Cell motility driving mediolateral intercalation in explants of Xenopus laevis

In Xenopus, convergence and extension are produced by active intercalation of the deep mesodermal cells between one another along the mediolateral axis (mediolateral cell intercalation), to form a narrower, longer array. The cell motility driving this intercalation is poorly understood. A companion paper shows that the endodermal epithelium organizes the outermost mesodermal cells immediately beneath it to undergo convergence and extension, and other evidence suggests that these deep cells are the most active participants in mediolateral intercalation (Shih, J. and Keller, R. (1992) Development 116, 887–899). In this paper, we shave off the deeper layers of mesodermal cells, which allows us to observe the protrusive activity of the mesodermal cells next to the organizing epithelium with high resolution video microscopy. These mesodermal cells divide in the early gastrula and show rapid, randomly directed protrusive activity. At the early midgastrula stage, they begin to express a characteristic sequence of behaviors, called mediolateral intercalation behavior (MIB): (1) large, stable, filiform and lamelliform protrusions form in the lateral and medial directions, thus making the cells bipolar; (2) these protrusions are applied directly to adjacent cell surfaces and exert traction on them, without contact inhibition; (3) as a result, the cells elongate and align parallel to the mediolateral axis and perpendicular to the axis of extension; (4) the elongate, aligned cells intercalate between one another along the mediolateral axis, thus producing a longer, narrower array. Explants of essentially a single layer of deep mesodermal cells, made at stage 10.5, converge and extend by mediolateral intercalation. Thus by stage 10.5 (early midgastrula), expression of MIB among deep mesodermal cells is physiologically and mechanically independent of the organizing influence of the endodermal epithelium, described previously (Shih, J. and Keller, R. (1992) Development 116 887–899), and is the fundamental cell motility underlying mediolateral intercalation and convergence and extension of the body axis

The epithelium of the dorsal marginal zone of Xenopus has organizer properties

We have investigated the properties of the epithelial layer of the dorsal marginal zone (DMZ) of the Xenopus laevis early gastrula and found that it has inductive properties similar to those of the entire Spemann organizer. When grafts of the epithelial layer of the DMZ of early gastrulae labelled with fluorescein dextran were transplanted to the ventral sides of unlabelled host embryos, they induced secondary axes composed of notochord, somites and posterior neural tube. The organizer epithelium rescued embryos ventralized by UV irradiation, inducing notochord, somites and posterior neural tube in these embryos, while over 90% of ventralized controls showed no such structures. Combinations of organizer epithelium and ventral marginal zone (VMZ) in explants of the early gastrula resulted in convergence, extension and differentiation of dorsal mesodermal tissues, whereas similar recombinants of nonorganizer epithelium and the VMZ did none of these things. In all cases, the axial structures forming in response to epithelial grafts were composed of labelled graft and unlabelled host cells, indicating an induction by the organizer epithelium of dorsal, axial morphogenesis and tissue differentiation among mesodermal cells that otherwise showed non-axial development. Serial sectioning and scanning electron microscopy of control grafts shows that the epithelial organizer effect occurs in the absence of contaminating deep cells adhering to the epithelial grafts. However, labelled organizer epithelium grafted to the superficial cell layer contributed cells to deep mesodermal tissues, and organizer epithelium developed into mesodermal tissues when deliberately grafted into the deep region. This shows that these prospective endodermal epithelial cells are able to contribute to mesodermal, mesenchymal tissues when they move or are moved into the deep environment. These results suggest that in normal development, the endodermal epithelium may influence some aspects of the cell motility underlying the mediolateral intercalation (see Shih, J. and Keller, R. (1992) Development 116, 901–914), as well as the tissue differentiation of mesodermal cells. These results have implications for the analysis of mesoderm induction and for analysis of variations in the differentiation and morphogenetic function of the marginal zone in different species of amphibians

No-Vacation Nation

This report reviewed international vacation and holiday laws and found that the United States is the only advanced economy that does not guarantee its workers any paid vacation or holidays. As a result, 1 in 4 U.S. workers do not receive any paid vacation or paid holidays. The lack of paid vacation and paid holidays in the U.S. is particularly acute for lower-wage and part-time workers, and for employees of small businesses. This report also includes a comparative appendix with information on paid leave and holiday laws in Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Japan, the Netherlands, New Zealand, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom

Bus Rapid Transit Systems: Expanding Transit Investment Alternatives at the Federal Level

Policymakers are looking at Bus Rapid Transit as a complement to current mass transit investment options, including light- and heavy-rail. The Transportation Equity Act for the 21st Century (TEA- 21), which provides six-year funding for highway and transit programs, including Major Capital Investment "New Starts" Projects, expired on September 30th. An extension was approved through February 29th. New Starts Projects are defined under current legislation as fixed-guideway systems (like rail systems) that significantly restructure land use and congestion patterns in an urban area. As policymakers look for ways to invest in BRT, the Department of Transportation is encouraging BRT eligibility under the New Starts Program. The proposal included in the "Safe, Accountable, Flexible and Efficient Transportation Equity Act of 2003" (SAFETEA), the policy recommendation for TEA-21 reauthorization put forth by the Bush Administration, provides funding under the New Starts Program to "non-fixed guideway" systems. The Congress will consider this as it debates SAFETEA through 2003 and 2004

Survey of anadromous fish spawning areas: completion report, project AFC-8 July 1970 - January 1975 for Potomac River drainage, Upper Chesapeake Bay drainage

This report summarizes Project AFC-8 ("Survey of Anadromous Fish Spawning Areas") stream investigation and improvement activities for the entire study period of July 1970 to January 1975. The four and one-half year study included four inter-related jobs or types of stream investigation: Literature and Data Review, Fishery Investigation, Stream Investigation, Data Summarization and Storage/Preparation of Report. (Document has 229 pages.

No-Vacation Nation Revisited

This report reviews the most recently available data from a range of national and international sources on statutory requirements for paid vacations and paid holidays in 21 rich countries (16 European countries, Australia, Canada, Japan, New Zealand, and the United States). In addition to our finding that the United States is the only country in the group that does not require employers to provide paid vacation time, we also note that several foreign countries offer additional time off for younger and older workers, shift workers, and those engaged in community service including jury duty. Five countries even mandate that employers pay vacationing workers a small premium above their standard pay in order to help with vacation-related expenses. Most other rich countries have also established legal rights to paid holidays over and above paid vacation days. We distinguish throughout the report between paid vacation -- or paid annual leave, terms we use interchangeably -- and paid holidays, which are organized around particular fixed dates in the calendar. Our analysis does not cover paid leave for other reasons such as sick leave, parental leave, or leave to care for sick relatives

The patterning and functioning of protrusive activity during convergence and extension of the Xenopus organiser

We discuss the cellular basis and tissue interactions regulating convergence and extension of the vertebrate body axis in early embryogenesls of Xenopus. Convergence and extension occur in the dorsal mesoderm (prospective notochord and somite) and in the posterior nervous system (prospective hindbrain and spinal cord) by sequential cell intercalations. Several layers of cells intercalate to form a thinner, longer array (radial intercalation) and then cells intercalate in the mediolateral orientation to form a longer, narrower array (mediolateral intercalation). Fluorescence microscopy of labeled mesodermal cells in explants shows that protrusive activity is rapid and randomly directed until the midgastrula stage, when it slows and is restricted to the medial and lateral ends of the cells. This bipolar protrusive activity results in elongation, alignment and mediolateral intercalation of the cells. Mediolateral intercalation behavior (MIB) is expressed in an anterior- posterior and lateral-medial progression in the mesoderm. MIB is first expressed laterally in both somitic and notochordal mesoderm. From its lateral origins in each tissue, MIB progresses medially. If convergence does not bring the lateral boundaries of the tissues closer to the medial cells in the notochordal and somitic territories, these cells do not express MIB. Expression of tissue-specific markers follows and parallels the expression of MIB. These facts argue that MIB and some aspects of tissue differentiation are induced by signals emanating from the lateral boundaries of the tissue territories and that convergence must bring medial cells and boundaries closer together for these signals to be effective. Grafts of dorsal marginal zone epithelium to the ventral sides of other embryos, to ventral explants and to UV-ventralized embryos show that it has a role in organising convergence and extension, and dorsal tissue differentiation among deep mesodermal cells. Grafts of involuting marginal zone to animal cap tissue of the early gastrula shows that convergence and extension of the hindbrain-spinal cord are induced by planar signals from the involuting marginal zone