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PubMed2001

Formation of the middle ear: recent progress on the developmental and molecular mechanisms.

The middle ear allows animals to hear while moving in an aerial medium. It is composed of a cavity harbouring a chain of three ossicles that transmit vibrations produced by airborne sound in the tympanic membrane into the inner ear, where they are converted into neural impulses. The middle ear develops in the branchial arches, and this requires sequential interactions between the epithelia and the underlying mesenchyme. Gene-inactivation experiments have identified genes required for the formation of different middle ear components. Some encode for signalling molecules, including Endothelin1 and Fgf8, probable mediators of epithelial-mesenchymal interactions. Other genes, including Eya1, Prx1, Hoxa1, Hoxa2, Dlx1, Dlx2, Dlx5, and Gsc, are most likely involved in patterning and morphogenetic processes in the neural crest-derived mesenchyme. Mechanisms controlling formation of a functional tympanic membrane are also discussed. Basically, the tympanic ring, which serves as support for the tympanic membrane, directs invagination of the first pharyngeal cleft ectoderm to form the external acoustic meatus (EAM), which provides the outer layer of the membrane. Gsc and Prx1 are essential for tympanic ring development. While invaginating, the EAM controls skeletogenesis in the underlying mesenchyme to form the manubrium of the malleus, the link between the membrane and the middle ear ossicles.

Animals↗

PubMed1980

Morphology, ultrastructure, and development of the parasitic larva and its surrounding trophamnion of Polypodium hydriforme Ussov (Coelenterata).

The larval stage of Polypodium hydriforme is planuliform and parasitic inside the growing oocytes of acipenserid fishes. The larva has inverted germ layers and a special envelope, the trophamnion, surrounding it within the host oocyte. The trophamnion is a giant unicellular provisory structure derived from the second polar body and performing both protective and digestive functions, clearly a result of adaptation to parasitism. The trophamnion displays microvilli on its inner surface, and irregular protrusions anchoring it to the yolk on its outer surface. Its cytoplasm contains long nuclear fragments, ribosomes, mitochondria, microtubules, microfilaments, prominent Golgi bodies, primary lysosomes, and secondary lysosomes with partially digested inclusions. The cells of the larva proper are poorly differentiated. No muscular, glandular, neural, interstitial, or nematocyst-forming cells have been found. The entodermal (outer layer) cells bear flagella and contain rough endoplasmic reticulum; the ectodermal (inner layer) cells lack cilia and contain an apical layer of acid mucopolysaccharid granules. The cells of both layers contain mitochondria, microtubules, and Golgi bodies; their nuclei display large nucleoli with nucleolonema-like structure, decondensed chromatin, and some perichromatin granules. At their apical rims, the ectodermal cells from septate junctions; laterally, the cells of both layers form simple contacts and occasional interdigitations. The lateral surfaces of entodermal cells are strengthened by microtubules.

Animals↗

PubMed1997

Murine gastrulation requires HNF-4 regulated gene expression in the visceral endoderm: tetraploid rescue of Hnf-4(-/-) embryos.

Immediately prior to gastrulation the murine embryo consists of an outer layer of visceral endoderm (VE) and an inner layer of ectoderm. Differentiation and migration of the ectoderm then occurs to produce the three germ layers (ectoderm, embryonic endoderm and mesoderm) from which the fetus is derived. An indication that the VE might have a critical role in this process emerged from studies of Hnf-4(-/-) mouse embryos which fail to undergo normal gastrulation. Since expression of the transcription factor HNF-4 is restricted to the VE during this phase of development, we proposed that HNF-4-regulated gene expression in the VE creates an environment capable of supporting gastrulation. To address this directly we have exploited the versatility of embryonic stem (ES) cells which are amenable to genetic manipulation and can be induced to form VE in vitro. Moreover, embryos derived solely from ES cells can be generated by aggregation with tetraploid morulae. Using Hnf-4(-/-) ES cells we demonstrate that HNF-4 is a key regulator of tissue-specific gene expression in the VE, required for normal expression of secreted factors including alphafetoprotein, apolipoproteins, transthyretin, retinol binding protein, and transferrin. Furthermore, specific complementation of Hnf-4(-/-) embryos with tetraploid-derived Hnf-4(+/+) VE rescues their early developmental arrest, showing conclusively that a functional VE is mandatory for gastrulation.

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PubMed1985

Muscle development in the grasshopper embryo. II. Syncytial origin of the extensor tibiae muscle pioneers.

The extensor tibiae muscle (ETi) in the metathoracic leg of the grasshopper, which powers the jump, is among the most studied insect muscles. In contrast to many insect muscles which are simple (consisting of only a single bundle of muscle fibers), the ETi is a complex muscle which consists of an array of bundles of muscle fibers, each with a separate site of insertion on the body wall ectoderm and on the ETi apodeme ectoderm. Here we describe the embryonic development of this complex muscle. The ETi muscle develops from a single muscle pioneer (MP) which connects the initial invagination of the ETi apodeme to the wall of the femur. This MP then dramatically expands around the developing apodeme to form a large horseshoe-shaped, multinucleate cell, called the supramuscle pioneer (supra-MP); the number of nuclei in the supra-MP increases by cell fusion rather than by nuclear division. The arms of the supra-MP grow steadily longer and their outer edges begin to appear scalloped, certain areas remaining tightly apposed to the ectoderm of the wall of the leg while adjacent areas lose their adhesion and are pulled away. By about 50% of embryonic development the ETi supra-MP consists of a periodic series of bridges (cytoplasmic extensions) connecting the leg wall ectoderm with the apodeme, and linked into a giant syncytium near their inner, apodeme surface by a thin layer of cytoplasm containing hundreds of nuclei. Each bridge is surrounded by a cluster of many smaller mesoderm cells. Next the syncytium begins to divide such that by 60% the periodic bridges of the supra-MP have lost syncytial contact with each other and now themselves form an array of smaller, individual, multinucleate MPs connecting the body wall to the apodeme, each surrounded by a mass of undifferentiated mesoderm cells. This initial cycle of fusion and division is followed by a second similar cycle in which the individual mesoderm cells surrounding each MP fuse with the MP. At the same time, the MP divides into the initial bundle of smaller muscle fibers. Coincident with this division into muscle fibers is the further development of thick and thin filaments and the T-tubule system.

Animals↗

PubMed1987

Structure of the upper teeth of the filefish, Stephanolepis cirrhifer.

The teeth of the filefish were investigated with light and scanning electron microscopy. Anterior and posterior rows of pleurodont teeth are present in the maxilla. The teeth in the anterior row possess a large labial and small lingual surface and the teeth in the posterior row possess a small labial and large lingual surface. The enameloid consists of three layers-the outer, middle, and inner layer. The outer layer consists of fine, parallel crystallites oriented perpendicular to the surface. They are similar to the crystallites in enamel derived from ectoderm. In the middle layer the crystallites form parallel bundles that are oriented in the same direction as that of the crystallites in the outer layer. In the inner layer bundles of crystallites interlace with each other. The tubular structures and the terminal branches of the dentinal tubules are located among the crystallites in the inner layer. In the dentine numerous dentinal tubules radiate from the pulp cavity toward the periphery and are seen to curve slightly in an S-shaped course. At the occlusal tip the alternating parabolic layers are seen and these are considered to be mineralized lines. The openings of the dentinal tubules are round or oval, surrounded by interlacing fibrils. The teeth are tightly fixed to the jawbone by bundles of fibrils. At the lower part of the lingual surface of the teeth in the anterior row and the labial surface of the teeth in the posterior row the bundles of fibrils start at the dentine and some fibrils run through connective tissue, while others terminate in projections of the jawbones.(ABSTRACT TRUNCATED AT 250 WORDS)

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PubMed2000

Indian hedgehog signaling in extraembryonic endoderm and ectoderm differentiation in ES embryoid bodies.

We previously demonstrated that a member of the Hedgehog gene family, Indian hedgehog (Ihh), is expressed in the visceral endoderm of EC and ES cell embryoid bodies and mouse embryos. Overexpression studies suggested that Ihh was involved in visceral endoderm differentiation. We now provide evidence for a Hh response in the embryoid body core and in the mesothelial layer of the visceral yolk sac. We also demonstrate that treatment of ES embryoid bodies with the Hh antagonists cAMP and forskolin results in downregulation of the Hh response and altered embryoid body differentiation. The outer endoderm layer undergoes a transition to parietal endoderm while formation of an embryonic ectoderm layer surrounding a cavity is inhibited. These treatments also result in a decrease in the expression of markers for the mesoderm derivatives, blood and endothelial cells. We present a model to explain how Ihh and BMP signaling may regulate extraembryonic endoderm and embryonic ectoderm differentiation.

Animals↗

PubMed2002

Expression of the transcription factors GATA3 and Pax2 during development of the mammalian inner ear.

The transcription factors GATA3 and Pax2 are expressed throughout development of the mouse inner ear. We have used antibodies to study their temporal and spatial expression patterns from embryonic days E8-E16.5. The two factors show reciprocal relationships in the regional patterning of the early otocyst and cellular patterning within the sensory epithelia. GATA3 is expressed in the whole otic placode at E8. In the otocyst at E9.5-10.5, the distribution is lateral and complementary to the medial expression pattern of Pax2. Only Pax2 is expressed in the endolymphatic duct, but both factors are expressed in the cochlea. At E11.5-13.5, GATA3 is expressed strongly in the cochlea, but in the dorsal, vestibular region it is downregulated. In all sensory epithelia, downregulation coincides with sensory innervation. Pax2 is expressed in all sensory and some nonsensory epithelia, but within sensory epithelia at E16.5 it is restricted to hair cells. GATA3 is expressed throughout key periods of cell proliferation, fate determination, and differentiation and is not specifically associated with any of these processes. Expression persists most strongly in the main components of the developing auditory system. These include the auditory sensory epithelium, the afferent and efferent nerves, and the mesenchymal and ectodermal cells in regions that form key parts of the middle and outer ear. GATA3 is thus expressed in functionally distinct groups of cells that integrate to form a complete sensory system. The results suggest that both factors may be involved in tissue compartmentalisation, morphogenesis, and cell signalling.

Aging↗

PubMed1979

Scanning electron microscopy (SEM) of cranial neural crest migration in chick embryos.

This study describes migrating cranial neural crest cells and the microenvironment through which they migrate in chick embryos. Just prior to and during cell migration, an extensive fibrillar meshwork is observed, particularly on the outer surface of the neural tube and the inner surface of the ectoderm. This meshwork in general had a random orientation. This suggested to us that the meshwork does not provide a directive vector for cell migration but rather a substratum to promote or enhance crest cell filopodial attachment as the cells migrate. Much remains to be done in characterizing the composition of this meshwork. Based on other studies in which a smiliar meshwork has been observed, it is not unreasonable to consider it to be partly collagenous. Another major component in the relatively cell-free space through which avian crest cells migrate is hyaluronic acid. The migrating crest cells are characteristically bipolar and are generally oriented in the direction of migration, although little is known about the actual mechanism of motility. Alterations in the migrating cell or in the environment through which it migrates may interfere with normal craniofacial morphogenesis, as discussed elsewhere in this volume by Johnston and Sulik.

Animals↗

PubMed1997

Implication of OTX2 in pigment epithelium determination and neural retina differentiation.

The expression pattern of Otx2, a homeobox-containing gene, was analyzed from the beginning of eye morphogenesis until neural retina differentiation in chick embryos. Early on, Otx2 expression was diffuse throughout the optic vesicles but became restricted to their dorsal part when the vesicles contacted the surface ectoderm. As the optic cup forms, Otx2 was expressed only in the outer layer, which gives rise to the pigment epithelium. This early Otx2 expression pattern was complementary to that of PAX2, which localizes to the ventral half of the developing eye and optic stalk. Otx2 expression was always observed in the pigment epithelium at all stages analyzed but was extended to scattered cells located in the central portion of the neural retina around stage 22. The number of cells expressing Otx2 transcripts increased with time, following a central to peripheral gradient. Bromodeoxyuridine labeling in combination with immunohistochemistry with anti-OTX2 antiserum and different cell-specific markers were used to determine that OTX2-positive cells are postmitotic neuroblasts undergoing differentiation into several, if not all, of the distinct cell types present in the chick retina. These data indicate that Otx2 might have a double role in eye development. First, it might be necessary for the early specification and subsequent functioning of the pigment epithelium. Later, OTX2 expression might be involved in retina neurogenesis, defining a differentiation feature common to the distinct retinal cell classes.

Animals↗

PubMed1983

Development of the frontal bone and cranial meninges in the embryonic chick: an experimental study of tissue interactions.

The frontal region of the embryonic chick was studied to determine whether epithelial influences are necessary for frontal bone development. The frontal bone is a membrane bone, of neural crest and head mesodermal origin, which develops within mesenchyme sandwiched between two epithelia, neural ectoderm and epidermis. Rudiments were treated enzymatically to separate epithelial and mesenchymal tissues. Frontal mesenchyme then was grown as chorioallantoic membrane grafts either in the presence or absence of neural ectoderm and/or epidermis. The results indicate that neural ectoderm, though required during early stages of development to induce frontal bone development (Schowing, 1968), is not required during later stages (HH 22-30, the stages tested in this study) for osteogenesis. Epidermis, however, was shown to be required for frontal bone development during the stages tested. Frontal mesenchyme formed bone when epidermis was present on the outer aspect of the mesenchyme, and did not form bone when the epidermis had been removed prior to grafting, whether or not neural ectoderm was present. This dependence upon epidermis continues beyond the onset of meningeal differentiation. Once the outer ectomeninx-dermis is distinguishable from the inner endomeninx, osteogenic capabilities are confined to the ectomeninx-dermis layer. Furthermore, the ectomeninx-dermis layer attached to epidermis is able to form membrane bone in the absence of the endomeninx and neural ectoderm. The endomeninx, though normally nonchondrogenic, was shown to be capable of forming cartilage when the neural ectoderm is removed. Neural ectoderm, therefore, may have an inhibitory effect on chondrogenesis in the endomeninx.

Animals↗

PubMed1995

Signals for death and survival: a two-step mechanism for cavitation in the vertebrate embryo.

Conversion of a solid primordium to a hollow tube of cells is a morphogenetic process used frequently during vertebrate embryogenesis. In the early mouse embryo, this process of cavitation transforms the solid embryonic ectoderm into a columnar epithelium surrounding a cavity. Using both established cell lines and normal embryos, we provide evidence that cavitation in the early mouse embryo is the result of the interplay of two signals, one from an outer layer of endoderm cells that acts over short distances to create a cavity by inducing apoptosis of the inner ectodermal cells, and the other a rescue signal mediated by contact with the basement membrane that is required for the survival of the columnar cells that line the cavity. This simple model provides a paradigm for investigating tube morphogenesis in diverse developmental settings.

Amnion↗

PubMed1994

The development of cellular junctions in the Drosophila embryo.

The pattern and development of cellular junctions in the different tissues of the Drosophila embryo from the blastoderm stage until hatching were analyzed. The cellular junctions found include: gap junctions, two types of septate junctions, and several types of cell-cell and cell-substrate adherens junctions. During early and mid embryogenesis (stages 4 to 13) only spot adherens junctions, gap junctions, and zonulae adherentes prevail. Scattered spot adherens junctions are already formed at the blastoderm stage. During and shortly after gastrulation, spot adherens junctions become concentrated at the apical pole and fuse into continuous zonulae adherentes in the posterior endoderm and the ectoderm. In addition to the zonulae adherentes, ectodermally derived epithelia possess scattered gap junctions and form pleated septate junctions and hemiadherens junctions during late embryogenesis (stages 14 to 17). Mesenchymal tissues (i.e., all nonepithelial tissues of the embryo, including the neural primordium and, transiently, the mesoderm and endoderm) possess both spot adherens junctions and gap junctions at a low frequency. Initially, the midgut epithelium does not establish a junctional complex and possess only gap junctions and spot adherens junctions. Only late in development does a circumferential smooth septate junction develop; zonulae adherentes are missing. The various derivatives of the mesoderm express spot adherens junctions, hemiadherens junctions, and gap junctions, but never zonulae adherentes or septate junctions. After organogenesis, several different types of tissue-specific adherens junctions are formed, among them connecting hemiadherens junctions (between gut epithelium and visceral muscle and early during the formation of the muscle tendon junction), muscle tendon junctions (between somatic muscle and tendon cells), fasciae adherentes (between the cells of both the visceral muscle and the dorsal vessel), and autocellular nephrocyte junctions (in nephrocytes). Interesting exceptions to the general pattern of junctional development are provided by the outer epithelial layer of the proventriculus and the Malpighian tubules. Both tissues develop as typical ectodermal epithelia and possess zonulae adherentes. During late embryogenesis, both epithelia lose the zonulae adherentes and form smooth rather than pleated septate junctions, thereby expressing a junctional complex similar to that of the endodermally derived midgut epithelium.

Animals↗

PubMed1977

Artificial neural induction in amphibia. I. Sandwich explants.

1. Embryonic tissues (ectoderm, neural plate) of Ambystoma mexicanum and Tritus were killed with hot water and implanted into ectoderm sandwiches. They induced the ectoderm to form neural tissue, lentoids and unspecialized epidermis. Neural tissue always showed retina character. Egg pigment was eliminated and gathered at the outer retinal surface or in the centre of rosettes. Neural crest cells like mesenchyme or melanophores were completely lacking, retinal pigment did not develop. 2. The thus induced living retina tissue was reimplanted into fresh ectoderm after 2 days. It continued histogenetic and morphogenetic differentiation and formed ocular vesicles with numerous eye cups. It induced the enveloping ectoderm to again form retina, lentoids and unspecialized epidermis without neural crest derivatives or RPE. 3. This inductive process can be reproduced several times.

Ambystoma↗

PubMed2005

Lens-forming competence in the epidermis of Xenopus laevis during development.

In larval X. laevis the capacity to regenerate a lens under the influence of inductive factors present in the vitreous chamber is restricted to the outer cornea and pericorneal epidermis (Lentogenic Area, LA). However, in early embryos, the whole ectoderm is capable of responding to inductive factors of the larval eye forming lens cells. In a previous paper, Cannata et al. (2003) demonstrated that the persistence of lens-forming competence in the LA is the result of early signals causing lens-forming bias in the presumptive LA and of late signals from the eye causing cornea development. This paper analyzes 1) the decrease of the lens-forming capacity in ectodermal regions both near LA (head epidermis) and far from LA (flank epidermis) during development, 2) the capacity of the head epidermis and flank epidermis to respond to lens-competence promoting factors released by an eye transplanted below these epidermal regions, and 3) the eye components responsible for the promoting effect of the transplanted eye. Results were obtained by implanting fragments of ectoderm or epidermis into the vitreous chamber of host tadpoles and by evaluating the percentage of implants positive to a monoclonal antibody anti-lens. These results demonstrated that the lens-forming competence in the flank region is lost at the embryonic stage 30/31 and is weakly restored by eye transplantation; however, lens-forming competence in the head region is lost at the larval stage 48 and is strongly restored by eye transplantation. The authors hypothesize that during development the head ectoderm outside the LA is attained by low levels of the same signals that attain the LA and that these signals are responsible for the maintenance of lens-forming competence in the cornea and pericorneal epidermis of the larva. In this hypothesis, low levels of these signals slacken the decrease of the lens-forming competence in the head ectoderm and make the head epidermis much more responsive than the flank epidermis to the effect of promoting factors released by a transplanted eye. Results obtained after transplantation of eyes deprived of some components indicate that the lens and the retina are the main source of these promoting factors. The immunohistochemical detection of the FGFR-2 (bek variant) protein in the epidermis of stage 53 larvae submitted to eye transplantation at stage 46 showed that the eye transplantation increased the level of FGFR-2 protein in the head epidermis but not in the flank epidermis, indicating that the lens-forming competence in X. laevis epidermis could be related to the presence of an activated FGF receptor system in the responding tissue.

Animals↗

PubMed1990

Immunocytochemical analysis of embryonic compartmentation with a monoclonal antibody against a cytokeratin-related antigen.

Mab 113F4, a monoclonal antibody recognizing an antigen in the outer synaptic layer of the chick neural retina, also recognizes an antigen appearing in all three germ layers of the gastrulating chick embryo. However, as neurulation proceeds, the antigen is down-regulated in three distinct patterns. First, the antigen is lost specifically from those trunk ectodermal cells destined to form the neural plate and, later, the neural tube. It remains absent from any neural derivative until day 13 when it appears in the outer synaptic layer of the neural retina, coincident with synaptogenesis in this region. Second, the entirety of the head ectoderm loses this antigen as the head lifts off the blastoderm. This down-regulation is followed later by a similar loss of antigen expression in the trunk ectoderm. Third, expression in the mesoderm becomes limited to the lateral plate and extraembryonic epithelia. Endodermal derivatives continue to express the antigen throughout development. Antigen 113F4 is localized within the cytoplasm and is organized in a fibrillar pattern. The intracellular localization of this antigen and its characteristic spatio-temporal tissue distribution are consistent with the antigen being a cytokeratin or cytokeratin-related antigen. The changes in tissue distribution suggest a possible role in tissue modelling in response to inductive interactions during development.

Animals↗

PubMed2004

Nitric oxide regulates swimming in the jellyfish Aglantha digitale.

The cnidarian nervous system is considered by many to represent neuronal organization in its earliest and simplest form. Here we demonstrate, for the first time in the Cnidaria, the neuronal localization of nitric oxide synthase (NOS) in the hydromedusa Aglantha digitale (Trachylina). Expression of specific, fixative-resistant NADPH-diaphorase (NADPH-d) activity, characteristic of NOS, was observed in neurites running in the outer nerve ring at the base of the animal and in putative sensory cells in the ectoderm covering its tentacles. At both sites, diphenyleneiodonium (10(-4) M) abolished staining. Capillary electrophoresis confirmed that the NO breakdown products NO2- and NO3- were present at high levels in the tentacles, but were not detectable in NADPH-d-negative areas. The NADPH-d-reactive neurons in the tentacles send processes to regions adjacent to the inner nerve ring where swimming pacemaker cells are located. Free-moving animals and semi-intact preparations were used to test whether NO is involved in regulating the swimming program. NO (30-50 nM) and its precursor L-arginine (1 mM) stimulated swimming, and the effect was mimicked by 8-Br-cGMP (50-100 microM). The NO scavenger PTIO (10-100 microM) and a competitive inhibitor of NOS, L-nitroarginine methyl ester (L-NAME, 200 microM), significantly decreased the swimming frequency in free-moving animals, while its less-active stereoisomer D-nitroarginine methyl ester (D-NAME, 200 microM) had no such effect. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 5-20 microM), a selective inhibitor of soluble guanylyl cyclase, suppressed spontaneous swimming and prevented NO-induced activation of the swimming program. We suggest that an NO/cGMP signaling pathway modulates the rhythmic swimming associated with feeding in Aglantha, possibly by means of putative nitrergic sensory neurons in its tentacles.

Animals↗

PubMed1988

Accumulation and decay of DG42 gene products follow a gradient pattern during Xenopus embryogenesis.

The DG42 gene is expressed during a short window during embryogenesis of Xenopus laevis. The mRNA for this gene can be first detected just after midblastula, peaks at late gastrula, and decays by the end of neurulation. The sequence of the DG42 cDNA and genomic DNA predicts a 70,000-Da protein that is not related to any other known protein. Antibodies prepared against portions of the DG42 open reading frame that had been expressed in bacteria detected a 70,000-Da protein in the embryo with a temporal course of appearance and decay that follows that of the RNA by several hours. Localization of the mRNA in dissected embryos and immunohistochemical detection of the protein showed that DG42 expression moves as a wave or gradient through the embryo. The RNA is first detected in the animal region of the blastula, and by early gastrula is found everywhere except in the outer layer of the dorsal blastopore lip. By midgastrula DG42 protein is present in the inner ectodermal layer and the endoderm; it disappears from dorsal ectoderm as the neural plate is induced and later decays in a dorsoventral direction. The last remnants of DG42 protein are seen in ventral regions of the gut at the tailbud stage.

Animals↗

PubMed1984

In vitro development of isolated ectoderm from axolotl gastrulae.

The development of ectoderm isolated from the animal pole of axolotl gastrulae is monitored by light microscopy, electron microscopy and analysis of newly synthesized proteins, glycoproteins and glycolipids. When control embryos are undergoing neurulation it is shown that the explants autonomously begin to express epidermal markers and do not express mesodermal markers. However the results suggest that not all the cells become epidermal and electron microscope examination shows that only the outer layer does so, the inner cells remaining undifferentiated.

Ambystoma↗