During her research at Washington University in St. Louis, Missouri, she discovered nerve growth factor, the first substance known to regulate the growth of cells. She showed that when tumors from mice were transplanted to chicken embryos they induced rapid growth of the embryonic nervous system. She concluded that the tumor released a nerve growth-promoting factor that affected certain types of cells.

The research increased the understanding of many conditions, including tumors, developmental malformations, and senile dementia. It also led to the discovery by Stanley Cohen of another substance, epidermal growth factor, which stimulates the proliferation of epithelial cells. The two shared the Nobel Prize for medicine in 1986.

Levi-Montalcini returned to Italy to become the director of the laboratory of cell biology of the National Council of Scientific Research in Rome in 1969.

After retiring in the late 1970s, she continued to work as a guest professor and wrote several books to popularize science. She created the Levi-Montalcini Foundation to grant scholarships and promote educational programs worldwide, particularly for women in Africa.

In 2001 Levi-Montalcini was made a senator for life, one of the country’s highest honors.

She then became active in Parliament, especially between 2006 and 2008, when she and other life senators would cast their votes to back the thin majority of center-left Premier Romano Prodi.

Levi-Montalcini had no children and never married, fearing such ties would undercut her independence.

‘‘I never had any hesitation or regrets in this sense,’’ she said in a 2006 interview. ‘‘My life has been enriched by excellent human relations, work and interests. I have never felt lonely.’’

Italian mathematician Piergiorgio Odifreddi said he was always struck by the contrast of this ‘‘petite, frail woman and the power of her mind.’’ He recalled comments that Levi-Montalcini made when she turned 100. She mentioned that she would sleep no more than two or three hours a night because ‘‘I have no time to lose,’’ Odifreddi told Sky TG24.