Contribution of ventral blood island mesoderm to hematopoiesis in postmetamorphic and metamorphosis-inhibited Xenopus laevis

In an effort to label very early erythrocyte and lymphocyte populations and to follow their fate in normally developing postmetamorphic frogs and goitrogen-treated permanent larvae, diploid (2N) and triploid (3N) ventral blood island (VBI) mesoderm was exchanged between neurula stage embryos (about 16-22 hr old). Beginning at 15 days of age, half of the 2N or 3N hosts were treated with sodium perchlorate to prevent thyroxine-induced developmental changes. At larval stages 55-59 (41-48 days) and at 1-2 months postmetamorphosis (110-120 days), the untreated control chimeras and age-matched perchlorate-treated chimeras were killed for analysis of the VBI contribution to blood, spleen, and thymus populations by flow cytometry. The data suggest that grafting of ventral blood island mesoderm is an effective way to label an early larval erythrocyte population that declines after metamorphosis. In perchlorate-blocked permanent larvae this early VBI-derived erythrocyte population persists. In contrast, grafting of VBI mesoderm was less useful as a method to label a larvally distinct lymphocyte population in the thymus and spleen. At the late larval stages that we examined, the proportion of VBI-derived cells in thymus and spleen was not different from that observed after metamorphosis. Inhibition of metamorphosis interfered with the thymocyte expansion that normally occurs after metamorphosis, but the proportion of VBI-derived cells in thymus and spleen was not affected. This suggests that lymphopoiesis occurring in late larval life and after metamorphosis uses a stable persisting population of VBI-derived stem cells as well as dorsally derived stem cells.     

<Emphasis Type="Italic">AmphiFoxQ2</Emphasis>, a novel winged helix/forkhead gene,exclusively marks the anterior end of the amphioxus embryo

A full-length FoxQ-related gene (AmphiFoxQ2) was isolated from amphioxus. Expression is first detectable in the animal/anterior hemisphere at the mid blastula stage. The midpoint of this expression domain coincides with the anterior pole of the embryo and is offset dorsally by about 20 degrees from the animal pole. During the gastrula stage, expression is limited to the anterior ectoderm. By the early neurula stage, expression remains in the anterior ectoderm and also appears in the adjacent anterior mesendoderm. By the early larval stages, expression is detectable in the anteriormost ectoderm and in the rostral tip of the notochord. AmphiFoxQ2 is never expressed anywhere except at the anterior tip of amphioxus embryos and larvae. This is the first gene known that exclusively marks the anterior pole of chordate embryos. It may, therefore, play an important role in establishing and/or maintaining the anterior/posterior axis.     

文昌鱼tropomyosin基因的克隆、进化分析及其胚胎发育与成体中的表达模式

Tropomyosin是一种分布广泛而且在进化上十分保守的蛋白,是肌肉形成和收缩过程中重要的调节蛋白质。通过RT-PCR和RACE技术得到文昌鱼tropomyosin基因全长,编码一个含284个氨基酸残基的蛋白质,将文昌鱼Tropomyosin和在其他物种中的同源物进行比对建树,发现其在功能域上高度保守并且只有一个拷贝,符合动物分类学中各物种的进化地位。胚胎整体原位杂交实验得知,tropomyosin在文昌鱼早期发育的表达,最早从原肠胚末期神经胚早期开始,定位于分化中的中内胚层。到神经胚期,tropomyosin的表达出现在发育中的体节和脊索中。随着发育的进行,tropomyosin的表达稳定地集中在体节、脊索处。到72h幼虫阶段,tropomyosin的表达仍然在肌节内。成体的切片原位杂交结果显示,tropomyosin在肌节中的表达大幅度下调,而在神经管细胞、脊索和腮区腮瓣处仍然可以检测到明显的表达,在外胚层和表皮内没有发现杂交信号。研究结果表明,tropomyosin的表达与文昌鱼肌节、肌肉以及神经索的发生相关,参与文昌鱼胚胎躯体模式的构建,而且在成体的生命活动中发挥重要作用。     

Ciliated band structure in planktotrophic and lecithotrophic larvae of Heliocidaris species (Echinodermata: Echinoidea): a demonstration of conservation and change

The evolution of lecithotrophic (non-feeding) development in sea urchins is associated with reduction or loss of structures found in the planktotrophic (feeding) echinopluteus larvae. Reductions or losses of larval feeding structures include pluteal arms, their supporting skeleton and the ciliated band that borders them. The barrel-shaped lecithotrophic larva of Heliocidaris erythrogramma has, at its posterior end, two or three ciliated band segments comprised of densely packed, elongate cilia. These cilia may be expressions of the epaulettes that would have been present in an ancestral larval form, represented today by the feeding echinopluteus of H. tuberculata . We compared the development and cellular organization of the larval ciliary structures of both Heliocidaris species to assess whether the ciliary bands of H. erythrogramma are expressions of the feeding ciliated band or epaulettes of an echinopluteus. Epaulette development in feeding larvae of H. tuberculata involves separation of specific parts of the ciliated band from the rest of the feeding ciliated band, hyperplastic addition of ciliated cells and hypertrophic growth of the cilia. Like epaulettes, the ciliated bands of H. erythrogramma are composed of long spindle-shaped cells arranged in a cup-shaped collection that bulges into the blastocoel; and these cells have elongated cilia. In their developmental origin and topological arrangement however, the ciliated bands of H. erythrogramma correspond more closely with parts of the pluteal feeding ciliated band than with epaulettes. The larvae of this echinoid appear to develop epaulette-like bands from parts of the original (but reduced) feeding ciliated band. The evolution of development in H. erythrogramma has thus involved both conservation and change in echinopluteal ciliary structures.     

AmphiNk2-tin,an amphioxus homeobox gene expressed in myocardial progenitors: insights into evolution of the vertebrate heart

We isolated a full-length cDNA clone of amphioxus AmphiNk2-tin, an NK2 gene similar in sequence to vertebrate NK2 cardiac genes, suggesting a potentially similar function to Drosophila tinman and to vertebrate NK2 cardiac genes during heart development. During the neurula stage of amphioxus, AmphiNk2-tin is expressed first within the foregut endoderm, then transiently in muscle precursor cells in the somites, and finally in some mesoderm cells of the visceral peritoneum arranged in an approximately midventral row running beneath the midgut and hindgut. The peritoneal cells that express AmphiNk2-tin are evidently precursors of the myocardium of the heart, which subsequently becomes morphologically detectable ventral to the gut. The amphioxus heart is a rostrocaudally extended tube consisting entirely of myocardial cells (at both the larval and adult stages); there are no chambers, valves, endocardium, epicardium, or other differentiated features of vertebrate hearts. Phylogenetic analysis of the AmphiNk2-tin sequence documents its close relationship to vertebrate NK2 class cardiac genes, and ancillary evidence suggests a relationship with the Drosophila NK2 gene tinman. Apparently, an amphioxus-like heart, and the developmental program directing its development, was the foundation upon which the vertebrate heart evolved by progressive modular innovations at the genetic and morphological levels of organization.     

Phenoloxidase, a marker enzyme for differentiation of the neural ectoderm and the epidermal ectoderm during embryonic development of amphioxus Branchiostoma belcheri tsingtaunese

The development of phenoloxidase during amphioxus embryogenesis was spectrophotometrically and histochemically studied for the first time in the present study. It was found that (1) PO activity initially appeared in the general ectoderm including the neural ectoderm and the epidermal ectoderm at the early neurula stage but not in the mesoderm or the endoderm, and (2) PO activity disappeared in the neural plate cells but remained unchanged in the epidermal cells when the neural plate was morphologically quite distinct from the rest of the ectoderm. It is apparent that PO could serve as a marker enzyme for differentiation of the neural ectoderm from the epidermal ectoderm during embryonic development of amphioxus.     

Topographic changes in nascent and early mesoderm in amphioxus embryos studied by DiI labeling and by in situ hybridization for a Brachyury gene

 In amphioxus embryos, the nascent and early mesoderm (including chorda-mesoderm) was visualized by expression of a Brachyury gene (AmBra-2). A band of mesoderm is first detected encircling the earliest (vegetal plate stage) gastrula sub-equatorially. Soon thereafter, the vegetal plate invaginates, resulting in a cap-shaped gastrula with the mesoderm localized at the blastoporal lip and completely encircling the blastopore. As the gastrula stage progresses, DiI (a vital dye) labeling demonstrates that the entire mesoderm is internalized by a slight involution of the epiblast into the hypoblast all around the perimeter of the blastopore. Subsequently, during the early neurula stage, the internalized mesoderm undergoes anterior extension mid-dorsally (as notochord) and dorsolaterally (in paraxial regions where segments will later form). By the late neurula stage, AmBra-2 is no longer transcribed throughout the mesoderm as a whole; instead, expression is detectable only in the posterior mesoderm and in the notochord, but not in paraxial mesoderm where definitive somites have formed. Received: 28 November 1996 / Accepted: 2 January 1997     

The amphioxus T-box gene, AmphiTbx15/18/22, illuminates the origins of chordate segmentation

Amphioxus and vertebrates are the only deuterostomes to exhibit unequivocal somitic segmentation. The relative simplicity of the amphioxus genome makes it a favorable organism for elucidating the basic genetic network required for chordate somite development. Here we describe the developmental expression of the somite marker, AmphiTbx15/18/22, which is first expressed at the mid-gastrula stage in dorsolateral mesendoderm. At the early neurula stage, expression is detected in the first three pairs of developing somites. By the mid-neurula stage, expression is downregulated in anterior somites, and only detected in the penultimate somite primordia. In early larvae, the gene is expressed in nascent somites before they pinch off from the posterior archenteron (tail bud). Integrating functional, phylogenetic and expression data from a variety of triploblast organisms, we have reconstructed the evolutionary history of the Tbx15/18/22 subfamily. This analysis suggests that the Tbx15/18/22 gene may have played a role in patterning somites in the last common ancestor of all chordates, a role that was later conserved by its descendents following gene duplications within the vertebrate lineage. Furthermore, the comparison of expression domains within this gene subfamily reveals similarities in the genetic bases of trunk and cranial mesoderm segmentation. This lends support to the hypothesis that the vertebrate head evolved from an ancestor possessing segmented cranial mesoderm.     

The persistence of Golgi complexes during cell division in an insect epidermis

The Golgi complexes of animal cells are said to become vesicular during cell division in order to allow the equal partitioning of organelles between daughter cells (Warren, 1985). However, in the epidermis of fifth stage larval Calpodes ethlius (Lepidoptera, Hesperi idae), cutical deposition is concurrent with cell division in preparation for pupation. We therefore looked at the Golgi complexes of these epidermal cells to see if they maintained their interphase form to allow them to continue to function during cell division. Dividing cells were recognized by changes in the nucleus and nuclear envelope, the form of the cell cortex and cell surface, and by the disposition of microtubules. Epidermal Golgi complexes consist of 3-5 cisternae capped by endoplasmic reticulum with transfer vesicles and rings of GC beads next to the cis face, and secretory vesicles on the trans face. Golgi complexes of dividing cells are structurally indistinguishable from those in interphase, their beads are in the rings characteristic of active GCs, and cuticle continues in uninterrupted lamellae above the apical microvilli. The observations suggest that Golgi complexes in dividing insect cells differ from those of most vertebrates by remaining functional through mitosis.