Somitogenesis in amphibia

Development ◽  
1983 ◽  
Vol 76 (1) ◽  
pp. 157-176
Author(s):  
Tom Elsdale ◽  
Duncan Davidson
Keyword(s):  
Central Region ◽  
Tail Bud ◽  
Temperature Shock ◽  
Anteroposterior Axis ◽  
Tissue Change ◽  
Narrow Zone ◽  
Tail Tip

Following neurulation, the frog segments c.40 somites and concurrently undergoes a striking elongation along the anteroposterior axis. This elongation (excluding the head) is largely the result of a presegmental extension of posterior tissue with a lesser contribution from the extension of segmented tissue. Presegmental extension is entirely the result of activity within a narrow zone of extension that occupies the central region in the tail bud. Within the zone of extension, a minimum of six prospective somites undergo an eight- to ten-fold extension along the axis. The zone passes posteriorly across the tissue of the tail tip. The anterior of the tail bud contains three extended prospective somites in the course of segmentation. The anterior boundary of the zone of extension coincides in space exactly with the anterior boundary of the zone of abnormal segmentation that results from temperature shock. This means that extension ceases immediately before the sudden tissue change associated with segmentation.

A developmental pathway controlling outgrowth of the Xenopus tail bud

Development ◽  
1999 ◽  
Vol 126 (8) ◽  
pp. 1611-1620 ◽  
Author(s):  
C.W. Beck ◽  
J.M. Slack
Keyword(s):  
Normal Development ◽  
Posterior Part ◽  
Neural Plate ◽  
General Mechanism ◽  
Developmental Pathway ◽  
Tail Bud ◽  
Neural Tubes ◽  
Tailbud Stage ◽  
Tail Tip ◽  
Host Embryo

We have developed a new assay to identify factors promoting formation and outgrowth of the tail bud. A piece of animal cap filled with the test mRNAs is grafted into the posterior region of the neural plate of a host embryo. With this assay we show that expression of a constitutively active Notch (Notch ICD) in the posterior neural plate is sufficient to produce an ectopic tail consisting of neural tube and fin. The ectopic tails express the evenskipped homologue Xhox3, a marker for the distal tail tip. Xhox3 will also induce formation of an ectopic tail in our assay. We show that an antimorphic version of Xhox3, Xhox3VP16, will prevent tail formation by Notch ICD, showing that Xhox3 is downstream of Notch signalling. An inducible version of this reagent, Xhox3VP16GR, specifically blocks tail formation when induced in tailbud stage embryos, comfirming the importance of Xhox3 for tail bud outgrowth in normal development. Grafts containing Notch ICD will only form tails if placed in the posterior part of the neural plate. However, if Xwnt3a is also present in the grafts they can form tails at any anteroposterior level. Since Xwnt3a expression is localised appropriately in the posterior at the time of tail bud formation it is likely to be responsible for restricting tail forming competence to the posterior neural plate in our assay. Combined expression of Xwnt3a and active Notch in animal cap explants is sufficient to induce Xhox3, provoke elongation and form neural tubes. Conservation of gene expression in the tail bud of other vertebrates suggests that this pathway may describe a general mechanism controlling tail outgrowth and secondary neurulation.

Somitogenesis in amphibia

Development ◽  
1979 ◽  
Vol 53 (1) ◽  
pp. 245-267
Author(s):  
Tom Elsdale ◽  
Murray Pearson
Keyword(s):  
Refractory Period ◽  
Behavioural Change ◽  
Paraxial Mesoderm ◽  
Gastrula Stage ◽  
Tail Bud ◽  
Temperature Shock ◽  
Sensitive Periods ◽  
Set Up ◽  
Specific Length

A somite pre-pattern is established shortly before visible segmentation. The pre-pattern results from the interaction of two components: a wave of cell behavioural change that passes along the axis, and, an underlying co-ordination of the cells that is the basis for their association into large somite-sized groupings. The evidence is derived from studies of the zones of abnormal segmentation that follow temperature shocks delivered between the neurula and tail-bud stages (Pearson & Elsdale, 1979). Temperature shock given earlier at the mid-gastrula stage is however ineffective in inducingabnormalities in somitogenesis. Shocks given before the mid-gastrula stage reveala prior period of sensitivity stretching back into the blastula. Thus early and late sensitive periods can be defined separated by a short refactory period. Quite different patterns in the distribution of somite abnormalities characterize the results of shock during the two sensitive periods, suggesting different aetiologies. It is concluded that the wave of rapid cell change is set up early in embryogenesis during theblastula stage, and each cell of the prospective paraxial mesoderm carries a determination to change after a specific length of time, i.e. a countdown is set in each cell. As a result of the movements of gastrulation, the prospective paraxial mesoderm cells become laid out along the axis of the neurula in the order (antero-posterior sequence) in which they will change. The achievement of the correct redistribution of the cells depends crucially on the conservation of the sequence in the blastula by the maintenanceof topological integrity throughout gastrulation. It is suggested that early shock disturbs gastrulation movements, causing some mixing up of the cells resulting in incoherenceof the wavefront. Whereas early shocks are thus assumed to affect the wave, the evidencesuggests that late shock undoes co-ordination. It is concluded therefore that co-ordination is established later, after the refractory period, around the late gastrula stage.

Somitogenesis in amphibian embryos

Development ◽  
1980 ◽  
Vol 58 (1) ◽  
pp. 107-118
Author(s):  
Jonathan Cooke ◽  
Tom Elsdale
Keyword(s):  
Ambient Temperature ◽  
Relative Rate ◽  
Recovery Period ◽  
The Body ◽  
Tissue Temperature ◽  
Tail Bud ◽  
Cellular Activation ◽  
Temperature Shock ◽  
Spatial Periodicity ◽  
Post Shock

Temperature shocks of a few minutes duration at 37 °C to tail-bud embryos of Rana induce zones of abnormal segmentation along the somite files subsequently produced. The immediate result of a temperature shock is a temporary arrest of development as a whole, following which the schedule of somite determination and formation is resumed at the normal rate. It is during the period immediately following this that the zone of abnormal somite pattern is determined. Thus the length of the abnormal zone reflects the total time taken by the morphogenetic system to recover from the disturbance, and might depend upon variables affecting both the duration of the initial arrest and the duration of the recovery period itself. Observations are presented demonstrating how the lengths of abnormal zones, caused by a temperature shock of any particular severity, are affected by three variables: (1) the ambient temperature to which the embryos were adapted before shock, (2) the ambient temperature of post-shock development, (3) the stage in somitogenesis, i.e. the number of somites already formed at the time of shock. The data (in this and previous papers of the series) support models postulating that the spatial periodicity in cell behaviour, that is somite morphogenesis, reflects a normal interaction between two hidden aspects of development, one a wavefront of cellular activation passing down the body axis, and the other having the character of a temporal periodicity throughout the tissue. Temperature shock, as well as halting the wavefront (i.e. stopping development) temporarily, leads to a subsequent period during which there is only gradual recovery of normal co-ordination between the periodicity within cells of the tissue and the wavefront progress. It is the relative rate of this recovery, alone, that is responsible for variation in the length of the abnormal zone.

Interstellar gas in the central region of the Galaxy

1967 ◽  
Vol 31 ◽  
pp. 239-251 ◽  
Author(s):  
F. J. Kerr
Keyword(s):  
Central Region ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Interstellar Gas ◽  
Hydrogen Recombination ◽  
The Galaxy ◽  
Centre Region ◽  
The Relationship

A review is given of information on the galactic-centre region obtained from recent observations of the 21-cm line from neutral hydrogen, the 18-cm group of OH lines, a hydrogen recombination line at 6 cm wavelength, and the continuum emission from ionized hydrogen.Both inward and outward motions are important in this region, in addition to rotation. Several types of observation indicate the presence of material in features inclined to the galactic plane. The relationship between the H and OH concentrations is not yet clear, but a rough picture of the central region can be proposed.

Access to effective ABA treatment in Ohio: Central region

2004 ◽  
Author(s):  
Christine Averill ◽  
Jacquie Wynn
Keyword(s):  
Central Region

Bleeding Time in Rats: A Comparison of Different Experimental Conditions

1982 ◽  
Vol 48 (01) ◽  
pp. 108-111 ◽  
Author(s):  
Elisabetta Dejana ◽  
Silvia Villa ◽  
Giovanni de Gaetano
Keyword(s):  
Platelet Function ◽  
Bleeding Time ◽  
Platelet Dysfunction ◽  
Experimental Conditions ◽  
Platelet Number ◽  
Coagulation Defects ◽  
Tail Tip ◽  
And Function ◽  
Storage Pool Disease ◽  
Type Of Anaesthesia

SummaryThe tail bleeding time (BT) in rats definitely varies according to the method applied. Of the various variables that may influence BT, we have evaluated the position (horizontal or vertical) of the tail, the environment (air or saline), the temperature (4°, 23° or 37° C) and the type of anaesthesia. Transection of the tail tip cannot be used to screen drugs active on platelet function since it is sensitive to coagulation defects. Template BT in contrast is not modified by heparin and is sensitive to defects of platelet number and function (“storage pool disease”, dipyridamole-like drugs, exogenous prostacyclin). In contrast the test fails to detect aspirin-induced platelet dysfunction. The evidence reported indicates that thromboxane A2-prostacyclin balance is not a factor regulating BT. Aspirin treatment however may be a precipitating factor when associated with other abnormalities of platelet function.Template BT is a valid screening test for platelet disorders and for antiplatelet drugs.

DWI identifies tissue change due to prolonged ictal activity in stroke-related epilepsy

2005 ◽  
Vol 32 (S 4) ◽  
Author(s):  
K Szabo ◽  
J Hirsch ◽  
B Pohlmann-Eden ◽  
O Sedlaczek ◽  
M Griebe ◽  
...  
Keyword(s):  
Tissue Change ◽  
Ictal Activity

The excavation of a disused military road at Buchlyvie, Central Region.

1994 ◽  
Vol 19 (1) ◽  
pp. 101-106 ◽  
Author(s):  
R. Page ◽  
C. Page
Keyword(s):  
Central Region ◽  
The Road

Summary Excavation of a road suspected to be Roman revealed a massive foundation surmounted by a flimsy upper road that had been little used except for cart traffic. The road was apparently part of the Stirling to Dumbarton military road, constructed between 1771 and 1780, one ofthe last military roads built in Scotland.

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