Corning Transwell® Membrane Inserts are polycarbonate, polyester (PET) microporous membranes held in a well by an insert. The Transwell® system permits the study of cell migration and invasion through the membrane pores. The membranes are available in various diameters (e.g. 12 mm or 24 mm) and pore sizes (e.g. 12.0 µm or 8.0 µm), and come with an adapted multiwell plate (e.g. 6-well plate for 24 mm and 12-well plate for 12 mm). The smallest pore sizes (smaller than 3.0 µm) are preferably dedicated to drug transport studies.
I regularly use this product to study migration and invasion of mammary carcinoma cells MCF-7 and MDA-MB-231 cells. For cell migration, I first coat the membranes with a specific solution to strengthen cell attachment. Then I seed the cells on the membrane and incubate them in a normal media while the lower compartment is filled with media containing various chemoattractants (e.g. growth factors which are either added to the media or secreted by cells grown in the bottom of the well). To study cell invasion, I perform the same protocol used for the migration assay, however, I coat the membranes with Matrigel™ (from BD Biosciences) prior to seeding the cells. Coating the Matrigel™ myself instead of using Corning Transwell® membranes pre-coated with Matrigel™ allows me to control the Matrigel™ thickness and concentration. For both migration and invasion assays, I fix and stain the cells on the membrane following a proper incubation period. I next remove the cells on the upper side of the membrane, and finally count on the membrane the cells that have moved through the pores to the lower side.
The Transwell® Membrane Inserts are well-designed allow for a large range of experimental set-ups. For example, it is easy to change media in both the lower and upper compartments without removing the insert from the well. Moreover, the wide choice in membrane and pore sizes provides additional flexibility. Although they are well-designed, the inserts can be tricky to handle and manipulate; removing them from the well when wearing gloves is especially tricky. A simple piece of light plastic on the top edge of the insert would make it much easier to pull out the inserts.
12-well plates allow 4 experiments to be run in triplicate simultaneously, but produce only a small number of counted cells due to the small size of the membrane; these low counts can result in statistical inaccuracies. On the other hand, the 6-well plates give a larger number of counted cells, which is statistically more accurate, but reduces the number of simultaneous experiments. Furthermore, setting Matrigel™ on 12.0 µm pores membranes is tricky since the Matrigel™ can leak through the pores before gelling. This last point confers a pessimistic view concerning the isolation between the lower and the upper compartments when using this pore size, but seems an inherent difficulty in these types of assays and not specific to Corning Transwell® Membranes.
Finally, Corning includes membranes, inserts and multiwell plates in the pack, when other brands don’t always include the adapted multiwell plates. However, ordered plates are usually delivered in 8 to 11 weeks, which is 4 to 6 times longer than other manufacturers.