A novel energy filter using semiconductor superlattices and itsapplication to tunneling time calculations

An artificial quantum-mechanical filter using superlattice structures is proposed in this paper. By gradually changing the barrier widths of a superlattice according to a Gaussian function, a broad-band and almost zero sidelobe transmission profile can be obtained. The WKB approximation is applied to demonstrate the phenomena of abrupt change of transmission profile. The proposed structure allows the incident electrons to be nearly totally transmitted when the impinging electron energy is in the passband. On the other hand, a complete reflection occurs when the impinging energy is in the stopband. By adjusting the structure parameters, the desired passband and stopband of such a filter can be obtained. Time evolution of an electron wavepacket moving through the structure is calculated by numerically solving the time-dependent Schrodinger equation. Numerical results clearly demonstrate the characteristics of total transmission and reflection. By simulating the movement of a totally transmitted wavepacket, ambiguity results from the nature of the wavepacket in the determination of electron tunneling time can be avoided. The generalized concept of matched quantum-mechanical wave impedance (QMWI) analogous to transmission line theory is presented to explain the occurrence of total transmission of the proposed structure. The tunneling time (τ_QMWI) calculated based on the concept of QMWI is compared with the accurate tunneling time obtained by our simulation     

Allelic deletions on chromosome 11q13 in multiple endocrine neoplasia type 1-associated and sporadic gastrinomas and pancreatic endocrine tumors

Endocrine tumors (ETs) of pancreas and duodenum occur sporadically and as a part of multiple endocrine neoplasia type 1 (MEN1). The MEN1 tumor suppressor gene has been localized to chromosome 11q13 by linkage analysis but has not yet isolated. Previous allelic deletion studies in enteropancreatic ETs suggested MEN1 gene involvement in tumorigenesis of familial pancreatic ETs (nongastrinomas) and sporadic gastrinomas. However, only a few MEN1-associated duodenal gastrinomas and sporadic pancreatic nongastrinomas have been investigated. We used tissue microdissection to analyze 95 archival pancreatic and duodenal ETs and metastases from 50 patients for loss of heterozygosity (LOH) on 11q13 with 10 polymorphic markers spanning the area of the putative MEN1 gene. Chromosome 11q13 LOH was detected in 23 of 27 (85%) MEN1-associated pancreatic ETs (nongastrinomas), 14 of 34 (41%) MEN1-associated gastrinomas, 3 of 16 (19%) sporadic insulinomas, and 8 of 18 (44%) sporadic gastrinomas. Analysis of LOH on 11q13 showed different deletion patterns in ETs from different MEN1 patients and in multiple tumors from individual MEN1 patients. The present results suggest that the MEN1 gene plays a role in all four tumor types. The lower rate of 11q13 LOH in MEN1-associated and sporadic gastrinomas and sporadic insulinomas as compared to MEN1 nongastrinomas may reflect alternative genetic pathways for the development of these tumors or mechanisms of the MEN1 gene inactivation that do not involve large deletions. The isolation of the MEN1 gene is necessary to further define its role in pathogenesis of pancreatic and duodenal ETs.     

Cryopreservation of single human spermatozoa

A procedure is described that allows cryopreservation and efficient post-thaw recovery of either a single or a small group of human spermatozoa. This is achieved by injecting them into cell-free human, mouse or hamster zonae pellucidae before the addition of cryoprotectant. The method involves a combination of physical micromanipulation procedures and glycerol-mediated cryoprotection. Zonae were tracked by positioning them in straws between two small air bubbles prior to freezing. Spermatozoa from poor specimens were cryopreserved and their fertilizing ability after thawing was compared with that of fresh spermatozoa from fertile men. Human eggs used for fertilization testing were either 1 day old or in-vitro matured. Only 2% of the frozen zonae were lost and >75% of spermatozoa cryopreserved in this manner were recovered and prepared for intracytoplasmic sperm injection. The feasibility of cryopreserving a single spermatozoon was assessed. Fifteen motile spermatozoa were frozen in 15 zonae, of which 14 were recovered after thawing. Ten were injected into spare eggs, of which eight became fertilized. Spermatozoa recovered mechanically from human zonae fertilized the same proportion of oocytes as fresh fertile control spermatozoa. The recovery and fertilization rates with spermatozoa frozen in animal zonae were 87 and 78% respectively. The fertilization rate was marginally higher (P < 0.05) than that for spermatozoa frozen in human zonae, perhaps because the latter may have acrosome reacted more frequently. The zona pellucida appears to be an ideally suited sterile vehicle for storage of single spermatozoa.     

Complement-inhibiting peptides identified by proximity to indels in the C3/4/5 protein family

To find protein-protein interactive sites in complement component C3, we examined regions of C3 that are proximal to sites of length polymorphism or indels in the C345 protein family. We reasoned that indels probably mark protein interactive sites because they usually involve residues at protein surfaces. To test for the involvement of individual indels, we examined the effects on complement function of synthetic peptides corresponding to indel-proximal segments of C3. We inferred that if such a segment made direct contact with a C3 binding protein, then the corresponding peptide might also bind to that protein and inhibit binding to C3. Twenty-one peptides were tested; four of these inhibited complement-mediated erythrocyte lysis at < or =100 microM and complement-mediated killing of Escherichia coli at about threefold higher levels. These results indicate that the four peptides act as specific inhibitors of complement. They also suggest that indels can be effective guides for locating interactive sites in C3 and in any protein that is a member of a protein family. Because only a linear sequence is required, a focus on indels may be particularly useful for identifying interactive sites in proteins for which a three-dimensional structure is unavailable.     

Endovascular surgical approach to intracranial vascular diseases

PURPOSE: The endovascular surgical approach to complex disorders of the central nervous system has made rapid and significant advancements over the past decade. Patients with intracranial arterial aneurysms, traumatic carotid and vertebral artery lesions, including fistulas and pseudoaneurysms, hemodynamically significant atherosclerotic lesions, vasospasm, and acute stroke are now being approached and treated by newer and less invasive techniques, including cerebral angioplasty and thrombolytic therapy. METHODS: All procedures are usually performed from a transfemoral approach utilizing a variety of occlusion devices, including detachable silicone balloons, microcoils, electrolytic detachable coils, liquid tissue adhesives, and particulate emboli for vessel occlusion. For dilatation and reperfusion of vessels, balloon angioplasty catheters, stents, and thrombolytic drugs are being used. RESULTS: For the treatment of traumatic vascular injuries, such as carotid cavernous sinus fistulas and vertebral arteriovenous fistulas and pseudoaneurysms, endovascular therapy has become the treatment of choice. The endovascular approach for intracranial aneurysms is emerging as a therapeutic option in selected cases. For occlusive disorders in patients presenting with acute cerebral ischemia, extracranial angioplasty and cerebral thrombolysis techniques are currently under investigation. CONCLUSIONS: As these techniques continue to evolve, the field of interventional neuroradiology will expand the therapeutic options for managing complex cerebrovascular disorders and improve patient outcome in acute stroke therapy.