Since the discovery of nerve growth factor, it has been thought that neurotrophic factors are released or secreted from target cells. However, more recently it has been suggested that a specific neurotrophic factor known as brain-derived neurotrophic factor (BDNF) may reach target cells directly from presynaptic axons. It has not been known how these molecules get from the neuron in which they are produced to the target cells. Keigo Kohara, Akihiko Kitamura, Mieko Morishima, and Tadaharu Tsumoto have demonstrated that BDNF is transported anterogradely from presynaptic neurons to target neurons.

Some may have read my previous articles in Microscopy Today and know that I am a strong proponent of digital imaging for photomicrographs and photomacrographs. I require that the digital images match the resolution and field size of traditional film images, with the 4X5 Polaroid film most commonly used in metallurgical laboratories where I worked before early retirement. My development of an affordable universal student microscope for home microscopy resulted in a need for color digital images that would meet my requirements at an affordable cost. This need was met by recording on 35 mm film with subsequent scanning of selected film images to a Kodak Master Photo CD. More recently I purchased a CanoScan FS2710 film scanner for digitizing my family's collection of slides and negatives for saving on CD's.

Gluteraldehyde (GA) has proven to be an extraordinarily versatile product. Its many uses range from softening leather and paper to disinfecting and sterilizing hospital instruments. It is used, of course, as a fixative in preparing biological samples for microscopic investigations that most readers of this publication will be familiar with. Even though quality questions relating to GA have been well researched and documented, there is still enough disagreement between industry experts to stimulate an enthusiastic discussion on microscopic list servers.

Recently, there was a thread on the microscopy listserver about focusing TEMs, in particular whether to focus at crossover, or at the illumination level used for recording images. When setting the image focus there are a number of problems that may complicate matters.

Many of the SEM specimens which we are asked to deal with are highly three-dimensional non-conductive objects, such as silica beads, pollen grains and spores, small inorganic and organic crystals such as zeolites and drug preparations, starch grains, etc. These often cause severe charging problems which result in all of the familiar and tiresome charge-related image artefacts, such as flare, excessive contrast, specimen movement, and variable scan lines. Charging can also make it very difficult to obtain high-resolution images or do satisfactory analysis.

It is most important that any test specimens used to audit an instrument are from a traceable source and are certified by a recognised body.

Traditional methods for the evaluation of resolution in the scanning electron microscope rely upon a high-density particle upon a low-density substrate, the most popular method being evaporated gold on a carbon substrate. Although most manufacturers use this method, it is subject to abuse as there is no inbuilt magnification standard and therefore the evaluation of the image may only be made through measurement. To simply judge the specimen by appearance as most scientists do, could lead to a misleading result.

An ideal specimen for the evaluation of scanning electron microscope performance is dried polystyrene latex sputter coated with gold, or gold palladium for the evaluation of higher performance (e.g., field emission instruments) (Figure1). The specimen requires very pure polystyrene latex that is allowed to settle and dry down over a period of time sufficient for it to form a solid block. The block is fixed to a specimen stub with silver Dag and the adhesive allowed to dry.

Pressure scales can be extremely confusing to new operators. This is not surprising. To my mind, there are three primary areas of confusion.

Firstly, the pressure of gas inside an instrument changes over many orders of magnitude during pump-down. The change is about 9 orders of magnitude for a traditional Scanning Electron Microscope and about 13 orders of magnitude for an ultra-high vacuum instrument such as a Scanning Auger Microprobe.

Recently there was an exchange on the Confocal Listserver about the proper method for cleaning objective lenses. Some suggestions were useful and were some fraught with danger. This article is adapted from my reply to the list.

There are only three hard and fast rules about lens cleaning:

1. 1) Never “dry” clean a lens.

2. 2) Use a solvent or dispersant that is likely to remove whatever is on the lens.

3. 3) Use the best quality lens tissue available.

The most likely source of damage to glass surfaces and fine optical coatings is abrasion. The particles that cause the abrasion are the particles on the objective lens surface and the particles in whatever is used to wipe the lens.

The synthesis of inorganic materials of a specific size and shape is a key aspect in modern powder technology. During the past two decades, solution-based precipitation reactions have been explored for controlled particle formation. A wide variety of particle shapes, sizes, and compositions has been demonstrated. However, several questions, especially concerning particle nucleation, remain. Some of these questions are currently addressed by investigating precipitate (ex situ) at different reaction stages by means of powder x-ray diffraction (PXRD), scanning (SEM) and transmission electron microscopy (TEM).

I'm somewhat amazed by how many varied techniques there are for rotary shadowing, and how few seem to agree with what works for us. So, here is our method.

We shadow biological molecules from the connective tissue matrix, usually ranging in size from 16 to 300 kilodaltons. Many are linear but some are globular. We spray the molecules in solution with 70% glycerol. The other 30% is 100 microgram/mL of protein, preferably in a volatile buffer such as 1% acetic acid or 0.1M ammonium bicarbonate, pH 7.8. Other buffers can be used, but salt crystals can be a big problem.

Many scanning electron microscope (SEM) laboratories use double sticky carbon tape applied directly to SEM stubs as samplers for small particles. The SEM samplers used by law enforcement to test the hands of a suspect for gunshot residue (GSR) are constructed in this manner.

A few years ago, during a presentation at a professional meeting, I presented spectra of particles that were mounted on carbon double sticky tape that was applied directly to the aluminum- alloy SEM stubs. After the presentation a colleague posited that small peaks of copper, which appeared in some of my spectra, were due to a copper support frame for the backscatter detector at the top of the SEM chamber and not the particles. Indeed, I have frequently noticed small peaks of copper in spectra of particles where these should not be present.

Many organisms are mostly soft tissue, with hard bits embedded within them. This includes many invertebrates, and vertebrates with calcified areas in their tissues. This procedure works well for soft-bodied corals which have hard pieces of calcium carbonate (sclerites) embedded in their tissues. It should also work for similar soft-tissue/calcified bits in other animals.

Questions have arisen recently on the Microscopy Listserver about the best way to decapsulate integrated circuit chips. Here is the method that has been developed at the Dallas Device Analysis Operation of Texas Instruments, inc., and is used by many laboratories here. We find that it works very well.

This note concerns two very important parts of most beam instrument systems; diffusion pumps and water chillers. As we'll see below, the two can be intimately connected.

Many SEMs, TEMs and other electron beam instruments contain one or more diffusion pumps as part of their vacuum systems. These are usually vertically-oriented cylindrical objects, perhaps 30 cm high, wrapped in several turns of copper tubing. They are usually placed behind or below the instrument's column, and typically handle high vacuums for tungsten filaments, or backing for ion pumps with other emitter types. Generally, these units are fairly maintenance-free; a change of oil every few years may be all that is required.

There are two possibilites for plunge freezing in LN2: one is “normal” liquid nitrogen -- as it comes from the dewar, the other is slush nitrogen.

If using liquid nitrogen without slush, then something like isopentane, ethane, or propane (ordinary cooking propane will do) must be used. The caveat is that using these liquid gases is a serious fire and explosion hazard, especially since liquid oxygen forms at liquid nitrogen temperatures, and dissolves into the liquid hydrocarbon. These gases can be used safely (1 have done so) but they take care and understanding of what's happening, and a safe place to dispose of the cryogen.