Roll-your-own endothelial tubes

Just one ingredient can make the difference between a delicious meal and an inedible mess even the dog won't touch. Likewise, 20 years ago the recipe for cultivating endothelial cells lacked a vital factor, and many a dish of cells ended up in the trash. The vascular biologists who nabbed the missing ingredient not only simplified the process of growing endothelial cells; their discovery was also a boon for angiogenesis research.

The early to mid-1970s saw a spate of cell-rearing successes, as biologists nurtured primary cultures of endothelial cells (Gimbrone et al., 1974), liver parenchymal cells (Bissell et al., 1973), sympathetic neurons (Mains and Patterson, 1973), and smooth muscle cells (Ross, 1971). Other workers managed to raise secondary cultures of smooth muscle cells (Schubert et al., 1974) and pioneered two- and three-dimensional collagen substrates (Elsdale and Bard, 1972). But as Michael Stemerman (University of California, Riverside) recalls, human endothelial cells remained tricky to cultivate—particularly cells from the most readily available source: the umbilical vein. Researchers tried hard to be good hosts, tempting the cells with sumptuous beds of fibronectin, a tantalizing broth of calf's serum, and a mixture called medium 199. But the ungrateful cells usually died after two or three passages.

Finding what the cells craved was a matter of plying them with one growth factor after another, Stemerman says. One additive the scientists tried was endothelial cell growth factor (ECGF), which they isolated from the hypothalamuses of cattle. Previous work had suggested that ECGF, now known as fibroblast growth factor 1, stimulated endothelial cells, and indeed it galvanized the cultures. Instead of perishing after three passages, the cultures were vibrant after more than 20 (Maciag et al., 1981). “For the first time, we showed that you could propagate these cells almost indefinitely,” Stemerman says. “We were shocked.”

The cultures would surprise the team again. Withholding ECGF and fibronectin, the researchers discovered, spurred the cells to roll up into tiny tubes (Maciag et al., 1982). Within a month to six weeks, the tubes would branch into a complex network, creating the beginnings of a capillary tree right there in the culture dish. Following up on the finding, other researchers sought to pin down the conditions that promoted this behavior. Madri and Williams (1983) showed that collagens from the basement membrane, which sheaths the endothelial cells in a capillary, prompted rapid tube formation. Further work indicated that laminin, a basement membrane protein, stimulates cells to roll up (Kubota et al., 1988) and that the stickiness and strength of the extracellular matrix supporting the cells might also determine whether they proliferate or get tubular (Ingber and Folkman, 1989).