西伯利亚鲟仔鱼侧线系统的发育

鲟鱼属软骨硬鳞鱼,在电感受器的进化中占据着极为重要的地位。该文以光镜和扫描电镜手段研究了西伯利亚鲟侧线系统早期发育,包括侧线基板发育及感觉嵴的形成、侧线感受器的发育和侧线管道的形成。1日龄,听囊前后外胚层增厚区域出现6对侧线基板;除后侧线基板细胞向躯干侧面迁移外,其他侧线基板均形成感觉嵴结构;每一侧线基板中均有神经丘原基形成。7日龄,壶腹器官在吻部腹面两侧出现,壶腹器官的发育比神经丘晚一周左右。9日龄,神经丘下的表皮略有凹陷,侧线管道开始形成。29日龄,在吻部腹面两侧可见少数个别的壶腹器官表皮细胞覆盖壶腹器官中央区域留下3～4个小的开口;壶腹管内可见大量的微绒毛存在,在其他鲟形目鱼类、软骨鱼类中也存在类似的结构。57日龄,躯干侧线管道已完全埋于侧骨板中;壶腹器官主要分布在吻部腹面,3～4个聚集在一起,呈"梅花状",分布紧密,并且该部分皮肤表面凹陷,形成花朵状凹穴;侧线系统发育完善。     

Development and evolution of lateral line placodes in amphibians I. Development

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.     

Retinoic acid signaling is essential for formation of the heart tube in Xenopus

Retinoic acid is clearly important for the development of the heart. In this paper, we provide evidence that retinoic acid is essential for multiple aspects of cardiogenesis in Xenopus by examining embryos that have been exposed to retinoic acid receptor antagonists. Early in cardiogenesis, retinoic acid alters the expression of key genes in the lateral plate mesoderm including Nkx2.5 and HAND1, indicating that early patterning of the lateral plate mesoderm is, in part, controlled by retinoic acid. We found that, in Xenopus, the transition of the heart from a sheet of cells to a tube required retinoic acid signaling. The requirement for retinoic acid signaling was determined to take place during a narrow window of time between embryonic stages 14 and 18, well before heart tube closure. At the highest doses used, the lateral fields of myocardium fail to fuse, intermediate doses lead to a fusion of the two sides but failure to form a tube, and embryos exposed to lower concentrations of antagonist form a heart tube that failed to complete all the landmark changes that characterize looping. The myocardial phenotypes observed when exposed to the retinoic acid antagonist resemble the myocardium from earlier stages of cardiogenesis, although precocious expression of cardiac differentiation markers was not seen. The morphology of individual cells within the myocardium appeared immature, closely resembling the shape and size of cells at earlier stages of development. However, the failures in morphogenesis are not merely a slowing of development because, even when allowed to develop through stage 40, the heart tubes did not close when embryos were exposed to high levels of antagonist. Indeed, some aspects of left-right asymmetry also remained even in hearts that never formed a tube. These results demonstrate that components of the retinoic acid signaling pathway are necessary for the progression of cardiac morphogenesis in Xenopus.     

Electrosensory ampullary organs are derived from lateral line placodes in cartilaginous fishes

Ampullary organ electroreceptors excited by weak cathodal electric fields are used for hunting by both cartilaginous and non-teleost bony fishes. Despite similarities of neurophysiology and innervation, their embryonic origins remain controversial: bony fish ampullary organs are derived from lateral line placodes, whereas a neural crest origin has been proposed for cartilaginous fish electroreceptors. This calls into question the homology of electroreceptors and ampullary organs in the two lineages of jawed vertebrates. Here, we test the hypothesis that lateral line placodes form electroreceptors in cartilaginous fishes by undertaking the first long-term in vivo fate-mapping study in any cartilaginous fish. Using DiI tracing for up to 70 days in the little skate, Leucoraja erinacea, we show that lateral line placodes form both ampullary electroreceptors and mechanosensory neuromasts. These data confirm the homology of electroreceptors and ampullary organs in cartilaginous and non-teleost bony fishes, and indicate that jawed vertebrates primitively possessed a lateral line placode-derived system of electrosensory ampullary organs and mechanosensory neuromasts.     

Homeobox genes in axolotl lateral line placodes and neuromasts

 Gene expression has been studied in considerable detail in the developing vertebrate brain, neural crest, and some placode-derived organs. As a further investigation of vertebrate head morphogenesis, expression patterns of several homeobox-containing genes were examined using whole-mount in situ hybridization in a sensory system primitive for the vertebrate subphylum: the axolotl lateral lines and the placodes from which they develop. Axolotl Msx-2 and Dlx-3 are expressed in all of the lateral line placodes. Both genes are expressed throughout development of the lateral line system and their expression continues in the fully developed neuromasts. Expression within support cells is highly polarized. In contrast to most other observations of Msx genes in vertebrate organogenesis, expression of Msx-2 in developing lateral line organs is exclusively epithelial and is not associated with epithelial-mesenchymal interactions. A Hox-complex gene, Hoxb-3, is shown to be expressed in the embryonic hindbrain and in a lateral line placode at the same rostrocaudal level, but not in other placodes nor in mature lateral line organs. A Hox gene of a separate paralog group, Hoxa-4, is expressed in a more posterior hindbrain domain in the embryo, but is not expressed in the lateral line placode at that rostrocaudal level. These data provide the first test of the hypothesis that the neurogenic placodes develop in two rostrocaudal series aligned with the rhombomeric segments and patterned by combinations of Hox genes in parallel with the central nervous system. Received: 2 April 1997 / Accepted: 2 July 1997     

Evolution of electrosensory ampullary organs: conservation of Eya4 expression during lateral line development in jawed vertebrates

The lateral line system of fishes and amphibians comprises two ancient sensory systems: mechanoreception and electroreception. Electroreception is found in all major vertebrate groups (i.e. jawless fishes, cartilaginous fishes, and bony fishes); however, it was lost in several groups including anuran amphibians (frogs) and amniotes (reptiles, birds, and mammals), as well as in the lineage leading to the neopterygian clade of bony fishes (bowfins, gars, and teleosts). Electroreception is mediated by modified “hair cells,” which are collected in ampullary organs that flank lines of mechanosensory hair cell containing neuromasts. In the axolotl (a urodele amphibian), grafting and ablation studies have shown a lateral line placode origin for both mechanosensory neuromasts and electrosensory ampullary organs (and the neurons that innervate them). However, little is known at the molecular level about the development of the amphibian lateral line system in general and electrosensory ampullary organs in particular. Previously, we identified Eya4 as a marker for lateral line (and otic) placodes, neuromasts, and ampullary organs in a shark (a cartilaginous fish) and a paddlefish (a basal ray‐finned fish). Here, we show that Eya4 is similarly expressed during otic and lateral line placode development in the axolotl (a representative of the lobe‐finned fish clade). Furthermore, Eya4 expression is specifically restricted to hair cells in both neuromasts and ampullary organs, as identified by coexpression with the calcium‐buffering protein Parvalbumin3. As well as identifying new molecular markers for amphibian mechanosensory and electrosensory hair cells, these data demonstrate that Eya4 is a conserved marker for lateral line placodes and their derivatives in all jawed vertebrates.     

The development of lateral line placodes: Taking a broader view

The lateral line system of anamniote vertebrates enables the detection of local water movement and weak bioelectric fields. Ancestrally, it comprises neuromasts – small sense organs containing mechanosensory hair cells – distributed in characteristic lines over the head and trunk, flanked on the head by fields of electroreceptive ampullary organs, innervated by afferent neurons projecting respectively to the medial and dorsal octavolateral nuclei in the hindbrain. Given the independent loss of the electrosensory system in multiple lineages, the development and evolution of the mechanosensory and electrosensory components of the lateral line must be dissociable. Nevertheless, the entire system arises from a series of cranial lateral line placodes, which exhibit two modes of sensory organ formation: elongation to form sensory ridges that fragment (with neuromasts differentiating in the center of the ridge, and ampullary organs on the flanks), or migration as collectives of cells, depositing sense organs in their wake. Intensive study of the migrating posterior lateral line placode in zebrafish has yielded a wealth of information concerning the molecular control of migration and neuromast formation in this migrating placode, in this cypriniform teleost species. However, our mechanistic understanding of neuromast and ampullary organ formation by elongating lateral line placodes, and even of other zebrafish lateral line placodes, is sparse or non-existent. Here, we attempt to highlight the diversity of lateral line development and the limits of the current research focus on the zebrafish posterior lateral line placode. We hope this will stimulate a broader approach to this fascinating sensory system.     

The evolutionary history of placodes: a molecular genetic investigation of the larvacean urochordate Oikopleura dioica

The evolutionary origin of vertebrate placodes remains controversial because divergent morphologies in urochordates, cephalochordates and vertebrates make it difficult to recognize organs that are clearly homologous to placode-derived features, including the olfactory organ, adenohypophysis, lens, inner ear, lateral line and cranial ganglia. The larvacean urochordate Oikopleura dioica possesses organs that morphologically resemble the vertebrate olfactory organ and adenohypophysis. We tested the hypothesis that orthologs of these vertebrate placodes exist in a larvacean urochordate by analyzing the developmental expression of larvacean homologs of the placode-marking gene families Eya, Pitx and Six. We conclude that extant chordates inherited olfactory and adenohypophyseal placodes from their last common ancestor, but additional independent proliferation and perhaps loss of placode types probably occurred among the three subphyla of Chordata.     

Development and evolution of lateral line placodes in amphibians. - II. Evolutionary diversification

The amphibian lateral line system develops from a series of lateral line placodes. The different phases of development from early induction, to pattern formation, differentiation, morphogenesis, and metamorphic fate were summarized in the first part of this review (Schlosser, 2002a). Here, a survey of the diversity of lateral line systems in amphibians is presented indicating that most phases of lateral line development have been subject to evolutionary changes. Several trends suggest important roles for both adaptive changes and internal constraints in amphibian lateral line evolution. Many of these trends involved the coordinated modification of different derivatives of lateral line placodes suggesting that these placodes are not only autonomous developmental modules, but also units of evolutionary variation that tend to be modified in a coherent and largely context-independent fashion.     

Loss of ectodermal competence for lateral line placode formation in the direct developing frog Eleutherodactylus coqui.

In the direct-developing frog Eleutherodactylus coqui neuromasts and ganglia of the lateral line system never develop. We show here that this absence of the lateral line system, which is evolutionarily derived in anurans, is due to very early changes in development. Ectodermal thickenings, which are typical of lateral line placodes, and from which neuromasts and ganglion cells of the lateral line originate, never form in E. coqui, although other neurogenic placodes are present. Moreover, although NeuroD is expressed in the lateral line placodes of Xenopus laevis, corresponding expression sites are lacking in E. coqui. Heterospecific transplantation experiments show that axolotl ectoderm can be induced to form lateral line placodes after transplantation to E. coqui hosts but that E. coqui ectoderm does not form lateral line placodes on axolotl hosts. This suggests that the loss of the lateral line system in E. coqui is due to the specific loss of ectodermal competence to form lateral line placodes in response to inductive signals. Our results (1) indicate that the competence for lateral line placode formation is distinct and dissociable from the competence to form other neurogenic placodes and (2) support the idea that the lateral line system acts as a module in development and evolution.     

Stepwise aggregation, compaction, and differentiation of uncompacted F9 cells

To study the relationship between compaction and differentiation in aggregates of F9 embryonal carcinoma cells, a subline was developed which grows mostly uncompacted in monolayer culture in medium containing a low concentration of calcium (about 0.05 mM). When these cells were trysinized and cultured in suspension in the same medium, they formed loose, open aggregates, which failed to differentiate into embryoid bodies after exposure to 10 nM retinoic acid, confirming the requirement of compaction for differentiation. If, after culture for 3 days, the uncompacted F9 aggregates were exposed to additional calcium (4 nM), all compacted within an hour. The number of days necessary for aggregates to acquire this ability to compact rapidly was reduced if the monolayer of cells from which the aggregates were derived had been exposed to additional calcium to cause compaction for several days prior to trypsinization and aggregation. Next, treatment of the compacted F9 aggregates with 10 nM retinoic acid was followed by differentiation into embryoid bodies. The number of days required for this was also reduced if the aggregates were formed from previously compacted cells, presumably because compaction of the aggregates occurred sooner. The acceleration in compaction and differentiation in aggregates formed from previously compacted cells suggests that some of the proteins important for compaction, which are synthesized in a monolayer of compacted cells, persist through trypsinization and are carried over from monolayer to aggregates. Alternatively, an inhibitor of compaction is decreased in the compacted monolayer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Molecular analysis of neurogenic placode development in a basal ray-finned fish

Neurogenic placodes are transient, thickened patches of embryonic vertebrate head ectoderm that give rise to the paired peripheral sense organs and most neurons in cranial sensory ganglia. We present the first analysis of gene expression during neurogenic placode development in a basal actinopterygian (ray-finned fish), the North American paddlefish (Polyodon spathula). Pax3 expression in the profundal placode confirms its homology with the ophthalmic trigeminal placode of amniotes. We report the conservation of expression of Pax2 and Pax8 in the otic and/or epibranchial placodes, Phox2b in epibranchial placode-derived neurons, Sox3 during epibranchial and lateral line placode development, and NeuroD in developing cranial sensory ganglia. We identify Sox3 as a novel marker for developing fields of electrosensory ampullary organs and for ampullary organs themselves. Sox3 is also the first molecular marker for actinopterygian ampullary organs. This is consistent with, though does not prove, a lateral line placode origin for actinopterygian ampullary organs.     

Retinoic acid is a modulator of thyroid hormone activation of Ca2+-ATPase in the human erythrocyte membrane

The thyroid hormones thyroxine (T4) and 3,3',5-L-triiodothyronine (T3) stimulate plasma membrane Ca2+-ATPase (EC 3.6.1.3) activity in human erythrocytes by a mechanism independent of the cell nucleus. The current studies were conducted to determine the effect of retinoic acid on the extranuclear activation by T4 and T3 of Ca2+-ATPase in the human red cell. The retinoid inhibited basal and T4-stimulatable activity of that enzyme in a dose-dependent manner. At the highest tested concentration (10(-6) M), retinoic acid inhibited basal enzyme activity by 25% and T4-stimulated activity by 72%. A concentration as low as 5 x 10(-10) M retinoic acid shifted the dose-response curve of both T4 and T3 so that the concentration of each associated with maximal enzyme stimulation was 10(-9) M instead of 10(-10) M. Retinoic acid displaced [125I]T4 binding to red cell membranes as effectively as unlabeled T4. Retinol failed to influence either basal or T4-stimulated enzyme activity or to displace T4 binding. These results indicate that retinoic acid can partially block the T4 and T3 stimulation of Ca2+-ATPase in human red cell membranes and suggest a physiologic role for the retinoid as a modulator of this peripheral action of thyroid hormone. They suggest that the red cell membrane is an important site of action for this active retinoid.     

The response of embryonal carcinoma cells to retinoic acid depends on colony size

Single cells of the feeder-layer-dependent mouse embryonal carcinoma (EC) cell line, NG2, can spontaneously give rise to colonies containing a wide variety of differentiated cell types in vitro. When cultured with retinoic acid at a concentration of 10(-7) M, single NG2 cells irreversibly differentiated to parietal endoderm, as identified by morphological criteria and immunohistochemical staining. Parietal endoderm was also the first product of spontaneous differentiation. However, when retinoic acid was added to monolayer groups of NG2 cells, not all of the cells were induced to differentiate. The parietal-endoderm cells which did form were generally found at the periphery of cell colonies, as is the case during spontaneous differentiation. Differentiation in the centre of these colonies yielded a variety of cell types over a 21-day period. These results are consistent with the hypothesis that retinoic acid induces the differentiation of EC cells by accelerating cellular response to intrinsic stimuli, rather than by overriding these stimuli.     

Cell-surface changes during in vitro differentiation of pluripotent embryonal carcinoma cells

When aggregates of HM-1 embryonal carcinoma (EC) cells were exposed to 10(-6) M retinoic acid for 2 days and cultured in medium lacking retinoic acid, they differentiated to nerve cells, endoderm cells, and myoblasts. Cells 2 days after initial exposure to retinoic acid were not significantly different from the parental EC cells, as judged by cell-surface architecture and by reactivity to lectins. On the fourth day, the surface of the aggregates was covered with two kinds of cells distinguishable from the parental cells. The round cells with short villi seemed to be precursors to endoderm cells. Receptors for Dolichos biflorus agglutinin (DBA) newly appeared and receptors for peanut agglutinin (PNA) were still expressed on their surfaces. The other cells, which were round cells with a few processes, might be precursors to nerve cells. PNA receptors had disappeared from their surfaces, and DBA receptors were not expressed. On the sixth day of differentiation, possible precursors to myoblasts were detected; they were flat cells with smooth surfaces. These cells lacked cell-surface receptors for the two lectins, while the precursor cells and the myoblasts excreted intercellular fibers reacting with PNA. HM-1 cells synthesized much embryoglycan, the structure of which was similar to that of the glycan isolated from quasinullipotent F9 cells. The only difference was that the glycan from HM-1 cells lacked DBA binding sites. Synthesis of fucosylated embryoglycan mainly decreased between the second and fourth day of differentiation. As above, cell-surface changes occurred mainly between the second and fourth day. The period seems to be important in determining the fate of the cells, since endoderm cells were scarcely seen among differentiated cells which had been continuously exposed to 10(-6) M retinoic acid during the period.     

Hedgehog参与西伯利亚鲟侧线机械和电感受器分化的初始证据

为揭示Hedgehog（Hh）信号与神经丘和壶腹器官分化的关系,研究以西伯利亚鲟（Acipenser baerii Brandt）为模型,首先对再生过程中的神经丘和壶腹器官的转录组进行比较分析,发现Hh信号通路关键基因（Shh、Patched 1）在两类感受器中差异表达,且它们的表达在再生过程中呈现动态性。然后用环巴胺（Cyclopamine,Hh信号抑制剂）处理西伯利亚鲟胚胎（st29）,用扫描电镜和FM1-43荧光染色对西伯利亚鲟仔鱼（st43-st44）分析发现环巴胺显著抑制了壶腹器官的发育。整体原位杂交表明,Shh、Patched1、Smoothened、Gli2在腹面侧线区域的表达受到了环巴胺的抑制。以上结果暗示Hh信号通路与神经丘和壶腹器官的发育有关,推测Hh信号在神经丘和壶腹器官的分化过程中起到了重要作用。