Signalling between the hindbrain and paraxial tissues dictates neural crest migration pathways.

Cranial neural crest cells are a pluripotent population of cells derived from the neural tube that migrate into the branchial arches to generate the distinctive bone, connective tissue and peripheral nervous system components characteristic of the vertebrate head. The highly conserved segmental organisation of the vertebrate hindbrain plays an important role in patterning the pathways of neural crest cell migration and in generating the distinct or separate streams of crest cells that form unique structures in each arch. We have used focal injections of DiI into the developing mouse hindbrain in combination with in vitro whole embryo culture to map the patterns of cranial neural crest cell migration into the developing branchial arches. Our results show that mouse hindbrain-derived neural crest cells migrate in three segregated streams adjacent to the even-numbered rhombomeres into the branchial arches, and each stream contains contributions of cells from three rhombomeres in a pattern very similar to that observed in the chick embryo. There are clear neural crest-free zones adjacent to r3 and r5. Furthermore, using grafting and lineage-tracing techniques in cultured mouse embryos to investigate the differential ability of odd and even-numbered segments to generate neural crest cells, we find that odd and even segments have an intrinsic ability to produce equivalent numbers of neural crest cells. This implies that inter-rhombomeric signalling is less important than combinatorial interactions between the hindbrain and the adjacent arch environment in specific regions, in the process of restricting the generation and migration of neural crest cells. This creates crest-free territories and suggests that tissue interactions established during development and patterning of the branchial arches may set up signals that the neural plate is primed to interpret during the progressive events leading to the delamination and migration of neural crest cells. Using interspecies grafting experiments between mouse and chick embryos, we have shown that this process forms part of a conserved mechanism for generating neural crest-free zones and contributing to the separation of migrating crest populations with distinct Hox expression during vertebrate head development.     

植物的遗传转化及其应用(英文)

近年来,植物遗传转化研究有了长足的发展。已经达到能够通过简单的遗传控制手段研究具有新表现型的植物,甚至达到进入商业化的程度。这些手段包括植物生物学的主要研究技术以及植物组织培养和树种改良的一些实用方法。尽管采用农瘤杆菌和鸟枪法等技术的植物遗传转化系统已经得到了广泛的应用,但是在如何开发具有能够得到控制表达的转基因高产植物方面,在如何使所得到的转基因植物远离遗传危害等方面,目前的转化系统遇到了极大的技术挑战。已经提出了各种各样的方法用于将新基因稳定地导入120多种不同植物的核基因组。本文将讨论这些遗传转化系统所需的生物学要求和实际应用方面的需求、基因转化和转基因表达的研究策略、遗传转化植物的鉴定以及转基因植物与大众的认可。本文将分为七个部分加以讨论:一、导言;二 、基因转化到细胞里的方法;三、植物遗传转化策略;四、植物遗传转化的鉴定;五、植物遗传转化的实际应用;六、转基因植物与环境;七、未来植物遗传转化的需求与发展方向。     

Novel bacteriolytic activity associated with the style microflora of the mussel Mytilus edulis (L.)

Previous studies have shown that bacterioplankton are retained by the mussel Mytilus edulis (L.), and that lysozyme-like enzymes are associated with the crystalline style of the mussels. This study has demonstrated the presence of a lytic agent which is produced by bacteria associated with the crystalline style. The production of the agent is inhibited by chloramphenicol, and anaerobic growth conditions. The agent is heat-sensitive and nonfilterable. Attempts at the isolation of the agent in a cell-free extract have been unsuccessful. The presence of bacteria-like cells in the outer laminae of the crystalline style, coupled with the nonfilterable nature of the lytic agent, make it seem possible that these bacteria are responsible for lysis of gram-negative bacteria which are taken in with the food supply.     

Concerns of patients,MDs are transforming transfusion medicine.

Fear of HIV and AIDS has been the driving force in reducing physicians'' use of blood and blood products. Nancy Robb interviewed doctors across the country to determine steps they are taking to lower the number of transfusions and discovered that transfusion medicine in Canada has undergone a sea change.     

Offshore energy boom providing opportunities outside medicare's umbrella

Physicians upset by limits imposed by the medicare system are getting a chance to spread their entrepreneurial wings on the East Coast. A boom in offshore exploration, led by Newfoundland''s massive Hibernia project, has led to numerous business opportunities for physicians.     

Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates

Original studies leading to the gravitationalmodel of pulmonary blood flow and contemporary studies showinggravity-independent perfusion differ in the recent use of laboratoryanimals instead of humans. We explored the distribution ofpulmonary blood flow in baboons because their anatomy, serialdistribution of vascular resistances, and hemodynamic responses tohypoxia are similar to those of humans. Four baboons wereanesthetized with ketamine, intubated, and mechanically ventilated.Different colors of fluorescent microspheres were given intravenouslywhile the animals were in the supine, prone, upright (repeated), andhead-down (repeated) postures. The animals were killed, and their lungswere excised, dried, and diced into~2-cm³ pieces with the spatialcoordinates recorded for each piece. Regional blood flow was determinedfor each posture from the fluorescent signals of each piece. Perfusionheterogeneity was greatest in the upright posture and least when prone.Using multiple-stepwise regression, we estimate that 7, 5, and 25% ofperfusion heterogeneity is due to gravity in the supine, prone, andupright postures, respectively. Although important, gravity is not thepredominant determinant of pulmonary perfusion heterogeneity in uprightprimates. Because of anatomic similarities, the same may be true for humans.