Biomedical subjects

J Jellies

Publications and source records attributed to J Jellies.

30 records · Page 2Linked to original sources

PubMed1993

Multimorphic growth cones in the embryonic medicinal leech: relationship between shape changes and outgrowth transitions.

Comparative studies of growth cone morphology may provide insight into the mechanisms underlying motility and navigation in vivo. Here we analyzed the morphology of a unique set of growth cones in the embryonic medicinal leech, Hirudo medicinalis. The comb or C-cell is a transient cell found as a bilateral pair in each midbody segment. Early in development, from embryonic day (E)7 to E11, each C-cell adds and orients about 70 parallel growth cones that remain relatively nonmotile until E12 when rapid process outgrowth is initiated. Individual C-cells from E10 to E14 were injected with Lucifer yellow and growth cones were traced with a camera lucida. Growth cone morphology was quantified from the drawings. Lamellar regions increased in area with age and change in extension rate. Young, relatively nonmotile growth cones had numerous short filopodia in many orientations, while at highly motile stages filopodial number decreased, length increased, and orientation became more restricted in the direction of outgrowth. Thus, while filopodia were distributed symmetrically, such that the average filopodial angle was predictive of the direction of outgrowth at all stages, younger (relatively nonmotile) growth cones project more filopodia in many directions than do older more motile growth cones. These results suggest that: (1) alterations in morphology may reflect developmentally regulated changes in extension and the local environment, (2) these growth cones maintain a large area for environmental sampling as they increase extension rate, even as filopodia become more restricted in orientation, and (3) C-cell growth cones might progressively alter their affinity for local cellular cues as they initiate rapid and directed outgrowth. The C-cell of embryonic leech may provide a relatively simple system in which to test these ideas experimentally.

Animals↗

PubMed1993

Developmental regulation of a glycolipid epitope on actively extending growth cones and central and peripheral projections in the medicinal leech.

The monoclonal antibody, A2B5, that recognizes vertebrate gangliosides also recognizes embryonic cells in the medicinal leech (Hirudo medicinalis) in a spatially and temporally regulated way. Furthermore, A2B5-positive glycolipids could be isolated from embryonic leeches. Early in development A2B5 labeled axon tracts within the central nervous system coincident with the initial growth of axons, while later in embryogenesis, A2B5 also labeled the forming peripheral nerves. This central and peripheral staining disappeared during the latter third of embryogenesis. A2B5 also labeled the growing tips of an identified myo-organizing cell. This cell, the C-cell, projects an array of parallel processes and exhibits a discrete transition in the rate of growth cone extension (J. Jellies, and W. B. Kristan, Jr. 1991. Dev. Biol. 148, 334-354.). A2B5 failed to recognize the relatively non motile growth cones of the C-cell during early embryogenesis. The C-cell growth cones began to exhibit A2B5 labeling as they became more rapidly extending and this labeling persisted throughout the later motile phase of C-cell growth. In addition to its widespread distribution on embryonic (but not mature) cellular processes, A2B5 also labeled the mitotic profiles of dividing cells in all tissues. Thus, the A2B5 epitope may be presented intracellularly, or both intra- and extracellularly. When glycolipids were extracted from embryonic leeches, partitioned by elution from a silicic acid column, and analyzed using high-performance thin-layer chromatography, at least one of the major glycolipid bands was resorcinol-positive, consistent with presentation of sialic acid residues. The fraction enriched for the resorcinol-positive band was recognized by A2B5 on dot blots, as was ganglioside GQ1b. While potential mechanisms remain to be examined, on the basis of the distinct distribution of the A2B5 epitope and our finding of A2B5-positive glycolipids in leech embryos, we suggest that complex polar glycolipids may play a role in the extension of cellular projections in the medicinal leech.

Animals↗

PubMed1992

Tract formation and axon fasciculation of molecularly distinct peripheral neuron subpopulations during leech embryogenesis.

In leech, the central projections of peripheral sensory neurons segregate into specific axonal tracts, which are distinguished by differential expression of surface antigens recognized by the monoclonal antibodies Lan3-2 and Lan4-2. Lan3-2 recognizes an epitope expressed on axons that segregate into three distinct axon fascicles. In contrast, the Lan4-2-positive axons selectively project into only one of the Lan3-2-positive axon tracts. These observations provide evidence for a hierarchy of guidance cues mediating specific pathway formation in this system. Since the Lan3-2 antibody has been shown to perturb this process and since, as shown here, the Lan3-2 and Lan4-2 antigens are closely molecularly interrelated, these antibodies may help define molecules and epitopes mediating neuronal recognition and axonal guidance.

Afferent Pathways↗

PubMed1992

Development of segment- and target-related neuronal identity in the medicinal leech.

The rhythmic pumping of the paired heart tubes in the medicinal leech Hirudo medicinalis offers an excellent system for studying the development of a simple behavior in terms of its neuronal and muscular components. The present experiments examined the development of identified heart excitor (HE) motor neurons during normal embryogenesis. Using intracellular impalements and dye-filling, we found that the HE motor neurons could be identified at an early stage of development and that they initially elaborated axonal arborizations in inappropriate target fields in the ventral body wall. These inappropriate projections were retracted as those at the appropriate target (developing heart tube muscle) extended. This remodelling occurred at least 4 days before the HEs acquired the adult phenotype of being driven to fire action potentials in a rhythmic pattern. Although the HEs exhibited centrally driven rhythmic oscillations late in embryogenesis, at earlier stages they exhibited largely a tonic discharge interrupted by bursts of inhibitory potentials in a periodic, but not a rhythmic, pattern. We also found what appeared to be non-rhythmic HE homologs in anterior and posterior segments where HE neurons have not been previously described. These homologs may project along similarly patterned guidance cues early in development, since they are at first indistinguishable from the definitive HEs, but they continued to elaborate both lateral and medial body wall projections over the same period that definitive HEs were expanding their arborizations over the developing heart tube and retracting their body wall projections. In both adult and embryonic leeches the homologs exhibited mostly tonic activity that was interrupted by pronounced, but non-rhythmic, hyperpolarizing postsynaptic potentials. Thus, there appears to be early segmental specification directing the final phenotype of the iterated neuron that, in most segments, becomes the HE motor neuron.

Animals↗

PubMed1991

The oblique muscle organizer in Hirudo medicinalis, an identified embryonic cell projecting multiple parallel growth cones in an orderly array.

The oblique muscle layer in the leech body wall is built upon the processes of a unique identified embryonic cell, the Comb- or C-cell. Each C-cell is composed of a spindle-shaped soma that projects approximately 70 parallel processes through the developing body wall at an angle oblique to the long axis. The morphogenesis of this cell and the navigation of its growth cones were examined by intracellular dye filling and antibody staining. At the earliest stages described each C-cell had about six processes, with those near the center of the cell oriented obliquely. As processes were added at the axial ends of the soma they often projected along previously developed longitudinal or circular muscle founder cells and then secondarily aligned themselves parallel to the older processes from the same C-cell. All growth cones initially extended to a particular location in the body wall, where they ceased growing until all 70 processes had been added (over the course of about 5 days). As adjacent segmental homologs met, their growth cones intermingled, eventually sorting out to align parallel. When one of these cells was ablated early--but not later--in development, the remaining adjacent segmental homologs expanded into the vacant territory, consistent with a hypothesis of mutual avoidance between segmental homologs. Most processes that expanded into the experimentally induced vacancy remained correctly oriented and parallel; the few exceptions projected instead along the mirror-image trajectory. Thus, expression of specific avoidance between adjacent C-cell processes is developmentally regulated and functions as a guidance mechanism in vivo, in that it serves to restrict possible trajectories. After aligning its growth cones, each cell stopped adding processes and the processes rapidly extended in concert along relatively precise trajectories. Processes of contralateral homologs cross to form the orthogonal grid used as a scaffold by myocytes to form the oblique muscles. The advancing fronts of growth cones reached the dorsal midline at about the same time as body closure occurs (at about Embryonic Day 20) at which time the C-cells became granular, lost processes, and presumably died. This sequence of developmental events is consistent with temporal and spatial regulation of different morphogenetic strategies, including--but not limited to--specific avoidance, and further suggests testable hypotheses of mechanisms of growth cone navigation in the intact embryo.

Animals↗

PubMed1990

Muscle assembly in simple systems.

Muscle development has been the subject of intense scrutiny at cellular, biochemical and molecular levels, yet little is known about the factors that generate anatomically distinct muscles during embryogenesis. We now know that at least some muscles are initially organized by interactions with particular cells that appear early in development, the muscle organizers. These organizers have been described in both arthropods and annelids, and serve similar functions in both groups: they provide cellular scaffolding during the early and relatively simple anatomical stages of embryogenesis, which is then used to pattern the assembly of large numbers of pre-myocytes. Thus, single cells provide an early framework that is retained even as the embryo becomes vastly more complex. Furthermore, studies have shown that the muscle organizer is necessary for motor neuron growth cones to locate (or recognize) the appropriate target region. In other words, the motor neurons extend toward muscles during muscle histogenesis and can use the muscle organizer for guidance, rather than the definitive muscle which has not yet emerged. The discovery of these identifiable tissue organizers has opened several intriguing avenues by which the roles of cell-cell interactions in development can be further addressed. Additionally, the discovery of these cells implies that in order to understand the ways in which motor neurons are initially matched to particular muscle targets we should also study the organizers that may provide positional introductions between future synaptic partners.

Animals↗

PubMed1988

An identified cell is required for the formation of a major nerve during embryogenesis in the leech.

Investigations of the cues by which axonal growth cones navigate long distances to their targets have revealed the use of a rich and complex diversity of cellular and extracellular information. In the present study we describe one of the most conceptually simple pathfinding cues: a single identified cell in the leech, Hirudo medicinalis, that may guide axons several hundred micrometers to innervate a particular target. One of the stereotyped nerves of H. medicinalis is a "sex nerve" that projects from the anterior root of ganglion 6 [SNA (6)] to the male reproductive structures in the adjacent anterior segment. The pathway for SNA (6) is completely underlain by a single peripheral cell, here called the axonal runway cell (ARC), before axons enter the pathway. The ARC is apparently a nonneuronal cell that stains with a monoclonal antibody that recognizes leech muscle cells. The importance of the ARC for establishing SNA(6) was tested by ablating it before axons entered the pathway. When the ARC was killed either by physical disruption with a microelectrode, or by photoablation after filling it with the fluorescent dye Lucifer yellow, SNA(6) always failed to form, whereas all other nerves formed normally. Killing other peripheral cells in proximity to the ARC did not interfere with SNA(6) formation. Ablation of possible "pioneer neurons" for SNA(6) also did not prevent its formation. These results show that formation of a particular nerve requires only a single cell to serve as a guide for outgrowing processes.

Animals↗

PubMed1988

Embryonic assembly of a complex muscle is directed by a single identified cell in the medicinal leech.

The present study examines the morphological development of a highly organized muscle layer in the leech Hirudo medicinalis, in an effort to characterize those factors that are important in directing its assembly. The tubular body wall of the leech contains 3 major muscle layers that are anatomically distinct: an inner layer of longitudinal muscle, an outer layer of circular muscle, and a grid of oblique muscle sandwiched between them. The oblique muscle layer appears later in development than the other 2 and is preceded by several days by the development of a single, complex cell (here called the comb, or C-cell) whose shape strongly resembles the organization of the oblique muscle grid. There is a bilateral, mirror-image pair of C-cells in each segment. The C-cell has a central, longitudinally oriented soma and projects about 35 fine, parallel processes both medially and laterally at approximately 45 degrees to the long axis. Using a combination of intracellular and antibody labels, it was found that individual muscle cells align themselves with these processes to form correctly oriented fascicles during development. Photoablation of the C-cell at early stages resulted in the complete absence of all oblique muscle fascicles that would have corresponded to that cell; therefore, this discrete muscle-associated cell is considered to be an identified "muscle organizer." Such cellular organizers may direct muscular and neuromuscular assembly in many species.

Animals↗

PubMed1987

Morphological changes in leech Retzius neurons after target contact during embryogenesis.

Segmental variation in identified neurons may provide an opportunity to examine extrinsic influences on neuronal phenotype, since segmentally homologous neurons must contain much the same intrinsic information, having arisen from very similar or identical precursors. Two large serotonergic Retzius (Rz) cells are found in each segmental ganglion of the leech Hirudo medicinalis. While most Rz cells innervate the body wall in their own segment and, by way of axons in the interganglionic connectives, the body wall of adjacent segments, the Rz cells in ganglia 5 and 6 [Rz(5,6)] lack interganglionic axons and innervate only the reproductive tissue (Glover and Mason, 1986). Here we describe and quantify the development of differences between Rz(5,6) and other Rz cells in peripheral innervation, neuropilar arborization, and soma size. We filled individual Rz cells with Lucifer yellow or HRP in adults and in staged embryos. During the first 72 hr of outgrowth of Rz cell processes, the morphology of Rz(5,6) was indistinguishable from that of other Rz cells. Only after the processes of Rz(5,6) reached the reproductive tissue did they begin to differ from their segmental homologs. This temporal correlation suggests that these morphological differences arise because of some interaction between Rz(5,6) and their target tissue.

Animals↗

PubMed1987

Segment-specific morphogenesis of leech Retzius neurons requires particular peripheral targets.

In most segments of the leech, a pair of Retzius (Rz) cells innervate the body wall musculature and skin; however, in the segments specialized for reproduction (midbody segments 5 and 6), these neurons innervate the reproductive tissue instead. Whereas all Rz cells have the same morphology early in embryogenesis, those in the reproductive segments [Rz(5,6)] become considerably different from their segmental homologs. Unlike standard Rz cells, Rz(5,6) do not have axons in the interganglionic connectives or in the body wall (Glover and Mason, 1986). Rz(5,6) also have significantly smaller somata and fewer branches in the ganglionic neuropil than do standard Rz cells (Jellies et al., 1987). Since these differences between Rz cells do not become apparent until after Rz(5,6) processes appear to contact the reproductive tissue primordia, interactions between Rz(5,6) processes and the reproductive tissue may determine the segmental specializations of these neurons. We have tested this possibility by ablating the reproductive tissue primordia early in embryogenesis and subsequently examining Rz(5,6) morphology. In the absence of reproductive tissue, Rz(5,6) became more like standard Rz cells: they retained axons in the interganglionic connectives, they projected into the body wall, and the density of their arborization within the neuropil increased. These results indicate that the development of some segmental specializations of Rz(5,6) involves an interaction with their unique target tissue.

Animals↗

PubMed1986

Activity of crayfish abdominal-positioning interneurones during spontaneous and sensory-evoked movements.

The premotor interneurones that produce coordinated abdominal movements in crayfish (Procambarus) when stimulated directly, are also 'sensorimotor'. Sets of these interneurones respond in predictable ways to touching the body surface. One set of interneurones (type I) is activated to spiking by touch, while another (type II) receives only subthreshold influences. Several of these interneurones have overlapping receptive fields on the body surface. Touching areas of overlap activates groups of interneurones which discharge at low to moderate frequencies, rather than producing a high-frequency discharge of a single cell. No single positioning interneurone has been identified which is solely responsible for a "voluntary' (spontaneous) motor programme. When active, the positioning interneurones contribute to the production of the behaviour as a member of a constellation of such cells. The results show that this motor system comprises interneurones with sensory as well as motor properties. Although single cells can produce coordinated movements when stimulated at high frequencies, these positioning interneurones appear to function as 'command elements' within a large 'command system' and not as individual units.

Abdomen↗

PubMed1983

The organization of flexion-evoking interneurons in the abdominal nerve cord of the crayfish, Procambarus clarkii.

Intracellular recording and dye injection was used to investigate the flexion-evoking interneurons in the isolated abdominal nerve cord of the crayfish, Procambarus clarkii. The interneurons described in this study have all of the characteristics of cells called flexion command fibers in earlier works. These interneurons show cell bodies in all of the abdominal ganglia, all are interganglionic, and those with posteriorly directed axons enter the terminal caudal ganglion. Some cells were found in as many as eight different preparations, supporting the idea that these interneurons are identified cells. In addition to presenting evidence for identities, evidence was obtained for serial homology in this premotor system. A summary of the general organization of the flexion premotor apparatus is offered. It consists primarily of parallel elements (command interneurons) that are either bipolar or monopolar whose axonal projections course to the terminal caudal ganglion or rostrally into the thoracic region, perhaps into the cerebral ganglion. When activated singly, these parallel elements do not recruit additional interneurons but there are data to suggest that several of these elements could act in concert and interact in a low-gain fashion to produce coordinated positioning behavior.

Action Potentials↗