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M Shankland

Publications and source records attributed to M Shankland.

35 records · Page 2Linked to original sources
PubMed1990

Interactions between adjacent ganglia bring about the bilaterally alternating differentiation of RAS and CAS neurons in the leech nerve cord.

Antibodies to small cardioactive peptide (SCP) label a segmentally iterated subset of cells in the leech nerve cord, including the previously identified alternating SCP (AS) neurons. Unlike the majority of leech neurons, these cells are asymmetrically distributed in the adult nerve cord. Moreover, each AS neuron shows a strong tendency to lie on alternate right and left sides in successive ganglia. Previous work has shown that these unpaired neurons arise from bilaterally paired embryonic homologues, only 1 of which takes on the mature immunoreactive phenotype. The 2 AS homologues within a ganglion compete for this fate, in that either the right or the left homologue will become a mature AS neuron with a high degree of reliability if its contralateral homologue is ablated during embryogenesis. In this paper, we demonstrate the existence of interactions between neurons in adjacent ganglia that could account for the alternation of sides observed during normal development. The unilateral ablation of a single AS homologue neuron forced its contralateral homologue to take on the mature AS fate, and this consistently biased the side of AS development in adjacent, unlesioned ganglia both anterior and posterior to the lesion. One of the AS neurons, the caudal alternating SCP (CAS) cell, was injected with Lucifer yellow in adult nerve cords and was shown to have a large primary axon that extends into more anterior ganglia, as well as other, finer axons that are variable in number and arrangement. If the interganglionic interaction of AS neuron homologues is mediated by their primary axons, signals of developmental import must be transmitted both anterogradely and retrogradely along the axon's length. The present results indicate that the development of individual AS neurons is influenced by homologous cells located in the same and neighboring ganglia and suggest that the final, multisegmental patterning of the AS neuron distribution is not predetermined, but rather, arises as an emergent property of the cell interactions that occur during nervous system differentiation.

Animals↗
PubMed1989

Segmental specificity and lateral asymmetry in the differentiation of developmentally homologous neurons during leech embryogenesis.

This paper describes the embryonic development of three leech neurons which undergo spatially regulated patterns of differentiation. In leeches, the nervous system arises from an iterated array of embryonic cell lineages, and each neuron is represented by a set of bilaterally symmetric and segmentally repeated homologs. Two of the cells discussed here, the neurons nz4 and mz3, stain with antibodies to the neuropeptides SCP and FMRFamide during the course of their embryonic differentiation, but only a subset of the initially immunoreactive homologs continue to express this immunoreactivity into postembryonic life. Those nz4 cells which retain immunoreactivity are referred to as RAS neurons, and the persistently immunoreactive mz3 cells referred to as CAS neurons. The subset of homologs which show persistent expression is segment specific, such that the mature RAS and CAS neurons occupy different segmental domains. In addition, both neurons display a final pattern of expression which is laterally asymmetric, with only one of the two homologs in each segment maintaining the RAS or CAS phenotype. Asymmetric differentiation can occur in either orientation for any given segment, although there is a very strong tendency for the persistently immunoreactive cells to lie on opposite sides of successive segments. The fate of the transiently immunoreactive homologs is unclear, but labeling with intracellular lineage tracers suggests that there are some mz3 neurons which survive late into postemobryonic life and never express detectable levels of immunoreactivity. Intracellular lineage tracers also allowed us to follow the development of a third neuron, mz4, which does not stain for either peptide. The mz4 neuron is initially paired, but undergoes an asymmetric pattern of cell death which also shows a strong tendency to alternate sides in successive segments. These spatially coordinated patterns of neuronal survival and/or differentiation suggest that cell interactions play a role in determining the developmental choices made by individual neurons, and a subsequent paper will characterize those interactions through experimental manipulation.

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PubMed1988

Developmental origin of segmental differences in the leech ectoderm: survival and differentiation of the distal tubule cell is determined by the host segment.

The body plan of the adult leech is metameric, with each hemisegmental complement of ectodermal and mesodermal tissues being produced from a set of seven serially repeated embryonic blast cells. Previous studies have shown that homologous o blast cells give rise to an almost identical complement of descendant cells in each of the 21 abdominal segments, but that one o blast cell derivative--the distalmost cell of the nephridial tubule--is only present in 15 abdominal segments in the mature leech. Here we show that all o blast cells generate a presumptive distal tubule cell and that this cell migrates to its normal position in all abdominal segments. However, in segments which normally do not contain the mesodermal portion of the nephridium, the distal tubule cell dies before undergoing its terminal morphological differentiation. To ascertain whether the fate of the distal tubule cell is determined by its lineage history or by the segmental environment into which it is born, we utilized a previously described procedure for altering the segmental register between different embryonic cell lines. This procedure allowed us to effectively transplant o blast cells into more posterior segments prior to the cell divisions which generate their descendant clones. The results indicate that the survival or death of the distal tubule cell is determined by the identity of the host segment and that a given distal tubule cell could be effectively murdered or rescued by slipping its blast cell precursor into an appropriate segment. These findings suggest that the segment-specific pattern of distal tubule cell survival is not inherent to the O cell line, but arises from interactions with surrounding tissues.

Abdomen↗
PubMed1987

Determination of cleavage pattern in embryonic blast cells of the leech.

The o blast cells of the leech embryo become committed to one of two alternative cleavage geometries shortly before they divide. Cleavage geometry depends upon the presence or absence of the adjoining p bandlet, and if that bandlet is ablated, the pattern of o blast cell cleavages will undergo an abrupt transition several hours later. Previous work has shown that the oblast cell becomes committed to the formation of a particular complement of postmitotic descendants early in its differentiation, but the present findings suggest that cleavage pattern and descendant fate are determined at separate commitment events.

Animals↗
PubMed1987

Differentiation of the O and p cell lines in the embryo of the leech. I. Sequential commitment of blast cell sublineages.

The o blast cells are a group of embryonic precursors found in the ectodermal cell layer of the leech germinal band. At the time of their birth these blast cells have the potential to follow either an O or a P developmental pathway and normally become committed to the O pathway as a result of positional cues encountered during the course of their differentiation. The present study characterizes the normal pattern of o blast cell differentiation, including a description of the first six cell divisions in the stereotyped lineage by which the o blast cell gives rise to its clonal descendants. Injection of fluorescent lineage tracers reveals that this clone consists of a precisely defined set of uniquely identifiable neuronal, epidermal, and nephridial descendants and that each of the first three o blast cell divisions brings about a reproducible segregation of these descendant cell fates. Previous work has shown that the o blast cell's descendant clone becomes committed to the O pathway in a stepwise sequence of at least three discrete events which occur, for the most part, many cell divisions prior to histotypic differentiation. The present findings suggest that (i) each of those commitment events is associated with a particular blast cell division, and (ii) each commitment event independently determines the fate of a different blast cell sublineage. The first two commitment events occur just prior to cell divisions which segregate the committed sublineage from the remainder of the blast cell clone, suggesting that the committed state is manifested by only one of the two daughter cells produced at those divisions.

Animals↗
PubMed1987

Differentiation of the O and P cell lines in the embryo of the leech. II. Genealogical relationship of descendant pattern elements in alternative developmental pathways.

The p blast cells are a group of embryonic precursors found in the ectodermal cell layer of the leech germinal band. Each p blast cell normally undergoes the same invariant sequence of cell divisions and gives rise to a precisely defined set of uniquely identifiable neuronal and epidermal descendants in the mature leech. In the present paper, various of the p blast cell progeny were injected with a fluorescent lineage tracer in order to characterize the cellular composition of their descendant clones, and the results show that there is a stereotyped segregation of descendant cell fates through the first three p blast cell divisions. Previous work has shown that neurons and epidermal specializations which normally descend from the p blast cell will arise from a different precursor--the o blast cell--in response to ablation of the neighboring P cell line and that if the o blast cell is at a certain stage of differentiation when the ablation is performed it will produce only a subset of the normal P descendants. Comparison with the present findings indicates that under those conditions the o blast cell clone is not simply recapitulating a branch of the normal p blast cell lineage, but rather manifests an alternative lineage in which P descendants exhibit an abnormal genealogical relationship. Thus, even though normal leech development comprises a nearly invariant cell lineage, lineage relationships are open to considerable reorganization under experimental conditions.

Animals↗
PubMed1985

Cell lineage and segmentation in the leech.

Segments in the leech arise by the proliferation of longitudinally arrayed bandlets of blast cells derived from ten identifiable embryonic stem cells, two M, two N, four O/P and two Q teloblasts. In each bandlet, older blast cells lie ahead of those born later. By using microinjected cell lineage tracers it was shown previously that the teloblasts give rise to characteristic cell patterns made up of segmentally iterated complements of progeny designated as M, N, O, P and Q kinship groups. When a teloblast is injected after it has begun generating blast cells, a boundary is observed later in development between anterior, unlabelled progeny of blast cells produced before injection and posterior, labelled progeny of blast cells produced after injection. We have examined such boundaries in detail to establish the precise relationship between blast cell clones and segments, with the following conclusions: (i) in the M, O and P cell lines, one blast cell generates one segmental complement of progeny, but serially homologous blast clones intermix so that no segment boundaries can be defined based on primary blast cell clones; (ii) in the N and Q cell lines, two blast cells are required to generate a complete segmental complement of progeny; (iii) in the process of forming the germinal plate, cells derived from the N and Q teloblasts move past those derived from the M and O/P teloblasts, so that consegmental blast cell clones do not come into register until well after the establishment of segmentally iterated units within each bandlet.

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PubMed1984

Stepwise commitment of blast cell fates during the positional specification of the O and P cell lines in the leech embryo.

The o and p bandlets of the leech embryo are parallel columns of ectodermal blast cells which are identified by their relative positions, and which during normal embryogenesis follow distinct developmental pathways. A previous study showed that o blast cells are initially capable of following either the O or P pathway, and suggested that commitment to the O pathway depends upon interaction with the adjacent p bandlet. To better understand the nature and timing of this interaction we examined the fate of o blast cells whose p blast cell neighbors had been selectively ablated by photoexcitation of a fluorescent lineage tracer. If an o blast cell has not yet begun its secondary divisions, its normal commitment to the O pathway can be effectively prevented by ablation of the adjacent p bandlet. Comparing the outcome of progressively later lesions reveals that the progeny of the o blast cell become committed to the O pathway in a series of three discrete steps, and that these steps occur around the time of the first three blast cell divisions. Each of the three events affects a different subset of elements within the blast cell clone, and apparently commits those elements to either the O or P pathway depending upon the presence or absence of the other bandlet. These changes in blast cell fate are coextensive with the lesion along the bandlet's length, suggesting that the interaction of the two bandlets is localized to neighboring cells.

Animals↗
PubMed1983

Sensory receptor differentiation and axonal pathfinding in the cercus of the grasshopper embryo.

An immunological probe selective for insect neurons (L.Y. Jan and Y.N. Jan, 1982, Proc. Nat. Acad. Sci. USA 79, 2700-2704) was used to characterize the genesis of sensory neurons and the formation of the peripheral nerves in the grasshopper cercus. During embryogenesis the cercal ectoderm produces a characteristic set of sensory neurons in a precise spatiotemporal order. The first neurons migrate from the epidermis into the lumen and send out axons toward the CNS along the epidermal wall. These luminal neurons arise in four distinct groups, each of which establishes a separate branch of the cercal nerve, with the axons of the three distal groups converging onto the cell bodies of the more proximal neurons and thus seeming to use them as an intermediate target on route to the CNS. Epidermal neurons, whose cell bodies remain within the epithelium, begin to appear at a later stage. These cells come to innervate external sensory hairs, and in general their axons grow to the CNS along the preexisting nerves. Each sensory hair possesses two nonneuronal cells--the trichogen (shaft-forming) and tormogen (socket-forming) cells--which also stain with the antibody and begin to display immunoreactivity at the same time as the allied sensory neuron. The trichogen and tormogen cells do not form the hair shaft and socket until much later, with outgrowth occurring in an order quite distinct from that in which the receptors undertook their initial, biochemical differentiation. Thus, these two aspects of trichogen/tormogen differentiation appear to be under separate developmental control.

Animals↗
PubMed1981

Development of a sensory afferent projection in the grasshopper embryo. I. Growth of peripheral pioneer axons within the central nervous system.

The grasshopper's cercal nerve is established early in embryogenesis by an identified pair of peripheral neurons called the cercal pioneers. Like the peripheral pioneer neurons in other insect appendages, these two cells send their axons from the periphery to the rudimentary CNS and thus lay the foundation for a nerve that will later be followed by a large number of sensory axons. In this paper, cobalt fills of the primordial cercal nerve were used to characterize the disposition of these peripheral pioneer axons within the embryonic CNS. The pioneer axons stained by this technique terminate in ellipsoidal growth cones which have filopodia radiating from the leading edge and a single long terminal filament pointing along the path the axon is taking. The growing axons also bear filopodia along their sides, but these structures disappear as the cells mature. The pioneer axons of the cercal nerve make an abrupt turn where they first enter the ganglion rudiment and join the axons of the primary longitudinal tract. The pioneers then grow along this tract for several hundred microns without forming secondary growth cones or branches. This prolonged absence of central arborization distinguishes the peripheral pioneer axons from the axons of later-arising epidermal sensory neurons.

Animals↗
PubMed1981

Development of a sensory afferent projection in the grasshopper embryo. II. Growth and branching of peripheral sensory axons within the central nervous system.

The morphogenesis of several types of sensory axon branching patterns has been described by cobalt filling the cercal nerve of the grasshopper embryo at a series of different stages in development, thus staining the earliest sensory axons as they grow through the CNS. This embryonic sensory projection contains all five types of cercal afferents seen in the adult, and no new sensory tracts are added during postembryonic life. When the embryonic sensory axons first follow their pioneer axons into the neuropil they choose pathways which are characteristic of the adult sensory tracts. Since the afferents follow these paths without sending collaterals into the other tracts, it appears that the growth axon chooses its specific pathway without extensive exploration of alternative routes. Likewise, nearly all of the branches which arise from the embryonic sensory axons remain within the eventual domain characteristic of each cell type. This precise, determinate pattern of initial growth implies that the sensory axons are guided through the neuropil and achieve their final branching patterns with a minimum of overgrowth and pruning. The fact that initial growth is so precise also suggests that the parameters which guide the growing axon may help to determine its eventual pattern of synaptic connectivity by limiting its physical access to large portions of the neuropil which contain potentially compatible synaptic partner cells. Two different types of neurons may be supplying the sensory afferents with guidance cues: (i) Although most of the cercal sensory axons diverge from the cercal pioneer axons within the CNS, some sensory afferents continue to follow the pioneers through several ganglia. (ii) In the adult, a large number of the cercal sensory axons form a hollow shell of arborization around the main dendrite of an identified synaptic target cell, the Medical Giant Interneuron (MGI). This structure, the interneuron dendrite and the shell of sensory arbor, is called the cercal glomerulus. Since the MGI's dendrite is already present at the stage when the first sensory axons enter the CNS, interactions between these cells could serve to guide the glomerular sensory axons away from the pioneers into their future tracts.

Animals↗
PubMed1979

Quantitative staging of embryonic development of the grasshopper, Schistocerca nitens.

During development of the grasshopper embryo, it is feasible to examine the structure, pharmacology, and physiology of uniquely identified cells. These experiments require a fast, accurate staging system suitable for live embryos. We present a system comprising (1) subdivision of embryogenesis into equal periods, (2) expression of stage in percent of complete embryogenesis time, (3) characterization of stages by light micrographs (and descriptive test), and (4) illustration of stages at the egg, embryo, and limb levels of resolution. Advantages of a percent-system include communicability, flexibility in temporal resolution, accurate assignment of elapsed time in developmental processes, and uniform coverage of the period of embryogenesis. The stages described are at 5% intervals with an estimated error of +/- 1%.

Animals↗
PubMedDate not supplied

Positional determination of supernumerary blast cell death in the leech embryo.

Segmented animals are divided into a longitudinal array of developmentally homologous subunits known as metameres. The embryonic origin of the segmental body plan has been studied in a variety of organisms, with particular emphasis on the mechanisms underlying the delineation of the individual metameres and their secondary diversification. I have examined the embryonic events which determine the total number of segments in the glossiphoniid leech Helobdella triserialis. The germinal band of the leech consists of chains of segmental founder cells, called blast cells, and is normally reduced to 32 segments by the degeneration of supernumerary blast cells located at its caudal end. By using a novel technique for selective cell ablation, the segmental register of the four ectodermal cell lines can be altered so that lineally identifiable blast cells take part in the formation of ectopic segments. I show here that the survival or death of ectodermal blast cells is determined by position independent of their cell lineage identities, implying that the final number of segments is imposed on the ectoderm by interactions with the other embryonic tissues.

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