The essentials of electron microscopy were invented between 1920 and 1955, and developed to a useful level in the 1960's. As a biological discipline, ultrastructure research emerged in the 1970's, and continues to evolve to this day. The basic premise is that photons of light that travel through the lenses of a light microscope are replaced by electrons. Electrons offer superior resolution, so the limit of resolution of a light microscope (around 0.2 micrometers or 0.2 µm) is increased by about 1000 times to around 0.2 nanometers (or 0.2 nm). Biological specimens do not allow a microscopist to use this limit of resolution very often, but resolution in the nanometer range is common in medical research and diagnostics today.

Electrons do not travel through matter, so a high vacuum is needed. Electrons are focussed by passing them through lenses that are hollow electromagnets. Electron microscopes are thus a confusing array of vacuum pumps, magnets, complex electronics and cooling systems to maintain an even temperature, but we are not interested in how they work, just that they produce very high resolution images of tissues and cells. They create images either by bouncing electrons off the surface of specimens (scanning electron microscopy or SEM), or by sending them through the specimen (transmission electron microscopy or TEM).

The images from an electron microscope are produced by the presence or absence of electrons, so they are only created in black and white. Any color seen in electron micrographs is artificially added either by computer enhancement of grey levels, or for artistic purposes. Some of the electron micrographs in this web site have had color added to simulate hematoxylin and eosin stains.

Electron microscopy is not routinely used in veterinary practice, but it is an essential part of research and diagnostics. The electron micrographs within this web site are selected to help you appreciate the roles of organelles within cells, and the integration of cells and extracellular materials into tissues.

	An arteriole is a small artery. The wall is composed of an internal endothelium, a middle smooth muscle cell layer, and outer connective tissue layer. At about 2 o'clock in the image is a small, non-myelinated nerve accompanying the vessel. Arterial blood pressure is maintained by arterioles such as this one; in turn arteriolar diameter is controlled by sympathetic nerves which affect the state of smooth muscle contraction.
	This micrograph was published in 1989 by Drs. Singh and Sims and a colleague. It shows an eosinophil within a hemal node. The eosonophil is actively engulfing a red blood cell, and merging it with a lysosome, to digest the red blood cell. Reference: Acta Anatomica, 134:341-345, 1989.
	If a secreting cell is making a product that is predominantly protein, the dominant organelle would be rough endoplasmic reticulum. In this case, the cell that is labelled is a goblet cell, which produces mucus. Mucus molecules have a protein core, but most of the side chains are complex carbohydrates. As a result, the dominant organelle is the golgi, from which maturing granules can be seen migrating toward the top of the cell for export. At the top (or apical end) of the cell, it is possible to see the brush border of microvilli arising from the other epithelial cells surrounding goblet cells.
	Hepatocytes (cells of the liver)store lipids within their cytoplasm. In this micrograph, a large lipid droplet is seen almost totally surrounded by the nucleus. This is an unusual, but not impossible, presentation. During osmication to preserve the lipid droplets, a dark border to lipid droplets may form. There is, however, no membrane covering lipid inclusions.
	This micrograph shows a small peripheral nerve. The outer margin or edge of the nerve is a connective tissue wrap of collagen and fibroblasts, the epineurium. This is the white material you see when dissecting a nerve. Within the epineurium is the collection of axons that comprise the anatomical nerve. In this case, most of the axons are surrounded by myelin sheaths. Myelin is a lipid-rich plasma membrane, which stains deep black with osmication. A few non-myelinated axons can also be seen.
	In contrast to the small peripheral nerve described above, this one is composed entirely of non-myelinated nerve axons. Epineurium still surrounds the nerve, but there are no myelin sheaths around individual axons.
	Here we have transmission electron micrographs of mammalian trachea. The epithelial surface of trachea has kinocilia which beat in a rhythmic manner to move mucus out of the lungs and into the pharynx for swallowing. The top photo is at lower magnification, and shows the mucus layer on top, a watery, fluid-filled zone with cross-sectioned cilia, & a microvillous zone at the surface of the epithelial cells. The lower photo is taken at higher magnification, and enables us to resolve organelles more easily. Basal bodies (or centrioles) are seen at the bases of the cilia, microtubules are seen within the cilia, microvilli are located between the cilia, and small mitochondria are within the epithelial cell cytoplasm.

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