that each cell membrane retains its anatomical identity throughout the desmosome. Structural changes begin to take place in the intracellular yolk drops of the area vasculosa just before the establishment of the yolk sac circulation. These changes are considered to be the result of enzyme activity and it is suggested that partly digested yolk is carried in the blood stream to the area pellucida. These events exemplify the nicety of timing involved in many embryonic processes, for just as the intracellular yolk drops of the area pellucida become used up, so, it seems, new supplies of yolk start to arrive from the area vasculosa. Other changes which take place as the area opaca becomes colonized by mesoderm are described. They include: the re-arrangement of the ectoderm cells and changes in their shape; the loss of dorsal villi from the ectoderm cells and the appearance of more desmosomes and to nofilaments in them; changes in shape and arrangement of the endoderm cells, which become epithelial, acquiring desmosomes and tonofilaments. Pinocytic-like indentations appear at the lower border of the endoderm cells. The mesenchyme cells are described and it is noted that collagen-like fibrils appear at the same time.
Studies on Transplantations of Amphibian Anterior Pituitary into Chick Embryo. Analysis of Induction Capacity by Differential Solubility and Precipitation Method
Authors: Sherbet Gajanan V.
Published: April 2021

An attempt at analysing the inductive capacity of the anterior pituitary gland of Rana tigrina, in order to understand the nature of its active principle, was made, having recourse to differential solubility and precipitation of the hormonal and the ribonucleic acid factors using half-saturated ammonium sulphate, 50 per cent pyridine and 2·5 per cent trichloroacetic acid. The study of the inductive capacity of the gland in its normal state and after a 24-hour treatment with the solvents was made by grafting experiments on chick primitive-streak embryos. The behaviour of control pituitary grafts taken from donors at different seasons, namely breeding and non-breeding, and those taken from female and male donors at the same season, and also the relationship found to exist between the volume of the graft and that of the induced neural tissue, have suggested that the active principle of induction is probably FSH glycoprotein, and that the RNA contained in the gland has only a facilitating action.
An Experimental Analysis of the Development of the Haploid Syndrome in Embryos of Xenopus laevis
Authors: Hamilton Louie
Published: April 2021

The only observed effect of rearing haploid and diploid embryos of Xenopus laevis at 16°C instead of 22°C was retarded development. The same proportion of haploids (90–95 per cent) develop the syndrome (oedema, feeble heart, poorly coiled gut) and the stage expectancy is similar. Haploid tissues may live longer and develop more normally when telo-biosed to diploid tissues, but anterior diploid tissue promotes better development than the same quantity of posterior tissue. It is suggested that the haploid syndrome develops because haploids lack hybrid vigour completely, and cannot regulate the abnormalities caused by smaller cell size and/or unmasked recessive lethal genes. An hypothesis is put forward to explain the fact that haploid nuclei are more nearly spherical than diploid nuclei.
The Isolated Xenopus laevis Tail: A Preparation for Studying the Central Nervous System and Metamorphosis in Culture
Authors: Shaffer B. M.
Published: April 2021

The Xenopus tail preparation described here was initially developed for studying the action of substances on the vertebrate nervous system. For this both physiological preparations and whole animals have their disadvantages. The former are short-lived; the latter may die before any major chemical or structural changes have had time to occur. It was therefore thought desirable to seek a relatively long-lived preparation that possessed a functional central nervous system, but yet was not dependent on the conduction of nerve impulses for its survival. The tail of a Xenopus laevis tadpole seemed to be highly suitable for this purpose, as it contained complete reflex arcs and was sufficiently small to carry out its necessary exchanges by diffusion. Moreover, it was so transparent that nerves and muscle striations could be studied microscopically during life. Although agents to which the skin was impermeable could easily be injected into the tail, it was thought best to seek media that would be suitable for partially flayed tails.
The Differential Growth-Response of Embryonic Chick Limb-Bone Rudiments to Triiodothyronine in vitro. III. Hormone Concentration
Authors: Lawson Kirstie
Published: April 2021

Limb-bone rudiments from 6-day-old chick embryos were explanted in a medium of fowl plasma and embryo extract and the effect of various concentrations of T3 on the growth in length and wet weight of the rudiments was studied. Treatment with 0·01 to 0·32μg.T3/ml. produced a progressive retardation in the growth in length of the tibia, and a stimulation in the growth of the radius, which was maximal at intermediate concentrations. The growth in length of the ulna was increased by low levels of added hormone and retarded by high levels. The final wet weight of the tibia was slightly increased by low T3 concentrations and reduced by high doses. The final wet weights of the radius and ulna were greater than the controls at all T3 concentrations, but the greatest increase in the size of the ulna was reached at a lower dose level than that needed for the radius. These results suggested that the growth stimulation of the radius and retardation of the leg bones when exposed to the same concentration of T3 reflect a difference in sensitivity of the rudiments which may respond in a fundamentally similar way. The response of the ulna to 0 ·16μg. T3/ml. was a continuous process of growth stimulation followed by retardation. A difference in the threshold of response between the third metatarsus and radius was found at low concentrations of T3: the flattened cell zone of the metatarsus was affected by 0·0005 μg./ml. to which the radius was insensitive.
Functional Specificity of Spinal Cord Segments in the Control of Limb Movements
Authors: Székely George
Published: April 2021

Spinal cord segments from limb levels (both brachial and lumbo-sacral) were homoplastically grafted into the place of previously removed thoracic segments in newt embryos. In other embryos the brachial segments were replaced by lumbo-sacral segments or vice versa. In early larval stages a pair of supernumerary limbs were transplanted at the level of the grafted segments in the thoracic region. The function of normal and supernumerary limbs inner-vated by heterotopic spinal cord segments was studied. When brachial segments were substituted for thoracic segments the super-numerary limbs yielded co-ordinated stepping movements, perfectly synchro-nized with the normal forelimbs. Apart from minor discrepancies arising from differences in gross anatomical structure, the same rhythm and co-ordination of stepping movements were found whether fore- or hindlimbs were transplanted. Brachial segments in lumbo-sacral positions were able to bring the normal hindlimbs into co-ordinated movement. Although there was no correlation in the movements between the four limbs, the hindlimbs showed a definite ten-dency to move in parallel with the forelimbs on the same side. Supernumerary limbs, innervated by heterotopic lumbo-sacral segments grafted into thoracic regions, showed co-ordinated movements which accom-panied the movements of the normal hindlimbs either synchronously or, in most cases, with some delay. Essentially the same movement patterns were given both by transplanted fore- and hindlimbs except that the motility of the elbow was poorer than that of a normal forelimb. This was regarded as a ‘hindlimb character’ in the movement of forelimbs innervated by lumbo-sacral segments. Orthotopic forelimbs innervated by heterotopic lumbo-sacral segments grafted into brachial regions also disclosed co-ordinated movements with a tendency to move in parallel with the hindlimbs on the same side. Well-defined ‘hindlimb character’ was verified in both the motility and the posture of such limbs. The following conclusions were drawn, (i) A special segmental apparatus, already determined in early embryonic stages, exists at limb levels of the spinal cord for controlling co-ordinated limb movements, (ii) The rhythm and character of limb movements executed by these apparatus corresponded always to the nature of the innervating segments irrespective of the nature of the innervated limbs, (iii) A secondary process of determination was assumed to provide the fore- or hindlimb character of the differentiation of these segmental apparatus. The incomplete establishment of this determination might explain the observed time lag and irregularities in function of heterotopic lumbo-sacral segments under certain experimental conditions.
Viral Susceptibility and Embryonic Differentiation.
Authors: Rapola Juhani , Vainio Tapani , Saxén Lauri
Published: April 2021

The fact that viral susceptibility changes during embryogenesis has been pointed out by both experimental embryologists and clinical practitioners, not to mention virologists working with avian material. In attempts to find the fundamental factors which make embryonic tissue susceptible or resistant to a given virus, the metabolic and proliferative rate have been considered relevant (Williamson et al., 1953; Robertson et al., 1955; Tôndury, 1956). Experience accumulated in studies of the replication of various viruses in tissue culture has taught us that a high metabolic rate and active proliferation may not always enhance viral replication (Ginsberg, 1958). However, there seems to be justification for the view that an injurious agent leads to more severe tissue damage when it exercises its effect upon actively proliferating tissues than when it does so at the ‘resting stage’ .
Localization of specific retinoid-binding sites and expression of cellular retinoic-acid-binding protein (CRABP) in the early mouse embryo
Authors: Dencker L. , Annerwall E. , Busch C. , Eriksson U.
Published: April 2021

Retinoids (vitamin A derivatives) are important for normal embryogenesis and retinoic acid, an acidic derivative of vitamin A, was recently proposed to be an endogenous morphogen. Several retinoids are also potent teratogens. Using an autoradiographic technique, we have identified tissues and cells in early mouse embryos that are able to specifically accumulate a radiolabelled synthetic derivative of retinoic acid. Strong accumulation of radioactivity was seen in several neural crest derivatives and in specific areas of the CNS. Gel filtration analyses of cytosols from embryos that received the radiolabelled retinoid in utero suggested that cellular retinoic acid-binding protein (CRABP) was involved in the accumulation mechanism. Immunohisto-chemical localization confirmed that cells accumulating retinoids also expressed CRABP. Strong CRABP immunoreactivity was found in neural crest-derived mesenchyme of the craniofacial area, in visceral arches, in dorsal root ganglia and in cells along the gut and the major vessels of the trunk region. In CNS, CRABP expression and retinoid binding was largely restricted to the hindbrain, to a single layer of cells in the roof of the midbrain and to cells in the mantle layer of the neural tube. Our data suggest that cells in the embryo expressing CRABP are target cells for exogenous retinoids as well as endogenous retinoic acid. Retinoic acid may thus play an essential role in normal development of the CNS and of tissues derived from the neural crest. We propose that the teratogenic effects of exogenous retinoids are due to an interference with mechanisms by which endogenous retinoic acid regulates differentiation and pattern formation in these tissues.
Treatment with β bungarotoxin blocks muscle spindle formation in fetal rats
Authors: Kucera Jan , Waldro Jon M.
Published: April 2021

Sensory and motor fibers of peripheral nerves were irreversibly destroyed in fetal rats by administering β bungarotoxin (BTX) on embryonic day 16 or 17, after assembly of primary myotubes, but before the formation of muscle spindles. Soleus muscles of toxin-treated fetuses and their untreated littermates were removed just prior to birth and were examined by light microscopy of serial transverse sections for the presence of spindles and immunocytochemical expression of several isoforms of myosin heavy chains (MHC). Untreated muscles exhibited numerous spindles that were innervated by branches of intramuscular nerves and contained muscle fibers expressing a slow-tonic MHC isoform characteristic of the intrafusal but not extrafusal fibers. Toxin-treated muscles were devoid of intramuscular nerve bundles and perineurial structures. Encapsulations of muscle fibers resembling spindles were absent and no myotubes expressed the slow-tonic MHC isoform associated with intrafusal fibers in β BTX-treated muscles. Thus, the assembly of muscle spindles, formation of the spindle capsule, and transformation of undifferentiated myotubes into the intrafusal fibers that contain spindle-specific myosin isoforms all depend on the presence of innervation in prenatal rat muscles.
Cardiac troponin T in developing, regenerating and denervated rat skeletal muscle
Authors: Saggin Leopoldo , Gorza Luisa , Ausoni Simonetta , Schiaffino Stefano
Published: April 2021

Fetal rat skeletal muscles express a troponin T (TnT) isoform similar to the TnT isoform expressed in the embryonic heart with respect to electrophoretic mobility and immunoreactivity with cardiac TnT-specific monoclonal antibodies. Immunoblotting analyses reveal that both the embryonic and the adult isoforms of cardiac TnT are transiently expressed during the neonatal stages. In addition, other TnT species, different from both cardiac TnTs and from the TnT isoforms expressed in adult muscles, are present in skeletal muscles during the first two postnatal weeks. By immunocytochemistry, cardiac TnT is detectable at the somitic stage and throughout embryonic and fetal development, and disappears during the first weeks after birth, persisting exclusively in the bag fibers of the muscle spindles. Cardiac TnT is re-expressed in regenerating muscle fibers following a cold injury and in mature muscle fibers after denervation. Developmental regulation of this TnT variant is not coordinated with that of the embryonic myosin heavy chain with respect to timing of disappearance and cellular distribution. No obligatory correlation between the two proteins is likewise found in regenerating and denervated muscles.
Insulin increases the cell number of the inner cell mass and stimulates morphological development of mouse blastocysts in vitro
Authors: Harvey Mark B. , Kaye Peter L.
Published: April 2021

Previous studies showed that insulin promotes cell proliferation and morphological development of preimplantation mouse embryos. In this report, the receptor responsible for these actions and the cell populations that are affected were investigated. Insulin’s 9 % stimulation of blastocyst cell number was entirely due to a 23 % increase in ICM cell number with an ECS0 of 0.54 pM. This and the similar degrees of stimulation of immunosurgically isolated ICMs by both physiological and supraphysiological insulin concentrations suggest that insulin receptors are present on the ICM and respond to exogenous insulin transcytosed through the TE to promote expansion of the ICM cell numbers. In morphological studies, insulin increased the number of blastocysts and decreased the number of morulae by 10 % after 54 h culture from 2-cell embryos with ECsos of about 0.95 pM. The equivalence of these ECsos suggests mediation of insulin’s stimulation of blastocyst formation via insulin receptors which are functionally expressed around the time of compaction at the 8-cell stage. These results support our hypothesis that insulin has an important role in the regulation of growth during preimplantation development.
Stem cells: attributes, cycles, spirals, pitfalls and uncertainties. Lessons for and from the crypt
Authors: Potten C.S. , Loeffler M.
Published: April 2021

We consider some of the problems involved in current discussions on stem cells in adult mammalian tissues. The present concepts involve a number of pitfalls, weaknesses and logical, semantic and classification problems. This indicates the necessity for new and well-defined concepts that are amenable to experimental analysis. One of the major difficulties in considering stem cells is that they are defined in terms of their functional capabilities which can only be assessed by testing the abilities of the cells, which itself may alter their characteristics during the assay procedure: a situation similar to the uncertainty principle in physics. The terms that describe stem cell functions are often not well defined and are used loosely, which can lead to confusion. If such context-dependent interactions exist between the manipulation and measurement process and the challenged stem cells, the question of, for example, the number of stem cells, in a tissue has to be posed in a new way. Rather than obtaining a single number one might end up with various different numbers under different circumstances, all being complementary. This might suggest that stemness is not a property but a spectrum of capabilities from which to choose. This concept might facilitate a reconciliation between the different and sometimes opposing experimental results. Given certain experimental evidence, we have attempted to provide a novel concept to describe structured cell populations in tissues involving stem cells, transit cells and mature cells. It is based on the primary assumption that the proliferation and differentiation/maturation processes are in principle independent entities in the sense that each may proceed without necessarily affecting the other. Stem cells may divide without maturation while cells approaching functional competence may mature but do not divide. In contrast, transit cells divide and mature showing intermediate properties between stem cells and mature functional cells. The need to describe this transition process and the variable coupling between proliferation and maturation leads us to formulate a spiral model of cell and tissue organisation. This concept is illustrated for the intestinal epithelium. It is concluded that the small intestinal crypts contain 4–16 actual stem cells in steady state but up to 30–40 potential stem cells (clonogenic cells) which may take over stem cell properties following perturbations. This implies that transit cells can under certain circumstances behave like actual stem cells while they undergo maturation under other conditions. There is also evidence that the proliferation and differentiation/maturation processes are subject to controls that ultimately lead to a change in the spiral trajectories.(ABSTRACT TRUNCATED AT 400 WORDS)
Homeotic genes regulate the spatial expression of putative growth factors in the visceral mesoderm of Drosophila embryos
Authors: Reuter Rolf , Panganiban Grace E. F. , Hoffmann F. Michael , Scott Matthew P.
Published: April 2021

During Drosophila embryogenesis homeotic genes control the developmental diversification of body structures. The genes probably coordinate the expression of as yet unidentified target genes that carry out cell differentiation processes. At least four homeotic genes expressed in the visceral mesoderm are required for midgut morphogenesis. In addition, two growth factor homologs are expressed in specific regions of the visceral mesoderm surrounding the midgut epithelium. One of these, decapentaplegic (dpp), is a member of the transforming growth factor β (TGF-β) family; the other, wingless (wg), is a relative of the mammalian protooncogene int-1. Here we show that the spatially restricted expression of dpp in the visceral mesoderm is regulated by the homeotic genes Ubx and abd-A. Ubx is required for the expression of dpp while abd-A represses dpp. One consequence of dpp expression is the induction of labial (lab) in the underlying endoderm cells. In addition, abd-A function is required for the expression of wg in the visceral mesoderm posterior to the dpp- expressing cells. The two growth factor genes therefore are excellent candidates for target genes that are directly regulated by the homeotic genes.
Gonadal expression of c-kit encoded at the W locus of the mouse
Authors: Manova Katya , Nocka Karl , Besmer Peter , Bachvarova Rosemary F.
Published: April 2021

Recently, it has been shown that the c-kit proto-oncogene is encoded at the white spotting ( W) locus in mice. Mutations of this gene cause depletion of germ cells, some hematopoietic cells and melanocytes. In order to define further the role of c-kit in gametogenesis, we have examined its expression in late fetal and postnatal ovaries and in postnatal testis. By RNA blot analysis, c-kit transcripts were not detected in late fetal ovaries but appeared at birth. The relative amount reached a maximum in ovaries of juvenile mice, and decreased in adult ovaries, c-kit transcripts were present in increasing amounts in isolated primordial, growing and full-grown oocytes, as well as in ovulated eggs. Little was detected in early 2-cell embryos and none in blastocysts. In situ hybridization revealed c-kit transcripts in a few oocytes of late fetal ovaries and in all oocytes (from primordial to full-grown) in ovaries from juvenile and adult mice. Expression was also observed in ovarian interstitial tissue from 14 days of age onward. Using indirect immunofluorescence, the c-kit protein was detected on the surface of primordial, growing and full-grown oocytes, as well as on embryos at the 1- and 2-cell stages; little remained in blastocysts.

In situ hybridization analysis of testes from mice of different ages demonstrated expression in spermatogonia from 6 days of age onward. Using information provided by determining the stage of the cycle of the seminiferous epithelium for a given tubule and by following the age dependence of labeling, it was concluded that the period of expression of c-kit extends from at least as early as type A2 spermatogonia through type B spermatogonia and into preleptotene spermatocytes. Leydig cells were labelled at all ages examined.

The expression pattern in oocytes correlates most strongly with oocyte growth and in male germ cells with gonial proliferation.
Expression of the murine homeobox-containing gene Hox-2.3 suggests multiple time-dependent and tissue-specific roles during development
Authors: Vogels Ronald , De Graaff Wim , Deschamps Jacqueline
Published: April 2021

This study reports the expression pattern of the murine homeobox-containing gene Hox-2.3 during development. Using in situ hybridization, we first detect Hox-2.3 transcripts in the allantois primordium at 7.5 days post coitum (p.c.). One day later transcripts are found in embryonic ectoderm and mesoderm. In 9.5- and 10.5-day embryos Hox-2.3 expression is observed in the central nervous system (CNS) from a rostral boundary in the upper spinal cord to the caudal end. Within this anteroposterior domain, Hox-2.3 expression is also found in the peripheral nervous system, in the mesoderm and in the hindgut epithelium. The rostral boundary in the mesoderm is located at the level of the 11th somite and thus shifted posteriorwards compared to the rostral boundary in the neural tube. During subsequent development, the initially broad expression pattern in the somitic, lateral plate and intermediate mesoderm becomes restricted to structures in the urogenital system. In adults, the spinal cord and the derivatives of the Wolffian and Müllerian ducts continue to express the gene at a high level. The described temporal and tissue-specific changes in expression of Hox-2.3 are suggestive of several levels of regulation as reported for Drosophila homeotic genes and argue for more than one role of the gene during development and in adults.
Diverse expression of dsrc29A, a gene related to src, during the life cycle of Drosophila melanogaster
Authors: Katzen ALisa L. , Kornberg Thomas , Bishop J. Michael
Published: April 2021

We used in situ hybridization to study the RNA expression of the dsrc29A gene during Drosophila development. This gene encodes two proteins differing at their amino termini. Both gene products contain a protein-tyrosine kinase domain and resemble the protein encoded by vertebrate src. We examined most stages of development in the Drosophila life cycle: embryos, third instar larvae, pupae and adults. Our results revealed that dsrc29A expression is specialized throughout development, being prominent at various times in neural tissue, phagocytic cells, dorsal vessel, ovaries, gut, developing salivary glands, imaginai discs and disc derivatives. These findings confirm and extend previous results for the distribution of dsrc29A protein, indicating that the regulation of this gene is primarily at the level of transcription. In some tissues expression is transient, whereas in others, it is continuous, and expression occurs in proliferative, differentiating and differentiated tissue. These patterns of expression demonstrate how a single protein-tyrosine kinase might play diverse roles at different times during development. Comparison of the expression of dsrc29A and other members of the proteintyrosine kinase gene superfamily reveals that the genes are expressed in distinctive but sometimes overlapping patterns.
A cytosolic sperm factor stimulates repetitive calcium increases and mimics fertilization in hamster eggs
Authors: Swann Karl
Published: April 2021

Microinjection of cytosolic sperm extracts into unfertilized golden hamster eggs caused a series of increases in cytoplasmic free calcium, Ca2+i, and membrane hyperpolarizing responses, HRs. These HRs and Ca2+i, transients are similar to those seen during in vitro fertilization of hamster eggs. The sperm factor that is responsible for causing these effects appears to be of high molecular weight and protein based. Injection of sperm factor activated eggs and mimicked fertilization in causing repetitive HRs in the presence of phorbol esters and in sensitizing the egg to calcium-induced calcium release. Since these effects cannot be mimicked by injecting G-protein agonists or calcium-containing solutions, it seems unlikely that a receptor-G-protein signalling system is involved at fertilization. These data instead suggest a novel signal transduction system operates during mammalian fertilization in which a protein factor is transferred from the sperm into the egg cytoplasm after gamete membrane fusion.
Estimation of neuroblast numbers in insect neurogenesis using the lateral inhibition hypothesis of cell differentiation
Authors: Honda Hisao , Tanemura Masaharu , Yoshida Akihiro
Published: April 2021

Cells in the neurogenic region of an insect ectoderm have two alternative fates, making neurons or epidermis. The fates seem to be determined through a laterally inhibitory interaction among cells. That is, initially homogeneous cells are all competent to differentiate into neuroblasts. Once a cell has differentiated as a neuroblast, it inhibits its immediate neighbors from following this pathway. The differentiation process is simulated by a digital computer in a planar array of polygonal domains similar to a cell pattern. We find that the number of cells differentiating as neuronal precursors in insect neurogenesis is that expected under the hypothesis of lateral inhibition of cell differentiation between immediate neighbors.
The distribution of E-cadherin during Xenopus laevis development
Authors: Levi Giovanni , Gumbiner Barry , Thiery Jean Paul
Published: April 2021

A vast amount of experimental evidence suggests that cell surface molecules involved in cell-to-cell and/or cell-to-substrate interactions participate in the control of basic events in morphogenesis. E-cadherin is a cell adhesion molecule directly implicated in the control of Ca2+-dependent interactions between epithelial cells. We report here the patterns of expression of E-cadherin in developmental stages of Xenopus laevis ranging from early embryo to adult using immunofluorescence microscopy. Although its distribution shares some similarities with those of L-CAM in the chicken and E-cadherin/Uvomorulin in the mouse, the distribution of E-cadherin in Xenopus presents several peculiar and unique features.

In early stages of Xenopus development, E-cadherin is not expressed. The molecule is first detectable in the ectoderm of late gastrulas (stage 13–13.5 NF). At this time both the external and the sensory layer of the non-neural ectoderm accumulate high levels of E-cadherin while the ectoderm overlying the neural plate and regions of the involuting marginal zone (IMZ) not yet internalized by the movements of gastrulation are E-cadherin-negative. Unlike most other species, endodermal cells express no or very low levels of E-cadherin up to stage 20 NF. Endodermal cells become strongly E-cadherin-positive only when a well-differentiated epithelium forms in the gut. No mesodermal structures are stained during early development. In the placodes, in contrast to other species, E-cadherin disappears very rapidly after placode thickening.

During further embryonic development E-cadherin is present in the skin, the gut epithelium, the pancreas, many monostratified epithelia and most glands. Hepatocytes are stained weakly while most other tissues, including the pronephros, are negative. In the mesonephros, the Wolffian duct and some tubules are positive.

During metamorphosis a profound restructuring of the body plan takes place under the control of thyroid hormones, which involves the degeneration and subsequent regeneration of several tissues such as the skin and the gut. All newly formed epithelia express high levels of E-cadherin. Surprisingly, degenerating epithelia of both skin and intestine maintain high levels of the protein even after starting to become disorganized and to degenerate.

In the adult, staining is strong in the skin, the glands, the lungs, the gut epithelium and the pancreas, weak in the liver and absent from most other tissues.

Our results show that the expression of E-cadherin in Xenopus is strongly correlated with the appearance of differentiated epithelia.
The homeobox gene Hox 7.1 has specific regional and temporal expression patterns during early murine craniofacial embryogenesis, especially tooth development in vivo and in vitro
Authors: Mackenzie Alasdair , Leeming Gareth L. , Jowett Adrian K. , Ferguson Mark W. J. , Sharpe Paul T.
Published: April 2021

Hox 7.1 is & murine homeobox-containing gene expressed in a range of neural-crest-derived tissues and areas of putative epithelial-mesenchymal interactions during embryogenesis. We have examined the expression of Hox 7.1 during craniofacial development in the mouse embryo between days 8 and 16 of development. Whereas facial expression at day 10 of gestation is broadly localised in the neural-crest-derived mesenchyme of the medial nasal, lateral nasal, maxillary and mandibular processes, by day 12 expression is restricted to the mesenchyme immediately surrounding the developing tooth germs in the maxillary and mandibular processes. Hox 7.1 expression in the mesenchyme of the dental papilla and follicle is maximal at the cap stage of development and progressively declines in the bell stage prior to differentiation of odontoblasts and ameloblasts.

Hox 7.1 expression in tooth germs is independent of overall embryonic stage of development but is dependent on stage of development of the individual tooth. Similar patterns of transient Hox 7.1 expression can also be detected in tooth germs in vitro in organ cultures of day 11 first branchial arch explants cultured for up to 7 days.

Hox 7.1 is also expressed early in development (days 10/11) in the epithelium of the developing anterior pituitary (Rathke’ s pouch), the connective tissue capsule and meninges of the developing brain, and specific regions of neuroepithelium in the developing brain.
Molecular definition of the morphogenetic and regulatory functions and the cis-regulatory elements of the Drosophila Abd-B homeotic gene
Authors: Boulet Anne M. , Lloyd Alan , Sakonju Shigeru
Published: April 2021

The Abdominal-B (Abd-B) gene, a member of the Drosophila bithorax complex, is required during development to specify the identity of parasegments 10– 14. Based on genetic studies, Casanova, J., Sánchez-Herrero, E. and Morata, G. (1986) Cell 47, 627-636, proposed that the Abd-B gene consists of two distinct elements that provide a morphogenetic (m) function in PS 10-13 and a regulatory (r) function in PS 14, where it represses m function. Here we present molecular confirmation of this genetic model. . Using specific antibodies, we show that the 55x103Mr and 30x103MrAbd-B proteins, predicted by cDNA analysis, are indeed present in PS 10-13 and PS 14, respectively. We also examine Abd-B mRNA and protein expression patterns in embryos mutant for either the m or r function. These data allow us to unambiguously assign m function to the 55x103Mr protein and r function to the 30x103Mr protein. Furthermore, as postulated by the model, transcription of the mRNA encoding the m protein is derepressed in PS 14 in the absence of r function. We have also studied the effect of mutations mapping in the infra-abdominal (iab) region located downstream of the Abd-B gene. Genetic studies suggest that the iab region contains crv-acting regulatory elements controlling Abd-B expression in PS 10-12. We present molecular evidence for the presence of downstream crs-regulatory elements by analyzing Abd-B mRNA and protein patterns in iab-6 and iab-7 embryos. Our analysis reveals the presence of parasegment and cell-specific regulatory elements of the Abd-B gene within each iab region. The Abd-B gene may provide a model for the understanding of similarly complex homeotic genes in higher organisms.
The large upstream control region of the Drosophila homeotic gene Ultrabithorax
Authors: Irvine Kenneth D. , Helfand Stephen L. , Hogness David S.
Published: April 2021

Ultrabithorax (Ubx) is a Drosophila homeotic gene that determines the segmental identities of parts of the thorax and abdomen. Appropriate Ubx transcription requires a long upstream control region (UCR) that is defined genetically by the bithoraxoid (bxd) and postbithorax (pbx) subfunction mutations. We have directly analyzed UCR functions by the examination of β-galactosidase expression in flies containing Ubx-lacZ fusion genes. 35 kb of UCR DNA confers upon β-galactosidase an expression pattern that closely parallels normal Ubx expression throughout development. In contrast, 22 kb of UCR DNA confers fewer features of normal Ubx expression, and with 5 kb of UCR DNA the expression pattern has no resemblance to Ubx expression except in the visceral mesoderm. We have also shown that bxd chromosome breakpoint mutants form a comparable 5’ deletion series in which the severity of the effect on Ubx expression correlates with the amount of upstream DNA remaining in the mutant. In Ubx-lacZ fusions containing 22 kb of UCR DNA, and in comparable bxd mutants, there is a persistent pair-rule pattern of metameric expression in early development, demonstrating that there are distinct mechanisms with different sequence requirements for the initial activation of Ubx in different metameres. The correction of this pair-rule pattern later in embryogenesis shows that there are also distinct mechanisms for the activation of Ubx at different times during development.
Expression of FGF receptor gene in rat development
Authors: Wanaka Akio , Milbrandt Jeffrey , Johnson Eugene M.
Published: April 2021

We examined the expression of FGF-receptor (FGF-R) mRNA during rat development with in situ hybridization histochemistry. Embryonic tissues (E9, E12, E14, E17) and postnatal neural tissues (Pl, P7, P14, adult) were examined. We detected significant levels of FGF-R mRNA in various tissues at different developmental stages. As postulated by previous studies using other methods, FGF-R gene expression was observed primarily in mesoderm- and neuroectodenn-derived tissues. In the nervous system, the pattern of gene expression was developmentally regulated; in embryos, FGF-R mRNA was mainly detected in the ependymal layer of the central nervous system (CNS). Postnatally, FGF-R transcripts were observed in specific neuronal populations, such as hippocampal neurons. FGF-R mRNA was also found in sensory systems such as trigeminal and dorsal root ganglia in late stage embryos; however, FGF-R mRNA decreased in the postnatal period. FGF-R mRNA expression was modulated in the developing retina: FGF-R messages were observed in the pigment epithelium and neuroblast layer at embryonic stages; in the postnatal period, they were found in the ganglion cell and inner granular layer. In non-neuronal embryonic tissues, a wide variety of organs expressed FGF-R message. Particularly, the prevertebral column, bone, kidney and skin showed high levels of expression. These observations reinforce the idea that FGF exerts effects on the development of various tissues.
Mutually repressive interactions between the gap genes giant and Krüppel define middle body regions of the Drosophila embryo
Authors: Kraut Rachel , Levine Michael
Published: April 2021

The gap genes play a key role in establishing pair-rule and homeotic stripes of gene expression in the Dros-ophila embryo. There is mounting evidence that overlapping gradients of gap gene expression are crucial for this process. Here we present evidence that the segmentation gene giant is a bona fide gap gene that is likely to act in concert with hunchback, Krüppel and knirps to initiate stripes of gene expression. We show that Krüppel and giant are expressed in complementary, non-overlapping sets of cells in the early embryo. These complementary patterns depend on mutually repressive interactions between the two genes. Ectopic expression of giant in early embryos results in the selective repression of Krüppel, and advanced-stage embryos show cuticular defects similar to those observed in Krüppel− mutants. This result and others suggest that the strongest regulatory interactions occur among those gap genes expressed in nonadjacent domains. We propose that the precisely balanced overlapping gradi-ents of gap gene expression depend on these strong regulatory interactions, coupled with weak interactions between neighboring genes.
‘Unmasking’ of stored maternal mRNAs and the activation of protein synthesis at fertilization in sea urchins
Authors: Kelso-Winemiller Leslie C. , Winkler Matthew M.
Published: April 2021

The isolation and in vitro assay of maternal mRNPs has led to differing conclusions as to whether maternal mRNAs in sea urchin eggs are in a repressed or ‘masked’ form. To circumvent the problems involved with in vitro approaches, we have used an in vivo assay to determine if the availability of mRNA and/or components of the translational machinery are limiting protein synthesis in the unfertilized egg. This assay involves the use of a protein synthesis elongation inhibitor to create a situation in the egg in which there is excess translational machinery available to bind mRNA. Eggs were fertilized and the rate of entry into polysomes of individual mRNAs was measured in inhibitor-treated and control embryos using 32P-labeled cDNA probes. The fraction of ribosomes in polysomes and the polysome size were also determined. The results from this in vivo approach provide strong evidence for the coactivation of both mRNAs and components of the translational machinery following fertilization. The average polysome size increases from 7.5 ribosomes per message in 15 min embryos to approximately 10.8 ribosomes in 2h embryos. This result gives additional support to the idea that translational machinery, as well as mRNA, is activated following fertilization. We also found that individual mRNAs are recruited into polysomes with different kinetics, and that the fraction of an mRNA in polysomes in the unfertilized egg correlates with the rate at which that mRNA is recruited into polysomes following fertilization.
Hβ58, an insertional mutation affecting early postimplantation development of the mouse embryo
Authors: Radice Glenn , Lee James J. , Costantini Frank
Published: April 2021

The generation and analysis of insertional mutations affecting mouse embryogenesis provides a powerful method to identify new genes that function in early development. In this paper, we describe an insertional mutation that inteferes with postimplantation mouse development beginning at the time of gastrulation. Embryos homozygous for the Hβ58 transgenic insertion developed normally through the early postimplantation, egg cylinder stage (day 6.5 of development). At the primitive streak stage (day 7.5), however, they began to display characteristic abnormalities, including a retardation in the growth of the embryonic ectoderm (the earliest identifiable defect), and in some cases abnormalities of the amnion and chorion. Homozygotes continued to develop for 2–3 more days, reaching the size of a normal 8.5 day embryo, and formed tissues representative of all three germ layers, including several differentiated cell types. The site of insertion was mapped, by a combination of cytogenetic and genetic methods, to chromosome 10, and it appeared to define a new genetic locus. The inserted transgene provided a probe to clone and characterize the mutant locus, as well as the corresponding wild-type locus. In addition to an insertion of 10–20 copies of the transgene, the mutant locus contained a deletion of 2–3 kb of DNA found at the wild-type locus, and possibly an insertion of mouse repetitive DNA. However, genomic sequences on both sides of the insertion site remained co-linear in the wildtype and mutant genomes, and no chromosomal abnormalities could be detected. Five single copy DNA probes spanning the insertion site were tested for their ability to hybridize to RNA from 8.5 day embryos; one of the probes (located within the region deleted from the mutant chromosome) hybridized to a 2.7 kb mRNA encoded at the H/158 locus, thus identifying a gene whose disruption appears to be responsible for the mutant phenotype.
Differential expression of two cadherins in Xenopus laevis
Authors: Angres B. , Müller A. H. J. , Kellermann J. , Hausen P.
Published: April 2021

Using a cadherin fraction from Xenopus tissue culture cells as an immunogen, two monoclonal antibodies were obtained that allowed the characterization of two distinct cadherins in the Xenopus embryo.

The two cadherins differ in molecular weight, in thentime of appearance during development and in their spatial pattern of expression.

One of the antigens was identified as E-cadherin. It appears in the embryonic ectoderm during gastrulation when epidermal differentiation commences and it disappears from the neural plate area upon neural induction.

The second antigen could not be allocated to any of the known cadherin subtypes and was termed U-cadherin. It is present in the egg and becomes deposited in newly formed inner cell membranes during cleavage, the outer apical membranes of the embryo remaining devoid of the cadherin throughout development. U-cadherin is found on membranes of all cells up to the late neurula stages. A conspicuous polarized expression of the antigen on the membranes of individual inner cells suggests its participation in the segregation of cell layers and organ anlagen.

These findings are discussed in the context of current hypotheses on the role of cadherins in establishing the spatial structure of the embryo.
Two homologous regulatory genes, lin-12 and glp-1, have overlapping functions
Authors: Lambie Eric J. , Kimble Judith
Published: April 2021

Two homologous genes, lin-12 and glp-1, encode transmembrane proteins required for regulatory cell interactions during C. elegans development. Based on their single mutant phenotypes, each gene has been thought to govern a distinct set of cell fates. We show here that lin-12 and glp-1 are functionally redundant during embryogenesis: Unlike either single mutant, the lin-12 glp-1 double mutant dies soon after hatching. Numerous cellular defects can be observed in these Lag (for lin-12 and glp-1) double mutants. Furthermore, we have identified two genes, lag-1 and lag-2, that appear to be required for both lin-12 and glp-1 -mediated cell interactions. Strong loss-of-function lag. mutants are phenotypically indistinguishable from the lin-12 glp-1 double; weak lag mutants have phenotypes typical of lin-12 and glp-1 single mutants. We speculate that the lin-12 and glp-1 proteins are biochemically interchangeable and that their divergent roles in development may rely largely on differences in gene expression.
The murine Hox-2 genes display dynamic dorsoventral patterns of expression during central nervous system development
Authors: Graham Anthony , Maden Malcolm , Krumlauf Robb
Published: April 2021

This report demonstrates that the genes in the murine Hox-2 cluster display spatially and temporally dynamic patterns of expression in the transverse plane of the developing CNS. All of the Hox-2 genes exhibit changing patterns of expression that reflect events during the ontogeny of the CNS. The observed expression correlates with the timing and location of the birth of major classes of neurons in the spinal cord. Therefore, it is suggested that the Hox-2 genes act to confer rostrocaudal positional information on each successive class of newly born neurons. This analysis has also revealed a striking dorsal restriction in the patterns of Hox-2 expression in the spinal cord between 12.5 and 14.5 days of gestation, which does not appear to correlate with any morphological structure. The cellular retinol binding protein (CRBP) shows a complementary ventral staining pattern, suggesting that a number of genes are dorsoventrally restricted during the development of the CNS. The expression of Hox-2 genes has also been compared with the Hox-3.1 gene, which exhibits a markedly different dorsoventral pattern of expression. This suggests that, while genes in the different murine Hox clusters may have similar A-P domains of expression, they are responding to different dorsoventral patterning signals in the developing spinal cord.