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T C Lacalli

Publications and source records attributed to T C Lacalli.

17 recordsLinked to original sources
PubMed2003

Ventral neurons in the anterior nerve cord of amphioxus larvae. I. An inventory of cell types and synaptic patterns.

Serial sections were used to map the ventrally positioned neurons of the anterior nerve cord of a 12.5-day amphioxus larva from the infundibular region to the end of somite 2. Synaptic patterns reveal five categories of descending pathways, four of which are associated with the ventral compartment (VC) motoneurons responsible for escape swimming. 1) Pre-, para-, and postinfundibular (tegmental) neurons with large varicosities and mixed vesicle populations provide both synaptic and paracrine input to various components of the tegmental neuropile and primary motor center. Four categories of these neurons are distinguished on the basis of their vesicles. 2) Multiple anterior sensory pathways converge on the large paired neurons (LPNs) located near the junction of somites 1 and 2. LPN synaptic output is almost exclusively contralateral. This, together with the evidence for cross-innervation between the third pair of LPNs, is consistent with the latter acting as locomotory pacemakers. 3) Axons from several classes of tegmental neurons converge in the paraxial region on each side of the cord where they form distinct tracts, the upper paraxial bundles. The right bundle is larger than the left, which suggests a role during early development when myotome contractions are biased to one side. 4) Fibers in the ventral tracts from ipsilateral projection neurons, sensory neurons, and additional ascending fibers synapse repeatedly with VC motoneurons. This may be how the overall level of excitation of the latter is controlled so as to modulate their response to pacemaker input. The fifth pathway consists of fibers involved in controlling the dorsal compartment (DC) motoneurons responsible for slow swimming, which are largely isolated from inputs to the VC locomotory system. The ventral neurons of the primary motor center form a more or less continuous file on either side of the floor plate, with certain cell types showing a tendency to cluster. There are, however, few obvious patterns of the kind expected if development were controlled by a rigid, lineage-based mechanism. The evolutionary implications of the involvement of a midbrain-level pacemaker in controlling larval swimming in amphioxus is discussed.

Animals↗
PubMed2001

New perspectives on the evolution of protochordate sensory and locomotory systems, and the origin of brains and heads.

Cladistic analyses generally place tunicates close to the base of the chordate lineage, consistent with the assumption that the tunicate tail is primitively simple, not secondarily reduced from a segmented trunk. Cephalochordates (i.e. amphioxus) are segmented and resemble vertebrates in having two distinct locomotory modes, slow for distance swimming and fast for escape, that depend on separate sets of motor neurons and muscle cells. The sense organs of both amphioxus and tunicate larvae serve essentially as navigational aids and, despite some uncertainty as to homologies, current molecular and ultrastructural data imply a close relationship between them. There are far fewer signs of modification and reduction in the amphioxus central nervous system (CNS), however, so it is arguably the closer to the ancestral condition. Similarities between amphioxus and tunicate sense organs are then most easily explained if distance swimming evolved before and escape behaviour after the two lineages diverged, leaving tunicates to adopt more passive means of avoiding predation. Neither group has the kind of sense organs or sensory integration centres an organism would need to monitor predators, yet mobile predators with eyes were probably important in the early Palaeozoic. For a predator, improvements in vision and locomotion are mutually reinforcing. Both features probably evolved rapidly and together, in an 'arms race' of eyes, brains and segments that left protochordates behind, and ultimately produced the vertebrate head.

Animals↗
PubMed2001

Characterization of Amphioxus AmphiVent, an evolutionarily conserved marker for chordate ventral mesoderm.

Structure and developmental expression are described for amphioxus AmphiVent, a homolog of vertebrate Vent genes. In amphioxus, AmphiVent-expressing ventral mesoderm arises at midneurula by outgrowth from the paraxial mesoderm, but in vertebrates, Vent-expressing ventral mesoderm originates earlier, at the gastrula stage. In other embryonic tissues (nascent paraxial mesoderm, neural plate, endoderm, and tailbud), AmphiVent and its vertebrate homologs are expressed in similar spatiotemporal domains, indicating conservation of many Vent gene functions during chordate evolution. The ventral mesoderm evidently develops precociously in vertebrates because their relatively large embryos probably require an early and extensive deployment of the mesoderm-derived circulatory system. The vertebrate ventral mesoderm, in spite of its strikingly early advent, still resembles the nascent ventral mesoderm of amphioxus in expressing Vent homologs. This coincidence may indicate that Vent homologs in vertebrates and amphioxus play comparable roles in ventral mesoderm specification.

Amino Acid Sequence↗
PubMed2000

Cell morphology in amphioxus nerve cord may reflect the time course of cell differentiation.

Amphioxus embryos elongate following neurulation, and this lengthens the developing nerve cord. Most neurons and support cells remain attached at their apices to the neuroepithelium, and the apices themselves become correspondingly longer. In consequence, apex length can be used in some instances as a measure of whether a given cell last divided before elongation or after, and approximately when. The data indicate that most floorplate, ependymoglial and infundibular cells are generated comparatively early, before most neurons. Among the neurons, the segmentally arranged DC (dorsal compartment) motoneurons appear to be among the first to develop, which accords with molecular data on the time course of neural development, using neurogenin and islet as markers.

Animals↗
PubMed1999

Tunicate tails, stolons, and the origin of the vertebrate trunk.

Tunicates are primitive chordates that develop a transient 'tail' in the larval stage that is generally interpreted as a rudimentary version of the vertebrate trunk. Not all tunicates have tails, however. The groups that lack them, salps and pyrosomes, instead have a trunk-like reproductive stolon located approximately where the tail would otherwise be. In salps, files of blastozooids are formed along the sides of the stolon. The tail and caudal trunk in more advanced chordates could have evolved from a stolon of this type, an idea referred to here as the 'stolon hypothesis'. This means the vertebrate body could be a composite structure, since there is the potential for each somite to incorporate elements originally derived from a complete functional zooid. If indeed this has occurred, it should be reflected in some fashion in gene expression patterns in the vertebrate trunk. Selected morphological and molecular data are reviewed to show that they provide some circumstantial support for the stolon hypothesis. The case would be stronger if it could be demonstrated that salps and/or pyrosomes are ancestral to other tunicates. The molecular phylogenies so far available generally support the idea of a pelagic ancestor, but offer only limited guidance as to which of the surviving pelagic groups most closely resembles it. The principal testable prediction of the stolon hypothesis is that head structures (or their homologues) should be duplicated in series in the trunk in advanced chordates, and vice versa, i.e. trunk structures should occur in the head. The distribution of both rhabdomeric photoreceptors and nephridia in amphioxus conform with this prediction. Equally striking is the involvement of the Pax2 gene in the development of both the inner ear and nephric ducts in vertebrates. The stolon hypothesis would explain this as a consequence of the common origin of otic capsules and excretory ducts from atrial rudiments: from the paired rudiments of the parent oozooid in the case of the otic capsule (these express Pax2 according to recent ascidian data), and from tubular rudiments in the stolon in the case of the excretory ducts.

Animals↗
PubMed1990

Modeling the Drosophila pair-rule pattern by reaction-diffusion: gap input and pattern control in a 4-morphogen system.

Various reaction-diffusion models will produce striped patterns, but the most effective models so far devised to do this require two matched pairs of interacting morphogens, i.e. four substances in all. This paper examines the behavior of one such model, of a fairly generalized type, and its application to the process of pair-rule pattern formation during Drosophila embryogenesis. It is assumed that the two self-activating morphogens required by the model, expressed in complementary out-of-phase stripes, are products of early-acting pair-rule genes. Possible candidates include the primary pair-rule genes, hairy and runt. The conditions under which regular stripes are generated by the model are then examined, with emphasis on the way pre-existing patterns act to control stripe formation via their effect on rates of reaction within the pair-rule system, specifically rates of pair-rule gene transcription, to show how gap gene products may act during pattern formation. A fully symmetrical set of reactions, in which rates of formation, self- and cross-activation are exactly matched, gives unaligned stripes. Pronounced asymmetries in this regard, e.g. differential rates of formation or self-activation, destabilize stripes or produce local interruptions in the pattern like those seen in gap mutants. A limited degree of asymmetry, coupled with a gradient in the value of one or more parameters will give a correctly aligned, well-controlled pattern of stripes. The experimental evidence indicates that gap genes could be responsible for both of these effects: they activate the pair-rule system asymmetrically and, when first expressed, generate a sufficiently complex landscape of concentration peaks and gradations to provide the local cues needed to correctly position and align the pair-rule stripes. In this respect, the pair-rule system can be viewed as having an intrinsic pattern-forming capability, but it depends on the input of gap genes for pattern control. Gradients are involved, but from this analysis, it is the graded distribution of gap products that is important, not the overall antero-posterior gradient. The uniform spacing of stripes, despite underlying peaks and troughs of gap gene expression, shows that pattern wavelength is relatively insensitive to parameter change, also a property of the model.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Theoretical aspects of stripe formation in relation to Drosophila segmentation.

Many aspects of Drosophila segmentation can be discussed in one-dimensional terms as a linear pattern of repeated elements or cell states. But the initial metameric pattern seen in the expression of pair-rule genes is fully two-dimensional, i.e. a pattern of stripes. Several lines of evidence suggest a kinetic mechanism acting globally during the syncytial blastoderm stage may be responsible for generating this pattern. The requirement that the mechanism should produce stripes, not spots or some other periodic pattern, imposes preconditions on this act, namely (1) sharp anterior and posterior boundaries that delimit the pattern-forming region, and (2) an axial asymmetrizing influence in the form of an anteroposterior gradient. Models for Drosophila segmentation generally rely on the gradient to provide positional information in the form of concentration thresholds that cue downstream elements of a hierarchical control system. This imposes restrictions on how such models cope with experimental disturbances to the gradient. A shallower gradient, for example, means fewer pattern elements. This need not be the case if the gradient acts through a kinetic mechanism like reaction-diffusion that involves the whole system. It is then the overall direction of the gradient that is important rather than specific concentration values.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Ciliary band formation in the doliolaria larva of Florometra. II. Development of anterior and posterior half-embryos and the role of the mesentoderm.

We report the results of cutting experiments on embryos of the crinoid Florometra serratissima, which produce, in the doliolaria stage, a striped pattern of ciliary bands. Embryos at gastrula and post-hatching elongation stages were divided into anterior and posterior fragments. Complementary fragments express parts of the total pattern without adding extra pattern elements, i.e. the pattern is a mosaic. Some fragments elongate which, from an examination of internal structures, we interpret as due to the elongation and displacement of the mesenteric sac. The number of pattern elements expressed correlates with degree of elongation and internal landmarks correlate with certain external pattern features. This suggests that the pattern mosaic may reside in the internal tissues, i.e. in the mesentoderm, but we are as yet unable to prove this. The results are discussed with reference to the roles of tissues of different germ layer origin in related embryos, including vertebrates, in which the mesentoderm has a significant instructive role.

Animals↗
PubMed1986

Ciliary band formation in the doliolaria larva of Florometra. I. The development of normal epithelial pattern.

The development of ciliary band pattern in the doliolaria larva of Florometra serratissima is described based on scanning and transmission electron microscopy. The uniformly ciliated epithelium of the post-hatching larva develops four regularly spaced bands over a period of approx. 20 h generating an epithelial pattern that is, essentially, a series of stripes. The first visible events of pattern formation progress over the larval surface in a posterior-to-anterior and dorsal-to-ventral sequence, but the initial pattern is not, in fact, striped. It instead consists of a close-packed array of oval interband domains separated and surrounded by belts of band cells. Secondarily the interband domains expand laterally and coalesce to form continuous, broad stripes, while the bands remain as narrow stripes between them. Two possible explanations for this unusual sequence of events are discussed: that it can be understood in evolutionary terms with reference to band pattern in other echinoderm larvae, and that it is a morphogenetic necessity because limitations inherent in the patterning mechanism prevent the direct formation of regular stripes.

Animals↗
PubMed1975

Morphogenesis in Micrasterias. II. Patterns of morphogenesis.

The final form of the polar lobe and lateral wings of developing semicells of M. rotata results from combined action of three growth processes: tip growth, branching and lobe broadening. Tip growth unaccompanied by branching or broadening occurs during normal development in M. radiata, but is observed only under abnormal conditions (e.g. raised temperature) in M. rotata. When branching and broadening do occur, they occur together and for this reason may be causally related. Autoradiograms demonstrate that specific patterns of cell wall incorporation can be associated with each of the three processes in M. rotata. Autoradiographic patterns found in the polar lobe differ from those found in wings. The growing polar lobe also responds to laser irradiation differently from the wings; lasings occasionally cause duplication of the polar lobe.

Autoradiography↗
PubMed1975

Morphogenesis in Micrasterias. I. Tip growth.

Observations on lobe growth in the lateral wings of the developing primary cell wall in the desmid Micrasterias rotata are reported and discussed. Patterns of incorporation of methyl-[3H]-methionine and C-1[3H] glucose into the primary wall as revealed in autoradiograms indicate that formation of the new wall is concentrated at the tips of lobes. Patterns follow the predictions of Robertson's model for tip growth in fungal hyphae; thus they link growth in M. rotata lobes with mechanisms of cell elongation found in other cells. Damage done to selected regions of the cell surface with a laser microbeam demonstrates that only certain regions are required for continued growth and morphogenesis while much of the surface plays only a passive role. In growth stages at which lobes are already well defined (stage 4 and later) continued growth of each lobe requires the participation of an area no more than 4-5 mum in diameter, here termed a singularity, at its tip. At early stages (prior to stage 3) singularities per se cannot be demonstrated. At these stages the capacity to intitiate lobes and hence to form singularities is not fixed at specific points, but is distributed over an area of the surface no less than 10 mum in diameter. Singularities, by their persistence and repeated duplication, are directly responsible for the spatial form of the two lateral wings.

Autoradiography↗