In Vitro Techniques for Studies of Malpighian Tubules
Insect Malpighian tubules are extrordinarily useful for studies of virtually all facets of epithelial ion and water transport. Their greatest experimental advantage results from their geometry: tubules consist of a single layer of squamous epithelial cells that form a blind-ended cylinder. Secretion products, which form the luminal contents of the cylinder, can thus easily be kept separate from the bathing fluids in vitro. If a drop of bathing fluid is placed in 5–6 mm paraffin oil on wax, and the open end of the tubule is pulled out of the drop, the secretion issuing from the open end will form a second droplet that will adhere to a steel or glass pin by surface tension (Ramsay, 1954). The volume of the spherical droplet can be calculated from the formula 4/3πr 3, where r, the radius of the drop, is measured by an eyepiece graticule fitted to a dissecting microscope. Such straightforward methods circumvent much of the experimental interference and the need for elaborate experimental chambers entailed when studying fluid or solute movements across a flat sheet of epithelial cells.
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